Shortwave radiative forcing and efficiency of key aerosol types using AERONET data

The shortwave radiative forcing (delta F) and the radiative forcing efficiency (delta F-eff) of natural and anthropogenic aerosols have been analyzed using estimates of radiation both at the Top (TOA) and at the Bottom Of Atmosphere (BOA) modeled based on AERONET aerosol retrievals. Six main types of atmospheric aerosols have been compared (desert mineral dust, biomass burning, urban-industrial, continental background, oceanic and free troposphere) in similar observational conditions (i.e., for solar zenith angles between 55 degrees and 65 degrees) in order to compare the nearly same solar geometry. The instantaneous delta F averages obtained vary from -122 +/- 37 Wm(-2) (aerosol optical depth, AOD, at 0.55 mu m, 0.85 +/- 0.45) at the BOA for the mixture of desert mineral dust and biomass burning aerosols in West Africa and -42 +/- 22 Wm(-2) (AOD = 0.9 +/- 0.5) at the TOA for the pure mineral dust also in this region up to -6 +/- 3 Wm(-2) and -4 +/- 2 Wm(-2) (AOD = 0.03 +/- 0.02) at the BOA and the TOA, respectively, for free troposphere conditions. This last result may be taken as reference on a global scale. Furthermore, we observe that the more absorbing aerosols are overall more efficient at the BOA in contrast to at the TOA, where they backscatter less solar energy into the space. The analysis of the radiative balance at the TOA shows that, together with the amount of aerosols and their absorptive capacity, it is essential to consider the surface albedo of the region on which they are. Thus, we document that in regions with high surface reflectivity (deserts and snow conditions) atmospheric aerosols lead to a warming of the Earth-atmosphere system.