Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 6 10 2006 10.5194/acp-6-3131-2006 http://www.atmos-chem-phys.net/6/3131/2006/ http://www.atmos-chem-phys.net/6/3131/2006/acp-6-3131-2006.html http://www.atmos-chem-phys.net/6/3131/2006/acp-6-3131-2006.pdf 3131 3148 2006-07-28 Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols M. O. Andreae andreae@mpch-mainz.mpg.de A. Gelencsér Max Planck Institute for Chemistry, Biogeochemistry Department, P.O._space_Box 3060, 55020 Mainz, Germany Air Chemistry Group of the Hungarian Academy of Sciences, University of Veszprém, P.O. Box 158, H-8201 Veszprém, Hungary Although the definition and measurement techniques for atmospheric "black carbon" ("BC") or "elemental carbon'' ("EC") have long been subjects of scientific controversy, the recent discovery of light-absorbing carbon that is not black ("brown carbon, C_brown") makes it imperative to reassess and redefine the components that make up light-absorbing carbonaceous matter (LAC) in the atmosphere. Evidence for the atmospheric presence of C_brown comes from (1) spectral aerosol light absorption measurements near specific combustion sources, (2) observations of spectral properties of water extracts of continental aerosol, (3) laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and (4) indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that brown carbon may severely bias measurements of "BC" and "EC" over vast parts of the troposphere, especially those strongly polluted by biomass burning, where the mass concentration of C_brown is high relative to that of soot carbon. Chemical measurements to determine "EC" are biased by the refractory nature of C_brown as well as by complex matrix interferences. Optical measurements of "BC" suffer from a number of problems: (1) many of the presently used instruments introduce a substantial bias into the determination of aerosol light absorption, (2) there is no unique conversion factor between light absorption and "EC" or "BC" concentration in ambient aerosols, and (3) the difference in spectral properties between the different types of LAC, as well as the chemical complexity of C_brown, lead to several conceptual as well as practical complications. 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