Monster Media 1994 #2/Monster Media No. 2 (Monster Media)(1994).ISO/freq_qa/ozone05.zip/OZONE05.TXT

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* Copyright 1994 Robert Parson                                        *
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This is the first of four FAQ files dealing with stratospheric ozone
depletion. This part deals with basic scientific questions about the
ozone layer, and serves as an introduction to the remaining parts which
are more specialized. Part II deals with sources of stratospheric
chlorine and bromine, part III with the Antarctic Ozone Hole, and Part
IV with the properties and effects of ultraviolet radiation. The later
parts are mostly independent of each other, but they all refer back.
to Part I.  I emphasize physical and chemical mechanisms
rather than biological effects,  although I make a few remarks about
the latter in part IV. I have little to say about legal and policy
issues other than a very brief summary at the end of part I.

The overall approach I take is conservative. I concentrate on what
is known and on most probable, rather than worst-case, scenarios.
For  example, I have relatively little to say about the effects
of UV radiation on terrestrial plants - this does not mean that the
effects are small, it  means that they are as yet not well
quantified (and moreover, I am not well qualified to interpret the
literature.)  Policy decisions must take into account not only the
most probable scenario, but also a range of less probable ones.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future.

| _Caveat_: I am not a specialist. In fact, I am not an atmospheric
| chemist at all - I am a physical chemist studying gas-phase
| reactions who talks to atmospheric chemists. These files are an
| outgrowth of my own efforts to educate myself about this subject
| I have discussed some of these issues with specialists but I am
| solely responsible for everything written here, including all errors.
| This document should not be cited in publications off the net;
| rather, it should be used as a pointer to the published literature.

*** Corrections and comments are welcomed.

- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry
  University of Colorado (for which I do not speak)

  [e-mail]
  [e-mail]

CONTENTS

1.  THE STRATOSPHERE

 1.1) What is the stratosphere?
 1.2) How is the composition of air described?
 1.3) How does the composition of the atmosphere change with altitude?
     (Or, how can CFC's get up to the stratosphere when they are heavier
      than air?)

2.  THE OZONE LAYER

 2.1) How is ozone created?
 2.2) How much ozone is in the layer, and what is a "Dobson Unit"?
 2.3) How is ozone distributed in the stratosphere?
 2.4) How does the ozone layer work?
 2.5) What sorts of natural variations does the ozone layer show?
 2.6) What are CFC's?  [See Part II for more detail]
 2.7) How do CFC's destroy ozone?
 2.8) What about HCFC's and HFCs? Do they destroy ozone?
 2.9) *IS* the ozone layer getting thinner (outside antarctica)?
 2.10) Is middle-latitude ozone loss due to CFC emissions?
 2.11) If the ozone is lost, won't the UV light just penetrate
       deeper into the atmosphere and make more ozone?
 2.12) Do Space Shuttle launches damage the ozone layer?
 2.13) Will commercial supersonic aircraft damage the ozone layer?
 2.14) What is being done about ozone depletion, and what can we
       expect to see?

3. REFERENCES
_________________________________________________________________


1.  THE STRATOSPHERE

1.1) What is the stratosphere?

The stratosphere extends from about 15 km to 50 km. In the
stratosphere temperature _increases_ with altitude, due to the
absorption of UV light by oxygen and ozone.  This creates a global
"inversion layer"  which impedes vertical motion into and within
the stratosphere - since warmer air lies above colder air, convection
is inhibited.  The word "stratosphere" is related to the word
"stratification" or layering.

The stratosphere is often compared to the "troposphere", which is
the atmosphere below about 15 km.  The boundary - called the
"tropopause" - between these regions is quite sharp, but its
precise location varies between ~10 and ~17 km, depending upon
latitude and season. The prefix "tropo" refers to change:  the
troposphere is the part of the atmosphere in which weather occurs.
This results in relatively rapid mixing of tropospheric air.
[Wayne] [Wallace and Hobbs]

Above the stratosphere lie the "mesosphere", ranging from ~50 to
~100 km, in which temperature decreases with altitude; the
"thermosphere", ~100-400 km, in which temperature increases
with altitude again, and the "exosphere", beyond ~400 km, which
fades into the background of interplanetary space. In the upper
mesosphere and thermosphere electrons and ions are abundant, so
these regions are also referred to as the "ionosphere". In technical
literature the term "lower atmosphere" is synonymous with the
troposphere, "middle atmosphere" refers to the stratosphere
and mesosphere, while "upper atmosphere" is usually reserved for the
thermosphere and exosphere. This usage is not universal, however,
and one occasionally sees the term "upper atmosphere" used to
describe everything above the troposphere (for example, in NASA's
Upper Atmosphere Research Satellite, UARS.)

1.2) How is the composition of air described?
    (What is a 'mixing ratio'?)

The density of the air in the atmosphere depends upon altitude, and
in a complicated way because the temperature also varies with
altitude.  It is therefore awkward to report concentrations of
atmospheric species in units like g/cc or molecules/cc.  Instead,
it is convenient to report the "mole fraction", the relative
number of molecules of a given type in an air sample. Atmospheric
scientists usually call a mole fraction a "mixing ratio". Typical
units for mixing ratios are parts-per-million, billion, or
trillion by volume, designated as "ppmv", "ppbv", and "pptv"
respectively. (The expression "by volume" reflects Avogadro's Law -
for an ideal gas mixture, equal volumes contain equal numbers of
molecules - and serves to distinguish mixing ratios from "mass
fractions" which are given as parts-per-million by weight.)  Thus
when it is said that the mixing ratio of hydrogen chloride at 3 km
is 0.1 ppbv, it means that 1 out of every 10 billion molecules in
an air sample collected at that altitude will be an HCl molecule.
[Wayne] [Graedel and Crutzen]

1.3) How does the composition of the atmosphere change with altitude?
     (Or, how can CFC's get up to the stratosphere when they are
        heavier than air?)

In the earth's troposphere and stratosphere, most _stable_ chemical
species are "well-mixed" - their mixing ratios are independent of
altitude.  If a species' mixing ratio changes with altitude, some
kind of physical or chemical transformation is taking place. That
last statement may seem surprising - one might expect the heavier
molecules to dominate at lower altitudes.  The mixing ratio of
Krypton (mass 84), then, would decrease with altitude, while that
of Helium (mass 4) would increase.  In reality, however, molecules
do not segregate by weight in the troposphere or stratosphere.
The relative proportions of Helium, Nitrogen, and Krypton are
unchanged up to about 100 km.

Why is this? Vertical transport in the troposphere takes place by
convection and turbulent mixing.  In the stratosphere and in the
mesosphere, it takes place by "eddy diffusion" - the gradual mechanical
mixing of gas by motions on small scales. These mechanisms do not
distinguish molecular masses.  Only at much higher altitudes do mean
free paths become so large that _molecular_ diffusion dominates and
gravity is able to separate the different species, bringing hydrogen
and helium atoms to the top.
[Chamberlain and Hunten] [Wayne] [Wallace and Hobbs]

Experimental measurements of the fluorocarbon CF4 verify this
homogeneous mixing.  CF4 has an extremely long lifetime in the
stratosphere - probably many thousands of years.  The mixing ratio
of CF4 in the stratosphere  was found to be 0.056-0.060 ppbv
from 10-50 km, with no overall trend.  [Zander et al. 1992]

An important trace gas that is *not* well-mixed is water vapor. The
lower troposphere contains a great deal of water - as much as 30,000
ppmv in humid tropical latitudes. High in the troposphere, however,
the water condenses and falls to the earth as rain or snow, so that
the stratosphere is extremely dry, typical mixing ratios being about
4 ppmv. Indeed, the transport of water vapor from troposphere to
stratosphere is even more inefficient than this would suggest, since
much of the small amount of water in the stratosphere is actually
produced _in situ_ by the oxidation of methane.

Sometimes that part of the atmosphere in which the chemical
composition of stable species does not change with altitude is
called the "homosphere". The homosphere includes the troposphere,
stratosphere, and mesosphere. The upper regions of the atmosphere
- the "thermosphere" and the "exosphere" - are then referred to as
the "heterosphere". [Wayne] [Wallace and Hobbs]


2. THE OZONE LAYER

2.1) How is ozone created?

Ozone is formed naturally in the upper stratosphere by short
wavelength ultraviolet radiation. Wavelengths less than ~240
nanometers are absorbed by oxygen molecules (O2), which dissociate to
give O atoms. The O atoms combine with other oxygen molecules to
make ozone:

 O2 + hv -> O + O  (wavelength < 240 nm)
 O + O2 -> O3

2.2) How much ozone is in the layer, and what is a "Dobson Unit" ?

A Dobson Unit (DU) is a convenient scale for measuring the total
amount of ozone occupying a column overhead.  If the ozone layer
over the US were compressed to 0 degrees Celsius and 1 atmosphere
pressure, it would be about 3 mm thick. So, 0.01 mm thickness at
0 C and 1 at is defined to be 1 DU; this makes the ozone layer over
the US come out to ~300 DU. In absolute terms, 1 DU is about
2.7 x 10^16 molecules/cm^2.

In all, there are about 3 billion metric tons, or 3x10^15 grams,
of ozone in the earth's atmosphere; about 90% of this is in the
stratosphere.

The unit is named after G.M.B. Dobson, who carried out pioneering
studies of atmospheric ozone between ~1920-1960. Dobson designed
the standard instrument used to measure ozone from the ground. The
Dobson spectrometer measures the intensity solar UV radiation at
four wavelengths, two of which are absorbed by ozone and two of
which are not. These instruments are still in use in many places,
although they are gradually being replaced by the more elaborate
Brewer spectrometers. Today ozone is measured in many ways, from
aircraft, balloons, satellites, and space shuttle missions, but the
worldwide Dobson network is the only source of long-term data. A
station at Arosa in Switzerland has been measuring ozone since the
1920's, and some other stations have records that go back nearly as
long (although many were interrupted during World War II). The
present worldwide network went into operation in 1956-57.

2.3) How is ozone distributed in the stratosphere?

In absolute terms: about 10^12 molecules/cm^3 at 15 km, rising to
nearly 10^13 at 25 km, then falling to 10^11 at 45 km.

In relative terms: ~0.5 parts per million by volume (ppmv) at 15 km,
rising to ~8 ppmv at ~35 km, falling to ~3 ppmv at 45 km.

Even in the thickest part of the layer, ozone is a trace gas.

2.4) How does the ozone layer work?

UV light with wavelengths between 240 and 320 nm is absorbed by
ozone, which then falls apart to give an O atom and an O2 molecule.
The O atom soon encounters another O2 molecule, however (at all times,
the concentration of O2 far exceeds that of O3), and recreates O3:

 O3 + hv -> O2 + O
 O + O2 -> O3

Thus _ozone absorbs UV radiation without itself being consumed_;
the net result is to convert UV light into heat. Indeed, this is
what causes the temperature of the  stratosphere to increase with
altitude, giving rise to the inversion layer that traps molecules in
the troposphere. The ozone layer isn't just _in_ the stratosphere; the
ozone layer is responsible for the _existence_ of the stratosphere.

Ozone _is_ destroyed if an O atom and an O3 molecule meet:

 O + O3 -> 2 O2  ("recombination").

This reaction is slow, however, and if it were the only mechanism
for ozone loss, the ozone layer would be about twice as thick
as it is. Certain trace species, such as the oxides of Nitrogen (NO
and NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2)
can catalyze the recombination.  The present ozone layer is a
result of a competition between photolysis and recombination;
increasing the recombination rate, by increasing the
concentration of catalysts, results in a thinner ozone layer.

Putting the pieces together, we have the set of reactions proposed
in the 1930's by Sidney Chapman:

 O2 + hv -> O + O  (wavelength < 240 nm)  : creation of oxygen atoms
 O + O2 -> O3                             : formation of ozone
 O3 + hv -> O2 + O (wavelength < 320 nm)  : absorption of UV by ozone
 O + O3 -> 2 O2                           : recombination .

Since the photolysis of O2 requires UV radiation while
recombination does not, one might guess that ozone should increase
during the day and decrease at night. This has led some people to
suggest that the "antarctic ozone hole" is merely a result of the
long antarctic winter nights. This inference is incorrect, because
the recombination reaction requires oxygen atoms which are also
produced by photolysis. Throughout the stratosphere the concentration
of O atoms is orders of magnitude smaller than the concentration of
O3 molecules, so both the production and the destruction of ozone by
the above mechanisms shut down at night. In fact, the thickness of the
ozone layer varies very little from day to night, and above 70 km
ozone concentrations actually _increase_ at night.

(The unusual catalytic cycles that operate in the antarctic ozone
hole do not require O atoms; however, they still require light to
operate because they also include photolytic steps. See Part III.)

2.5) What sorts of natural variations does the ozone layer show?

There are substantial variations from place to place, and from
season to season. There are smaller variations on time scales of
years and more. [Wayne] [Rowland 1991]

 a. Regional and Seasonal Variation

Since solar radiation makes ozone, one expects to see the
thickness of the ozone layer vary during the year. This is so,
although the details do not depend simply upon the amount of solar
radiation received at a given latitude and season - one must also
take atmospheric motions into account. (Remember that
both production and destruction of ozone require solar radiation.)

The ozone layer is thinnest in the tropics, about 260 DU, almost
independent of season. Away from the tropics seasonal variations
become important, but in no case (outside the Antarctic ozone hole)
does the layer become appreciably thinner than in the tropics. For
example:

    Location                          Column thickness, Dobson Units

                                        Jan     Apr     Jul     Oct

 Huancayo, Peru (12 degrees S) :        255     255     260     260
 Aspendale, Australia (38 deg. S):      300     280     335     360
 Arosa, Switzerland  (47 deg. N):       335     375     320     280
 St. Petersburg, Russia (60 deg. N):    360     425     345     300

These are monthly averages. Interannual standard deviations amount
to ~5 DU for Huancayo, 25 DU for St. Petersburg. [Rowland 1991].
Notice that the highest ozone levels are found in the _spring_,
not, as one might guess, in summer, and the lowest in the fall,
not winter. Indeed, at high latitudes in the Northern Hemisphere
there is more ozone in January than in July! Most of the ozone is
created over the tropics, and then is carried to higher latitudes
by prevailing winds (the general circulation of the stratosphere.)
[Dobson] [Garcia] [Salby and Garcia] [Brasseur and Solomon]

The antarctic ozone hole, discussed in detail in Part III, falls
*far outside* this range of natural variation. Mean October ozone
at Halley Bay on the Antarctic coast was 117 DU in 1993, down
from 321 DU in 1956.

b. Year-to-year variations.

Since ozone is created by solar UV radiation, one expects to see
some correlation with the 11-year solar sunspot cycle. Higher
sunspot activity corresponds to more solar UV and hence more rapid
ozone production. This correlation has been verified, although
its effect is small, about 2% from peak to trough averaged over the
earth, about 4% in polar regions. [Stolarski et al.]

Another natural cycle is connected with the "quasibiennial
oscillation", in which tropical winds in the lower stratosphere
switch from easterly to westerly every 26 months. This leads to
variations of the order of 3% at a given latitude, although the
effect tends to cancel when one averages over the entire globe.

Episodes of unusual solar activity ("solar proton events") can
also affect ozone levels, as can major volcanic eruptions such as
Agung in 1963, El Chichon in 1982, and Pinatubo in 1991. (The
principal mechanism for this is _not_ injection of chlorine into
the stratosphere, as discussed in  Part II, but rather the
injection of sulfate aerosols which change the radiation balance
in the stratosphere by scattering light, and which convert
inactive chlorine compounds to active, ozone-destroying forms.)
These are all small effects, however, (a few % at most in a global
average), and persist for short periods, 3 years or less.

2.6) What are CFC's?

CFC's - ChloroFluoroCarbons - are a class of volatile organic compounds
that have been used as refrigerants, aerosol propellants, foam blowing
agents, and as solvents in the electronic industry. They are chemically
very unreactive, and hence safe to work with. In fact, they are so inert
that the natural reagents that remove most atmospheric pollutants do not
react with them, so after many years they drift up to the stratosphere
where short-wave UV light dissociates them. CFC's were invented in 1928,
but only came into large-scale production after ~1950. Since that year,
the total amount of chlorine in the stratosphere has increased by
a factor of 4. [Solomon]

The most important CFC's for ozone depletion are:

  CF2Cl2 (CFC-12),
  CFCl3 (CFC-11), and
  CF2ClCFCl2 (CFC-113).

In discussing ozone depletion, "CFC" is occasionally used to
refer to a somewhat broader class of chlorine-containing organic
compounds that have similar properties - unreactive in the
troposphere, but readily  photolyzed in the stratosphere. These
include:

 HydroChloroFluoroCarbons such as CHClF2 (HCFC-22),
 Carbon Tetrachloride, CCl4,
 Methyl Chloroform, CH3CCl3,
 and Methyl Chloride, CH3Cl.

(The more careful publications always use phrases like "CFC's and
related compounds", but this gets tedious.)

Only methyl chloride has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning.
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years.  Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a large fraction to reach the stratosphere.

Most of Part II is devoted to stratospheric chlorine chemistry;
look there for more detail.

2.7) How do CFC's destroy ozone?

CFC's themselves do not destroy ozone; certain of their decay products
do.  After CFC's are photolyzed, most of the chlorine eventually ends
up as Hydrogen Chloride, HCl, or Chlorine Nitrate, ClONO2. These are
called "reservoir species" - they do not themselves react with ozone.
However, they do decompose to some extent, giving, among other things,
a small amount of atomic chlorine, Cl, and Chlorine Monoxide, ClO,
which can catalyze the destruction of ozone by a number of mechanisms.
The simplest is:

  Cl + O3 -> ClO + O2

  ClO + O -> Cl + O2

 Net effect: O3 + O -> 2 O2

Note that the Cl atom is a _catalyst_ - it is not consumed by the
reaction. Each Cl atom introduced into the stratosphere can
destroy thousands of ozone molecules before it is removed.
The process is even more dramatic for Bromine - it has no stable
"reservoirs", so the Br atom is always available to destroy ozone.
On a per-atom basis, Br is 10-100 times as destructive as Cl.
On the other hand, chlorine and bromine concentrations in
the stratosphere are very small in absolute terms. The mixing ratio
of chlorine from all sources in the stratosphere is about 3 parts
per billion, (most of which is in the form of CFC's that have not
yet fully decomposed) whereas ozone mixing ratios are measured in
parts per million.  Bromine concentrations are about 100 times
smaller still. (See Part II.)

