Comparisons of the TOMS aerosol index with Sun-photometer aerosol optical thickness: Results and applications

A nearly 20-year global data set (1979-1994 and 1996 to the present) of tro pospheric absorbing aerosols has been developed from total ozone mapping sp ectrometer (TOMS) backscattered radiance measurements in the range from 331 to 380 nm. The occurrence of aerosols is derived directly from measured ba ckscattered radiances and is represented by a quantity known as the aerosol index. Previous theoretical model simulations have demonstrated that the a erosol index depends on aerosol optical thickness (AOT), single scattering albedo, and aerosol height and that the AOT can be determined provided that the microphysical properties and height of aerosols are known. In this pap er we show that the TOMS aerosol index measurements are linearly proportion al to the AOT derived independently from ground-based Sun-photometer instru ments over regions of biomass burning and regions covered by African dust. We also show how this linear relationship can be used to directly convert t he aerosol index into AOT for smoke and dust aerosols for the regions near the Sun-photometer sites and how information about aerosol height can be in ferred from the results. Finally, we apply this method to the TOMS data ove r the last two decades and find a significant increase in the amount of bio mass burning smoke in the African savanna regions during the 1990s in addit ion to the more obvious increase in South America.