Chemistry of Atmospheric Brown Carbon

The chemistry of atmospheric brown carbon (BrC) is reviewed. BrC is now recognized as an important component in the atmosphere that affects climate forcing through a combination of direct effects on the transmission of solar and terrestrial radiation and indirect effects resulting from changes in cloud formation and microphysics. In addition, long-range transport and deposition of BrC most likely play a role in carbon and nitrogen cycling between atmosphere, land, and water and contribute to the formation of colored dissolved organic matter (CDOM). The existing evidence suggests that even a very small weight fraction of strongly absorbing BrC chromophores may have a distinct effect on organic aerosols (OA) optical properties. Because of the low concentrations of light-absorbing molecules in complex organic mixtures composing both laboratory-generated and ambient OA, identification of BrC chromophores is a very challenging task. Despite the analytical difficulties, several classes of compounds have been identified as potential contributors to light absorption by BrC. These include nitroaromatic compounds, such as nitrophenols, imidazole-based and other N-heterocyclic compounds, and quinines. The identification and structural characterization of BrC chromophores clearly require highly sensitive molecular characterization approaches capable of detecting both strongly and weakly absorbing species.