The complete chemistry is very complicated - more than 100
distinct species are involved. The rate of ozone destruction at any
given time and place depends strongly upon how much Cl is present
as Cl or ClO, and thus upon the rate at which Cl is released from
its reservoirs.  This makes quantitative _predictions_ of future
ozone depletion difficult. [Rowland 1989, 1991] [Wayne]

2.8) What about HCFC's and HFC's? Do they destroy ozone?

HCFC's (hydrochlorofluorocarbons) differ from CFC's in that only
some, rather than all, of the hydrogen in the parent hydrocarbon
has been replaced by chlorine or fluorine. The most familiar
example is CHClF2, known as "HCFC-22", used as a refrigerant and
in many home air conditioners (auto air conditioners use CFC-12).
The hydrogen atom makes the molecule susceptible to attack by the
hydroxyl (OH) radical, so a large fraction of the HCFC's are
destroyed before they reach the stratosphere. Molecule for
molecule, then, HCFC's destroy much less ozone than CFC's, and
they were suggested as CFC substitutes as long ago as 1976.

The impact of a compound on stratospheric ozone is usually
measured by its "ozone depletion potential", defined as the
steady-state limit of the amount of ozone destroyed by the
halocarbon, relative to the amount destroyed by CFC-12. HCFC's
generally have ozone depletion potentials around 0.01-0.1, so that
during its lifetime a typical HCFC will have destroyed 1-10% as
much ozone as the same amount of CFC-12. This measure can sometimes
be misleading, however. Since the HCFC's are more reactive in the
troposphere, fewer of them reach the stratosphere. However, they are
also more reactive in the stratosphere, so they release chlorine
more quickly. Just as short-lived radioisotopes are more intensely
radioactive than long-lived ones, those HCFC's that do reach the
stratosphere deplete ozone more quickly than CFC's. The short-term
effects are therefore larger than one would predict from the ozone
depletion potential alone, and the long-term effects correspondingly
smaller. This must be taken into account when substituting HCFC's
for CFC's. [Solomon and Albritton]

HFC's, hydrofluorocarbons, contain no chlorine at all, and hence
have an ozone depletion potential of zero. (In 1993 there were
tentative reports that the fluorocarbon radicals produced by
photolysis of HFC's could catalyze ozone loss, but this has now
been shown to be negligible [Ravishankara et al.]) A familiar
example is CF3CH2F, known as HFC-134a, which is being used in some
automobile air conditioners and refrigerators. HFC-134a is more
expensive and more difficult to work with than CFC's, and while it
has no effect on stratospheric ozone it is a greenhouse gas (though
somewhat less potent than the CFC's). Some engineers have argued
that non-CFC fluids, such as propane-isobutane mixtures, are better
substitutes for CFC-12 in auto air conditioners than HFC-134a.

2.9) *IS* the ozone layer getting thinner (outside antarctica) ?

So it seems, although so far the effects are small. After
carefully accounting for all of the known natural variations, a
net decrease of about 3% per decade for the period 1978-1991
remains.  This is a global average over latitudes from 66 degrees
S to 66 degrees N (i.e. the arctic and antarctic are excluded in
calculating the average). The depletion increases with latitude,
being somewhat larger in the Southern Hemisphere. There is no
significant depletion in the tropics; over the US, Europe, and
Australia 4%/decade is typical. The depletion is larger in the
winter months, smaller  in the summer. [Stolarski et al.]

The following table, extracted from a much more detailed one in
[Herman et al.], illustrates the seasonal and regional trends in
_percent per decade_ for the period 1979-1990:

 Latitude      Jan     Apr     Jul     Oct      Example

  65 N        -3.0    -6.6    -3.8    -5.6      Iceland
  55 N        -4.6    -6.7    -3.1    -4.4      Moscow, Russia
  45 N        -7.0    -6.8    -2.4    -3.1      Minneapolis, USA
  35 N        -7.3    -4.7    -1.9    -1.6      Tokyo
  25 N        -4.2    -2.9    -1.0    -0.8      Miami, FL, USA
   5 N        -0.1    +1.0    -0.1    +1.3      Somalia

   5 S        +0.2    +1.0    -0.2    +1.3      New Guinea
  25 S        -2.1    -1.6    -1.6    -1.1      Pretoria, S. Africa
  35 S        -3.6    -3.2    -4.5    -2.6      Buenos Aires
  45 S        -4.8    -4.2    -7.7    -4.4      New Zealand
  55 S        -6.1    -5.6    -9.8    -9.7      Tierra del Fuego
  65 S        -6.0    -8.6   -13.1   -19.5      Palmer Peninsula

(These are longitudinally averaged satellite data, not individual
measurements at the places listed in the right-hand column. There
are longitudinal trends as well.)

After 1991 these trends accelerated. Satellite and
ground-based measurements show a remarkable decline for 1992
and early 1993, a full 4% below the average value for the
preceding twelve years and 2-3% below the _lowest_ values observed
in the earlier period. In Canada the spring ozone levels were 11-17%
below normal [Kerr et al.]. This decline overwhelms the
effect of the solar cycle; 1991 was a solar maximum, while the
1992 results are already below those for the 1986 solar minimum.
Sulfate aerosols from the July 1991 eruption of Mt. Pinatubo may
be the cause of this latest spike; these aerosols can convert
inactive "reservoir" chlorine into active ozone-destroying forms,
and can also interfere with the production and transport of ozone
by changing the solar radiation balance in the stratosphere.
[Brasseur and Granier] [Hofmann and Solomon] [Hofmann et al. 1994]
Another cause may be the unusually strong arctic polar vortex in
1992-93, which made the arctic stratosphere more like the antarctic
than is usually the case. [Gleason et al.] [Waters et al.]
Most likely all of these mechanisms are working in concert.
In any event, the rapid ozone loss after 1991 appears to have been
a transient phenomenon, superimposed upon the much slower downward
trend identified for 1970-1991.

2.10) Is the middle-latitude ozone loss due to CFC emissions?

That's the majority opinion, although not everyone agrees. The
present trends are too small to allow a watertight case to be made
(as _has_ been made for the far larger, but localized, depletion
in the Antarctic Ozone hole; see Part III.). Other possible causes
are being investigated. To quote from [WMO 1991], p. 4.1:

 "The primary cause of the _Antarctic ozone hole_ is firmly
established to be halogen chemistry....There is not a full
accounting of the observed downward trend in _global ozone_.
Plausible mechanisms include heterogeneous chemistry on sulfate
aerosols [which convert reservoir chlorine to active chlorine -
R.P.] and the transport of chemically perturbed polar air to middle
latitudes.  Although other mechanisms cannot be ruled out, those
involving the catalytic destruction of ozone by chlorine and
bromine appear to be largely responsible for the ozone loss and
_are the only ones for which direct evidence exists_."

 (emphases mine - RP)

 The recent UARS measurements of ozone and ClO in the Northern
Hemisphere find a correlation between enhanced ClO and depleted
ozone, which further supports this hypothesis. [Waters et al.]
A legal analogy might be useful here - the connection between
_antarctic_ ozone depletion and CFC emissions has been proved beyond
a reasonable doubt, while at _middle latitudes_ there is only
probable cause for such a connection.

One must remember that there is a natural 10-20 year time lag
between CFC emissions and ozone depletion. Ozone depletion today is
(probably) due to CFC emissions in the '60's and '70's. Present
controls on CFC emissions are designed to avoid possibly large
amounts of ozone depletion 30 years from now, not to remediate the
depletion that has taken place up to now.

2.11) If the ozone is lost, won't the UV light just penetrate
      deeper into the atmosphere and make more ozone?

 This does happen to some extent - it's called "self-healing" - and
has the effect of moving ozone from the upper to the lower
stratosphere. Recall that ozone is _created_ by UV with wavelengths
less than 240 nm, but functions by _absorbing_ UV with wavelengths
greater than 240 nm. The peak of the ozone absorption band is at
~250 nm, and the cross-section falls off at shorter wavelengths.
The O2 and O3 absorption bands do overlap, though, and UV radiation
between 200 and 240 nm has a good chance of being absorbed by
_either_ O2 or O3. (Below 200 nm the O2 absorption cross-section
increases dramatically, and O3 absorption is insignificant in
comparison.) Since there is some overlap, a decrease in ozone does
lead to a small increase in absorption by O2. This is a weak feedback,
however, and it does not compensate for the ozone destroyed. Negative
feedback need not imply stability, just as positive feedback need not
imply instability.

Numerical calculations of ozone depletion take the "self-healing"
phenomenon into account, by letting the perturbed ozone layer come
into equilibrium with the exciting radiation.

2.12) Do Space Shuttle launches damage the ozone layer?

No. In the early 1970's, when very little was known about the role
of chlorine radicals in ozone depletion, it was suggested that HCl
from solid-fueled rocket motors might have a significant effect upon
the ozone layer - if not globally, perhaps in the immediate vicinity
of the launch. It was quickly shown that the effect was negligible,
and this has been repeatedly demonstrated since. Each shuttle
launch produces about 68 metric tons of chlorine as HCl; a full
year's worth of shuttle and solid-fueled rocket launches produces
about 725 tons. This is negligible compared to chlorine emissions in
the form of CFC's and related compounds (1.2 million tons/yr in
the 1980's, of which ~300,000 tons reach the stratosphere each
year. It is also negligible in comparison to natural sources, which
produce about 75,000 tons per year. [Prather et al.] [WMO 1991].

See also the sci.space FAQ, Part 10, "Controversial Questions",
available by anonymous ftp from rtfm.mit.edu in the directory
pub/usenet/news.answers/space/controversy.

2.13) Will commercial supersonic aircraft damage the ozone layer?

Short answer:  Probably not. This problem is very complicated,
and a definite answer will not be available for several years,
but present model calculations indicate that a fleet of high-speed
civil transports would deplete the ozone layer by < 1%. [WMO 1991]

Long answer (this is a tough one):

Supersonic aircraft fly in the stratosphere. Since vertical transport
in the stratosphere is slow, the exhaust gases from a supersonic jet
can stay there for two years or more. The most important exhaust gases
are the nitrogen oxides, NO and NO2, collectively referred to as "NOx".
NOx is produced from ordinary nitrogen and oxygen by electrical
discharges (e.g. lightning) and by high-temperature combustion (e.g. in
automobile and aircraft engines).

The relationship between NOx and ozone is complicated. In the
troposphere, NOx _makes_ ozone, a phenomenon well known to residents
of Los Angeles and other cities beset by photochemical smog. At high
altitudes in the troposphere, similar chemical reactions produce ozone
as a byproduct of the oxidation of methane; for this reason ordinary
subsonic aircraft actually increase the thickness of the ozone layer
by a very small amount.

Things are very different in the stratosphere. Here the principal
source of NOx is nitrous oxide, N2O ("laughing gas"). Most of the
N2O in the atmosphere comes from bacteriological decomposition of
organic matter - reduction of nitrate ions or oxidation of ammonium
ions. (It was once assumed that anthropogenic sources were negligible
in comparison, but this is now known to be false. The total
anthropogenic contribution is now estimated at 8 Tg (teragrams)/yr,
compared to a natural source of 18 Tg/yr. [Khalil and Rasmussen].)
N2O, unlike NOx, is very unreactive - it has an atmospheric lifetime
of more than 150 years - so it reaches the stratosphere, where most of
it is converted to nitrogen and oxygen by UV photolysis.  However, a
small fraction of the N2O that reaches the stratosphere reacts instead
with oxygen atoms (to be precise, with the very rare electronically
excited singlet-D oxygen atoms), and this is the major natural source
of NOx in the stratosphere. About 1.2 million tons are produced each
year in this way.  This source strength would be matched by 500 of the
SST's designed by Boeing in the late 1960's, each spending 5 hours per
day in the stratosphere. (Boeing was intending to sell 800 of these
aircraft.) The Concorde, a slower plane, produces less than half as
much NOx and flies at a lower altitude; since the Concorde fleet is
small, its contribution to stratospheric NOx is not significant. Before
sending large fleets of high-speed aircraft into the stratosphere,
however, one should certainly consider the possible effects of
increasing the rate of production of an important stratospheric trace
gas by as much as a factor of two. [CIC 1975]

In 1969, Paul Crutzen discovered that NOx could be an efficient
catalyst for the destruction of stratospheric ozone:

       NO + O3  -> NO2 + O2
       NO2 + O -> NO + O2
-------------------------------
 net:  O3 + O -> 2 O2

This sequence was rediscovered two years later by H. S. Johnston, who
made the connection to SST emissions.  Until then it had been thought
that the radicals H, OH, and HO2 (referred to collectively as "HOx")
were the principal catalysts for ozone loss; thus, investigations of
the impact of aircraft exhaust on stratospheric ozone had focussed on
emissions of water vapor, a possible source for these radicals. (The
importance of chlorine radicals, Cl, ClO, and ClO2, referred to as -
you guessed it - "ClOx", was not discovered until 1973.) It had been
argued - correctly, as it turns out - that water vapor injection was
unimportant for determining the ozone balance.  The discovery of
the NOx cycle threw the question open again.

Beginning in 1972, the U.S. National Academies of Science and
Engineering and the Department of Transportation sponsored an
intensive program of stratospheric research. [CIC 1975] It soon
became clear that the relationship between NOx emissions and the
ozone layer was very complicated.  The stratospheric lifetime of
NOx is comparable to the timescale for transport from North to
South, so its concentration depends strongly upon latitude.  Much
of the NOx is injected near the tropopause, a region where
quantitative modelling is very difficult, and the results of
calculations depend sensitively upon how troposphere-stratosphere
exchange is treated. Stratospheric NOx chemistry is _extremely_
complicated, much worse than chlorine chemistry. Among other
things, NO2 reacts rapidly with ClO, forming the inactive chlorine
reservoir ClONO2 - so while on the one hand increasing NOx leads
directly to ozone loss, on the other it suppresses the action
of the more potent chlorine catalyst. And on top of all of this, the
SST's always spend part of their time in the troposphere, where NOx
emissions cause ozone increases. Estimates of long-term ozone
changes due to large-scale NOx emissions varied markedly from year
to year, going from -10% in 1974, to +2% (i.e. a net ozone _gain_)
in 1979, to -8% in 1982. (In contrast, while the estimates of the
effects of CFC emissions on ozone also varied a great deal in these
early years, they always gave a net loss of ozone.) [Wayne]

The discovery of the Antarctic ozone hole added a new piece to the
puzzle. As described in Part III, the ozone hole is caused by
heterogeneous chemistry on the surfaces of stratospheric cloud
particles. While these clouds are only found in polar regions,
similar chemical reactions take place on sulfate aerosols which are
found throughout the lower stratosphere. The most important of the
aerosol reactions is the conversion of N2O5 to nitric acid:

 N2O5 + H2O -> 2 HNO3 (catalyzed by aerosol surfaces)

N2O5 is in equilibrium with NOx, so removal of N2O5 by this
reaction lowers the NOx concentration.  The result is that in the
lower stratosphere the NOx catalytic cycle contributes much less to
overall ozone loss than the HOx and ClOx cycles. Ironically, the
same processes that makes chlorine-catalyzed ozone depletion so
much more important than was believed 10 years ago, also make
NOx-catalyzed ozone loss less important.

In the meantime, there has been a great deal of progress in
developing jet engines that will produce much less NOx - up to a
factor of 10 - than the old Boeing SST. The most recent model
calculations indicate that a fleet of the new "high-speed civil
transports" would deplete the ozone layer by less than 1%. Caution
is still required, since the experiment has not been done - we have
not yet tried adding large amounts of NOx to the stratosphere. The
forecasts, however, are good. [WMO 1991, Ch. 10]

..................................................................
_Aside_: One sometimes hears that the US government killed the SST
project in 1971 because of concerns raised by H. S. Johnston's work
on NOx. This is not true. The US House of Representatives had already
voted to cut off Federal funding for the SST when Johnston began
his calculations. The House debate had centered around economics and
the effects of noise, especially sonic booms, although there were
some vague remarks about "pollution" and one physicist had testified
about the possible effects of water vapor on ozone. About 6 weeks
after both houses had voted to cancel the SST, its supporters
succeeded in reviving the project in the House. In the meantime,
Johnston had sent a preliminary report to several professional
colleagues and submitted a paper to _Science_. A preprint of
Johnston's report leaked to a small California newspaper which
published a highly sensationalized account. The story hit the press
a few days before the Senate voted, 58-37, not to revive the SST.
(The previous Senate vote had been 51-46 to cancel the project. The
reason for the larger majority in the second vote was probably the
statement by Boeing's chairman that at least $500 million more would
be needed to revive the program.)
....................................................................


2.14) What is being done about ozone depletion?

The 1987 Montreal Protocol specified that CFC emissions should be
reduced by 50% by the year 2000 (they had been _increasing_ by 3%
per year.)  This agreement was amended in London in 1990, to state
that production of CFC's, CCl4, and halons should cease entirely by
the year 2000.  Restrictions have also been applied to other Cl
sources such as methylchloroform. (The details of the protocols are
complicated, involving different schedules for different compounds,
delays for developing nations, etc. See the book by [Benedick].)
The phase-out schedule was accelerated by four years by the 1992
Copenhagen agreements. A great deal of effort has been devoted to
recovering and recycling CFC's that are currently being used in
closed-cycle systems.

