Articles | Volume 15, issue 11
Atmos. Chem. Phys., 15, 6087–6100, 2015
Atmos. Chem. Phys., 15, 6087–6100, 2015

Research article 04 Jun 2015

Research article | 04 Jun 2015

Photochemical processing of aqueous atmospheric brown carbon

R. Zhao1, A. K. Y. Lee1, L. Huang2, X. Li3, F. Yang4, and J. P. D. Abbatt1 R. Zhao et al.
  • 1Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, Canada
  • 2Climate Research Division/Atmospheric Science and Technology Directorate/Science and Technology Branch, Environment Canada, Toronto, Canada
  • 3School of Chemistry and Environment, Beihang University, Beijing, China
  • 4Chongqing Institute of Green and Intelligent Technology, Chongqing, China

Abstract. Atmospheric brown carbon (BrC) is a collective term for light absorbing organic compounds in the atmosphere. While the identification of BrC and its formation mechanisms is currently a central effort in the community, little is known about the atmospheric removal processes of aerosol BrC. As a result, we report on a series of laboratory studies of photochemical processing of BrC in the aqueous phase, by direct photolysis and OH oxidation. Solutions of ammonium sulfate mixed with glyoxal (GLYAS) or methylglyoxal (MGAS) are used as surrogates for a class of secondary BrC mediated by imine intermediates. Three nitrophenol species, namely 4-nitrophenol, 5-nitroguaiacol and 4-nitrocatechol, were investigated as a class of water-soluble BrC originating from biomass burning. Photochemical processing induced significant changes in the absorptive properties of BrC. The imine-mediated BrC solutions exhibited rapid photo-bleaching with both direct photolysis and OH oxidation, with atmospheric half-lives of minutes to a few hours. The nitrophenol species exhibited photo-enhancement in the visible range during direct photolysis and the onset of OH oxidation, but rapid photo-bleaching was induced by further OH exposure on an atmospheric timescale of an hour or less. To illustrate the atmospheric relevance of this work, we also performed direct photolysis experiments on water-soluble organic carbon extracted from biofuel combustion samples and observed rapid changes in the optical properties of these samples as well. Overall, these experiments indicate that atmospheric models need to incorporate representations of atmospheric processing of BrC species to accurately model their radiative impacts.

Short summary
Aqueous-phase photochemical decay of light absorbing organic compounds, or atmospheric brown carbon (BrC), is investigated in this study. The absorptive change of laboratory surrogates of BrC, as well as biofuel combustion samples, were monitored during photolysis and OH oxidation experiments. The major finding is the rapid change in the absorptivity of BrC during such photochemical processing. This change should be taken into account to evaluate the importance of BrC in the atmosphere.
Final-revised paper