Algorithm

The TOMS total ozone data are processed using the TOMS Version 8 algorithm. This algorithm has been optimized for studies of short-term to seasonal variations in the data, but these data can also be used in trend analysis, particularly when combined with the SBUV data, which are optimized for long-term trend studies. The TOMS instrument measures backscattered solar UV radiation at several distinct wavelengths, some of which are absorbed by atmospheric ozone. Solar radiation at these wavelengths penetrates into the troposphere, and is reflected from the dense tropospheric atmosphere, clouds, or the surface back to the satellite, thereby traversing the atmospheric column twice. Total column ozone is derived by comparing the amount of backscattered radiation measured by the instrument at an ozone-absorbing wavelength, relative to the amount of radiation measured at a non-absorbing wavelength. The Version 8 algorithm uses only two wavelength measurements (317.5 and 331.2 nm) to derive total ozone.

Improvements in the Version 8 algorithm include:

• Updated a priori profile ozone climatology based on SAGE II and balloonsonde profiles
• Used climatological temperature profiles to account for seasonal and latitudinal temperature dependence in ozone absorption properties
• Updated tropospheric ozone zonal-mean climatology 
• More accurate radiative transfer in forward model
  
• Improved corrections for cloud, tropospheric aerosol, sea glint and rotational Raman scattering effects
• Improved treatment of surface reflectance over persistent snow/ice

For more information on the TOMS Version 8 algorithm, please see the V8 Algorithm Theoretical Basis Document and the Version 8 TOMS Algorithm Web Page .

Calibration

The TOMS instrument measures direct solar irradiance and radiation backscattered from the Earth. A solar diffuser plate is used to reflect diffuse solar radiation into the instrument.  Each wavelength measurement is expressed as the ratio of the Earth radiance to the solar irradiance at that wavelength. In this ratio, any instrument error effects the solar irradiance and backscattered radiance measurements equally, such that the error cancels in the ratio. This is not true for a calibration error in the solar diffuser plate, which effects only the solar radiation. Time-dependent changes in the TOMS calibration are primarily a result of diffuser plate degradation.

Nimbus 7 TOMS:
The Nimbus 7 TOMS single diffuser plate suffered substantial degradation after 10 years of daily exposure to UV. Therefore, a non-diffuser-based technique termed spectral discrimination was developed to calibrate Nimbus 7 TOMS. In this technique, instrument calibration is maintained by stabilizing the time dependence of the ratio of two long wavelength (ozone independent) channels (McPeters and Labow, Geophys. Res. Lett., Vol 23, 1996). This procedure is completed internally in the Version 8 algorithm. 

The Nimbus 7 TOMS instrument experienced chopper wheel synchronization fluctuations beginning in mid-1984. During that time, the instrument seemed to "toggle" between two preferred synchronization states. While the timing anomalies were below the synchronization error threshold, they did effect the ozone retrieval. In the Version 8 reprocessing, one of these states was chosen as the most likely, based on comparisons with Nimbus 7 SBUV data. The other state was assumed in the Version 7 processing, which lead to a time-dependent difference with the Nimbus 7 SBUV. This difference is significantly reduced using the Version 8 synchronization state assumption.

For specific details on the Nimbus 7 TOMS instrument, see the Nimbus 7 TOMS Users Guide (McPeters et al., 1996).

The Earth Probe TOMS time-dependent calibration is maintained by using a series of three on-board diffuser plates. The cover diffuser is exposed to UV radiation continuously, while the working and reference diffusers are exposed for a period of minutes once every week and once every 15 weeks, respectively. The calibration is directly maintained to high accuracy by analyzing the degradation of the cover diffuser relative to the working and reference diffusers.

The absolute calibration of the Earth Probe TOMS data was determined from extensive pre-launch instrument testing.

In 2001 a bias in measurements made on one side of the orbital track relative to measurements from the opposite side of the scan was discovered in the Earth Probe TOMS data, and this problem became worse with time.  The problem is believed a result of changing optical characteristics of the front scan mirror, possibly from the build-up of a thin film on the mirror. An empirical correction was implemented in the Version 8 Earth Probe TOMS calibration, extending the data period through December 2005. In this study we only use data through 1999 to avoid the period of instrument problems.

For specific details on the Earth Probe TOMS instrument, see the Earth Probe TOMS Users Guide (McPeters et al., 1998).

Aura OMI TO3:

The OMI instrument employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation in the visible and ultraviolet. The hyperspectral capabilities will improve the accuracy and precision of the total ozone amounts and will also allow for accurate radiometric and wavelength self calibration over the long term. With the additional wavelength measurements, OMI can directly measure more constituents than its predecessor. For more information see http://aura.gsfc.nasa.gov/instruments/omi/index.html

The OMI TO3 data product is produced with the Laboratory for Atmospheres Branch at NASA GSFC. This product is the OMI data processed using the TOMS Version 8 algorithm. The Dutch OMI team has developed and produces OMI total ozone data using a DOAS technique. We do not use this data currently, but intend to compare the products in the future.

For specific details on the Aura OMI Instrument, TO3 and DOAS products, see the OMI TO3 ReadMe File  (Bhartia, 2005),  the OMI TO3 Data Guide,  or the EOS AURA home page.

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