Changes in Light Absorption and Chemical Properties for Biomass Burning Organic Aerosol over Long Time Scales

Photolysis driven fragmentation is known to decrease the light absorbing properties of brown carbon (BrC) molecules in organic aerosol particles, but chemical changes and the lifetimes for the more photorecalcitrant fraction are not well understood. In this study, we probe the photoaging behavior of biomass burning organic aerosol (BBOA) particles collected on filters during the two Fire Influence on Regional and Global Environments Experiment (FIREX) campaigns in 2016 and 2019. We evaluate changes in the chemical properties during direct photolysis on the filters and probe the photobleaching rates over extended time periods in dilute aqueous solutions. We find that most of the laboratory burns (FIREX 2016) increase the average carbon oxidation state of the BBOA over 1–3 days of photolysis. We also introduce a new characterization method called Solar Flux-Weighted Mass Absorption Cross-section (SFW-MAC) that describes the full absorption properties of the mixture. We find that estimates from our photolysis rates are in the same range as ambient lifetimes. Our results show that longer term aging studies of more than 4 days are needed to fully capture the bleaching rate of atmospheric BrC aerosols, especially for photorecalcitrant BrC that will play a larger role farther away from the original emission source.