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Total Column Ozone from Nimbus-7 Total Ozone Mapping Spectrometer (TOMS)
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Readme Contents

Data Set Overview
Sponsor
Original Archive
Future Updates

The Data
Characteristics
Source

The Files
Format
Name and Directory Information
Companion Software

The Science
Theoretical Basis of Data
Processing Sequence and Algorithms
Scientific Potential of Data
Validation of Data

Contacts
Points of Contact

References

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Data Set Overview

The dataset consists of 1 x 1 degree gridded monthly averaged total column ozone. It is derived from the Nimbus-7 TOMS monthly averaged total column ozone data gridded at 1.25 x 1 degree (lon/lat). The Nimbus-7 TOMS data is the only source of high resolution global information about the total ozone content of the atmosphere for the period November 1, 1978 - May 6, 1993. The ozone dataset was produced by the Ozone Processing Team (OPT) using final Nimbus-7, Version 7, OPT data reduction algorithm and was released in the Spring of 1996. This ozone data set is important in studies involving atmospheric chemistry and upper air dynamics on both short (interannual) and long (decadal) time scales.

Sponsor
The production and distribution of this data set are being funded by NASA's Earth Science enterprise. The data are not copyrighted, however, we request that when you publish data or results using these data please acknowledge as follows:

The authors wish to thank the Ozone Processing Team (OPT) of the Atmospheric Chemistry & Dyamics Brach(Code 916) and the Distributed Active Archive Center (Code 902) at Goddard Space Flight Center, Greenbelt, MD, 20771 for the production and distribution of these data. These activities were sponsored by NASA's Earth Science enterprise.

Original Archive
The total ozone data from which this data set is derived were produced by the Ozone processing team under the direction of Dr. Richard McPeters (code 916), the Nimbus Project Scientist of NASA's Goddard Space Flight Center. The original data, which includes daily, and monthly gridded products, are currently available from the Goddard DAAC's Atmospheric Chemistry Site

Future Updates
In the Spring of 1996, the old Version 6 of the Nimbus-7 TOMS data was replaced with the new Version 7. No future updates of the Nimbus-7 product are planned. However the ozone data from follow- on TOMS instruments such as those on the Meteor-3, Earth Probe (EP), Advanced Earth Observation Satellite (ADEOS) will be added to this data set collection as they become available.

The Data

Characteristics

Source
The Total Ozone Mapping Spectrometer (TOMS) was designed and built in the mid-1970s as part of a comprehensive package of scientific instruments to be flown aboard NASA's Nimbus-7 spacecraft. It was one of eight instruments designed to provide continuous, long-term monitoring of atmospheric, ocean, and surface parameters on a global basis throughout most of the 1980s.

Nominal orbit parameters for the Nimbus-7 spacecraft were

Launch date: 10/24/78
Orbit: Sun synchronous, near polar
Nominal altitude: 955 km
Inclination: 99.3 degrees
Nodal period: 104 minutes
Equatorial crossing time: 12:00 PM (ascending)
Nodal Increment: 26.1 degrees

The TOMS instrument is a single stage fixed-grating Ebert-Fastie monochromater with a rotating chopper wheel to resolve the incoming light into 6 wavelength bands with a 1 nanometer (nm) bandpass. These wavelengths are

        312.5 nm        317.5 nm        331.3 nm
        339.9 nm        360.0 nm        380.0 nm

TOMS scans in the cross-track direction in 3 degree steps from 51 degrees on one side of nadir to 51 degrees on the other, for a total of 35 samples. The instantaneous field of view (IFOV) of 3 degrees by 3 degrees results in a footprint varying from a 50 km x 50 km square at nadir to a 125 km x 280 km diamond at the scan extremes. The total swath width is 3000 km, implying that consecutive orbits overlap to create a contiguous mapping of ozone data. Approximately 200,000 measurements are made on a daily basis during the sunlit portions of the orbits.

