Effect of heterogeneous oxidative aging on light absorption by biomass-burning organic aerosol

Light-absorbing organic aerosol (brown carbon; BrC) impacts the radiative balance of the earth’s atmosphere; however, the magnitude of this impact is poorly constrained due to uncertainties in BrC sources, composition, and lifetime. In particular, the role of chemical “aging” on the optical properties of BrC particles is poorly understood. Here we carry out laboratory studies aimed at understanding how one such aging process, heterogeneous oxidation, may affect the chemical and optical properties of biomass-burning-derived BrC. We generate BrC from smoldering ponderosa pine needles, oxidize the BrC in a flow reactor, and use simultaneous measurements of aerosol optical properties and chemical composition to monitor changes upon oxidation. Under the set of conditions investigated here, we find that with increased oxidant exposure the aerosol becomes more oxidized and less absorbing, presumably due to oxidative degradation of the chromophores. Both the kinetics and evolution of this process are oxidant dependent. While heterogeneous oxidation by ozone results in a rapid “bleaching” of the BrC (i.e. decrease in absorptivity), a substantial fraction of the BrC is resistant to bleaching by this mechanism. In contrast, bleaching due to heterogeneous oxidation by OH in the presence of ozone, remains active over long timescales (timescale of days), suggesting a sustained evolution of BrC optical properties throughout the aerosol atmospheric lifetime.