Chemical and physical transformations of organic aerosol from the photo-oxidation of open biomass burning emissions in an environmental chamber

Smog chamber experiments were conducted to investigate the chemical and physical transformations of or- ganic aerosol (OA) during photo-oxidation of open biomass burning emissions. The experiments were carried out at the US Forest Service Fire Science Laboratory as part of the third Fire Lab at Missoula Experiment (FLAME III). We investi- gated emissions from 12 different fuels commonly burned in North American wildfires. The experiments feature atmo- spheric and plume aerosol and oxidant concentrations; aging times ranged from 3 to 4.5 h. OA production, expressed as a mass enhancement ratio (ratio of OA to primary OA (POA) mass), was highly variable. OA mass enhancement ratios ranged from 2.9 in experiments where secondary OA (SOA) production nearly tripled the POA concentration to 0.7 in ex- periments where photo-oxidation resulted in a 30 % loss of the OA mass. The campaign-average OA mass enhancement ratio was 1.7± 0.7 (mean± 1 ); therefore, on average, there was substantial SOA production. In every experiment, the OA was chemically transformed. Even in experiments with net loss of OA mass, the OA became increasingly oxygenated and less volatile with aging, indicating that photo-oxidation transformed the POA emissions. Levoglucosan concentra-