Impact of tropospheric aerosol absorption on ozone retrieval from backscattered ultraviolet measurements

The impact of tropospheric aerosols on the retrieval of column ozone amounts using space-borne measurements of backscattered ultraviolet radiation is examined. Using radiative transfer calculations, we show that the radiative effects of UV-absorbing aerosols produce spurious ozone column reductions. The magnitude of the total ozone underestimation depends on the aerosol type, optical depth, and aerosol layer height. Desert dust can produce errors as large as 10%. Smaller errors are produced by carbonaceous aerosols that result from biomass burning. Ozone retrieval errors associated with nonabsorbing aerosols are typically +/-1% of the total ozone amount depending on aerosol optical depth and viewing geometry. Though the error is produced by complex interactions between ozone absorption (both stratospheric and tropospheric), aerosol scattering, and aerosol absorption, a surprisingly simple correction procedure reduces the error to similar to 1% for a variety of aerosols and for a wide range of aerosol loading. Comparison of the corrected Total Ozone Mapping Spectrometer (TOMS) data with operational data indicates that though the zonal mean total ozone derived from TOMS is not significantly affected by these errors, localized effects in the tropics can be large enough to affect seriously the studies of tropospheric ozone that are currently undergoing using the TOMS data.