ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-10505-2012 Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS Corr C. A. 1 Hall S. R. 2 Ullmann K. 2 Anderson B. E. 3 Beyersdorf A. J. 3 Thornhill K. L. 3 Cubison M. J. 4 Jimenez J. L. 4 Wisthaler A. 5 Dibb J. E. 1 Earth Systems Research Center, University of New Hampshire, Durham, NH, 03824, USA Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, 80301, USA NASA Langley Research Center, Hampton, VA, 23681, USA Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, 80309, USA Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria 12 11 2012 12 21 10505 10518 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/10505/2012/acp-12-10505-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/10505/2012/acp-12-10505-2012.pdf

Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350–550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350–500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 &pm; 0.38 for 17 April and 3.34 &pm; 0.11 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAE<sub>OA</sub>) for 17 April (11.15 &pm; 0.59) and 29 June (4.94 &pm; 0.19) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (&plus;0.16 vs. &minus;0.32). Differences in the AAE<sub>OA</sub>, as well as the overall AAE, were thus also possibly due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption.

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