Relationships between Particulate Matter, Ozone, and Nitrogen Oxides during Urban Smoke Events in the Western US.

Urban ozone (O) pollution is influenced by the transport of wildfire smoke but observed impacts are highly variable. We investigate O impacts from smoke in 18 western US cities during July-September, 2013-2017, with ground-based monitoring data from air quality system sites, using satellite-based hazard mapping system (HMS) fire and smoke product to identify overhead smoke. We present four key findings. First, O and PM (particulate matter <2.5 μm in diameter) are elevated at nearly all sites on days influenced by smoke, with the greatest mean enhancement occurring during multiday smoke events; nitrogen oxides (NO) are not consistently elevated across all sites. Second, PM and O exhibit a nonlinear relationship such that O increases with PM at low to moderate 24 h PM, peaks around 30-50 μg m, and declines at higher PM. Third, the rate of increase of morning O is higher and NO/NO ratios are lower on smoke-influenced days, which could result from additional atmospheric oxidants in smoke. Fourth, while the HMS product is a useful tool for identifying smoke, O and PM are elevated on days before and after HMS-identified smoke events implying that a significant fraction of smoke events is not detected.