Recent NOAA measurements [Elkins et al.] show that the _rate of
increase_ of halocarbon concentrations in the atmosphere has decreased
markedly since 1987, by a factor of 4 for CFC-11 and a factor of 2
for CFC-12. It appears that the Protocols are being observed. Under
these conditions total stratospheric chlorine is predicted to peak
in the first decade of the 21st century, and to slowly decline
thereafter.

Model calculations predict that ozone levels, averaged over the
year and over the Northern hemisphere, will fall to about 4% below
1980 levels in the first decade of the 21st century if the
protocols are obeyed. Very little depletion is expected in the
tropics, so correspondingly larger losses - more than 6% - are
expected at middle and high latitudes. These same models have
systematically _underestimated_ ozone depletion in the past, so
significantly larger losses are expected. In fact, 4% global
year-averaged ozone depletion was _already_ measured in 1993
[Gleason et al.] although this is in part a transient caused by
Mt. Pinatubo's eruption in July 1991. After 2010 the ozone layer
will slowly recover over a period of 20 years or so, although the
form of the recovery is strongly model-dependent. [WMO 1991]
I have no results at hand for the southern hemisphere; if current
trends continue ozone depletion will be more serious there. The
antarctic ozone hole is expected to last until 2050 or so. This
does not take into account the possibility of global warming,
which by cooling the stratosphere could make ozone depletion more
serious both at mid latitudes and in polar regions.

Some scientists are investigating ways to replenish stratospheric
ozone, either by removing CFC's from the troposphere or by tying up
the chlorine in inactive compounds. This is discussed in Part III.

___________________________________________________________________

3.  REFERENCES FOR PART I

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. Where possible I have limited myself to papers that
are (1) available outside of University libraries (e.g. _Science_
or _Nature_ rather than archival journals such as _J. Geophys. Res._)
and (2) directly related to the "frequently asked questions".
I have not listed papers whose importance is primarily historical.
Readers who want to see "who did what" should consult the review
articles listed below, or, if they can get them, the WMO reports
which are extensively documented.


Introductory Reading:

[Garcia] R. R. Garcia, _Causes of Ozone Depletion_, _Physics World_
April 1994 pp 49-55.

[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY 1993.

[Rowland 1989] F.S. Rowland, "Chlorofluorocarbons and the depletion
of stratospheric ozone", _American Scientist_ _77_, 36, 1989.

[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993,  pp. 9-18.

----------------------------
Books and Review Articles:

[Benedick] R. Benedick, _Ozone Diplomacy_, Harvard, 1991.

[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of
the Middle Atmosphere_, 2nd. Edition, D. Reidel, 1986

[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,
_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987

[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
 Oxford, 1968.

[CIC 1975] Climate Impact Committee, National Research Council,
_Environmental Impact of Stratospheric Flight_,
 National Academy of Sciences, 1975.

[Johnston 1992] H. S. Johnston, "Atmospheric Ozone",
_Annu. Rev. Phys. Chem._ _43_, 1, 1992.

[McElroy and Salawich] M. McElroy and R. Salawich,
"Changing Composition of the Global Stratosphere",
_Science_ _243, 763, 1989.

[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.

[Salby and Garcia] M. L. Salby and R. R. Garcia, "Dynamical Perturbations
 to the Ozone Layer", _Physics Today_ _43_, 38, March 1990.

[Solomon] S. Solomon, "Progress towards a quantitative understanding
 of Antarctic ozone depletion", _Nature_ _347_, 347, 1990.

[Wallace and Hobbs] J. M. Wallace and P. V. Hobbs,
_Atmospheric Science: an Introductory Survey_, Academic Press, 1977.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_,
        2nd.  Ed., Oxford, 1991.

[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_,
 Global Ozone Research and Monitoring Project - Report #18.

[WMO 1989] World Meteorological Organization,
_Scientific Assessment of Stratospheric Ozone: 1991_
 Global Ozone Research and Monitoring Project - Report #20.

[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_
 Global Ozone Research and Monitoring Project - Report #25.

-----------------------------------
More Specialized:

[Brasseur and Granier] G. Brasseur and C. Granier, "Mt. Pinatubo
 aerosols, chlorofluorocarbons, and ozone depletion", _Science_
 _257_, 1239,1992.

[Elkins et al.] J. W. Elkins, T. M. Thompson, T. H. Swanson,
J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and
A. G. Raffo, "Decrease in Growth Rates of Atmospheric
Chlorofluorocarbons 11 and 12", _Nature_ _364_, 780, 1993.

[Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P.
Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C.
Wellemeyer, W. Komhyr, A. Miller, and W. Planet, "Record Low Global
Ozone in 1992", _Science_ _260_, 523, 1993.

[Herman et al.] J. R. Herman, R. McPeters, and D. Larko,
 "Ozone depletion at northern and southern latitudes derived
 from January 1979 to December 1991 TOMS data",
 J. Geophys. Res. _98_, 12783, 1993.

[Hofmann and Solomon] D. J. Hofmann and S. Solomon, "Ozone
destruction through heterogeneous chemistry following the
eruption of El Chichon", J. Geophys. Res. _94_, 5029, 1989.

[Hofmann et al. 1994] D. J. Hofmann, S. J. Oltmans, W. D. Komhyr,
J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler,
B. J. Johnson, A. Torres, and W. A. Matthews,
"Ozone Loss in the lower stratosphere over the United States in
1992-1993: Evidence for heterogeneous chemistry on the Pinatubo
aerosol", Geophys. Res. Lett. _21_, 65, 1994.

[Kerr et al.] J. B. Kerr, D. I. Wardle, and P. W. Towsick,
"Record low ozone values over Canada in early 1993",
Geophys. Res. Lett. _20_, 1979, 1993.

[Khalil and Rasmussen] M.A.K. Khalil and R. Rasmussen, "The Global
 Sources of Nitrous Oxide", _J. Geophys. Res._ _97_, 14651, 1992.

[Prather et al. ] M. J. Prather, M.M. Garcia, A.R. Douglass, C.H.
Jackman, M.K.W. Ko, and N.D. Sze, "The Space Shuttle's impact on
the stratosphere", J. Geophys. Res. _95_, 18583, 1990.

[Ravishankara et al.] A. R. Ravishankara, A. A. Turnipseed,
N. R. Jensen, S. Barone, M. Mills, C. J. Howard, and S. Solomon,
"Do Hydrofluorocarbons Destroy Stratospheric Ozone?",
  _Science_ _263_, 71, 1994.

[Solomon and Albritton] S. Solomon and D.L. Albritton,
"Time-dependent ozone depletion potentials for short- and long-term
forecasts", _Nature_ _357_, 33, 1992.

[Stolarski et al.]  R. Stolarski, R. Bojkov, L. Bishop, C. Zerefos,
 J. Staehelin, and J. Zawodny, "Measured Trends in Stratospheric
Ozone",  Science _256_, 342 (17 April 1992)

[Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L.
Elson, D. Flower, R. Jarnot, and R. Harwood, "Stratospheric ClO and
ozone from the Microwave Limb Sounder on the Upper Atmosphere
Research Satellite", _Nature_ _362_, 597, 1993.

[Zander et al. 1992] R. Zander, M. R. Gunson, C. B. Farmer, C. P.
Rinsland, F. W. Irion, and E. Mahieu, "The 1985 chlorine and
fluorine inventories in the stratosphere based on ATMOS
observations at 30 degrees North latitude",  J. Atmos. Chem. _15_,
171, 1992.




-------------------------------------------------------------------------------
Area # 2120  news.answers           05-26-94 15:52      Message # 13061
From    : [e-mail]
To      : ALL
Subj    : Ozone Depletion FAQ Part

 @SUBJECT:Ozone Depletion FAQ Part II: Stratospheric Chlorine and Brom
Message-ID: <[e-mail]>
Newsgroups: sci.environment,sci.answers,news.answers
Organization: University of Colorado, Boulder

Archive-name: ozone-depletion/stratcl
Last-modified: 8 May 1994
Version: 4.3


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***********************************************************************
* Copyright 1994 Robert Parson                                        *
*                                                                     *
* This file may be distributed, copied, and archived. All             *
* copies must include this notice and the paragraph below entitled    *
* "Caveat".  Reproduction and distribution for profit is              *
* NOT permitted. If this document is transmitted to other networks or *
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***********************************************************************


This part deals not with ozone depletion per se (that is covered
in Part I) but rather with the sources and sinks of chlorine and
bromine in the stratosphere. Special attention is devoted to the
evidence that most of the chlorine comes from the photolysis of
CFC's and related compounds. Instead of relying upon qualitative
statements about relative lifetimes, solubilities, and so forth, I
have tried to give a sense of the actual magnitudes involved.
Fundamentally, this Part of the FAQ is about measurements, and I
have therefore included some tables to illustrate trends; the
data that I reproduce is in all cases a small fraction of what
has actually been published. In the first section I state the
present assessment of stratospheric chlorine sources and trends,
and then in the next section I discuss the evidence that leads to
those conclusions. After a brief discussion of Bromine in section 3,
I answer the most familiar challenges that have been raised in
section 4. Only these last are actually "Frequently Asked Questions";
however I have found the Question/Answer format to be useful
in clarifying the issues in my mind even when the questions are
rhetorical, so I have kept to it.

| Caveat: I am not a specialist. In fact, I am not an atmospheric
| chemist at all - I am a physical chemist studying gas-phase
| processes who talks to atmospheric chemists. These files are an
| outgrowth of my own efforts to educate myself about this subject.
| I have discussed some of these issues with specialists but I am
| solely responsible for everything written here, especially errors.

*** Corrections and comments are welcomed.

- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado  (for which I do not speak)

  [e-mail]
  [e-mail]


CONTENTS

1.  CHLORINE IN THE STRATOSPHERE - OVERVIEW

1.1) Where does the Chlorine in the stratosphere come from?
1.2) How has stratospheric chlorine changed with time?
1.3) How will stratospheric chlorine change in the future?


2.  THE CHLORINE CYCLE

2.1) What are the sources of chlorine in the troposphere?
2.2) In what molecules is _stratospheric_ chlorine found?
2.3) What happens to organic chlorine in the stratosphere?
2.4) How do we know that CFC's are photolyzed in the stratosphere?
2.5) How is chlorine removed from the stratosphere?
2.6) How is chlorine distributed in the stratosphere?
2.7) What happens to the fluorine from the CFC's?
2.8) Summary of evidence


3.  BROMINE IN THE STRATOSPHERE

3.1) Is bromine important to the ozone destruction process?
3.2) How does bromine affect ozone?
3.3) Where does the bromine come from?


4.  COMMONLY ENCOUNTERED OBJECTIONS

4.1) CFC's are much heavier than air...
4.2) CFC's are produced mostly in the Northern Hemisphere...
4.3) Sea salt puts more chlorine into the atmosphere than CFC's.
4.4) Volcanoes put more chlorine into the stratosphere than CFC's.
4.5) Space shuttles put a lot of chlorine into the stratosphere.


5.  REFERENCES

=================================================================

1.  CHLORINE IN THE STRATOSPHERE - OVERVIEW


1.1) Where does the Chlorine in the stratosphere come from?

~80% from CFC's and related manmade organic chlorine compounds,
 such as carbon tetrachloride and methyl chloroform

~15-20% from methyl chloride (CH3Cl), most of which is natural.

 A few % from inorganic sources, including volcanic eruptions.

 [WMO 1991] [Solomon] [AASE] [Rowland 1989,1991] [Wayne]

These estimates are based upon 20 years' worth of measurements of
organic and inorganic chlorine-containing compounds in the earth's
troposphere and stratosphere. Particularly informative is the
dependence of these compounds' concentrations on altitude and
their increase with time. The evidence is summarized in section 2
of this FAQ.

1.2) How has stratospheric chlorine changed with time?

The total amount of chlorine in the stratosphere has increased by
a factor of 2.5 since 1975 [Solomon] During this time period the
known natural sources have shown no major increases.  On the other
hand, emissions of CFC's and related manmade compounds have
increased dramatically, reaching a peak in 1987. Extrapolating
back, one infers that total stratospheric chlorine has increased
by a factor of 4 since 1950.

1.3) How will stratospheric chlorine change in the future?

Since the 1987 Montreal Protocol (see Part I) production of
CFC's and related compounds has been decreasing rapidly. While
CFC concentrations are still increasing, the rate of increase
has diminished:

                Growth Rate, pptv/yr     (From [Elkins et al.])

Year            CFC-12          CFC-11

1977-84          17               9
1985-88          19.5            11
1993             10.5             2.7

If this trend continues CFC concentrations in the troposphere will
peak before the end of the century. The time scale for mixing
tropospheric and lower stratospheric air is about 5 years, so
stratospheric chlorine is expected to peak in the next decade and
then slowly decline on a time scale of about 50 years.


2.  THE CHLORINE CYCLE

2.1) What are the sources of chlorine in the troposphere?

Let us divide the chlorine-containing compounds found in the
atmosphere into two groups, "organic chlorine" and "inorganic
chlorine". The most important inorganic chlorine compound in the
troposphere is hydrogen chloride, HCl. Its principal source is
acidification of salt spray - reaction of atmospheric sulfuric and
nitric acids with chloride ions in aerosols.  At sea level, this
leads to an HCl mixing ratio of 0.05 - 0.45 ppbv, depending strongly
upon location (e.g. smaller values over land.) However, HCl dissolves
very readily in water (giving hydrochloric acid), and condensation of
water vapor  efficiently removes HCl from the _upper_ troposphere.
Measurements show that the HCl mixing ratio is less than 0.1 ppbv at
elevations above 7 km,  and less than 0.04 ppbv at 13.7 km.
[Vierkorn-Rudolf et al.] [Harris et al.]

There are many volatile organic compounds containing chlorine, but
most of them are quickly decomposed by the natural oxidants in the
troposphere, and the chlorine atoms that were in these compounds
eventually find their way into HCl or other soluble species and are
rained out. The most important exceptions are:

 ChloroFluoroCarbons, of which the most important are
 CF2Cl2 (CFC-12), CFCl3 (CFC-11), and CF2ClCFCl2 (CFC-113);

 HydroChloroFluoroCarbons such as CHClF2 (HCFC-22);

 Carbon Tetrachloride, CCl4;

 Methyl Chloroform, CH3CCl3;

 and Methyl Chloride, CH3Cl (also called Chloromethane).

Only the last has a large natural source; it is produced
biologically in the oceans and chemically from biomass burning.
The CFC's and CCl4 are nearly inert in the troposphere, and have
lifetimes of 50-200+ years.  Their major "sink" is photolysis by UV
radiation. [Rowland 1989, 1991] The hydrogen-containing halocarbons
are more reactive, and are removed in the troposphere by reactions
with OH radicals. This process is slow, however, and they live long
enough (1-20 years) for a large fraction to reach the stratosphere.

As a result of this enormous difference in atmospheric lifetimes,
there is more chlorine present in the lower atmosphere in
halocarbons than in HCl, even though HCl is produced in much larger
quantities. Total tropospheric organic chlorine amounted to
~3.8 ppbv in 1989 [WMO 1991], and this mixing ratio is very nearly
independent of altitude throughout the troposphere. Methyl Chloride,
the only ozone-depleting chlorocarbon with a major natural source,
makes up 0.6 ppbv of this total. Compare this to the tropospheric HCl
mixing ratios given above: < 0.5 ppbv at sea level, < 0.1 ppbv at 3 km,
and < 0.04 ppbv at 10 km.

2.2) In what molecules is _stratospheric_ chlorine found?

The halocarbons described above are all found in the stratosphere,
and in the lower stratosphere they are the dominant form of chlorine.
At higher altitudes inorganic chlorine is abundant, most of it in
the form of HCl or of _chlorine nitrate_, ClONO2.  These are called
"chlorine reservoirs"; they do not themselves react with ozone, but
they generate a small amount of chlorine-containing radicals - Cl,
ClO, ClO2, and related species, referred to collecively as the
"ClOx family" - which do. An increase in the concentration of
chlorine reservoirs leads to an increase in the concentration of
the ozone-destroying radicals.

2.3) What happens to organic chlorine in the stratosphere?

The organic chlorine compounds are dissociated by UV radiation
having wavelengths near 230 nm. Since these wavelengths are also
absorbed by oxygen and ozone, the organic compounds have to rise
high in the stratosphere in order for this photolysis to take
place. The initial (or, as chemists say, "nascent") products are
a free chlorine atom and an organic radical, for example:

 CFCl3 + hv -> CFCl2 + Cl

The chlorine atom can react with methane to give HCl and a methyl
radical:

 Cl + CH4 -> HCl + CH3

 Alternatively, it can react with ozone and nitrogen oxides:

 Cl + O3 -> ClO + O2
 ClO + NO2 -> ClONO2

 (There are other pathways, but these are the most important.)

The other nascent product (CFCl2 in the above example) undergoes
a complicated sequence of reactions that also eventually leads to
HCl and ClONO2. Most of the inorganic chlorine in the stratosphere
therefore resides in one of these two "reservoirs". The immediate
cause of the Antarctic ozone hole is an unusual sequence of
reactions, catalyzed by polar stratospheric clouds, that "empty"
these reservoirs and produce high concentrations of ozone-destroying
ClOx radicals. [Wayne] [Rowland 1989, 1991]

2.4) How do we know that CFC's _are_ photolyzed in the stratosphere?

The UV photolysis cross-sections for the halocarbons have been
measured in the laboratory; these tell us how rapidly they will
dissociate when exposed to light of a given wavelength and intensity.
We can combine this with the measured intensity of radiation in the
stratosphere and deduce the way in which the mixing ratio of a
given halocarbon should depend upon altitude. Since there is almost
no 230 nm radiation in the troposphere or in the lowest parts of
the stratosphere, the mixing ratio should be independent of altitude
there. In the middle stratosphere the mixing ratio should drop off
quickly, at a rate which is determined by the photolysis cross-section.
Thus each halocarbon has a characteristic "signature" in its mixing
ratio profile, which can be calculated. Such calculations (first
carried out in the mid 1970's) agree well with the distributions
presented in the next section.

