Origin and Evolution of Brown Carbon Aerosol in the Atmosphere

Abstract:
Wildfires emit enormous amounts of carbonaceous particles, including black carbon (BC), composed of aggregates of elemental carbon, and brown carbon (BrC), composed of light-absorbing semi-volatile organic compounds. These particles play a critical role in climate change, in part by scattering and absorbing solar radiation, but the effects of aerosols are more poorly constrained than those of all other trace constituents in the atmosphere, stemming from uncertainties, for example, in the formation and aging of BrC. Though many primary and secondary organic compounds have been identified in BrC, they cannot account for the total absorptivity of BrC; consequently, organic charge-transfer (CT) complexes have been proposed to contribute to the absorptivity of BrC. Furthermore, BrC undergoes drastic changes in its composition and absorptivity through photolysis and reactions with atmospheric oxidants. Here, investigations of the contribution of CT complexes to the absorptivity of ambient and laboratory-generated BrC and of the evolution of surrogates of primary and secondary BrC exposed to hydroxyl radical in a series of smog chamber experiments, providing insights into the lifetime of BrC in the atmosphere and its dependence on particle viscosity, will be presented. An ongoing investigation of the evolution of BrC at low temperatures representative of the upper troposphere will also be introduced