MODELING OF PARTICLE-SIZE DISTRIBUTION AND ITS INFLUENCE ON THE RADIATIVE PROPERTIES OF MINERAL DUST AEROSOL

The radiative parameters of mineral aerosols are strongly dependent on particle size. Therefore explicit modeling of particle size distribut ion is needed to calculate the radiative effects and the climate impac t of mineral dust. We describe a parameterization of the global minera l aerosol size distribution in a transport model using eight size clas ses between 0.1 and 10 mu m. The model prescribes the initial size dis tribution using soil texture data and aerosol size measurements close to the ground. During transport, the size distribution changes as larg er particles settle out faster than smaller particles. Results of Mie scattering calculations of radiative parameters (extinction efficiency , single scattering albedo, asymmetry parameter) of mineral dust are s hown at wavelengths between 0.3 and 30 mu m for effective particle rad ii between 0.1 and 10 mu m. Also included are radiative properties (re flection, absorption, transmission) calculated for a dust optical thic kness of 0.1. Preliminary studies with the Goddard Institute for Space Studies (GISS) general circulation model (GCM), using two particle si ze modes, show regional changes in radiative flux at the top of the at mosphere as large as +15 W m(-2) at solar and +5 W m(-2) at thermal wa velengths in the annual mean, indicating that dust forcing is an impor tant factor in the global radiation budget.