Impacts of Wildfires on Air Quality: Not Just Particulate Matter

Recent decades saw increasing wildfire activities in the western US. Climate change is expected to increase the frequency of fire-prone weather conditions. While the impacts of wildfires on particulate matter air quality are well known, the extent to which biomass burning emissions affect gaseous air pollutants, such as ozone and its precursors, is less clear. In California, the national standard for ozone has been broadly violated. Wildfires emit substantial amounts of ozone precursors, nitrogen oxides (NO_x) and volatile organic compounds (VOCs), which react in the presence of sunlight to produce ozone. Here we present an observational approach that integrates smoke products, satellite retrievals of ozone precursors, and in-situ measurements to explore how wildfire emissions, when mixed with anthropogenic emissions, affect the ozone-NO_x-VOC chemistry from source to downwind urban areas. Satellite observations show that fires cause widespread increase of ozone precursors across California, which partially offset the reduction of NO_x and VOCs from anthropogenic emission control programs. The impacts of wildfire NO_x emissions are concentrated near-field over NO_x-limited region, and impacts of VOC emissions are more widespread, which extend to the VOC-limited urban areas, both contributing to more efficient O₃ production. The effects of precursor emissions outweigh the competing effects of smoke aerosols, which is evidenced from in-situ observed ozone enhancement under wildfire smoke.