Decoupling the Evolution of the Light-Absorption Properties of Primary and Secondary Organic Aerosol Produced from Duff Burning

This study investigated the coevolution of the light-absorption properties of biomass-burning primary and secondary organic aerosol (POA and SOA) during photochemical aging. We performed smoldering combustion of duff and photochemically aged the emissions in an oxidation flow reactor (OFR). We retrieved the imaginary part of the refractive index (<i>k</i>) of the POA, aged POA (APOA), SOA, and aged OA (AOA), which includes both APOA and SOA. Photochemical aging induced competing effects on AOA absorption: (1) slight photoenhancement in POA, and (2) formation of very weakly absorbing SOA, with midvisible <i>k</i> an order of magnitude smaller than that of the POA, that photobleached rapidly with further oxidation. The latter effect dominated, resulting in a net decrease in AOA absorption. Changes in chemical composition corroborated the evolution in light-absorption properties. While POA exhibited minimal change in chemical composition due to photochemical aging, SOA underwent significant chemical transformation consistent with the observed photobleaching. We also demonstrated that the previously used indirect method, which estimates SOA absorption by subtracting fresh POA absorption from AOA absorption, can lead to severe overestimation of SOA absorption. Our findings underscore the importance of considering the distinct optical evolution of SOA and POA during photochemical aging.