There is direct evidence as well. Photolysis removes a chlorine
atom, leaving behind a reactive halocarbon radical. The most likely
fate of this radical is reaction with oxygen, which starts a long
chain of reactions that eventually remove all the chlorine and
fluorine. Most of the intermediates are reactive free radicals, but
two of them, COF2 and COFCl, are fairly stable (they are analogs of
formaldehyde, H2CO) and live long enough to be detected. They
have been found, at precisely those altitudes at which the CFC
mixing ratios are dropping off rapidly (see below).

2.5) How is chlorine removed from the stratosphere?

Since the stratosphere is very dry, water-soluble compounds are
not quickly washed out as they are in the troposphere. The
stratospheric lifetime of HCl is about 2 years; the principal
sink is transport back down to the troposphere.

2.6) How is chlorine distributed in the stratosphere?

Over the past 20 years an enormous effort has been devoted to
identifying sources and sinks of stratospheric chlorine.  The
concentrations of the major species have been measured as a
function of altitude, by "in-situ" methods ( e.g. collection
filters carried on planes and balloons) and by spectroscopic
observations from aircraft, balloons, satellites, and the Space
Shuttle. From all this work we now have a clear and consistent
picture of the processes that carry chlorine through the stratosphere.


Let us begin by asking where inorganic chlorine is found. In the
troposphere, the HCl mixing ratio decreased markedly with increasing
altitude. In the stratosphere, on the other hand,  it _increases_ with
altitude, rapidly up to about 35 km, and then more slowly up to 55km
and beyond. This was noticed as early as 1976 [Farmer et al.]
[Eyre and Roscoe] and has been confirmed repeatedly since. Chlorine
Nitrate (ClONO2), the other important inorganic chlorine compound in
the stratosphere, also increases rapidly in the lower stratosphere, and
then falls off at higher altitudes.  These results strongly suggest
that HCl in the stratosphere is being _produced_ there, not drifting
up from below.

Let us now look at the organic source gases.  Here, the data show
that the mixing ratios of the CFC's and CCl4 are _nearly independent
of altitude_ in the troposphere, and _decrease rapidly with altitude_
in the stratosphere.  The mixing ratios of the more reactive
hydrogenated compounds such as CH3CCl3 and CH3Cl drop off somewhat
in the troposphere, but also show a much more rapid decrease in
the stratosphere.  The turnover in organic chlorine correlates
nicely with the increase in inorganic chlorine, confirming the
hypothesis that CFC's are being photolyzed as they rise high enough
in the stratosphere to experience enough short-wavelength UV. At
the bottom of the stratosphere almost all of the chlorine is
organic, and at the top it is all inorganic. [Fabian et al. ]
[Zander et al. 1987] [Zander et al. 1992] [Penkett et al.]

Finally, there are the stable reaction intermediates, COFCl and
COF2. These show up in the middle stratosphere, exactly where one
expects to find them if they are produced from organic source gases
and eventually react to give inorganic chlorine.

For example, the following is extracted from Tables II and III of
[Zander et al. 1992]; they refer to 30 degrees N Latitude in 1985.
I have rearranged the tables and rounded some of the numbers, and
the arithmetic in the second table is my own.

   Organic Chlorine and Intermediates, Mixing ratios in ppbv

Alt.,  CH3Cl CCl4 CCl2F2 CCl3F CHClF2 CH3CCl3 C2F3Cl3  ||  COFCl
km
12.5  .580  .100  .310  .205  .066     .096    .021    ||  .004
15    .515  .085  .313  .190  .066     .084    .019    ||  .010
20    .350  .035  .300  .137  .061     .047    .013    ||  .035
25    .120   -    .175  .028  .053     .002    .004    ||  .077
30     -     -    .030   -    .042      -       -      ||  .029
40     -     -     -     -     -        -       -      ||   -


     Inorganic Chlorine and Totals, Mixing ratios in ppbv

Alt., HCl  ClONO2   ClO  HOCl   ||   Total Cl,  Total Cl,  Total Cl
                                ||   Inorganic   Organic
km                              ||
12.5   -     -       -     -    ||       -       2.63        2.63
15    .065   -       -     -    ||     0.065     2.50        2.56
20    .566  .212     -     -    ||     0.778     1.78        2.56
25   1.027  .849    .028  .032  ||     1.936     0.702       2.64
30   1.452 1.016    .107  .077  ||     2.652     0.131       2.78
40   2.213 0.010    .234  .142  ||     2.607       -         2.61

(I have included the intermediate COFCl in the Total Organic column.)

This is just an excerpt. The original tables give results every 2.5km
from 12.5 to 55km, together with a similar inventory for Fluorine.
Standard errors on total Cl were estimated to be 0.02-0.04 ppbv.

Notice that the _total_ chlorine at any altitude is nearly constant
at ~2.5-2.8 ppbv. This is what we would expect if the sequence of
reactions that leads from organic sources to inorganic reservoirs
was fast compared to vertical transport. Our picture, then, would be
of a swarm of organic chlorine molecules slowly spreading upwards
through the stratosphere, being converted into inorganic reservoir
molecules as they climb. In fact this oversimplifies things -
photolysis pops off a single Cl atom which does reach its final
destination quickly, but the remaining Cl atoms are removed by a
sequence of slower reactions. Some of these reactions involve
compounds, such as NOx, which are not well-mixed; moreover,
"horizontal" transport does not really take place along surfaces of
constant altitude, so chemistry and atmospheric dynamics are in fact
coupled together in a complicated way.  These are the sorts of issues
that are addressed in atmospheric models.  Nevertheless, this simple
picture helps us to understand the qualitative trends, and
quantitative models confirm the conclusions [McElroy and Salawich].

We conclude that most of the inorganic chlorine in the stratosphere
is _produced_ there, as the end product of photolysis of the organic
chlorine compounds.

2.7) What happens to the Fluorine from the CFC's?

Most of it ends up as Hydrogen Fluoride, HF. The total amount of HF
in the stratosphere increased by a factor of 3-4 between 1978 and
1989 [Zander et al., 1990] [Rinsland et al.]; the relative increase
is larger for HF than for HCl (a factor of 2.2 over the same period)
because the natural source, and hence the baseline concentration,
is much smaller. For the same reason, the _ratio_ of HF to HCl has
increased, from 0.14 in 1977 to 0.23 in 1990. The fluorine budget,
as a function of altitude, adds up in much the same way as the
chlorine budget. [Zander et al. 1992].
There are some discrepancies in the lower stratosphere; model
calculations predict _less_ HF than is actually observed.


2.8) Summary of the Evidence

 a. Inorganic chlorine, primarily of natural origin, is efficiently
 removed from the troposphere; organic chlorine, primarily
 anthropogenic, is not, and in the upper troposphere organic
 chlorine dominates overwhelmingly.

 b. In the stratosphere, organic chlorine decreases with altitude,
 since at higher altitudes there is more short-wave UV available to
 photolyze it. Inorganic chlorine _increases_ with altitude.
 At the bottom of the stratosphere essentially all of the chlorine
 is organic, at the top it is all inorganic, and reaction
 intermediates are found at intermediate altitudes.

 c. Both HCl and HF in the stratosphere have been increasing steadily,

 in a correlated fashion, since they were first measured in the 1970's.


3.  BROMINE

3.1) Is bromine important to the ozone destruction process?

Br is present in much smaller quantities than Cl, but it is
much more destructive on a per-atom basis.  There is a large
natural source; manmade compounds contribute about 40% of the total.

3.2) How does bromine affect ozone?

Bromine concentrations in the stratosphere are ~150 times smaller
than chlorine concentrations.  However, atom-for-atom Br is 10-100
times as effective as Cl in destroying ozone.  (The reason for this
is that there is no stable 'reservoir' for Br in the stratosphere
- HBr and BrONO2 are very easily photolyzed so that nearly all of
the Br is in a form that can react with ozone.  Contrariwise, F is
innocuous in the stratosphere because its reservoir, HF, is
extremely stable.) So, while Br is less important than Cl, it must
still be taken into account.  Interestingly, the principal
pathway by which Br destroys ozone also involves Cl:

     BrO + ClO -> Br + Cl + O2
     Br + O3 -> BrO + O2
     Cl + O3 -> ClO + O2
   ----------------------------------
   Net:    2 O3 -> 3 O2

[Wayne p. 164] [Solomon]

so reducing stratospheric chlorine concentrations will, as a
side-effect, slow down the bromine pathways as well.


3.3) Where does the bromine come from?

The largest source of stratospheric Bromine is methyl bromide,
CH3Br.  Much of this is naturally produced in the oceans and in
wildfires [Mano and Andreae], but 30 - 60% is manmade [Khalil et al.]
It is widely used as a fumigant.

Another important source is the family of "halons", widely used in fire
extinguishers.  Like CFC's these compounds have long atmospheric
lifetimes (72 years for CF3Br) and very little is lost in the
troposphere.  [Wayne p. 167].  At the bottom of the stratosphere
the total Br mixing ratio is ~20 parts-per-trillion (pptv), of which ~
8 pptv is manmade.  [AASE] Uncertainties in these numbers are relatively
larger than for Cl, because the absolute quantities are so much smaller,
and we should expect to see these estimates change. Halons have been
restricted under the Montreal Protocol, and regulations on methyl
bromide use are under consideration.


4. COMMONLY ENCOUNTERED OBJECTIONS

4.1) CFC's are 4-8 times heavier than air, so how can they reach
 the stratosphere?

 This is answered in Part I of this FAQ, section 1.3. Briefly,
 atmospheric gases do not segragate by weight in the troposphere
 and the stratosphere, because the mixing mechanisms (convection,
 "eddy diffusion") do not distinguish molecular masses.

4.2) CFCs are produced in the Northern Hemisphere, so how do they get
 down to the Antarctic?

Vertical transport into and within the stratosphere is slow.  It
takes more than 5 years for a CFC molecule released at sea level to
rise high enough in the stratosphere to be photolyzed.  North-South
transport, in both troposphere and stratosphere, is faster - there is
a bottleneck in the tropics (it can take a year or two to get across
the equator) but there is still plenty of time.  CFC's are distributed
almost uniformly as a function of latitude, with a gradient of ~10%
from Northern to Southern Hemispheres. [Singh et al.]. [Elkins et al.]

4.3) Sea salt puts more chlorine into the atmosphere than CFC's.

True, but not relevant because this chlorine is in a form (HCl) that
is rapidly removed from the troposphere. Even at sea level there is
more chlorine present in organic compounds than in HCl, and in the
upper troposphere and lower stratosphere organic chlorine dominates
overwhelmingly. See section 2.1 above.

4.4) Volcanoes put more chlorine into the stratosphere than CFC's.

Short Reply: False. Volcanoes account for at most a few percent
of the chlorine in the stratosphere.

Long reply:  This is one of the most persistent myths in this
area. As is so often the case, there is a seed of truth at the
root of the myth.  Volcanic gases are rich in Hydrogen Chloride, HCl.
As we have discussed, this gas is very soluble in water and is
removed from the troposphere on a time scale of 1-7 days, so we can
dismiss quietly simmering volcanoes as a stratospheric source, just
as we can neglect sea salt and other natural sources of HCl. (In fact
tropospheric HCl from volcanoes is neglible compared to HCl from
sea salt.) However, we cannot use this argument to dismiss MAJOR
volcanic eruptions, which can in principle inject HCl directly into
the middle stratosphere.

What is a "major" eruption?  There is a sort of "Richter scale" for
volcanic eruptions, the so-called "Volcanic explosivity index" or
VEI.  Like the Richter scale it is logarithmic; an eruption with a
VEI of 5 is ten times "bigger" than one with a VEI of 4. To give a
sense of magnitude, I list below the VEI for some familiar recent
and historic eruptions:

Eruption           VEI             Stratospheric Aerosol,
                                   Megatons (Mt)

Kilauea             0-1            -
Erebus, 1976-84     1-2            -
Augustine, 1976     4              0.6
St Helen's, 1980    5 (barely)     0.55
El Chichon, 1982    5              12
Pinatubo, 1991      5-6            20 - 30
Krakatau, 1883      6              50 (est.)
Tambora, 1815       7              80-200 (est.)

[Smithsonian] [Symonds et al.] [Sigurdsson] [Pinatubo] [WMO 1988]
[Bluth et al.]

Roughly speaking, an eruption with VEI>3 can penetrate the
stratosphere. An eruption with VEI>5 can send a plume up to 25km,
in the middle of the ozone layer. Such eruptions occur about once
a decade. Since the VEI is not designed specifically to measure a
volcano's impact on the stratosphere, I have also listed the total
mass of stratospheric aerosols (mostly sulfates) produced by the
eruption. (Note that St. Helens produced much less aerosol than El
Chichon - you may remember that St. Helens blew out sideways, dumping
a large ash cloud over eastern Washington, rather than ejecting its
gases into the stratosphere.) Passively degassing volcanoes such as
Kilauea and Erebus are far too weak to penetrate the stratosphere, but
explosive eruptions like El Chichon and Pinatubo need to be considered
in detail.

Before 1982, there were no direct measurements of the amount of HCl
that an explosive eruption put into the stratosphere.  There were,
however, estimates of the _total_ chlorine production from an
eruption, based upon such geophysical techniques as analysis of
glass inclusions trapped in volcanic rocks.  [Cadle] [Johnston]
[Sigurdsson] [Symonds et al.] There was much debate
about how much of the emitted chlorine reached the stratosphere;
estimates ranged from < 0.03 Mt/year [Cadle] to 0.1-1.0 Mt/year
[Symonds et al.].  During the 1980's emissions of CFC's and related
compounds contributed >1.2 Mt of chlorine per year to the
atmosphere. [Prather et al.] This results in an annual flux of >0.3
Mt/yr of chlorine into the stratosphere. The _highest_ estimates
ofvolcanic emissions - upper limits calculated by assuming that
_all_ of the HCl from a major eruption reached and stayed in the
stratosphere - were thus of the same order of magnitude as human
sources.  (There is NO support whatsoever for the claim - found in
Dixy Lee Ray's _Trashing the Planet_  - that a _single_ recent
eruption produced ~500 times as much chlorine as a year's worth of
CFC production. This wildly inaccurate number appears to have arisen
from an editorial mistake in a scientific encyclopedia.)

It is very difficult to reconcile these upper limits with the
altitude and time-dependence of stratospheric HCl. The volcanic
contribution to the upper stratosphere should come in sudden bursts
following major eruptions, and it should initially be largest in
the vicinity of the volcanic plume. Since vertical transport in the
stratosphere is slow, one would expect to see the altitude profile
change abruptly after a major eruption, whereas it has maintained
more-or-less the same shape since it was first measured in 1975.
One would also not expect a strong correlation between HCl and
organochlorine compounds if volcanic injection were contributing
~50% of the total HCl.  If half of the HCl has an inorganic origin,
where is all that _organic_ chlorine going?

The issue has now been largely resolved by _direct_ measurements of
the stratospheric HCl produced by El Chichon, the most important
eruption of the 1980's, and Pinatubo, the largest since 1912. It
was found that El Chichon injected *0.04* Mt of HCl [Mankin
and Coffey].  The much bigger eruption of Pinatubo produced less
[Mankin, Coffey and Goldman], - in fact the authors were not sure
that they had measured _any_ significant increase. Analysis of
ice cores leads to similar conclusions for historic eruptions
[Delmas]. The ice cores show significantly enhanced levels of
sulfur following major historic eruptions, but no enhancement in
chlorine, showing that the chlorine produced in the eruption did
not survive long enough to be transported to polar regions. It is
clear, then, that even though major eruptions produce large amounts
of chlorine in the form of HCl, most of that HCl either never
enters the stratosphere, or is very rapidly removed from it.

Recent model calculations [Pinto et al.] [Tabazadeh and Turco]
have clarified the physics involved.  A volcanic plume contains
approximately 1000 times as much water vapor as HCl. As the plume
rises and cools the water condenses, capturing the HCl as it does
so and returning it to the earth in the extensive rain showers that
typically follow major eruptions. HCl can also be removed if it
is adsorbed on ice or ash particles. Model calculations show that
more than 99% of the HCl is removed by these processes, in good
agreement with observations.

------------------------------------------------------------------
In summary:

 * Older indirect _estimates_ of the contribution of volcanic
   eruptions to stratospheric chlorine gave results that ranged
   from much less than anthropogenic to somewhat larger than
   anthropogenic. It is difficult to reconcile the larger estimates
   with the altitude distribution of inorganic chlorine in the
   stratosphere, or its steady increase over the past 20 years.
   Nevertheless, these estimates raised an important scientific
   question that needed to be resolved by _direct_ measurements
   in the stratosphere.

 * Direct measurements on El Chichon, the largest eruption of
   the 1980's, and on Pinatubo, the largest since 1912, show
   that the volcanic contribution is small.

 * Claims that volcanoes produce more stratospheric chlorine than
   human activity arise from the careless use of old scientific
   estimates that have since been refuted by observation.

 * Claims that a single recent eruption injected ~500 times a year's
   CFC production into the stratosphere have no scientific basis
   whatsoever.

---------------------------------------------------------------

To conclude, we need to say something about Mt. Erebus.  In an
article in _21st Century_ (July/August 1989), Rogelio Maduro
claimed that this Antarctic volcano has been erupting constantly
for the last 100 years, emitting more than 1000 tons of chlorine
per day. This claim was repeated in Dixy Lee Ray's books.
"21st Century" is published by Lyndon LaRouche's political
associates, although LaRouche himself usually keeps a low profile
in the magazine.  Mt. Erebus has in fact been simmering quietly for
over a century but the estimate of 1000 tons/day of HCl only applied
to an especially active period between 1976 and 1983.  Moreover that
estimate [Kyle et al.] has been since been reduced to 167 tons/day
(0.0609 Mt/year). By late 1984 emissions had dropped by an order of
magnitude, and have remained at low levels since; HCl emissions
 _at the crater rim_ were 19 tons/day (0.007 Mt/year) in 1986,
and 36 tons/day (0.013 Mt/year) in 1991. [Zreda-Gostynska et al.]
Since this is a passively degassing volcano (VEI=1-2 in the active
period), very little of this HCl reaches the stratosphere. The
Erebus plume never rises more than 0.5 km above the volcano,
and in fact the gas usually just oozes over the crater rim. Indeed,
one purpose of the measurements of Kyle et al. was to explain high
Cl concentrations in Antarctic snow. The only places where I have
ever seen Erebus described as a source of stratospheric chlorine is
in LaRouchian publications and in articles and books that,
incredibly, consider such documents to be reliable sources.

