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Abstract. Fresh and atmospherically agedbiomass-burning (BB) aerosol mass is mostly comprised of stronglylight-absorbing black carbon (BC) and of organic carbon (OC) with itslight-absorbing fraction – brown carbon (BrC). There is a lack of data onthe physical and chemical properties of atmospheric BB aerosols, leading tohigh uncertainties in estimates of the BB impact on air quality and climate,especially for BrC. The polarity of chemical compounds influences their fatein the atmosphere including wet/dry deposition and chemical and physicalprocessing. So far, most of the attention has been given to the water-soluble(polar) fraction of BrC, while the non-polar BrC fraction has been largelyignored. In the present study, the light absorption properties of polar andnon-polar fractions of fresh and aged BB emissions were examined to estimatethe contribution of different-polarity organic compounds to the lightabsorption properties of BB aerosols. In our experiments, four globally and regionally important fuels were burnedunder flaming and smoldering conditions in the Desert Research Institute(DRI)combustion chamber. To mimic atmospheric oxidation processes (5–7 days), BBemissions were aged using an oxidation flow reactor (OFR). Fresh and OFR-agedBB aerosols were collected on filters and extracted with water and hexane tostudy absorption properties of polar and non-polar organic species. Resultsof spectrophotometric measurements (absorption weighted by the solar spectrumand normalized to mass of fuel consumed) over the 190 to 900 nm wavelengthrange showed that the non-polar (hexane-soluble) fraction is 2–3 times moreabsorbing than the polar (water-soluble) fraction. However, for emissionsfrom fuels that undergo flaming combustion, an increased absorbance wasobserved for the water extracts of oxidized/aged emissions while theabsorption of the hexane extracts was lower for the aged emissions for thesame type of fuels. Absorption Angstrom exponent (AAE) values,computed based on absorbance values from spectrophotometer measurements, werechanged with aging and the nature of this change was fuel dependent. Thelight absorption by humic-like substances (HULIS) was found to be higher in fuels characteristic of thesouthwestern USA. The absorption of the HULIS fraction was lower for OFR-agedBB emissions. Comparison of the light absorption properties ofdifferent-polarity extracts (water, hexane, HULIS) provides insight into thechemical nature of BB BrC and its transformation during oxidation processes.

Light absorption by polar and non-polar aerosol compounds from laboratory biomass combustion