Absorption characteristics of forest fire particulate matter

Abstract

Absorption properties of smokes from laboratory fires that represent prescription burns in the Southern states have been quantified to relate variations in measured absorption parameters to variation in fire conditions and to estimate emission factors for elemental carbon. Results showed significant differences in absorption of the smoke emissions between flaming and smoldering combustion with specific absorption coefficient B a values ranging from 0.04 m 2 g −1 for smoldering combustion to 1 m 2 g −1 at 632.8 nm for flaming combustion. These measured optical properties and previously measured size data were used in Mie calculations to determine the overall radiative properties for the smokes from these fires. Mie calculations for λ = 550 nm indicate that somewhat less than 50% of the extinction will be due to particulate matter absorption in flaming combustion, whereas only about 5% of the extinction will be due to absorption under purely smoldering conditions. Calculated mass concentration/scattering ( C m / σ s ) ratios for λ = 500 nm show only a slight variation with n IM , decreasing from 3.3 × 10 5 μg m −2 at an imaginary refractive index ( n IM ) = 0.07 to a value of 2.7 × 10 5 μgm −2 at n IM = 0.007. The ratios calculated for the low absorption cases are generally consistent with previous experimental observations, although these calculations suggest that some of the variations seen in this ratio are consistent with variation due to differences in the optical properties of the particulates.