Assessment of TOMS UV bias due to absorbing aerosols -: art. no. D23211

[1] We compared NASA Total Ozone Mapping Spectrometer ( TOMS) overpass irradiances against ground-based Brewer measurements at Ispra, Italy, and Thessaloniki, Greece, with the main objective of evaluating the effect of absorbing aerosols on TOMS UV bias using direct measurements of aerosol optical properties. Dependence of the TOMS/Brewer bias on aerosol absorption optical depth is significant. Our study demonstrates that the bias between TOMS and measured Brewer 324 nm irradiances increases with increasing aerosol absorption optical depth tau(abs). The correlation coefficients between the ratio TOMS/Brewer and tau(abs) at Ispra and Thessaloniki are similar to 0.7 or more and similar to 0.8 or more, respectively, depending on the range of solar zenith angle values selected for the analysis. We found that the correlation with single-scattering albedo omega or aerosol optical depth is significantly smaller than with tau(abs). If measurements or climatology of tau(abs) are available in the UVB range, the TOMS UV product can be postcorrected for absorbing aerosols using similar site-dependent regressions as established in our study. If no correction is applied, the mean positive biases between TOMS and Brewer 324 nm irradiances in Thessaloniki and Ispra are 19.2% and 29.7%, respectively, while the standard deviations are 8.9% and 16.5%. Depending on the level of correction, the mean positive bias can be essentially removed, and the standard deviations can be reduced down to similar to 6% and similar to 10%, respectively.