OMTO 3 README File

Overview This document provides a brief description of the OMTO3 data product. OMTO3 comprises total ozone, aerosol index, and ancillary information produced from the TOMS Version 8.5 (V8.5) algorithm applied to OMI " global mode " measurements. In the global mode each file contains a single orbit of data. In each orbit OMI measurements cover approximately 2600 km wide cross-track swath from pole to pole (sunlit portions only). Currently, OMTO3 products are not produced when OMI goes into the " zoom mode " for one day every 32 days. The accuracy and precision of the OMTO3 total ozone data is similar to the legacy Total Ozone Mapping Spectrometer (TOMS) data started in 1978, except over cloudy areas where OMTO3 data are more accurate than that of the TOMS. Since there are significant differences between the two products over cloudy areas, users should exercise caution in combining TOMS data with OMTO3 data for long-term trends. The TOMS project is planning to reprocess the entire TOMS record to make the dataset more consistent with OMTO3. The basic algorithm uses 2 wavelengths (317.5 and 331.2 nm under most conditions, and 331.2 and 360 nm for high ozone and high solar zenith angle conditions). The longer of the two wavelengths is used to derive effective cloud fraction (fc) based on the Mixed Lambert Equivalent Reflectivity (MLER) model [Ahmad et al 2004] that was developed to model the effect of clouds on Rayleigh scattering. When fc becomes less than zero or when there is snow/ice, we assume that no cloud is present and use the Lambert Equivalent Reflectivity (LER) model described by Ahmad et al [2004] to derive the clear scene reflectivity R. When fc exceeds 1 we assume100% cloud cover and derive cloud reflectivity using the LER model. Given the fc/R the shorter (stronger ozone-absorbing) wavelength is used to derive total ozone. An important difference between the V8.5 algorithm and previous versions, as well with the archived TOMS dataset, is the assignment of effective cloud height. It has been assumed in the previous algorithms that the absorption of backscattered solar radiation essentially stops at the cloud-top level when the clouds are optically thick. To estimate the total column amount, the " un-measured " column below the cloud-top (computed using climatology) is added to the measured column. Recent analysis of the OMI data in conjunction with CloudSat radar data [Vasilkov et al 2008] indicates that …