Dark aging of biomass burning as an overlooked source of oxidized organic aerosol

Oxidized organic aerosol (OOA) is a major component of ambient particulate matter, substantially affecting both climate and human health. A considerable body of evidence has established that OOA is readily produced in the presence of daylight (via the OH radical). However, this current mechanistic understanding fails to explain elevated OOA concentrations during night or wintertime periods of low photochemical activity, thus leading atmospheric models to under-predict OOA concentrations. Here we show that fresh emissions from biomass burning rapidly form OOA in the laboratory over a few hours and without any sunlight. The resulting OOA chemical composition is consistent with the observed OOA in field studies in major urban areas. By combining laboratory smog chamber experiments and ambient observations to inform a chemical-transport model, we present strong evidence that night-time oxidation of residential biomass burning plumes (proceeding through reactions with O3 and the NO3 radical) is an important source of OOA.