Comparable Organic Aerosol Mass Enhancement during Photochemical and Dark Oxidation of Biomass Burning Emissions

Secondary organic aerosol (SOA) formed from wildfire/biomass-burning emissions (BB) represents a significant fraction of global SOA production. However, there are large uncertainties in representing BB-SOA in climate models. We studied the evolution of organic aerosols (OA) from burning three biomass samples─savannah grass, savannah wood, and boreal forest surface─under different combustion conditions and during daytime (photochemical oxidation) and nighttime (dark oxidation) aging in an atmospheric chamber. OA dominated the BB emissions by contributing ∼82–99% of the total PM₁ mass. Atmospheric aging by both oxidation processes produced comparable amounts of net OA mass. We show, with PMF analysis, that this is connected to the more efficient loss of primary OA, which compensates for the more efficient gas-phase oxidation during daytime aging compared with nighttime. The observed moderate OA mass enhancement (0.75–1.3 times) agrees well with field observations, thereby addressing the discrepancy between laboratory and field studies. For both aging scenarios, total OA emission factors after aging are similar to those of primary OA, providing new insights into the evolution of BB emissions. Our results suggest a simplified treatment of OA in climate models in remote areas with low NO_x concentrations.