Biomass burning aerosol size distribution and modeled optical properties

Satellite remote sensing of smoke aerosol and estimates of aerosol forcing of climate require knowledge of the aerosol optical properties. A smoke aer osol physical and optical model is developed from a database of over 800 vo lume size distributions inverted from sky radiance data measured by the AER ONET radiometer network in Brazil over a 3-year period. The model represent s total column, ambient conditions during the burning season in the Amazon Basin and cerrado region of Brazil. The mean volume size distributions are bimodal and can be represented by two lognormals. Accumulation mode modal r adius is 0.13 +/- 0.02 mu m with sigma = 0.60 +/- 0.04, and coarse-mode mod al radius ranges from 6 to 40 mu m, with a mean of 11.5 mu m and sigma = 1. 26 +/- 0.23. The volume of each mode varies with optical thickness, causing the ratio of accumulation mode and coarse mode to vary as well, but the ef fect on the optical properties is negligible. Refractive index is taken to be 1.43-0.0035i, and single scattering albedo is assumed to be 0.90, which is modeled with an external mixing of black carbon. The optical properties in the visible are dominated by the accumulation mode. Accumulation mode ch aracteristics do not vary from year to year, from forest to cerrado region, with optical thickness or with precipitable water vapor. At one test locat ion, accumulation mode characteristics do vary with air mass trajectory ori gin. The model is tested with independent data and can accurately predict t he scattering phase function and path radiance in the backscattering direct ion that determines remote sensing properties and is responsible for scatte ring sunlight back to space.