ATMOSPHERIC CHEMISTRY
Troposphere and Stratosphere
The troposphere is the lowest region of the atmosphere, extending from the
surface to about 8-15 km, and is the portion in which life and weather exist.
The stratosphere is the region of the Earth's atmosphere between roughly
12 and 50 km above the surface and is a crucial component in the Earth's
environment because the protective ozone layer that blocks solar ultraviolet
radiation resides in the stratosphere.
Tropospheric Chemistry
The chemical composition of the troposphere, the lowest region of the atmosphere
extending from the surface to about 8-15 km (depending on latitude), is
changing at unprecedented rates. Gaseous and particulate emissions from
a wide variety of human activities, such as energy use, agricultural production,
and so on, now exceed emissions from natural sources. The rate at which
these air pollutants are put into the troposphere also exceeds the capacity
of removal processes, resulting in an accumulation of gases: carbon dioxide,
methane, chlorofluorocarbons, and nitrous oxide. These gases contribute
to the widely discussed potential for climate change due to the greenhouse
effect.
The four images above depict measurements of seasonal variations in total
tropospheric ozone in tropical regions. Reds indicate higher concentrations
of tropospheric ozone and blues indicate lower levels. In the air we breathe,
ozone is a harmful pollutant that causes damage to lung tissue and plants.
One Dobson Unit refers to a layer of ozone that would be 0.001 cm thick
under conditions of standard temperature (0 degress Celcius) and pressure.
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Carbon Dioxide
The major gases in the atmosphere, nitrogen and oxygen, are transparent
to both the radiation incoming from the sun and the radiation outgoing from
the Earth, so they have little or no effect on the greenhouse warming. Greenhouse
gases such as carbon dioxide, methane, and chlorofluorocarbons have been
increasing steadily in the atmosphere. These gases block heat reflected
from the Earth's surface into the atmosphere, preventing it from escaping
into space, thus contributing to global warming and increasing the surface
temperature of the Earth.
The chart shows that average atmospheric carbon dioxide concentrations observed
at Muana Loa, Hawaii increased approximately 40 parts per million by volume
(ppmv) between 1958 and 1995. The small fluctuations in the curve are seasonal
variations due primarily to the withdrawal and production of carbon dioxide
by terrestrial life. Notice that minimum values occur during the northern
hemisphere summers (when global photosynthetic activity is greatest) and
maximum values occur six months later.
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Natural CO2 Cycle
As plants and trees grow, photosynthesisinvolving the interaction of sunlight,
chlorophyll in green leaves, carbon dioxide, and waterresults in a net removal
of carbon dioxide from the air and the release of oxygen as a by-product.
Also, moisture is released to the air through evapotranspiration.
As seen in this illustration, carbon dioxide from the surrounding air is
absorbed by the tree and converted to oxygen which is then released back
into the air during photosynthesis.
CO2 Cycle with Biomass Burning
When forests die and decay or are burned, the biomass, built up as a result
of carbon dioxide fixation, is oxidized and carbon dioxide is returned to
the air. The burning of tropical forests has increased dramatically in recent
decades and can release about 10 to 20 percent as much carbon dioxide into
the atmosphere as the burning of fossil fuels.
Methane in the Atmosphere
Atmospheric methane is a greenhouse gas that has been measured directly
since 1983. It has increased steadily to present day levels; this increase
is highly correlated with human population growth and with related activities,
including agricultural practices.
This chart represents global average methane concentration in parts per
million by volume (ppmv). In 1983, the global averaged value of atmospheric
methane was 1.61 ppmv. A century ago, the average was 0.9 ppmv. Atmospheric
methane levels continue to rise.
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Carbon Monoxide
During April 9-19, and September 30- October 11, 1994, the Measurement of
Air Pollution from Satellites (MAPS) experiment flew on the Space Shuttle
Endeavour. The MAPS experiment measured the carbon monoxide in the atmosphere,
primarily in the mid-troposphere, between 57 North latitude and 57 South
latitude. The MAPS carbon monoxide measurements are in the form of volume
mixing ratios, which are the volume of carbon monoxide gas divided by the
corresponding volume of air and are expressed in units of parts per billion
by volume (ppbv).
The figures represent the ten-day averages of the carbon monoxide mixing
ratios for each mission. High values of carbon monoxide mixing ratios are
indicated by shades of red, while low values of carbon monoxide are indicated
by shades of blue in each of the five degree latitude by five degree longitude
areas. During April, the highest values of mid-tropospheric carbon monoxide
(about 120 ppbv) were found over the high northern latitudes. This pattern
is typical for the late winterearly spring distribution for the northern
hemisphere. During October the carbon monoxide over high northern latitudes
is about 90 ppbv, and peak values of 135-180 ppbv were found over the southern
Tropics where the seasonal burning in South America, Africa, and Australia
was ongoing. Together, these data show that the global distribution of tropospheric
carbon monoxide undergoes very large changes during the course of the year.
The changes are a consequence of the complex interaction of the source locations
and strengths, the global circulation, and the presence of other trace gases.
