Toms

TOMS is a 6-band (UV: 6) cross track scanning radiometer flown on many different polar-orbiting satellites (NIMBUS-7, Meteor-3, Earth Probe, ADEOS) since 1979. Swath width and spatial resolution are about 3000km and 50km. Although primarily designed to monitor ozone, measurements in channels with weak or no ozone absorption permit the retrieval of aerosol properties (Torres et al., 1998). The optical depth of scattering aerosol is derived from enhancements in molecular backscatter at The optical depths of absorbing aerosol and also the aerosol single scattering albedo are derived from changes to the background spectral dependency of molecular scattering between and These changes are a function of aerosol altitude so that altitude underestimates lead to overestimates in aerosol optical depth and vice versa. Also without sufficient background signal below the aerosol layer the detection of absorbing aerosol near the surface is difficult. The retrieval assumes surfaces at sea-level, which currently causes overestimates for absorbing aerosol optical depths at high altitude regions. For the properties of absorbing aerosol the current retrieval assumes an average altitude of 3km. Also a-priori assumptions for aerosol size (,shape) are necessary. The large 40km*40km pixel size of a TOMS image make it difficult to avoid contamination by clouds, which can have a strong impact on the retrieval of absorbing aerosol. Here a reflection threshold technique is assumed to remove clouds. Surface reflections, although small both over water and land in the ultraviolet, are based on minimum reflections for cases, where the spectral dependency of molecular scattering did not change. Data presented in the comparison are based on 1997 retrievals.

0 0

Post a comment