The ratios of backscattered to incident ultraviolet radiation at the four shortest TOMS wavelengths are used to infer total ozone, while the corresponding ratios at the two longer wavelengths are used for estimating the effective reflectivity caused by the combined influence of Earth's surface, clouds, and tropospheric aerosols. The new Version 7 User's Guide describes the algorithm in detail. It can be obtained from the Ozone Processing Team's Nimbus-7/TOMS page.

The Nimbus-7 TOMS instrument ceased to function on May 7, 1993.

The Files

Format

Name and Directory Information

Naming Convention:

The file naming convention for this data set is

tomsn7.o3.1nmegg.[yymm].ddd

where
tomsn7 = data product designator
o3 = parameter name
1 = number of levels
n = vertical coordinate, n = not applicable
m = temporal period, m = monthly
e = horizontal grid resolution, e = 1 x 1 degree
gg = spatial coverage, gg = global (land and ocean)
yy = year
mm = month
ddd = file type designation, (bin=binary, ctl=GrADS control file)

Directory Path:

/data/atmo_constituents/toms_ozone/yyyy/

where yyyy is year.

Companion Software
Several software packages have been made available on the CIDC CD-ROM set. The Grid Analysis and Display System (GrADS) is an interactive desktop tool that is currently in use worldwide for the analysis and display of earth science data. GrADS meta-data files (.ctl) have been supplied for each of the data sets. A GrADS gui interface has been created for use with the CIDC data. See the GrADS document for information on how to use the gui interface.

Decompression software for PC and Macintosh platforms have been supplied for datasets which are compressed on the CIDC CD-ROM set. For additional information on the decompression software see the aareadme file in the directory:

software/decompression/

Sample programs in FORTRAN, C and IDL languages have also been made available to read these data. You may also acquire this software by accessing the software/read_cidc_sftwr directory on each of the CIDC CD-ROMs

The Science

Theoretical Basis of Data
Incoming solar radiation undergoes absorption by gases such as ozone and Rayleigh scattering by molecules in the stratosphere. Radiation that penetrates to the troposphere is scattered by clouds and aerosols, with the radiation that reaches the ground being scattered by surfaces of widely different reflectivity. The two shortest wavelengths chosen for use in the TOMS ozone measurements were selected because of their high ozone absorption. Absorption by other atmospheric components, at these wavelengths, is negligible compared to that of ozone.

The backscattered radiance at a given wavelength depends, in principle, upon the entire ozone profile from the top of the atmosphere to the surface. At wavelengths longer than 310 nm, however, the backscattered radiance consists primarily of solar radiation that penetrates the stratosphere and is reflected back by the dense tropospheric air, clouds, aerosols and the Earth's surface. Because most of the ozone (about 90%) is in the stratosphere, the principal effect of total atmospheric ozone is to attenuate both the solar flux reaching the troposphere and the component reflected back to the satellite. This nearly complete spatial separation of the absorber elements in the stratosphere (i.e. ozone) from the "reflector" elements in the troposphere (i.e. aerosols, clouds, and Earth's surface) causes backscattered radiances longer than 310 nm to depend only weakly on the vertical distribution of ozone in the atmosphere. In the simplest case, whereby tropospheric and surface characteristics remain unchanged from one measurement to the next, and with no aerosols present in the stratosphere, a decrease (increase) in the backscattered radiance at the shortest TOMS wavelengths would signify an increase (decrease) in the total ozone amount below the satellite. Further discussion concerning the theory behind backscattered ultraviolet radiation and its relationship to atmospheric ozone can be found in Liou (1980) and Klenk et al. (1982).