4.5) Space shuttles put a lot of chlorine into the stratosphere.

Simply false. In the early 1970's, when very little was known about
the role of chlorine radicals in ozone depletion, it was suggested
that HCl from solid rocket motors might have a significant effect
upon the ozone layer -  if not globally, perhaps in the immediate
vicinity of the launch. It was immediately shown that the effect
was negligible, and this has been repeatedly demonstrated since.
Each shuttle launch produces about  68 metric tons of chlorine as
HCl; a full year's worth of shuttle and solid rocket launches
produces about 725 tons. This is negligible compared to chlorine
emissions in the form of CFC's and related compounds (1.2 million
tons/yr in the 1980's, of which ~0.3 Mt reach the stratosphere each
year). It is also negligible in comparison to natural sources, which
produce about 75,000 tons per year. [Prather et al.] [WMO 1991].

See also the sci.space FAQ, Part 10, "Controversial Questions".


5.  REFERENCES FOR PART II

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival sources such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions".  (In this part, I have
had to refer to archival journals more often than I would have
liked, since in many cases that is the only place where the
question is addressed in satisfactory detail.) Readers who want to
see "who did what" should consult the review articles listed below,
or, if they can get them, the extensively documented WMO reports.


Introductory Reading:

[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
 _Atmospheric Change: an Earth System Perspective_, Freeman, 1993.

[Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the
depletion of stratospheric ozone", _Am. Sci._ _77_, 36, 1989.

--------------------------------
Books and Review Articles:

[Brasseur and Solomon] G. Brasseur and S. Solomon, _Aeronomy of
the Middle Atmosphere_, 2nd Edition, D. Reidel, 1986.

[McElroy and Salawich] M. McElroy and R. Salawich, "Changing
Composition of the Global Stratosphere", _Science_ _243, 763, 1989.

[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.

[Solomon] S. Solomon, "Progress towards a quantitative
 understanding of Antarctic ozone depletion",
 _Nature_ _347_, 347, 1990.

[Wallace and Hobbs] J. M. Wallace and P. V. Hobbs,
_Atmospheric Science: an Introductory Survey_, Academic Press, 1977.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_,
        2nd.  Ed., Oxford, 1991.

[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_, Report # 18

[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_, Report # 25
-----------------------------

More specialized articles:

[AASE] End of Mission Statement, second airborne arctic
stratospheric expedition, NASA 30 April 1992.

[Bluth et al.] G. J. S. Bluth, C. C. Schnetzler, A. J. Krueger,
and L. S. Walter, "The contribution of explosive volcanism to
global atmospheric sulphur dioxide concentrations",
 _Nature_ _366_, 327, 1993.

[Cadle] R. Cadle, "Volcanic emissions of halides and sulfur
compounds to the troposphere and stratosphere", J. Geophys. Res.
_80_, 1651, 1975]

[Delmas] R. J. Delmas, "Environmental Information from Ice Cores",
_Reviews of Geophysics_ _30_, 1, 1992.

[Elkins et al.] J. W. Elkins, T. M. Thompson, T. H. Swanson,
J. H. Butler, B. D. Hall, S. O. Cummings, D. A. Fisher, and
A. G. Raffo, "Decrease in Growth Rates of Atmospheric
Chlorofluorocarbons 11 and 12", _Nature_ _364_, 780, 1993.

[Eyre and Roscoe] J. Eyre and H. Roscoe, "Radiometric measurement
of stratospheric HCl", _Nature_ _266_, 243, 1977.

[Fabian et al. 1979] P. Fabian, R. Borchers, K.H. Weiler, U.
Schmidt, A. Volz, D.H. Erhalt, W. Seiler, and F. Mueller,
"Simultaneously measured vertical profile of H2, CH4, CO, N2O,
CFCl3, and CF2Cl2 in the mid-latitude stratosphere and
troposphere", J. Geophys. Res.  _84_, 3149, 1979.

[Fabian et al. 1981] P. Fabian, R. Borchers, S.A. Penkett, and
N.J.D. Prosser, "Halocarbons in the Stratosphere", _Nature_ _294_,
733, 1981.

[Farmer et al.] C.B. Farmer, O.F. Raper, and R.H. Norton,
"Spectroscopic detection and vertical distribution of HCl in the
troposphere and stratosphere", Geophys. Res. Lett. _3_, 13, 1975.

[Harris et al.] G.W. Harris, D. Klemp, and T. Zenker,
"An Upper Limit on the HCl near-surface mixing ratio over the
Atlantic", J. Atmos. Chem. _15_, 327, 1992.

[Johnston] D. Johnston, "Volcanic contribution of chlorine to the
 stratosphere: more significant to ozone than previously
estimated?" _Science_ _209_, 491, 1980.

[Khalil et al.] M.A.K. Khalil, R. Rasmussen, and R. Gunawardena,
"Atmospheric Methyl Bromide: Trends and Global Mass Balance"
 J. Geophys. Res. _98_, 2887, 1993.

[Kyle et al.] P.R. Kyle, K. Meeker, and D. Finnegan,
"Emission rates of sulfur dioxide, trace gases, and metals from
 Mount Erebus, Antarctica", _Geophys. Res. Lett._ _17_, 2125, 1990.

[Mankin and Coffey] W. Mankin and M. Coffey, "Increased
stratospheric hydrogen chloride in the El Chichon cloud",
_Science_ _226_, 170, 1983.

[Mankin, Coffey and Goldman] W. Mankin, M. Coffey and A. Goldman,
"Airborne observations of SO2, HCl, and O3 in the stratospheric
plume of the Pinatubo volcano in July 1991", Geophys. Res. Lett.
_19_, 179, 1992.

[Mano and Andreae] S. Mano and M. O. Andreae, "Emission of Methyl
Bromide from Biomass Burning", _Science_ _263_, 1255, 1994.

[Penkett et al.] S.A. Penkett, R.G. Derwent, P. Fabian, R.
Borchers, and U. Schmidt, "Methyl Chloride in the Stratosphere",
_Nature_ _283_, 58, 1980.

[Pinatubo] Special Mt. Pinatubo issue, Geophys. Res. Lett. _19_,
#2, 1992.

[Pinto et al.] J. Pinto, R. Turco, and O. Toon, "Self-limiting
physical and chemical effects in volcanic eruption clouds",
 J. Geophys. Res. _94_, 11165, 1989.

[Prather et al. ] M. J. Prather, M.M. Garcia, A.R. Douglass, C.H.
Jackman, M.K.W. Ko, and N.D. Sze, "The Space Shuttle's impact on
the stratosphere", J. Geophys. Res. _95_, 18583, 1990.

[Sigurdsson] H. Sigurdsson, "Evidence of volcanic loading of the
 atmosphere and climate response",  _Palaeogeography,
Palaeoclimatology, Palaeoecology_ _89_, 277 (1989).

[Rinsland et al.] C. P. Rinsland, J. S. Levine, A. Goldman,
N. D. Sze, . K. W. Ko, and D. W. Johnson, "Infrared measurements
of HF and HCl total column abundances above Kitt Peak, 1977-1990:
Seasonal cycles, long-term increases, and comparisons with model
calculations", J. Geophys. Res. _96_, 15523, 1991.

[Singh et al.] H. Singh, L. Salas, H. Shigeishi, and E. Scribner,
"Atmospheric Halocarbons, hydrocarbons, and sulfur hexafluoride
global distributions, sources, and sinks", _Science_ _203_, 899, 1974.


[Smithsonian] Smithsonian Report, _Global Volcanism:1975-85_, p 14.

[Symonds et al.] R. B. Symonds, W. I. Rose, and M. H. Reed,
"Contribution of Cl and F-bearing gases to the atmosphere by
volcanoes", _Nature_ _334_, 415 1988.

[Tabazadeh and Turco] A. Tabazadeh and R. P. Turco, "Stratospheric
Chlorine Injection by Volcanic Eruptions: HCl Scavenging and
Implications for Ozone", _Science_ _260_, 1082, 1993.

[Vierkorn-Rudolf et al.] B. Vierkorn-Rudolf. K. Bachmann, B.
Schwartz, and F.X. Meixner, "Vertical Profile of Hydrogen Chloride
in the Troposphere", J. Atmos. Chem.  _2_, 47, 1984.

[Zander et al. 1987] R. Zander, C. P. Rinsland, C. B. Farmer, and
R. H. Norton, "Infrared Spectroscopic measurements of halogenated
source gases in the stratosphere with the ATMOS instrument", J.
Geophys. Res. _92_, 9836, 1987.

[Zander et al. 1990] R. Zander, M.R. Gunson, J.C. Foster, C.P.
Rinsland, and J. Namkung, "Stratospheric ClONO2, HCl, and HF
concentration profiles derived from ATMOS/Spacelab 3 observations
- an update", J. Geophys. Res.  _95_, 20519, 1990.

[Zander et al. 1992] R. Zander, M. R. Gunson, C. B. Farmer, C. P.
Rinsland, F. W. Irion, and E. Mahieu, "The 1985 chlorine and
fluorine inventories in the stratosphere based on ATMOS observations
at 30 degrees North latitude",  J. Atmos. Chem. _15_, 171, 1992.

[Zreda-Gostynska et al.] G. Zreda-Gostynska, P. R. Kyle, and
D. L. Finnegan, "Chlorine, Fluorine and Sulfur Emissions from
Mt. Erebus, Antarctica and estimated contribution to the antarctic
atmosphere", _Geophys. Res. Lett._ _20_, 1959, 1993.






-------------------------------------------------------------------------------
Area # 2120  news.answers           05-26-94 15:53      Message # 13064
From    : [e-mail]
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Subj    : Ozone Depletion FAQ Part

 @SUBJECT:Ozone Depletion FAQ Part III: The Antarctic Ozone Hole
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***********************************************************************
* Copyright 1994 Robert Parson                                        *
*                                                                     *
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***********************************************************************


 This part deals specifically with springtime antarctic ozone
depletion (and with the similar but smaller effects seen in the
Arctic spring). More general questions about ozone and ozone
depletion, including the definitions of many of the terms used
here, are dealt with in parts I and II. Biological effects of the
ozone hole are dealt with in part IV.


| Caveat: I am not a specialist. In fact, I am not an atmospheric
| chemist at all - I am a physical chemist who talks to atmospheric
| chemists. These files are an outgrowth of my own efforts to educate
| myself over the past two years. I have discussed some of these
| issues with specialists but I am solely responsible for everything
| written here, including any errors. This document should not be
| cited in publications off the net; rather, it should be used as a
| pointer to the published literature.

*** Corrections and comments are welcomed.


- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado  (for which I do not speak)

  [e-mail]
  [e-mail]


CONTENTS

1.   What is the antarctic ozone hole?

2.   How big is the hole, and is it getting bigger?

3.   When did the hole first appear?

4.   How far back do Antarctic ozone measurements go?

5.   But I heard that Dobson saw an ozone hole in 1956-58...

6.   Why is the hole in the antarctic?

7.   What is the evidence for the present theory?

8.   Will the ozone hole keep growing?

9.   Why be concerned about an ozone hole over antarctica?
     Nobody lives down there.

10.  Is there an ozone hole in the arctic? if not, why not?

11.  Can the hole be "plugged"?


1. What is the Antarctic ozone hole?

 For the past decade or so, ozone levels over Antarctica have fallen
to abnormally low values between late August and late November. At
the beginning of this period, ozone levels are already low, about
300 Dobson units (DU), but instead of slowly increasing as the
light comes back in the spring, they drop to 150 DU and below. In
the lower stratosphere, between 15 and 20 km, about 95% of the
ozone is destroyed. Above 25 km the decreases are small and the net
result is a thinning of the ozone layer by about 50%.  In the late
spring ozone levels return to more normal values, as warm,
ozone-rich air rushes in from lower latitudes. The precise duration
varies considerably from year to year; in 1990 the hole lasted well
into December.

 In some of the popular newsmedia, as well as many books, the
term "ozone hole" is being used far too loosely.  It seems that
any episode of ozone depletion, no matter how minor, now gets
called an ozone hole (e.g. 'ozone hole over Hamburg - but only for
one day'). This sloppy language trivializes the problem and blurs
the important scientific distinction between the massive ozone
losses in polar regions and the much smaller, but nonetheless
significant, ozone losses in middle latitudes. It is akin to
using "gridlock" to describe a routine traffic jam.


2. How big is the hole, and is it getting bigger?

 During the years 1978-1987 the hole grew, both in depth (total ozone
loss in a column) and in area. This growth was not monotonic but
seemed to oscillate with a two-year period (perhaps connected with
the "quasibiennial oscillation" of the stratospheric winds.) The
hole shrank dramatically in 1988 but in 1989-1991 was as large as in
1987, and in 1992-93 was larger still. In 1987 and 1989-93 it
covered the entire Antarctic continent and
part of the surrounding ocean. The exact size is determined
primarily by meteorological conditions, such as the strength of
the polar vortex in any given year. The boundary is fairly steep,
with decreases of 100-150 DU taking place in 10 degrees of
latitude, but fluctuates from day to day. On occasion, the
nominal boundary of the hole has passed over the tip of S. America,
(55 degrees S. Latitude). Australia and New Zealand are far outside
the hole, although they do experience ozone depletion, more than
is seen at comparable latitudes in the Northern hemisphere.  After
the 1987 hole broke up, December ozone levels over Australia and
New Zealand were 10% below normal.
[WMO 1991] [Atkinson et al.] [Roy et al.].


3. When did the hole first appear?

 It was first observed by ground-based measurements from Halley Bay
on the Antarctic coast, during the years 1980-84. [Farman, Gardiner
and Shanklin.] At about the same time, ozone decreases were seen at
the Japanese antarctic station of Syowa; these were less dramatic than
those seen at Halley (Syowa is about 1000 km further north) and did not
receive as much attention.  It has since been confirmed
by satellite measurements as well as ground-based measurements
elsewhere on the continent, on islands in the Antarctic ocean, and at
Ushaia, at the tip of Patagonia. With hindsight, one can see the hole
beginning to appear in the data around 1976, but it grew much more
rapidly in the 1980's. [Stolarski et al. 1992]


4. How far back do antarctic ozone measurements go?

 Ground-based measurements began in 1956, at Halley Bay. A few years
later these were supplemented by measurements at the South Pole and
elsewhere on the continent.  Satellite measurements began in the
early 70's, but the first really comprehensive satellite data came
in 1978, with the TOMS (total ozone mapping spectrometer) and SBUV
(solar backscatter UV) instruments on Nimbus-7.  The TOMS, which
finally broke down on May 7 1993, is the source for most of the
pretty pictures that one sees in review articles and the
popular press. Today there are several satellites monitoring ozone
and other atmospheric gases; the Russian Meteor-3 carries a new
TOMS, while instrument on NASA's UARS (Upper Atmosphere Research
Satellite) simultaneously measure ozone, chlorine monoxide (ClO),
and stratospheric pressure and temperature.


5. But I heard that Dobson saw an ozone hole in 1956-58...

 This is a myth, arising from a misinterpretation of an out-of-
context quotation from Dobson's paper. A glance at the original
suffices to refute it.

In his historical account [Dobson], Dobson mentioned that
when springtime ozone levels over Halley Bay were first measured,
he was surprised to find that they were about 150 DU below
corresponding levels (displaced by six months) in the Arctic.
Springtime arctic ozone levels are very high, ~450 DU; in the
Antarctic spring, however, Dobson's coworkers found ~320 DU, close
to winter levels. This was the first observation of the _normal_,
pre-1980 behavior of the Antarctic ozone layer: because of the
tight polar vortex (see below) ozone levels remain low until late
spring. In the Antarctic ozone hole, on the other hand, ozone
levels _decrease_ from these already low values. What Dobson
describes is essentially the _baseline_ from which the ozone hole
is measured. [Dobson] [WMO 1989]


 For those interested, here is how springtime antarctic
 ozone has developed from 1956 to 1991:

-------------------------------------------------------------
   Halley Bay Antarctic Ozone Data

   Mean October ozone column thickness, Dobson Units

   From J. D. Shanklin, personal communication, 1993.
   For a graphical representation see [Farman et al.],
   [Hamill and Toon], [Solomon], and [WMO 1991], p. 4.6

    1956      321                    1975      308
    1957      330                    1976      283
    1958      314                    1977      251
    1959      311                    1978      284
    1960      301                    1979      261
    1961      317                    1980      227
    1962      332                    1981      237
    1963      309                    1982      234
    1964      318                    1983      210
    1965      281                    1984      201
    1966      316                    1985      196
    1967      323                    1986      248
    1968      301                    1987      163
    1969      282                    1988      232
    1970      282                    1989      164
    1971      299                    1990      179
    1972      304                    1991      155
    1973      289                    1992      142
    1974      274                    1993      117


6. Why is the hole in the Antarctic?

 This was a mystery when the hole was first observed, but
it is now well understood. I shall limit myself to a
brief survey of the present theory, and refer the reader to two
excellent nontechnical articles [Toon and Turco] [Hamill and Toon]
for a more comprehensive discussion. Briefly, the unusual
physics and chemistry of the Antarctic stratosphere allows the
inactive chlorine "reservoir" compounds to be converted into ozone-
destroying chlorine radicals. While there is no more chlorine over
antarctica than anywhere else, in the antarctic spring most of
the chlorine is in a form that can destroy ozone.

