Strong spectral variation of biomass smoke light absorption and single scattering albedo observed with a novel dual‐wavelength photoacoustic instrument

A dual‐wavelength photoacoustic instrument operating at 405 and 870 nm was used during the 2006 Fire Lab at Missoula Experiment to measure light scattering and absorption by smoke from the combustion of a variety of biomass fuels. Simultaneous measurements of aerosol light scattering by reciprocal nephelometry within the instrument's acoustic resonator accompany photoacoustic aerosol light absorption measurements. Single scattering albedo values at 405 nm ranging from 0.37 to 0.95 were measured for different fuel types, and the spectral dependence of absorption was quantified using the Ångström exponent of absorption. An absorption Ångström exponent near unity is commonly observed for motor vehicle emission‐generated black carbon aerosol. For biomass smoke, Ångström exponents as high as 3.5 were found in association with smoke having single scattering albedo near unity. The measurements strongly suggest that light‐absorbing organic material is present in wood smoke. A second single‐wavelength photoacoustic instrument with reciprocal nephelometry was used to quantify aerosol scattering and absorption at 532 nm. Absorption Ångström exponents calculated using 532 and 870 nm data were as large as 2.5 for smoke with single scattering albedos near unity. The spectral variation in optical properties provides insight into the differentiation of aerosols from mobile or industrial sources versus those from biomass burning. Optical properties of biomass smokes could be classified by general fuel type such as flowering shrubs versus pine needle litter.