Derivation of atmospheric ozone content from measurements of the backscattered radiances requires a treatment of reflection from the Earth's surface and of scattering by clouds and other aerosols. In general, the scattered or reflected light depends on both incidence angle of the sunlight and viewing angle of the satellite. Studies (e.g., Dave, 1978) and ( Bhartia et al. 1993). show that, in practice, the contribution of clouds and tropospheric aerosols to the backscattered intensity can be treated by assuming that the effective lower boundary of the atmosphere is located at an average pressure in the troposphere, representing a "mix" of the estimated surface and cloud top pressures. Furthermore, this lower boundary will be assigned an effective Lambertian reflectivity which accounts collectively for the backscattering effects of clouds, tropospheric aerosols and the surface of the earth. This process of deriving an effective "scene reflectivity" and an associated effective "scene pressure" is performed for every instantaneous field-of-view (IFOV) along the TOMS scanline. In the presence of stratospheric aerosols (such as those resulting from volcanic eruptions), the concept of an average scene reflectivity and scene pressure break down, and the data must be flagged for this contamination accordingly ( Bhartia et al. 1993).

Processing Sequence and algorithms
The intensity of solar radiation backscattered by the earth-atmosphere system and received by a sensor aboard an earth-orbiting satellite can be expressed as: 

                I(i) = Ia(i) + Ig(i)
where

The ground contribution is given by: 

                Ig(i) = R*T(i) / (1-R*S(i))

where

In the above, I(i) and Ig(i) depend upon total ozone amount, the effective scene pressure level and reflectivity, the solar zenith angle and the satellite viewing angle. The purely atmospheric contribution Ia(i) as well as T(i) depend upon all of the above except the reflectivity R. The values of I(i), Ia(i) and to a lesser extent Ig(i) are also somewhat dependent upon the shape (i.e., vertical distribution) of the ozone profile.

Once the measured backscattered radiances have been corrected for the effects of wavelength drift and changes in the instrument optics and sensitivity (see McPeters et al., 1996); McPeters and Komhyr, 1991) a quantity called the "N-value", or N(i), is computed. it is defined as: 

        N(i) = -100 log[ I(i) / F(i) ]
where the ratio I/F denotes the backscattered radiance I(i) normalized by the direct solar radiation, F(i), incident at the level of the sensor.

Given the optical properties of the atmosphere at each TOMS wavelength, a set of tables is created relating total ozone to I/F (and thus N) for several independent variables. These include

The scene reflectivity R is not included as a table variable since the tabulated quantities of interest, S(i), T(i) and Ia(i), do not depend upon it. The theoretical values of I/F at each of the 6 TOMS wavelengths are calculated using the radiative transfer methodology of Dave (1964).

The computation of total column ozone is accomplished by computing radiance ratios called Pair values, which are ratios of I/F at a longer wavelength, which is relatively insensitive to ozone, to that of a shorter, ozone-sensitive wavelength. The pair values are defined as: 

        A-Pair = N(313 nm) - N(331 nm)
        B'-Pair = N(318 nm) - N(340 nm)
        C-Pair = N(331 nm) - N(340 nm)

Pairs are chosen about 20 nm apart or less, so that scattering effects are about the same, and the relative attenuation of the pair is sensitive mostly to ozone absorption. In addition, the ratios of the radiances help to minimize calibration errors and wavelength independent effects. Different pairs of wavelengths are used for different conditions, i.e., for large ozone amounts at low sun angles the A-pair becomes less sensitive to changes in total ozone since 313 nm senses higher in the atmosphere (Klenk et al., 1982). It also becomes more sensitive to ozone profile shape; thus more weight will be placed upon the derived B'- and C-Pair ozone values in this case.

Table interpolation is used to extract four total ozone estimates corresponding to each of three measured Pair values, i.e., at 1000 mb and 400 mb, and for two standard latitude bands on either side of the actual latitude measured for each pressure. Prior to this, surface reflectivities for the two pressures are computed from the two longest TOMS wavelengths, 360 nm and 380 nm, which are not sensitive to ozone, as follows: 

        R = (I - Ia)/(T - S*(I-Ia) )
where Ia, T, and S are obtained from the tables for the given sun and satellite angles and for each of the two pressures, and I is the satellite-measured radiance at 360 nm or 380 nm.