The story takes place in six acts, some of them occurring
simultaneously on parallel stages:

i. The Polar Vortex

 As the air in the antarctic stratosphere cools and descends during
the winter, the Coriolis effect sets up a strong westerly
circulation around the pole. When the sun returns in the spring the
winds weaken, but the vortex remains stable until November. The air
over antarctica is largely isolated from the rest of the
atmosphere, forming a gigantic reaction vessel. The vortex is not
circular, it has an oblong shape with the long axis extending out
over Patagonia.

(For further information about the dynamics of the polar vortex see
[Schoeberl and Hartmann], [Tuck 1989], [AASE], [Randel], [Plumb],
and [Waugh]). There is currently some controversy over just how isolated
the air in the vortex is. According to Tuck, the vortex is better
thought of as a flow reactor than as a containment vessel; ozone-rich
air enters the vortex from above while ozone-poor and ClO-rich air is
stripped off the sides. Recent tracer measurements lend some support
to this view, but the issue is unresolved. See [Randel] and [Plumb].)

ii. Polar Stratospheric Clouds ("PSC")

 The Polar vortex is extremely cold; temperatures in the lower
stratosphere drop below -80 C. Under these conditions large numbers
of clouds appear in the stratosphere. These clouds are composed
largely of nitric acid and water, probably in the form of crystals
of nitric acid trihydrate ("NAT"), HNO3.3(H2O). Stratospheric
clouds also form from ordinary water ice (so-called "Type II PSC"),
but these are much less common; the stratosphere is very dry and
water-ice clouds only form at the lowest temperatures.

iii. Reactions Catalyzed by Stratospheric Clouds

 Most of the chlorine in the stratosphere ends up in one of the
reservoir compounds, Chlorine Nitrate (ClONO2) or Hydrogen Chloride
(HCl). Laboratory experiments have shown, however, that these
compounds, ordinarily inert in the stratosphere, do react on the
surfaces of polar stratospheric cloud particles. HCl dissolves into
the particles as they grow, and when a ClONO2 molecule becomes
adsorbed the following reactions take place:

 ClONO2 + HCl -> Cl2 + HNO3

 ClONO2 + H2O -> HOCl + HNO3

The Nitric acid, HNO3, stays in the cloud particle.

In addition, stratospheric clouds catalyze the removal of Nitrogen
Oxides ("NOx"), through the reactions:

 N2O5 + H2O -> 2 HNO3

 N2O5 + HCl -> ClNO2 + HNO3

Since N2O5 is in (gas-phase) equilibrium  with NO2:

 2 N2O5 <-> 4 NO2 + O2

this has the effect of removing NO2 from the gas phase and
sequestering it in the clouds in the form of nitric acid, a process
called "denoxification" (removal of "NOx").

iv. Sedimentation and Denitrification

 The clouds may eventually grow big enough so that they settle out
of the stratosphere, carrying the nitric acid with them
("denitrification"). Denitrification enhances denoxification.
If, on the other hand, the cloud decomposes while in the
stratosphere, nitrogen oxides are returned to the gas phase.
Presumably this should be called "renoxification", but
I have not heard anyone use this term :-).

v. Photolysis of active chlorine compounds

 The Cl2 and HOCl produced by the heterogeneous reactions are
easily photolyzed, even in the antarctic winter when there is
little UV present. The sun is always very low in the polar winter,
so the light takes a long path through the atmosphere and the
short-wave UV is selectively absorbed.  Molecular chlorine,
however, absorbs _visible_  and near-UV light:

     Cl2 + hv -> 2 Cl

     Cl + O3 -> ClO + O2

The effect is to produce large amounts of ClO. This ClO would
ordinarily be captured by NO2 and returned to the ClONO2 reservoir,
but "denoxification" and "denitrification" prevent this by removing
NO2.

vi. The chlorine peroxide mechanism

 As discussed in Part I, Cl and ClO can form a catalytic cycle that
efficiently destroys ozone. This cycle uses free oxygen atoms,
however, which are only abundant in the upper stratosphere, whereas
the ozone hole forms in the lower stratosphere. Instead, the
principal mechanism involves chlorine peroxide, ClOOCl (often
referred to as the "ClO dimer"):

     ClO + ClO -> ClOOCl
     ClOOCl + hv -> Cl + ClO2
     ClO2 -> Cl + O2
     2 Cl + 2 O3 -> 2 ClO + 2 O2
    -------------------------------
    Net: 2 O3 -> 3 O2

 At polar stratospheric temperatures this sequence is extremely fast
and it dominates the ozone-destruction process. The second step,
photolysis of chlorine peroxide, requires UV light which only
becomes abundant in the lower stratosphere in the spring. Thus one
has a long buildup of ClO and ClOOCl during the winter, followed by
massive ozone destruction in the spring. This mechanism is believed
to be responsible for about 70% of the antarctic ozone loss.

Another mechanism that has been identified involves chlorine and
bromine:

     ClO + BrO -> Br + Cl + O2
     Br + O3 -> BrO + O2
     Cl + O3 -> ClO + O2
     -----------------------
    Net: 2 O3 -> 3 O2

 This is believed to be responsible for ~25% of the antarctic
ozone depletion. Additional mechanisms have been suggested, but
they seem to be less important. [WMO 1991]

(For further information on the "perturbed chemistry" of the
antarctic stratosphere, see [Solomon], [McElroy and Salawich],
and [WMO 1989, 1991]).


7. What is the evidence for the present theory?

The evidence is overwhelming - the results from a single 1987
expedition (albeit a crucial one) fill two entire issues of the
Journal of Geophysical Research. What follows is a very sketchy
summary; for more information the reader is directed to [Solomon]
and to [Anderson et al.].

 The theory described above (which is often called the
"PSC theory") was developed during the years 1985-87. At the same
time, others proposed completely different mechanisms, making no
use of chlorine chemistry. The two most prominent alternative
explanations were one that postulated large increases in nitrogen
oxides arising from enhanced solar activity, and one that
postulated an upwelling of ozone-poor air from the troposphere into
the cold stratospheric vortex.  Each hypothesis made definite
predictions, and a program of measurements was carried out to test
these. The solar activity hypothesis predicted enhanced NOx, whereas
the measurements show unusually _low_ NOx ("denoxification), in
accordance with the PSC hypothesis.  The "upwelling" hypothesis
predicted upward air motion in the lower stratosphere, which is
inconsistent with measurements of atmospheric tracers such as
N2O which show that the motion is primarily downwards.

 Positive evidence for the PSC theory comes from ground-based and
airborne observations of the various chlorine-containing compounds.
These show that the reservoir species HCl and ClONO2 are extensively
depleted in the antarctic winter and spring, while the concentration
of the active, ozone-depleting species ClO is strongly enhanced.
Measurements also show enormously enhanced concentrations of the
molecule OClO. This is formed by a side-reaction in the BrO/ClO
mechanism described above.

 Further evidence comes from laboratory studies. The gas-phase
reactions have been reproduced in the laboratory, and shown to
proceed at the rates required in order for them to be important in
the polar stratosphere. [Molina et al. 1990] [Sander et al.]
[Trolier et al.] [Anderson et al.]. The production of active
chlorine from reservoir chlorine on ice and sulfuric acid surfaces
has also been demonstrated in the laboratory [Tolbert et al.
1987,1988] [Molina et al. 1987]. (Recently evidence for these
reactions has been found in the arctic stratosphere as well: air
parcels that had passed through regions where the temperature
was low enough to form PSC's were found to have anomalously
low concentrations of HCl and anomalously high concentrations
of ClO [AASE].)

 The "smoking gun", however, is usually considered to be the
simultaneous in-situ measurements of a variety of trace gases from
an ER-2 stratospheric aircraft (a converted U2 spy plane) in
August-October 1987. [Tuck et al.] These measurements demonstrated a
striking "anticorrelation" between local ozone concentrations and ClO
concentrations. Upon entering the "hole", ClO concentrations
suddenly jump by a factor of 20 or more, while ozone concentrations
drop by more than 50%. Even the local fluctuations in the
concentrations of the two species are anticorrelated. [Anderson et al.]

 In summary, the PSC theory explains the following observations:

1. The ozone hole occupies the region of the polar vortex where
   temperatures are below -80 C and where polar stratospheric clouds
   are abundant.

2. The ozone hole is confined to the lower stratosphere.

3. The ozone hole appears when sunlight illuminates the vortex, and
   disappears soon after temperatures rise past -80 C, destroying PSC's.

4. The hole is associated with extremely low concentrations of NOx.

5. The hole is associated with very low concentrations of the chlorine
   "reservoirs", HCl and ClONO2, and very high concentrations of active
   chlorine compounds, ClO, and byproducts such as OClO.

6. Inside the hole, the concentrations of ClO and ozone are precisely
    anticorrelated, high ClO being accompanied by low ozone.

7. Laboratory experiments demonstrate that chlorine reservoir compounds
   do react to give active chlorine on the surfaces of ice particles.

8. Airborne measurements in the arctic stratosphere show that air
   which has passed through regions containing PSC's is low in
   reservoir chlorine and high in active chlorine.

The antarctic ozone hole, once a complete mystery, is now
one of the best understood aspects of the entire subject; it is
much better understood than the small but steadily growing ozone
depletion at mid latitudes, for example.


8. Will the ozone hole keep growing?

 To answer this, we need to consider separately the lateral
dimensions (the "area" of the hole), the vertical dimension (its
"depth") and the temporal dimension (how long the hole lasts.)

a.) Lateral Extent

 Let us define the "hole" to be the
region where the total ozone column is less than 200 DU,
i.e. where total ozone has fallen to less than 2/3 of normal
springtime antarctic values. Defined thus, the hole is always
confined to the south polar vortex, south of ~55 degrees. At
present it does not fill the whole vortex, only the central core
where stratospheric temperatures are less than ~-80 C. Typically
this region is south of ~65 degrees, although there is a great deal
of variation - in some years the center of the vortex is displaced
well away from the pole, and the nominal boundary of the hole has
on a few occasions passed over the tip of Chile. As stratospheric
chlorine continues to rise, the hole might "fill out" the vortex;
this could as much as double its area. [Schoeberl and Hartmann]. So
far this does not seem to be happening. The 1992 hole was 15-25%
larger than previous years, and the 1993 hole appears to be almost
as large. This increase is probably due to the
stratospheric sulfate aerosols from the July 1991 eruption of Mt.
Pinatubo, which behave in some respects like polar stratospheric
clouds. [Solomon et al. 1993] These aerosols settle out of the
stratosphere after 2-3 years, so the increases seen in 1992 are
expected to be temporary. In any case, it cannot grow beyond
~55 degrees without a major change in the antarctic wind patterns
that  would allow the vortex to grow. Such a change could
conceivably accompany global warming: the greenhouse effect warms
the earth's surface, but _cools_ the stratosphere. There is no
reason to expect the hole to expand out over Australia, S. Africa,
etc., although these regions could experience further ozone
depletion after the hole breaks up and the ozone-poor air drifts
north.

b. Vertical Depth

 The hole is confined to the lower stratosphere, where the
clouds are abundant. In this region the ozone is essentially
gone. The upper stratosphere is much less affected, however, so
that overall column depletion comes to ~50%. As stratospheric
chlorine concentrations continue to increase over the next 10
years or so, some penetration to higher altitudes may take place,
but large increases in depth are not expected.  (Once again,
aerosols from Mt. Pinatubo have allowed the 1992 and 1993 holes
to extend over a larger altitude range than usual, both higher
and lower, but this is probably a temporary effect.)

c. Duration of the hole

 Here we might see major effects. The hole is destroyed in late
spring/early summer when the vortex breaks up and warm, ozone-rich
air rushes in. If the stratosphere cools, the vortex becomes more
stable and lasts longer. As mentioned above, the greenhouse effect
actually cools the stratosphere. There is a more direct cooling
mechanism, however - remember that the absorption of solar UV by
ozone is the major source of heat in the stratosphere, and is the
reason that the temperature of the stratosphere increases with
altitude. Depletion of the ozone layer therefore cools the
stratosphere, and in this sense the hole is self-stabilizing. In
future years we might see more long-lived holes like that in 1990,
which survived into early December.

 (The relationship between ozone depletion and climate change is
 complicated, and best dealt with in a separate FAQ, preferably
 written by someone other than myself :-) )


9. Why be concerned about an ozone hole over antarctica?
   Nobody lives down there.

 First of all, even though the ozone hole is confined to the
antarctic, its effects are not. After the hole breaks up in the
spring, ozone-poor air drifts north and mixes with the air there,
resulting in a transient decrease at middle and high latitudes.
This has been seen as far north as Australia [WMO 1991][Roy et al.]
[Atkinson et al.] On a time scale of months short-wave UV
regenerates the ozone, but it is believed that this "dilution" may
be a major cause of the much smaller _global_ ozone depletion, ~3%
per decade, that has been observed. Moreover, the air from the
ozone hole is also rich in ClO and can destroy more ozone as it
mixes with ozone-rich air. Even during the spring, the air in
the vortex is not _completely_ isolated, although there is some
controversy over the extent to which the ozone hole acts as
a "chemical processor" for the earth's atmosphere.
([Tuck 1989] [Schoeberl and Hartmann] [AASE] [Randel] [Waugh].)

 From a broader standpoint, the ozone hole is a distant early
warning message. Because of its unusual meteorological properties
the antarctic stratosphere is especially sensitive to chemical
perturbations; the natural mechanisms by which chlorine is
sequestered in reservoirs fail when total stratospheric chlorine
reaches about 2 parts per billion. This suggests that allowing
CFC emissions to increase by 3% per year, as was occurring during
the 1980's, is unwise, to say the least. The emission reduction
schedules negotiated under the Montreal Protocol (as revised in
1990 and 1992) lead to a projected maximum of ~4 ppb total strat.
chlorine in the first decade of the 21st century, followed by a
gradual decrease. Letting emissions increase at 3%/year would have
led to >16 ppb total stratospheric chlorine by 2040, and even a
freeze at 1980 rates would have led to >10 ppb. [Prather et al.].


10. Is there an ozone hole in the arctic? if not, why not?

There is no _massive_ ozone loss in the arctic, although there _is_
unusually large springtime ozone depletion, so the word "hole" is
not appropriate. I like the expression "arctic ozone dimple" but
this is not canonical :-). The arctic polar vortex is much weaker
than the antarctic, arctic temperatures are several degrees higher,
and polar stratospheric clouds are much less common and tend to break
up earlier in the spring.) [Salby and Garcia] Thus even though
wintertime ClO gets very high, as high as antarctic ClO in 1991-2, it
does not remain high through the spring, when it counts. [AASE]

Recent UARS measurements, however, indicate that in 1993 arctic
stratosphere temperatures stayed low enough to retain PSC's until
late February, and ClO remained high into March. Large ozone
depletions, ~10-20%, were reported for high latitudes in the
Northern Hemisphere; these still do not qualify as an "ozone hole"
but they do seem to indicate that the same physics and chemistry
are operating, albeit with much less efficiency. [Waters et al.]
[Gleason et al.]

If "global warming" does indeed take place during the first
few decades of the next century, we may see a dramatic change in
arctic ozone. The greenhouse effect warms the surface of the
earth, but at the same time _cools_ the stratosphere. Since there
is much less air in the stratosphere,  2-3 degrees of surface
warming corresponds to a much larger decrease in stratospheric
temperatures, as much as 10 degrees. This could lead to a true
ozone hole in the arctic, although it would still probably be
smaller and weaker than the antarctic hole. [Austin et al.]

The 27 August issue of _Science_ magazine contains 8 papers devoted
to arctic ozone depletion in the winter of 1991-92. [AASE]


11. Can the hole be "plugged"?

The present ozone hole, while serious, is not in itself
catastrophic. UV radiation is always low in polar regions since the
sun takes a long path through the atmosphere and hence through the
ozone layer. There may be serious consequences for marine life in
the antarctic ocean, which is adapted to the normally low UV
levels. When the hole breaks up in summer, there may be temporary
increases in UV-b at high latitudes of the southern hemisphere as
air that is poor in ozone and rich in "active", ozone-destroying
forms of chlorine mixes with the air outside.

 Nevertheless it looks like we are stuck with the hole for the
next 50 years at least, and we don't know what new surprises the
atmosphere has in store for us. Thus, some atmospheric scientists
have been exploring the possibility of "fixing" the hole by
technological means. All such schemes proposed so far are highly
controversial, and there are no plans to carry any of them out
until the chemistry and dynamics of the stratosphere are much
better understood than they are at present.

 It should be made clear at the beginning that there is no
point in trying to replace the ozone directly. The amounts are far
too large to be transported to the stratosphere, and the antarctic
mechanisms are so fiendishly efficient that they will easily
destroy added ozone (recall that where the catalytic cycles
operate, ~95% of the ozone is gone, in spite of the fact that the
sun is generating it all the time.) It is far better to try to
remove the halogen catalysts.  One suggestion made a few years ago
was to release sodium metal into the stratosphere, in hopes that it
would form sodium chloride crystals which would settle out. The
problem is that the microcrystals remain suspended as long as they
are small, and can play the same role as clouds and aerosols in
converting reservoir chlorine to active chlorine.

 A second suggestion is to destroy the CFC's while they are
still in the troposphere, by photolyzing them with high-powered
infrared lasers installed on mountainsides. (CFC's and similar
molecules can absorb as many as 30 infrared photons
from a single laser pulse, a phenomonon known as infrared
multiphoton dissociation). The chlorine atoms released would
quickly be converted to HCl and rained out. The power requirements
of such a project are daunting, however, and it appears that much
of the laser radiation would be shifted out of the desired
frequency range by stimulated raman scattering. [Stix]

 A more serious possibility is being explored by one of the
discoverers of chlorine-catalyzed ozone depletion, Ralph Cicerone,
together with Scott Elliot and Richard Turco [Cicerone et al.
1991,1992]. They considered the effects of dumping ~50,000 tons of
ethane or propane, several hundred planeloads, into the antarctic
stratosphere every spring. The hydrocarbons would react rapidly
with the Cl-containing radicals to give back the reservoir HCl. The
hydrocarbons themselves are fairly reactive and would decompose by
the end of a year, so the treatment would have to be repeated
annually. The chlorine would not actually be removed from the
stratosphere, but it would be bound up in an inert form - in other
words, the catalyst would be "poisoned". There are
no plans to carry this or any other scheme out in the near future;
to quote from Cicerone et al. (1991), "Before any actual injection
experiment is undertaken there are many scientific, technical,
legal and ethical questions to be faced, not the least of which is
the issue of unintended side effects."


