MODELING THE RADIATIVE CHARACTERISTICS OF AIRBORNE MINERAL AEROSOLS AT INFRARED WAVELENGTHS

We explore the importance of the composition of airborne mineral aeros ols for assessments of their direct radiative forcing at infrared wave lengths. Our calculations employing Mie theory and data on spectral re fractive indices show that the existing variations in refractive indic es can cause large changes in the major aerosol optical characteristic s. Calculations of IR radiative forcings at the top of the atmosphere and IR downward and upward fluxes, based on an one-dimensional radiati on transfer code, give a wide range of results for varying optical mod els of the mineral aerosols. We estimate that for a ''low dust loading '' scenario the changes in IR downward flux at the surface relative to dust free conditions are in the range from 7 to 14 W/m(2) depending u pon the mineral aerosol selected. Under ''dry tropics'' atmospheric co nditions the IR forcing at the top of the atmosphere is in the range f rom 2 to 7 W/m(2). In turn, for a ''high dust loading'' scenario the c alculated changes, relative to dust free conditions, in IR downward fl ux at the surface vary from 50 to 80 W/m(2), and the IR forcing at the top of the atmosphere varies from 15 to 25 W/m(2). Therefore, we conc lude that incorporation of regionally and temporally varying dust mine ralogical composition into general circulation models could be benefic ial for decreasing the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of t he airborne mineral aerosols. Also the use of appropriate mineralogica l data is required for remote sensing of the atmospheric aerosols usin g satellite infrared observations.