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. Mainland and maritime Southeast Asia is home to more than 655 millionpeople, representing nearly 10 % of the global population. The dryseason in this region is typically associated with intense biomassburning activity, which leads to a significant increase in surface airpollutants that are harmful to human health, including ozone(O3). Latitude-based differences in the dry season and land usedistinguish two regional biomass burning regimes: (1) burning on thepeninsular mainland peaking in March and (2) burning across Indonesiapeaking in September. The type and amount of material burned in eachregime impact the emissions of nitrogen oxides (NOx = NO + NO2) andvolatile organic compounds (VOCs), which combine toproduce ozone. Here, we use the nested GEOS-Chem atmospheric chemistrytransport model (horizontal resolution of 0.25∘ × 0.3125∘),in combination with satellite observationsfrom the Ozone Monitoring Instrument (OMI) and ground-basedobservations from Malaysia, to investigate ozone photochemistryover Southeast Asia in 2014. Seasonal cyclesof tropospheric ozone columns from OMI and GEOS-Chempeak with biomass burning emissions.Compared to OMI, the model has a mean annual bias of −11 %but tends to overestimate tropospheric ozone near areas of seasonal fire activity.We find that outside these burning areas,the underlying photochemical environment is generallyNOx-limited and dominated by anthropogenic NOx andbiogenic non-methane VOC emissions. Pyrogenic emissions of NOx play a keyrole in photochemistry, shifting towards more VOC-limited ozone productionand contributing about 30 % of the regionalozone formation potential during both biomass burning seasons. Using theGEOS-Chem model, we find that biomass burning activity coincides withwidespread ozone exposure at levels that exceed world public healthguidelines, resulting in about 260 premature deaths acrossSoutheast Asia in March 2014 and another 160 deathsin September. Despite a positive model bias,hazardous ozone levels are confirmed by surface observations duringboth burning seasons.

Photochemical environment over Southeast Asia primed for hazardous ozone levels with influx of nitrogen oxides from seasonal biomass burning