REFERENCES FOR PART III

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival sources such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". This gives very short
shrift to much important work; for example, I say very little about
stratospheric NOx, even though a detailed accounting of chemistry
and transport of the nitrogen oxides is one of the major goals
of current research. Readers who want to see "who did what" should
consult the review articles listed below, or, if they can get them,
the extensively documented WMO reports.


Introductory Reading:

[Graedel and Crutzen] T. Graedel and P. Crutzen, _Atmospheric
 Change: an Earth System Perspective_, Freeman, 1993.

[Hamill and Toon] P. Hamill and O. Toon, "Polar stratospheric
clouds and the ozone hole", _Physics Today_ December 1991.

[Stolarski] Richard Stolarski, "The Antarctic Ozone Hole", _Sci.
American_ 1 Jan. 1988. (this article is now seriously out of date,
but it is still a good place to start).

[Toon and Turco] O. Toon and R. Turco, "Polar Stratospheric Clouds
and Ozone Depletion", _Sci. Am._ June 1991

[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993, pp. 9-18.

-----------------------------------------
Books and Review Articles:

[Anderson, Toohey and Brune] J.G. Anderson, D. W. Toohey, and W. H.
Brune,  "Free Radicals within the Antarctic vortex: the role of
CFC's in Antarctic Ozone Loss", _Science_ _251_, 39 (4 Jan. 1991).

[McElroy and Salawich] M. McElroy and R. Salawich, "Changing
Composition of the Global Stratosphere", _Science_ _243, 763, 1989.

[Solomon] S. Solomon, "Progress towards a quantitative
understanding of Antarctic ozone depletion",
_Nature_ _347_, 347, 1990.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd.  Ed.,
Oxford, 1991, Ch. 4.

[WMO 1989] World Meteorological Organization Global Ozone Research
and Monitoring Project - Report #20, "Scientific Assessment of
Stratospheric Ozone: 1989".

[WMO 1991] World Meteorological Organization Global Ozone Research
and Monitoring Project - Report #25, "Scientific Assessment of
Ozone Depletion: 1991".

-------------------------
More Specialized:

[AASE] Papers resulting from the Second Airborne Arctic Stratosphere
 Expedition, published in _Science_ _261_, 1128-1157, 27 Aug. 1993.

[Atkinson et al.] R. J. Atkinson, W. A. Matthews, P. A. Newman,
 and R. A. Plumb, "Evidence of the mid-latitude impact of Antarctic
 ozone depletion", _Nature_ _340_, 290, 1989.

[Austin et al.] J. Austin, N. Butchart, and K. P. Shine,
"Possibility of an Arctic ozone hole in a doubled-CO2 climate",
_Nature_ _360_, 221, 1992.

[Cicerone et al. 1991] R. Cicerone, S. Elliot, and R. Turco,
"Reduced Antarctic Ozone Depletions in a Model with Hydrocarbon
Injections", _Science_ _254_, 1191, 1991.

[Cicerone et al. 1992] R. Cicerone, S. Elliot, and R. Turco,
"Global Environmental Engineering", _Nature_ _356_, 472, 1992.

[Dobson] G. M. B. Dobson, "Forty Years' research on atmospheric
ozone at Oxford", _Applied Optics_, _7_, 387, 1968.

[Farman et al.] J. C. Farman, B. G. Gardiner, and J. D. Shanklin,
"Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx
interaction", _Nature_ _315_, 207, 1985.

[Frederick and Alberts] J. Frederick and A. Alberts, "Prolonged
 enhancement in surface ultraviolet radiation during the Antarctic
 spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.

[Gleason et al.] J. Gleason, P. Bhatia, J. Herman, R. McPeters, P.
Newman, R. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C.
Wellemeyer, W. Komhyr, A. Miller, and W. Planet, "Record Low Global
Ozone in 1992", _Science_ _260_, 523, 1993.

[Molina et al. 1987] M. J. Molina, T.-L. Tso, L. T. Molina, and
F.C.-Y. Yang, "Antarctic stratospheric chemistry of chlorine
nitrate, hydrogen chloride, and ice: Release of active chlorine",
_Science_ _238_, 1253, 1987.

[Molina et al. 1990] M. Molina, A. Colussi, L. Molina, R.
Schindler, and T.-L. Tso, "Quantum yield of chlorine atom formation
in the photodissociation of chlorine peroxide (ClOOCl) at 308 nm",
_Chem. Phys. Lett._ _173_, 310, 1990.

[Plumb] A. Plumb, "Mixing and Matching",
_Nature_ _365_, 489-90, 1993. (News and Views)

[Prather et al.] M.J. Prather, M.B. McElroy, and S.C. Wofsy,
"Reductions in ozone at high concentrations of stratospheric
 halogens", _Nature_ _312_, 227, 1984.

[Randel] W. Randel, "Ideas flow on Antarctic vortex",
_Nature_ _364_, 105, 1993 (News and Views)

[Roy et al.] C. Roy, H. Gies, and G. Elliott, "Ozone Depletion",
 _Nature_ _347_, 235, 1990. (Scientific Correspondence)

[Salby and Garcia] M. L. Salby and R. R. Garcia, "Dynamical Perturbations
to the Ozone Layer", _Physics Today_ _43_, 38, March 1990.

[Sander et al.] S.P. Sander, R.J. Friedl, and Y.K. Yung, "Role of
the ClO dimer in polar stratospheric chemistry: rate of formation
and implications for ozone loss", _Science_ _245_, 1095, 1989.

[Schoeberl and Hartmann] M. Schoeberl and D. Hartmann, "The
dynamics of the stratospheric polar vortex and its relation to
springtime ozone depletions", _Science_ _251_, 46, 1991.

[Solomon et al. 1993] S. Solomon, R. Sanders, R. Garcia, and J.
Keys, "Increased chlorine dioxide over Antarctica caused by
volcanic aerosols from Mt. Pinatubo", _Nature_ _363_, 245, 1993.

[Stix] T. H. Stix, "Removal of Chlorofluorocarbons from the
 earth's atmosphere", _J. Appl. Physics_ _60_, 5622, 1989.

[Stolarski et al. 1992]  R. Stolarski, R. Bojkov, L. Bishop, C.
Zerefos, J. Staehelin, and J. Zawodny, "Measured Trends in
Stratospheric Ozone",  Science _256_, 342 (17 April 1992)

[Tolbert et al. 1987] M.A. Tolbert, M.J. Rossi, R. Malhotra, and
D.M. Golden, "Reaction of chlorine nitrate with hydrogen chloride
and water at Antarctic stratospheric temperatures", _Science_
_238_, 1258, 1987.

[Tolbert et al. 1988] M.A. Tolbert, M.J. Rossi, and D.M. Golden,
"Antarctic ozone depletion chemistry: reactions of N2O5 with H2O
and HCl on ice surfaces", _Science_ _240_, 1018, 1988.

[Trolier et al.] M. Trolier, R.L. Mauldin III, and A. Ravishankara,
"Rate coefficient for the termolecular channel of the self-reaction
of ClO", _J. Phys. Chem._ _94_, 4896, 1990.

[Tuck 1989] A. F. Tuck, "Synoptic and Chemical Evolution of the
Antarctic Vortex in late winter and early spring, 1987: An ozone
processor", J. Geophys. Res. _94_, 11687, 1989.

[Tuck et al.] A. F. Tuck, R. T. Watson, E. P. Condon, and J. J.
Margitan, "The planning and execution of ER-2 and DC-8 aircraft
flights over Antarctica, August and September, 1987"
J. Geophys. Res. _94_, 11182, 1989.

[Waters et al.] J. Waters, L. Froidevaux, W. Read, G. Manney, L.
Elson, D. Flower, R. Jarnot, and R. Harwood, "Stratospheric ClO and
ozone from the Microwave Limb Sounder on the Upper Atmosphere
Research Satellite", _Nature_ _362_, 597, 1993.

[Waugh] D. W. Waugh, "Subtropical stratospheric mixing linked to
disturbances in the polar vortices", _Nature_ _365_, 535, 1993.









-------------------------------------------------------------------------------
Area # 2120  news.answers           05-26-94 15:54      Message # 13067
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Subj    : Ozone Depletion FAQ Part

 @SUBJECT:Ozone Depletion FAQ Part IV: UV Radiation and its Effects
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***********************************************************************
* Copyright 1994 Robert Parson                                        *
*                                                                     *
* This file may be distributed, copied, and archived. All             *
* copies must include this notice and the paragraph below entitled    *
* "Caveat". Reproduction and distribution for personal profit is      *
* not permitted. If this document is transmitted to other networks or *
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***********************************************************************


This file deals with the physical properties of ultraviolet
radiation and its biological consequences, emphasizing the
possible effects of stratospheric ozone depletion. It frequently
refers back to Part I, where the basic properties of the ozone
layer are described; the reader should look over that file first.

The overall approach I take is conservative. I concentrate on what
is known and on most probable, rather than worst-case, scenarios.
For example, I have relatively little to say about the
effects of UV radiation on plants - this does not mean that the
effects are small, it means that they are as yet not well
quantified (and moreover, I am not well qualified to interpret the
literature.) Policy decisions must take into account not only the
most probable scenario, but also  a range of less probable ones.
will probably do, but also the worst that he could possibly do.
There have been surprises, mostly unpleasant, in this field in the
past, and there are sure to be more in the future. In general,
_much_ less is known about biological effects of UV-B than about
the physics and chemistry of the ozone layer.


| _Caveat_: I am not a specialist. In fact, I am not an atmospheric
| scientist at all - I am a physical chemist studying gas-phase
| reactions who talks to atmospheric scientists. In this part in
| particular I am well outside the range of my own expertise.
| I have discussed some aspects of this subject with specialists,
| but I am solely responsible for everything written here, including
| any errors. This document should not be cited in publications off
| the net; rather, it should be used as a pointer to the published
| literature.

*** Corrections and comments are welcomed.


- Robert Parson
  Associate Professor
  Department of Chemistry and Biochemistry,
  University of Colorado (for which I do not speak)

  [e-mail]
  [e-mail]

CONTENTS

 1. What is "UV-B"?

 2. How does UV-B vary from place to place?

 3. *Is* UV-B increasing?

 4. What is the relationship between UV radiation and skin cancer?

 5. Is ozone loss responsible for the melanoma upsurge?

 6. Does UV Radiation cause cataracts?

 7. Are sheep going blind in Chile?

 8. What effects does increased UV have on agriculture?

 9. What effects does increased UV have on marine life?

10. Is UV-B responsible for the amphibian decline?

 References



1. What is "UV-B"?

 "UV-B" refers to UV light having a wavelength between 280 and
320 nm. These wavelengths are on the lower edge of ozone's UV
absorption band, in the so-called "Huggins bands". They are
absorbed by ozone, but less efficiently than shorter wavelengths
("UV-C").  (The absorption cross-section of ozone increases by more
than 2 orders of magnitude between 320 nm and the peak value at
~250 nm.)  Depletion of the ozone layer would first of all result
in increased UV-B. In principle UV-C would also increase, but it is
absorbed so efficiently that a very large depletion would have to
take place in order for significant amounts to reach the earth's
surface. UV-B and UV-C are absorbed by DNA and other biological
macromolecules, inducing photochemical reactions. UV radiation with
a wavelength longer than 320 nm is called "UV-A".  It is not
absorbed by ozone, but it is not believed to be especially
dangerous. (See, however, question #6.)


2. How does UV-B vary from place to place?

 A great deal.  It is strongest at low latitudes and high altitudes.
At higher latitudes, the sun is always low in the sky so that it takes
a longer path through the atmosphere and more of the UV-B is absorbed.
For this reason, ozone depletion is likely to have a greater impact on
_local_ ecosystems, such as terrestrial plants and the Antarctic marine
phytoplankton, than on humans or their livestock.
UV also varies with altitude and local cloud cover. These trends can
be seen in the following list of annually-averaged UV indices for
several US cities  [Roach] (units are arbitrary - I don't know
precisely how this index is defined though I assume it is
proportional to some integral over the UV-b region of the spectrum)

 Minneapolis, Minnesota         570
 Chicago, Illinois              637
 Washington, DC                 683
 San Francisco, California      715
 Los Angeles, California        824
 Denver, Colorado               951
 Miami, Florida                 1028
 Honolulu, Hawaii               1147

 It should be noted that skin cancer rates show a similar trend.
 (See below).



3. Is UV-B at the earth's surface increasing?

Yes, in some places; no, in others.

Very large increases - up to a factor of 2 - have been seen even
in the outer portions of the Antarctic hole. [Frederick and
Alberts]

Small increases, of order 1% per  year, have been measured in the
Swiss Alps. [Blumthaler and Ambach] These _net_ increases are small
compared to natural day-to-day fluctuations, but they are actually
a little larger than would be expected from the amount of ozone
depletion over the same period.

In urban areas of the US, UV-B
levels showed no significant increase (and in most cases actually
decreased a little)  between 1974 and 1985. [Scotto et al.].  This
is probably due to increasing urban pollution, including low-level
ozone and aerosols. [Grant] Tropospheric ozone is actually
somewhat more  effective at absorbing UV than stratospheric ozone,
because UV light is scattered much more in the troposphere, and
hence takes a longer path. [Bruehl and Crutzen]  Increasing
amounts of tropospheric aerosols, from urban and industrial
pollution, may also offset UV-B increases at the ground.  [Liu et
al.] [Madronich 1992, 1993] [Grant] There have been questions about
the suitability of the instruments used by Scotto et al.; they were
not designed for measuring long-term trends, and they put too much
weight on regions of the UV spectrum which are not appreciably
absorbed by ozone in any case. [WMO 1989] Nevertheless it seems
clear that so far ozone depletion over US cities is small enough to
be largely offset by competing factors.  Tropospheric ozone and aerosols
have increased in rural areas of the US and Europe as well, so
these areas may also be screened from the effects of ozone depletion.

A recent study [Kerr and McElroy] has found convincing evidence of
UV-B increases in Toronto, Canada during the period 1989-1993. The UV
intensity at 300 nm increased by 35% per year in winter and 7% per
year in summer. At this wavelength 99% of the total UV is absorbed,
so these represent large increases in a small number, and do not
represent a health hazard; nevertheless these wavelengths play a
disproportionately large role in skin carcinoma and plant damage
since DNA absorbs strongly there. Total UV-B irradiance, weighted
in such a way as to correlate with incidence of sunburn ("erythemally
active radiation"), increased by 5% per year in winter and 2% per year
in summer.  These are not long-term trends; they are dominated by
the unusually large, but temporary, ozone losses in these
regions in the years 1992-1993 (see part I) and should therefore not
be extrapolated into the future. However they do provide evidence
of a link between ozone at middle latitudes and total UV-B radiation.
Increases of similar magnitude between 1992 and 1993 were seen in
Germany [Seckmeyer et al.]

Indirect evidence for increases has been obtained in the Southern
Hemisphere, where stratospheric ozone depletion is larger and
tropospheric ozone (and aerosol pollution) is lower. Biologically
weighted UV-B irradiances at a station in New Zealand were 1.4-1.8
times higher than irradiances at a comparable latitude and season in
Germany, of which a factor of 1.3-1.6 can be attributed to differences
in the ozone column over the two locations [Seckmeyer and McKenzie].
In the southern hemisphere summer, the noontime UV-B irradiance
at Ushaia in Tierra del Fuego is 45% above what would be predicted
were there no ozone depletion. [Frederick et al. 1993]

In comparing UV-B estimates, one must pay careful attention to
exactly what is being reported. One wants to know not just whether
there is an increase, but how much increase there is at any given
wavelength, since the shorter wavelengths are more dangerous.
Different measuring instruments have different spectral responses,
and are more or less sensitive to various spectral regions. [Wayne,
Rowland 1991].  Wavelength-resolving instruments, such as the
spectroradiometers being used in Antarctica, Argentina, and Toronto,
are the most informative, as they allow one to distinguish the
effects of ozone trends from those due to clouds and aerosols.
[Madronich 1993] [Kerr and McElroy].


4. What is the relationship between UV radiation and skin cancer?

There are three kinds of skin cancer, basal cell carcinomas,
squamous cell carcinomas, and melanomas. In the US there were
500,000 cases of the first, 100,000 of the second, and 27,600 of
the third in 1990. [Wayne] More than 90% of the skin carcinomas in
the US are attributed to UV-b exposure: their frequency varies
sharply with latitude, just as UV does. The mechanism by which UV-B
induces carcinomas has been identified - the pyrimidine bases
in the DNA molecule form dimers when stimulated by UV-B radiation.
[Tevini]. Fortunately, these cancers are relatively easy to treat
if detected in time, and are rarely fatal. Skin carcinoma rates vary
sharply with latitude, just as UV-B does. Fair-skinned people of
North European ancestry are particularly susceptible. The highest
rates in the world are found in Queensland, a northerly province of
Australia.