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Stratospheric Chemistry
The stratosphere, the region of the Earth's atmosphere between roughly 12
and 50 km (8 and 30 miles) above the surface, is a crucial component in
the Earth's environment. Much of the importance of the stratosphere stems
from its absorption of the bulk of solar ultraviolet radiation.
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Stratospheric Ozone
High in the atmosphere, ozone acts as a shield, blocking harmful UV radiation
from the sun. Without this shield, we would be more susceptible to skin
cancer, cataracts, and impaired immune systems. Closer to Earth, in the
air we breathe, ozone is a harmful pollutant that causes damage to lung
tissue and plants.
Ozone, made up of three atoms of oxygen, is a relatively unstable molecule
found in the Earth's atmosphere. Most ozone is concentrated in the atmosphere
below a 50-km (30-mile) height. Although it represents only a tiny fraction
of the atmosphere, ozone is crucial for life on Earth because it absorbs
biologically harmful UV radiation.
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Ozone Balance
Ozone is created primarily by ultraviolet radiation. The air in the stratosphere
is bombarded continuously with ultraviolet radiation from the sun. A freed
oxygen atom then can react with an oxygen molecule and form a molecule of
ozone. In the presence of certain other molecules, however, ozone can be
destroyed.
Over the Earth's lifetime, natural processes have regulated the balance
of ozone in the stratosphere. As long as ozone is being created at the same
rate that it is being destroyed, the total amount of ozone will remain the
same.
Ozone Chemistry
Chlorine monoxide, the dominant ozone-destroying chlorine molecule, and
ozone were measured in the stratosphere of the Southern Hemisphere in September
of 1991 and 1992 by the Microwave Limb Sounder. Strong ozone minima over
Antarctica are seen on the right, corresponding to the chlorine monoxide
maxima on the left.
Small concentrations of chlorine monoxide (left globes) range from blues,
through greens and yellows, and up to the higher concentrations, shown in
reds and purples.
The total-column ozone measurements (right globes) are shown in violet (120
Dobson Units) through blue, green, yellow, and orange, to red (330 Dobson
Units).
Chlorofluorocarbons (CFCs) are the primary source of chlorine in the stratosphere.
Ozone destroying active chlorine (e.g., chlorine monoxide) is produced by
a series of photochemical reactions, including heterogeneous reactions on
the surfaces of polar stratospheric cloud particles.
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Interaction with Chlorine
Ultraviolet radiation strikes a chlorofluorocarbon (CFC) molecule and causes
a chlorine atom to break away .
The chlorine atom collides with an ozone molecule and steals an oxygen atom
to form chlorine monoxide and leave a molecule of ordinary oxygen.
When a free atom of oxygen collides with the chlorine monoxide the two oxygen
atoms form a molecule of oxygen. The chlorine atom is thus released and
free to destroy more ozone.
Fortunately, chlorine atoms do not remain in the stratosphere forever. Therefore,
if humans stop putting CFCs and other ozone-destroying chemicals into the
atmosphere, the ozone layer may eventually repair itself.
October Ozone Hole Over Antarctica
The springtime decline in total ozone over Antarctica produces what is widely
known as the "ozone hole". As summer approaches, the total ozone
increases and the "hole" disappears.
The images at the right clearly show that since 1979, the protective ozone
layer has declined in concentration and area. In fact, the ozone hole has
grown so much over the years that it is much larger than the entire Antarctic
continent, and is now approximately the size of North America. The color
bar and numbers represent Dobson Units, a measurement of total column ozone.
Red indicates high levels and purple indicates low levels.
Stratospheric clouds over Antarctica contain ice particles not found at
warmer latitudes. Reactions occurring on the surface of the ice particles
speed the ozone destruction caused by chlorine atoms from industrial chemicals,
mainly the chlorofluorocarbons (CFCs) used in refrigeration and other applications.
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Eight Marches in the Northern Hemisphere
High ozone values, shown in this figure with yellow and red colors are not
present in the 1993 data indicating a reduction of ozone thought to be caused
by the Mount Pinatubo eruption in 1991. These high values are again visible
in the 1994 data.
Ozone is being depleted worldwide partly due to the manufactured chemicals
called CFCs, or chlorofluorocarbons, used in refrigeration and other applications.
CFCs break down under ultraviolet light and release chlorine atoms that
attack ozone.
These images were obtained by NASA's TOMS instrument. They depict and compare
the total amount of ozone in a column of atmosphere from the ground to the
top of the ozone layer. Total column ozone is measured in Dobson Units.
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Decreasing Ozone Depletion in the Atmosphere
As shown in the graphs, up until 1994, increasing amounts of chlorine and
bromine were observed in the upper and lower atmosphere each year because
of use of refrigerants, foam-blowing agents, chlorinated solvents, and fire
retardants. Since 1994, the amount of ozone-depleting chemicals in the lower
atmosphere, or troposphere, has decreased as a result of overall adherence
to international environmental legislation.
(reference: Montzka et al., [1996], Science, 272, 1318-1322, [May 31, 1996]).
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