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Emitted by numerous primary sources and formed by secondary sources, atmospheric brown carbon (BrC) aerosol is chemically complex. As BrC aerosol ages in the atmosphere via a variety of chemical and physical processes, its chemical composition and optical properties change significantly, altering its impacts on climate. Research in the past decade has considerably expanded our understanding of BrC reactions in both the gas and condensed phases. We review these recent advances in BrC aging chemistry with a focus on gas phase reactions leading to BrC formation, aqueous and in-cloud processes, and aerosol particle reactions. Connections are made between single component BrC proxies and more complex chemical mixtures as well as between laboratory and field measurements of BrC chemistry. General conclusions are that chemical change can darken the BrC aerosol particles over short time scales of hours close to the source and that considerable photobleaching and oxidative whitening will occur when BrC is a day or more removed from its source.
Hems et al. (Mon,) studied this question.