Abstract
Abstract. Smoke particle emissions from the combustion of biomass fuels typical for the western and southeastern United States were studied and compared under high humidity and ambient conditions in the laboratory. The fuels used were Montana ponderosa pine (Pinus ponderosa), southern California chamise (Adenostoma fasciculatum), and Florida saw palmetto (Serenoa repens). Information on the non-refractory chemical composition of biomass burning aerosol from each fuel was obtained with an aerosol mass spectrometer and through estimation of the black carbon concentration from light absorption measurements at 870 nm. Changes in the optical and physical particle properties under high humidity conditions were observed for hygroscopic smoke particles containing substantial inorganic mass fractions that were emitted from combustion of chamise and palmetto fuels. Light scattering cross sections increased under high humidity for these particles, consistent with the hygroscopic growth measured for 100 nm particles in HTDMA measurements. Photoacoustic measurements of aerosol light absorption coefficients revealed a 20% reduction with increasing relative humidity, contrary to the expectation of light absorption enhancement by the liquid coating taken up by hygroscopic particles. This reduction is hypothesized to arise from two mechanisms: (1) shielding of inner monomers after particle consolidation or collapse with water uptake; (2) the lower case contribution of mass transfer through evaporation and condensation at high relative humidity (RH) to the usual heat transfer pathway for energy release by laser-heated particles in the photoacoustic measurement of aerosol light absorption. The mass transfer contribution is used to evaluate the fraction of aerosol surface covered with liquid water solution as a function of RH.
Highlights
Light absorbing particles often are formed by incomplete combustion of carbonaceous materials and are released by sources such as industrial plants, vehicle emissions, and biomass burning
Characterization of biomass burning emissions can make an important contribution to understanding properties of ambient aerosols because biomass burning provides a significant source of airborne particulate matter worldwide (Bond et al, 2004)
Additional variability in the radiative impact of biomass burning aerosol is provided by a number of factors including combustion conditions and morphological properties of the aerosol particles, as well as other factors discussed in this manuscript: the fractal dimension and collapse of the particles and effects of aerosol aging such as coating enhancement
Summary
Light absorbing particles often are formed by incomplete combustion of carbonaceous materials and are released by sources such as industrial plants, vehicle emissions, and biomass burning Such particles, generically referred to as black carbon or soot, form an important but not wellunderstood category of aerosols with dramatic radiative impact and climatic implications. Further description of the FLAME campaigns, including a detailed description of the experimental set up for the chamber burns and fuel list, can be found in other publications (Lewis et al, 2008; Moosmuller et al, 2007) and at the FLAME website (http://chem.atmos.colostate.edu/ FLAME/) This manuscript presents results concerning light scattering and absorption by the smoke from combustion of three representative biomass fuels under varying humidity conditions. Our work seeks to understand the changing optical properties of the smoke aerosol in light of particle morphology and measurement circumstances
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