ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-1155-2010 Absorption Angstrom Exponent in AERONET and related data as an indicator of aerosol composition Russell P. B. 1 Bergstrom R. W. 2 Shinozuka Y. 3 Clarke A. D. 4 DeCarlo P. F. 5 6 10 Jimenez J. L. 5 7 Livingston J. M. 8 Redemann J. 2 Dubovik O. 9 Strawa A. 1 NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000, USA Bay Area Environmental Research Institute, 560 3rd Street West, Sonoma, CA 95476, USA Oak Ridge Associated Universities, NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000, USA School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI, USA Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado, USA Dept. of Atmospheric and Oceanic Sciences University of Colorado, Boulder, Colorado, USA Dept. of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025, USA Laboratoire d'Optique Atmospherique, CNRS Universite de Lille, Bat 5, 59655 Villeneuve d'Ascq CEDEX, France now at: Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen, Switzerland 03 02 2010 10 3 1155 1169 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/10/1155/2010/acp-10-1155-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/1155/2010/acp-10-1155-2010.pdf

Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (often expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The main purpose of this paper is to show that AAE values for an Aerosol Robotic Network (AERONET) set of retrievals from Sun-sky measurements describing full aerosol vertical columns are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. These AERONET results are consistent with results from other, very different, techniques, including solar flux-aerosol optical depth (AOD) analyses and airborne in situ analyses examined in this paper, as well as many other previous results. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which seeks to provide retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Multidimensional cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.

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