An estimated scene pressure (Ps) is then computed using a climatological cloud top pressure (Pc), and actual terrain pressure (Pt) as follows: 

        Ps = (1-w)*Pc + w*Pt
where w is a weighting function based upon the measured surface reflectivity R at 1000 mb and on the presence or absence of snow/ice.

The Pair ozone values for the latitude and derived scene pressure (Ps) of the measurements are obtained by linear interpolation in latitude between the values for the two surrounding latitudes, followed by linear interpolation in pressure between the 400 mb and 1000 mb values.

Finally, a "best ozone" value is obtained as a weighted average of the total ozone values derived from the A, B', and C-pairs. The weighting takes into account the sensitivities of the individual Pair values to the profile shape, the solar zenith angle and to changes in the total ozone itself.

The above description is taken from the Version 6 User's Guide. The Version 7 data are produced using a revised instrument calibration based on analysis of the entire 14.5 year data record, as well as an improved algorithm. Improvements include:

The new Version 7 User's Guide describes the algorithm in detail. It can be obtained from the Ozone Processing Team's Nimbus-7/TOMS page.

In addition to deriving ozone values for every individual field-of-view over a day, the Ozone Processing Team has created a daily gridded product. Out of the approximately 200,000 measurements per day, only those values not contaminated by stratospheric aerosols, volcanic sulfur dioxide, inconsistent pair values of ozone, and/or implausible reflectivity or ozone values are included in this product. These individual measurements have been averaged into grid cells 1.25 degree in longitude by 1 degrees in latitude. These have been written as ASCII data files and images on a set of CD-ROMs, as well as in HDF format, and constitute the data source from which this 1 degree by 1 degree monthly average data set was constructed. The following steps were performed at the Goddard DAAC:

These data were further processed by the Goddard DAAC. Processing included, regridding, and reformatting the output data product. The regridding of these data from 1.25 degree x 1 degree to 1 degree x 1 degree was implemented as follows:

  1. For each monthly 1.25 x 1 data file, every data value in each latitude band was replicated by the target number of grid cells in a latitude band within the final output data file, 360, and assigned to a temporary array. Each original latitude band had 288 data values which when replicated 360 times produced a temporary array of 103680 data values for that latitude band.

  2. The first 288 (temporary array) data values were compared against the fill value for these data. Any values that were not fill values were then summed, and a count of data value and fill value occurrence was kept.

  3. A test for fill value occurrence was performed. If fill value constituted more than 50% of contributing values then the fill value was assigned to that grid cell. Otherwise, the average was computed for the target grid cell from only those points constituting data values. When assigning fill values, a new fill value was used to provide greater uniformity with other existing data sets held at the Goddard DAAC.

  4. Steps 2 and 3 were repeated for the next 288 values within the temporary array until all values were summed, tested for fill value occurrence, and assigned to a target grid cell.

  5. Steps 2, 3, and 4 were repeated for each of the next 179 latitude bands.

  6. To validate that the regridded data do not introduce any spurious artifacts into the original data, a visual comparison of the two data sets was performed and the ozone values in randomly selected, localized regions were examined to ensure spatial coherence in the regridded data set and a high degree of similarity with the original data set.

Scientific Potential of Data
Total ozone data as derived from the TOMS instrument are useful for understanding a variety of phenomena involving both short-term stratospheric fluctuations and long-term climate change. Stratospheric ozone modulates the incoming (and biologically harmful) solar ultraviolet radiation stream through absorption in much the same way as tropospheric carbon dioxide traps outgoing infrared radiation emitted by Earth's surface and atmosphere. Just as an increase or decrease of carbon dioxide in the lower atmosphere may have a climatic impact over the long term, so may changes in the ozone content of the upper atmosphere. Beside the study of long-term climate change, other specific examples of scientific applications of this data set include the following.

