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 aerosol physical and optical model is developed from a database of over 800 volume size distributions inverted from sky radiance data measured by the AERONET radiometer network in Brazil over a 3-year period. The model represents 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 radius is 0.13 ± 0.02 μm with σ = 0.60 ± 0.04, and coarse-mode modal radius ranges from 6 to 40 μm, with a mean of 11.5 μm and σ = 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 effect 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 characteristics do not vary from year to year, from forest to cerrado region, with optical thickness or with precipitable water vapor. At one test location, accumulation mode characteristics do vary with air mass trajectory origin. The model is tested with independent data and can accurately predict the scattering phase function and path radiance in the backscattering direction that determines remote sensing properties and is responsible for scattering sunlight back to space.