[Madronich and deGruiji] have estimated the expected increases in
skin carcinoma rates due to ozone depletion over the period 1979-1992:

 Lat.   % ozone loss    % increase in rate,     % increase in rate,
        1979-1992       basal cell carcinoma    squamous cell carcinoma

 55N    7.4 +-1.3       13.5 +-5.3              25.4 +-10.3
 35N    4.8 +-1.4        8.6 +-4.0              16.0 +-7.6
 15N    1.5 +-1.1        2.7 +-2.4               4.8 +-4.4

 15S    1.9 +-1.3        3.6 +-2.6               6.5 +-4.8
 35S    4.0 +-1.6        8.1 +-3.6              14.9 +-6.8
 55S    9.0 +-1.5       20.4 +-7.4              39.3 +-15.1

Of course, the rates themselves are much smaller at high latitudes,
where the relative increases in rates are large. These estimates do
not take changes in lifestyle into consideration.

Malignant melanoma is much more dangerous, but its connection
with UV exposure is not well understood. There seems to a correlation
between melanomas and brief, intense exposures to UV (long before
the cancer appears.) Melanoma incidence is definitely correlated with
latitude, with twice as many deaths (relative to state population)
in Florida or Texas as in Wisconsin or Montana, but this correlation
need not imply a causal relationship. Some claim that UV-A, which is
not absorbed by ozone, is involved. [Skolnick] [Setlow et al.]


5. Is ozone loss to blame for the melanoma upsurge?

A few physicians have said so, but most others think not.
[Skolnick]

First of all, UV-B has not, so far, increased very much, at least
in the US and Europe.

Second, melanoma takes 10-20 years to develop. There hasn't been
enough time for ozone depletion to play a significant role.

Third, the melanoma epidemic has been going on since the 1940's.
Recent increases in rates may just reflect better reporting, or
the popularity of suntans in the '60's and '70's. (This becomes
more likely if UV-A is in fact involved.)


6. Does UV-B cause cataracts?

While the evidence for this is indirect, it is very plausible.

The lens of the eye is a good UV-filter, protecting the delicate
structures in the retina. Too much UV results in short-term "snow
blindness", but the effects of prolonged, repeated exposure are
not known.  People living in naturally high UV environments such
as Bolivia or Tibet do have a high incidence of cataracts, and overall
cataracts are more frequently seen at lower latitudes. [Tevini]


7. Are sheep going blind in Chile?

If they are, it's not because of ozone depletion.

For a short period each year, the edge of the ozone hole passes
over Tierra del Fuego, at the southern end of the South American
continent. This has led to a flurry of reports of medical damage
to humans and livestock. Dermatologists claim that they are seeing
more patients with sun-related conditions, nursery owners report
damage to plants, a sailor says that his yacht's dacron sails have
become brittle, and a rancher declares that 50 of his sheep,
grazing at high altitudes, suffer "temporary cataracts" in the
spring. (_Newsweek_, 9 December 1991, p. 43; NY Times, 27 July
1991, p. C4; 27 March 1992, p. A7).

These claims are hard to believe. At such a high latitude,
springtime UV-B is naturally very low and the temporary increase
due to ozone depletion still results in a UV fluence that is well
below that found at lower latitudes. Moreover, the climate of
Patagonia is notoriously cold and wet. (There is actually more of
a problem in the summer, after the hole breaks up and ozone-poor
air drifts north. The ozone depletion is smaller, but the
background UV intensity is much higher.) There may well be effects
on _local_ species, adapted to low UV levels, but even these are
not expected to appear so soon. It was only in 1987 that the hole
grew large enough to give rise to significant UV increases
in southern Chile, and cataracts and malignant melanomas take many
years to develop. To be sure, people do get sunburns and
skin cancer even in Alaska and northern Europe, and all
else being equal one expects on purely statistical grounds such
cases to increase, from a small number to a slightly larger number.
All else is definitely not equal, however - the residents are now
intensely aware of the hazards of UV radiation and are likely to
protect themselves better. I suspect that the increase in
sun-related skin problems noted by the dermatologists comes about
because more people are taking such cases to their doctors.

As for the blind sheep, a group at Johns Hopkins has investigated
this and ascribes it to a local infection ("pink eye"). [Pearce]

This is _not_ meant to dismiss UV-B increases in Patagonia as
insignificant. Damage to local plants, for example, may well emerge
in the long term, as the ozone hole is expected to last for 50
years or more. The biological consequences of UV radiation are real,
but often very subtle; I personally find it hard to believe that
such effects are showing up so soon, and in such a dramatic fashion.
Ozone depletion is a real problem, but this particular story is a red
herring.


8. What effects does increased UV have upon plant life?

Generally harmful, but hard to quantify. Many experiments have
studied the response of plants to UV-B radiation, either by
irradiating the plants directly or by filtering out some of the UV
in a low-latitude environment where it is naturally high. The
artificial UV sources do not have the same spectrum as solar
radiation, however, while the filtering experiments do not
necessarily isolate all of the variables, even when climate
and humidity are controlled by growing the plants in a greenhouse.

 Out of some 200 agricultural plants tested, more than half show
sensitivity to UV-B increases. The measured effects vary markedly
from one species to another; some adapt very readily while others are
seriously damaged. Even within species there are marked differences;
for example, one soybean variety showed a 25% growth reduction under a
simulated ozone depletion of 16%, whereas another variety showed no
significant yield reduction. The general sense seems to be that
ozone depletion amounting to 10% or more could seriously affect
agriculture. Smaller depletions could have a severe impact on local
ecosystems, but very little is known about this at present.

I have not investigated the literature on this in detail, not
being a biologist. Interested readers should consult [Tevini and
Teramura] or the book by [Tevini] and the references therein.
If any botanist out there would like to write a summary for
this FAQ, please let me know.


9. What effects does increased UV have on marine life?

Again, generally harmful but hard to quantify. Seawater is
surprisingly transparent to UV-B. In clear waters radiation at 315
nm is attenuated by only 14% per meter depth. [Jerlov]. Many marine
creatures live in surface waters, and they have evolved a variety
of methods to cope with UV. Some simply swim to lower depths, some
develop protective coatings, some work at night to repair the
damage done during the day. These natural mechanisms however, are
often triggered by _visible_ light intensities, in which case they
do not protect against an increase in the _ratio_ of UV to visible
light. Also, if a photosynthesizing organism protects itself by
staying at lower depths, it will get less visible light and produce
less oxygen. An increase in UV-B can thus affect an ecosystem
without necessarily killing off individual organisms.

Many experiments have been carried out to determine the
response of various marine creatures to UV radiation; as with land
plants the effects vary a great deal from one species to another,
and it is difficult to draw general conclusions at this stage. We
can infer that organisms that live in tropical waters are safe,
since there is little or no ozone depletion there, and that
organisms that are capable of living in the tropics are probably
safe from large depletions at high latitudes since UV intensities
at high latitudes are always low. (One must be a little careful
with the second inference if the organism's natural defenses are
stimulated by visible light.) The problems arise with organisms
that have adapted to the naturally low UV levels of polar regions.

In this case, we have a natural laboratory for studying UV
effects:  the Antarctic Ozone hole. (Part III of the FAQ discusses
the hole in detail.) The outer parts of the hole extend far out
into the ocean,  beyond the pack ice, and these waters get
springtime UV-B doses equal to or greater than what is
seen in a normal antarctic summer. [Frederick and Alberts] [Smith
et al.]. The UV in shallow surface waters is effectively even
higher, because the sea ice is more transparent in spring than in
summer. There has been speculation that this UV could cause a
population collapse in the marine phytoplankton, the microscopic
plants that comprise the base of the food chain.

To my knowledge, only one field study has been published so far.
[Smith et al.]. These workers measured the photosynthetic
productivity of the phytoplankton in the "marginal ice zone" (MIZ),
the layer of relatively  fresh meltwater that lies over saltier
deep water.  Since the outer boundary of the ozone hole is
relatively sharp and fluctuates from day to day, they were able to
compare photosynthesis inside and outside the hole, and to
correlate photosynthetic yield with shipboard UV measurements.
They concluded that the UV-B increase brought about an overall
decrease of 6-12% in phytoplankton productivity. Since the "hole"
lasts for about 10-12 weeks, this corresponds to an overall decrease
of 2-4% for the year. The natural variability in phytoplankton
productivity from year to year is estimated to be about + or - 25%,
so the _immediate_ effects of the ozone hole, while real, are far
from catastrophic. To quote from [Smith et al.]:  "Our estimated
loss of 7 x 10^12 g of carbon per year is about three orders
of magnitude smaller than estimates of _global_ phytoplankton
production and thus is not likely to be significant in this
context. On the other hand, we find that the O3-induced loss to a
natural community of phytoplankton in the MIZ is measurable and the
subsequent ecological consequences of the magnitude and timing of
this early spring loss remain to be determined."  It appears, then,
that overall loss in productivity is not large - yet. (The
cumulative effects on the marine community are not known. The ozone
hole first became large enough to expose marine life to large UV
increases in 1987, and [Smith et al.] carried out their survey in
1990.) Ecological consequences - the displacement of UV-sensitive
species by UV-tolerant ones - are likely to be more important than
a decline in overall productivity, although they are poorly
understood at present.

10. Is UV-B responsible for the amphibian decline?

UV-B may be part of the story, although it is unlikely to be the
principal cause of this mysterious event.

During the past decade, there has been a widespread decline in
amphibian populations [Livermore] [Wake]. The decline appears to be
global in scope, although some regions and many species appear to be
unaffected.  While habitat destruction is undoubtedly an important
factor, many of the affected species are native to regions where
habitat is relatively undisturbed. This has led to speculation that
global perturbations, such as pesticide pollution, acid deposition,
and climate change, could be involved.

Recently, [Blaustein et al.] have investigated the effects of UV-B
radiation on the reproduction of amphibians living in the Cascade
Mountains of Oregon. In their first experiment, the eggs of several
amphibian species were analyzed for an enzyme that is known to
*repair* UV-induced DNA damage. The eggs of the Cascades frog,
R. cascadae, and of the Western toad, Bufo Boreas, showed low levels
of this enzyme; both species are known to be in serious decline
(R. Cascadae populations have fallen by ~80% since the 1970's [Wake].)
In contrast, much higher levels of the enzyme are found in the eggs of
the Pacific Tree Frog, _Hyla Regilla_, whose populations do not appear
to be in decline.

Blaustein et al. then studied the effects of UV-B upon the
reproductive success of these species in the field, by screening the
eggs with a filter that blocks the ambient UV. Two control groups were
used for comparison; in one no filter was present and in the other a
filter that *transmitted* UV-B was put in place.  They found that for
the two species that are known to be in decline, and that showed low
levels of the repair enzyme, filtering the UV dramatically increased
the proportion of eggs surviving until hatch, whereas for the species
that is not in decline and that produces high levels of the enzyme,
filtering the UV made little difference. Thus, both the laboratory and
the field experiments suggest a correlation between amphibian declines
and UV sensitivity, albeit a correlation that at present is based on a
very small number of species and a limited time period.

Contrary to the impression given by some media reports, Blaustein and
coworkers did *not* claim that ozone depletion is "the cause" of the
amphibian decline. The decline appears to be world-wide, whereas ozone
depletion is restricted to middle and high latitudes. Also, many
amphibian species lay their eggs under dense canopies or underground
where there is little solar radiation. So, UV should be regarded
as one of many stresses that may be acting on amphibian populations.


_____________________________________________________________________

 REFERENCES FOR PART IV

A remark on references: they are neither representative nor
comprehensive. There are _hundreds_ of people working on these
problems. For the most part I have limited myself to papers that
are (1) widely available (if possible, _Science_ or _Nature_ rather
than archival journals such as _J. Geophys. Res._) and (2) directly
related to the "frequently asked questions". Readers who want to
see "who did what" should consult the review articles listed below,
or, if they can get them, the WMO reports which are extensively
documented.


Introductory Reading:

[Graedel and Crutzen] T. E. Graedel and P. J. Crutzen,
_Atmospheric Change: an Earth System Perspective_, Freeman, NY
1993.

[Rowland 1989] F. S. Rowland, "Chlorofluorocarbons and the
depletion of stratospheric ozone", _American Scientist_ _77_, 36,
1989.

[Zurer] P. S. Zurer, "Ozone Depletion's Recurring Surprises
Challenge Atmospheric Scientists", _Chemical and Engineering News_,
24 May 1993,  pp. 9-18.

----------------------------
Books and Review Articles:

[Chamberlain and Hunten] J. W. Chamberlain and D. M. Hunten,
_Theory of Planetary Atmospheres_, 2nd Edition, Academic Press, 1987

[Dobson] G.M.B. Dobson, _Exploring the Atmosphere_, 2nd Edition,
 Oxford, 1968.

[Rowland 1991] F. S. Rowland, "Stratospheric Ozone Depletion",
_Ann. Rev. Phys. Chem._ _42_, 731, 1991.

[Tevini] M. Tevini, editor: "UV-B Radiation and Ozone Depletion:
Effects on humans, animals, plants, microorganisms, and materials"
Lewis Publishers, Boca Raton, 1993.

[Wayne] R. P. Wayne, _Chemistry of Atmospheres_, 2nd.  Ed.,
 Oxford, 1991.

[WMO 1988] World Meteorological Organization,
_Report of the International Ozone Trends Panel_,
 Global Ozone Research and Monitoring Project - Report #18.

[WMO 1989] World Meteorological Organization,
_Scientific Assessment of Stratospheric Ozone: 1989_
 Global Ozone Research and Monitoring Project - Report #20.

[WMO 1991] World Meteorological Organization,
_Scientific Assessment of Ozone Depletion: 1991_
 Global Ozone Research and Monitoring Project - Report #25.

-----------------------------------
More Specialized:

[Blaustein et al.] A. R. Blaustein, P. D. Hoffman, D. G. Hokit,
J. M. Kiesecker, S. C. Walls, and J. B. Hays, "UV repair and
resistance to solar UV-B in amphibian eggs: A link to population
declines?", _Proc. Nat. Acad. Sci._ _91_, 1791, 1994.

[Blumthaler and Ambach] M. Blumthaler and W. Ambach, "Indication of
increasing solar ultraviolet-B radiation flux in alpine regions",
_Science_ _248_, 206, 1990.

[Bruehl and Crutzen]  C. Bruehl and P. Crutzen, "On the
disproportionate role of tropospheric ozone as a filter against
solar UV-B radiation",_Geophys. Res. Lett._ _16_, 703, 1989.

[Frederick and Alberts] J.E. Frederick and A. Alberts, "Prolonged
 enhancement in surface ultraviolet radiation during the Antarctic
 spring of 1990", _Geophys. Res. Lett._ _18_, 1869, 1991.

[Frederick et al. 1993] J.E. Frederick, P.F. Soulen, S.B. Diaz,
I. Smolskaia, C.R. Booth, T. Lucas, and D. Neuschuler,
"Solar Ultraviolet Irradiance Observed from Southern Argentina:
 September 1990 to March 1991", J. Geophys. Res. _98_, 8891, 1993.

[Grant] W. Grant, "Global stratospheric ozone and UV-B radiation",
  _Science_ _242_, 1111, 1988. (a comment on [Scotto et al.])

[Jerlov] N.G. Jerlov, "Ultraviolet Radiation in the Sea",
_Nature_ _166_, 112, 1950.

[Kerr and McElroy] J. B. Kerr and C. T. McElroy, "Evidence for Large
 Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion",
 _Science_ _262_, 1032, 1993.

[Livermore] B. Livermore, "Amphibian alarm: Just where have all the
frogs gone?", _Smithsonian_, October 1992.

[Liu et al.] S.C. Liu, S.A. McKeen, and S. Madronich, "Effect of
 anthropogenic aerosols on biologically active ultraviolet
radiation", _Geophys. Res. Lett._  _18_,  2265, 1991.

[Madronich 1992] S. Madronich, "Implications of recent total
atmospheric ozone measurements for biologically active ultraviolet
radiation reaching the earth's surface",
_Geophys. Res. Lett. _19_, 37, 1992.

[Madronich 1993] S. Madronich, in [Tevini], above.

[Madronich and de Gruiji] S. Madronich and F. R. de Gruiji,
"Skin Cancer and UV radiation", _Nature_ _366_, 23, 1993.

[Pearce] F. Pearce, "Ozone hole 'innocent' of Chile's ills",
 _New Scientist_ #1887, 7, 21 Aug. 1993.

[Roach] M. Roach, "Sun Struck", _Health_, May/June 1992, p. 41.
 (See especially the sidebar by Steven Finch on p. 50).

[Scotto et al.] J. Scotto, G. Cotton, F. Urbach, D. Berger, and T.
Fears, "Biologically effective ultraviolet radiation: surface
measurements in the U.S.",  _Science_ _239_, 762, 1988.

[Seckmeyer et al.] G. Seckmeyer, B. Mayer, R. Erb, and G. Bernhard,
"UV-B in Germany higher in 1993 than in 1992", _Geophys. Res. Lett._
 _21_, 577-580, 1994.

[Seckmeyer and McKenzie] G. Seckmeyer and R. L. McKenzie,
"Increased  ultraviolet radiation in New Zealand (45 degrees S)
relative to Germany (48 degrees N.)", _Nature_ _359_, 135, 1992.

[Setlow et al.] R. B. Setlow, E. Grist, K. Thompson and
A. D. Woodhead, "Wavelengths effective in induction of Malignant
Melanoma", PNAS _90_, 6666, 1993.

[Skolnick] A. Skolnick,  "Is ozone loss to blame for melanoma
upsurge?" JAMA, _265_, 3218, June 26 1991.

[Smith et al.] R. Smith, B. Prezelin, K. Baker, R. Bidigare, N.
Boucher, T. Coley, D. Karentz, S. MacIntyre, H. Matlick, D.
Menzies, M. Ondrusek, Z. Wan, and K. Waters, "Ozone depletion:
Ultraviolet radiation and phytoplankton biology in antarctic
waters", _Science_ _255_, 952, 1992.

[Tevini and Teramura] M. Tevini and A. H. Teramura, "UV-B effects
on terrestrial plants", _Photochemistry and Photobiology_, _50_,
479, 1989. (This issue contains a number of other papers dealing
with biological effects of UV-B radiation.)

[Wake] D. B. Wake, "Declining Amphibian Populations", _Science_
 _253_, 860, 1991.




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