Ozone photochemistry in fresh biomass burning smoke over the United States

The first 5 hours of aging in biomass burning plumes can strongly affect ozone photochemistry. We examine how volatile organic compounds (VOCs), nitrogen oxides, and nitrous acid influence hydroxyl radical, ozone, and peroxyacetyl nitrate (PAN) based on three aircraft campaigns over the United States. Our analyses reveal variable, highly elevated hydroxyl radical concentrations in the first 2 hours, resulting in evident fire-to-fire variability in VOCs oxidation and in ozone and PAN production. About 40 to 70% of the variability is explained by chemical aging. Ozone production in the plumes is usually VOC-limited for the first 2 hours and then nitrogen oxide limited downwind. Box model results for hydroxyl radical, ozone, and most VOCs, using the full, explicit Master Chemical Mechanism (MCM) mechanism, suggest no major gaps in the current best knowledge of gas-phase chemistry. However, the MCM sometimes overestimates PAN due to underestimated nitrogen oxide sinks. GEOS-Chem, a widely used chemical transport model with a reduced mechanism, generally underperforms because of incomplete VOC representation. We identify these critical pathways to guide future model development.