Atmospheric Brown Carbon (BrC) refers to a set of UV and visible light absorbing organic compounds and can affect atmospheric radiative balance. Despite its potential importance, the atmospheric impact of BrC has not been considered in many models because the sources, identity and fate of these species remain unclear. Particularly uncertain is the role of atmospheric processing on the ability of BrC to absorb light. Recent laboratory and field studies have shown that organo-nitrogen compounds may constitute an important component of BrC. In this study, we focus on the aqueous-phase photochemistry of two subsets of such compounds: nitrophenols, which arise, from biomass burning emissions, and imidazole-type species formed in secondary organic aerosol particles. We chose 4-nitrophenol and 5-nitroguaiacol to represent nitrophenols and aqueous mixtures of glyoxal or methylglyoxal with ammonium sulfate to represent secondary organo-nitrogen compounds. Aqueous solutions containing each of these chemicals or mixtures were illuminated with UVB light, with the solution absorbance monitored over time using a capillary cell wave guide UV-visible spectrometer. A rapid decrease in absorbance was observed from all of these solutions upon illumination. In some cases, the decrease was accelerated when H2O2 was added to the solution as a photolytic source of OH radicals. By determining the rates of these processes in the laboratory, we propose that such aqueous phase photochemical reactions, i.e. both direct photolysis and indirect photo-oxidation, represent important atmospheric �photo-bleaching� processes.

Photo-bleaching of atmospheric brown carbon via direct photolysis and photo-oxidation in the aqueous phase

248th National Meeting (2014)

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