Modeling Refractive Index of Biomass Burning Aerosols*

Tthere is a significant uncertainty regarding the Refractive Index RI of biomass burning (BB) aerosols. Experimentally measured cross sections for white pine BB aerosols were modeled using T-matrix theory and RI's extracted from these calculations. Experimental measurements were conducted using a cavity ring-down spectrometer and a nephelometer. BB aerosols were obtained by burning white pine using (1) open fire in a burn drum, where the aerosols were collected in distilled water using an impinger and (2) a tube furnace to directly introduce the BB aerosols into an indoor smog chamber, where BB aerosols were then sampled directly. Filter samples were collected and electron microscopy images were used to obtain morphology and size information for the calculations. For the particles generated in the tube furnace, the real part of the RI decreased with particle size and the imaginary part was much smaller and nearly constant. RI is sensitive to particle size and sampling method, but there was no wavelength dependence over the range considered (500-680 nm). Our values for the RI of fresh (White Pine) biomass burning aerosol range from 1.33 + i0.008 to 1.74 + i0.008 for 200, 300, and 400 nm diameter particles.