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.
Copyright © 2019 American Association for Aerosol Research
Funding
This work was supported by EPA-STAR RD-83503301 and NSF CHE-1012809. Although the research described in this article has been funded in part by the US EPA, it has not been subjected to the Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. ECB gratefully acknowledges the NOAA Climate and Global Change fellowship and startup funds provided by the University of Colorado Boulder and the Cooperative Institute for Research in Environmental Sciences for financial support. The Advanced Light Source is supported by the Director, Office of Energy Research, Office of Basic Energy Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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