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

Urban ozone (O₃) pollution is influenced by the transport of wildfire smoke but observed impacts are highly variable. We investigate O₃ 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, O₃ and PM_2.5 (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_2.5 and O₃ exhibit a nonlinear relationship such that O₃ increases with PM_2.5 at low to moderate 24 h PM_2.5, peaks around 30-50 μg m^-3, and declines at higher PM_2.5. Third, the rate of increase of morning O₃ is higher and NO/NO₂ 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, O₃ and PM_2.5 are elevated on days before and after HMS-identified smoke events implying that a significant fraction of smoke events is not detected.