Modeling the spectral optical properties of ammonium sulfate and biomass burning aerosols: parameterization of relative humidity effects and model results

The importance of including the global and regional radiative effects of ae rosols in climate models has increasingly been realized. Accurate modeling of solar radiative forcing due to aerosols from anthropogenic sulfate and b iomass burning emissions requires adequate spectral resolution and treatmen t of spatial and temporal variability. The variation of aerosol spectral op tical properties with local relative humidity and dry aerosol composition m ust be considered. Because the cost of directly including Mie calculations within a climate model is prohibitive, parameterizations from off-line calc ulations must be used. Starting from a log-normal size distribution of dry ammonium sulfate, we developed optical properties for tropospheric sulfate aerosol at 15 relative humidities up to 99%. The resulting aerosol size dis tributions were then used to calculate bulk optical properties at wavelengt hs between 0.175 and 4 mu m. Finally, functional fits of optical properties were made for each of 12 wavelength bands as a function of relative humidi ty. Significant variations in optical properties occurred across the total solar spectrum. Relative increases in specific extinction and asymmetry fac tor with increasing relative humidity became larger at longer wavelengths. Significant variation in single-scattering albedo was found only in the lon gest near-IR band. This is also the band with the lowest single scattering albedo. A similar treatment was done for aerosols from biomass burning. In this case, two size distributions were considered. One was based on a distr ibution measured for Northern Hemisphere temperate forest fires while the s econd was based on a measured size distribution for tropical fires. Equilib rium size distributions and compositions were calculated for 15 relative hu midities and five black carbon fractions. Mie calculations and band average s of optical properties were done for each of the resulting 75 cases. Final ly, fits were made for each of 12 spectral bands as functions of relative h umidity and black carbon fraction. These optical properties result in globa l average forcing from anthropogenic sulfate aerosols of - 0.81 Wm(-2). The global average forcing for biomass aerosols ranged from - 0.23 to - 0.16 W m-2 depending on the assumed size distribution, while fossil fuel organic a nd black carbon are estimated to heat the atmosphere by about 0.16 Wm(-2). Published by Elsevier Science Ltd.