Validation of Data
Numerous comparisons were made with various ground based Dobson networks and with measurements taken from the summit of Mauna Loa. The later comparison is made due to the close coincidence of measurements by the two instruments, differing by 100 km in space and 1 hour in time. Differences range from 2% to 4% for Dobson network comparison, and 3 1/2% to 5% for the World Standard.

Contacts

Points of Contact
Science questions concerning the production and validation of this data set should be directed to:

Dr. Richard D. McPeters
Code 916
NASA Goddard Space Flight Center
Greenbelt, MD 20771
e-mail: mcpeters@wrabbit.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)

For information about or assistance in using any DAAC data, contact

EOS Distributed Active Archive Center (DAAC)
Code 902.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
e-mail: daacuso@daac.gsfc.nasa.gov
301-614-5224 (voice)
301-614-5268 (fax)

References

Bhartia, P.K., J. Herman, R.D. McPeters, and O. Torres, 1993. The effect of Mt. Pinatubo aerosols on total ozone measurements from backscatter ultra violet (BUV) Experiments. J. Geophys. Res., 98, 18547-18554.

Bowman, K.P., 1986, Interannual variability of total ozone during the breakdown of the Antarctic circumpolar vortex, Geophys. Res. Lett.,, 13, 1193-1196.

Bowman, K.P., 1988, Global trends in total ozone, Science, 239, 48-50.

Dave, J.V. 1964. Meaning of successive iteration of the auxiliary equation of radiative transfer. Astrophys. J., 140, 1292-1303.

Dave, J.V. 1978, Effect of aerosols on the estimate of total ozone in an atmospheric column from the measurements of its ultraviolet radiance, J. Atmos. Sci., 35, 899-911.

Eck, T.F., P.K. Bhartia, and J.B. Kerr, 1995, Satellite estimation of spectral UVB irradiance using TOMS derived total ozone and UV reflectivity, Geophys. Res. Lett., 22(5), 611-614.

Gordon, H.R., and D.K. Clark, 1981, Clear water radiances for atmospheric correction of Coastal Zone Color Scanner imagery, Appl. Optics, 20, 4175-4180.

Klenk, K.F., P.K. Bhartia, A.J. Fleig, V.G. Kaveeshwar, R.D. McPeters, and P.M. Smith, 1982, Total ozone determination from the Backscattered Ultraviolet (BUV) Experiment, J. Appl. Meteor., 21, 1672-1684.

Lait, L.R., M.R. Schoeberl, and P.A. Newman, 1989, Quasi-biennial modulation of the Antarctic ozone depletion, J. Geophy. Res., 94, 559-571.

Lienesch, J.H., and P.K.K. Pardey, 1985, "The use of TOMS data in evaluating and improving the total ozone from TOVS measurements", Rep. NOAA-TR-NESDIS-23, Issue 22, 3814-3828.

Liou, K.-N., 1980, An Introduction to Atmospheric Radiation, Academic Press, New York.

McPeters, R., and W.D Komhyr. 1991. Long-term changes in the Total Ozone Mapping Spectrometer relative to world standard Dobson Spectrometer 83. J. of Geophys. Res., 96, 2987-2993.

McPeters, R.D., P.K. Bhartia, A.J. Krueger, J. R. Herman, B. Schlesinger, C.G. Wellemeyer, C. J. Seftor, G. Jaross, S.L. Taylor, T. Swissler, O. Torres, G. Labow, W. Byerly, and R.P. Cebula, 1996. Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) Data Products User's Guide. NASA Reference Publication 1384.

Miller, A.J., R.M. Nagatani, K.B. Labitzke, E. Klinker, K. Rose, and D.F. Heath, 1976, Stratospheric ozone transport during the mid-winter warming of December 1970-January 1971, paper presented at Joint Symposium on Atmospheric Ozone, Dresden, Germany, August 9-16, 1976.

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