GCM aerosol radiative effects using geographically varying aerosol sizes deduced from AERONET measurements

Aerosol optical properties, and hence the direct radiative effects, are largely determined by the assumed aerosol size distribution. In order to relax the fixed aerosol size constraint commonly used in general circulation models (GCMs), measurements from the Aerosol Robotic Network (AERONET) and a new method to deduce a geographically and monthly varying aerosol size are used. Within the limitations of the GCMs' ability to predict aerosol mass concentrations, which are shown to be reasonable, the globally averaged modal radius of an internally mixed fine mode that best matches the AERONET-deduced Angstromngstrom exponent curve is found to be 0.04 mum. This corresponds to a direct top-of-the-atmosphere total aerosol solar forcing in clear skies computed using monthly averages of -2.2 W m(-2), which is about 0.5 W m(-2) greater than assuming a globally constant modal radius of 0.04 m m.