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Chemical composition and light absorbing properties of fresh and aged brown carbon aerosols from laboratory biomass burning
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Emissions from wildfires and use of biomass fuels can contribute not only to local air quality but to the global-scale climate. Both, fresh and atmospherically aged biomass-burning (BB) aerosols are mostly comprised of Black Carbon (BC) and Brown Carbon (BrC) and both significantly contribute towards overall light absorption by aerosols. Radiative forcing estimates for atmospheric biomass-burning aerosols have high uncertainties specially with BrC. The lack of chemical characterization for BrC and confounding interference of BC make it difficult to estimate only BrC contribution to light absorption with traditional particle phase characterization techniques. Moreover, there is still a large knowledge gap in the area of characterizing major organic compounds from BB emissions and understanding their fate during atmospheric photochemical transformations.In order to perform BB experiments in the combustion chamber under controlled conditions, we have selected representative fuels (e.g., Malaysian peats, Eucalyptus, Alaskan Peat), which have characteristics of both flaming and shouldering combustions. To mimic the atmospheric oxidation process (5-7 days) of biomass-burning smoke, we performed aging of gas- and particulate-phase emissions using an Oxidation Flow Reactor (OFR). Fresh and OFR-aged biomass-burning emissions were collected on both XAD-cartridges (gas phase) and filters (particulate phase) and extracted with different polarity solvents (water, hexane, dichloromethane). To estimate the contribution of BrC to the overall light absorption by biomass-burning aerosols, different extracts were analyzed with the UV-Vis spectrophotometry technique. The solvent extracted samples were analyzed for over 200 individual organic compounds using gas chromatography mass spectrometry (GC/MS). The polar fraction was derivatized prior to GC/MS analysis. 85 polar compounds were quantified including mono and dicarboxylic acids, methoxylated phenols, aromatic acids, anhydrosugars, resin acids and sterols.We found that the non-polar (hexane-soluble) fraction is 2-3 times more absorbing than the polar (water-soluble) fraction. However, for emissions from fuels that undergo flaming combustion, an increased absorbance was observed for the water extracts of oxidized/aged emissions while the absorption of the hexane extracts was lower for the aged emissions for the same type of fuels. High abundance of methoxyphenols and resin acids are found in peat burning emissions (smoldering combustion). Concentrations of higher molecular weight compounds decreased after OFR aging, while concentrations of low molecular weight compounds increased. This indicates a significant extent of fragmentation reactions in the OFR. Methoxyphenol concentrations decreased after OFR aging while a significant increase in dicarboxylic acids especially maleic acid was observed. These results can be used to perform source apportionments and predict the processes occurring during atmospheric transport. However, carbon content of the quantified polar compounds was found to constitute 4%-7% of the total organic carbon characterized by thermal optical technique which eventually leads to the motivation of characterizing other known compound classes (e.g., polycyclic aromatic hydrocarbons) and unknown compounds with full-scan analysis with same GC/MS technique.