Abstract
Although the definition and measurement methods of atmospheric ''black carbon'' (''BC'') have long been subjects of scientific controversy, the recent discovery of light-absorbing carbon that is not black (''brown carbon, Cbrown'') 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 Cbrown comes directly from aerosol absorption measurements near specific combustion sources, from observations of spectral properties of water extracts of continental aerosol, from laboratory studies indicating the formation of light-absorbing organic matter in the atmosphere, and indirectly from the chemical analogy of aerosol species to colored natural humic substances. We show that these species may severely bias measurements of ''BC'' and ''EC'' over vast parts of the troposphere, where mass concentration of Cbrown is high relative to that of combustion soot. We also imply that due to the strongly skewed absorption of Cbrown towards the UV, single-wavelength light absorption measurements may not be adequate for the assessment of absorption of solar radiation in the troposphere. The possible consequences of these effects on our understanding of tropospheric processes are discussed.