Wildfire Impacts on O₃ in the Continental United States Using PM_2.5 and a Generalized Additive Model (2018–2023)

We examined PM_2.5 and Hazard Mapping System smoke plume satellite data at ∼600 United States (US) air monitoring stations to identify surface smoke on 14.0% of all May-September days for 2018-2023, with large influences in 2020 and 2021, due to California fires, and 2023, due to Canadian fires. Days with smoke have an average of 11 μg m^-3 more PM_2.5 and 8 ppb higher maximum daily 8 h average (MDA8) O₃ concentrations than nonsmoke days, and they also account for 94% of all days that exceed the daily PM_2.5 health standard (35 μg m^-3) and 36% of all days that exceed the O₃ health standard (70 ppb). To estimate the smoke contributions to the O₃ MDA8, Generalized Additive Models (GAMs) were built for each site using the nonsmoke day data and up to 8 predictors. The mean and standard deviation of the residuals from the GAMs were 0 ± 6.1 ppb for the nonsmoke day data and 4.3 ± 7.9 ppb for the smoke day data, indicating a significant enhancement in the MDA8 O₃ on smoke days. We found positive residuals on 72% of the smoke days and for these days, we calculate an average smoke contribution to the O₃ MDA8 of 7.8 ± 6.0 ppb. Over the 6 year period, the percentage of exceedance days due to smoke in the continental US was 25% of all exceedance days, and the highest was in 2023 (38%). In 2023, the Central US experienced an unusually high number of exceedance days, 1522, with 52% of these impacted by smoke, while the Eastern US had fewer exceedance days, 288, with 78% of these impacted by smoke. Our results demonstrate the importance of wildland fires as contributors to exceedances of the health-based national air quality standards for PM_2.5 and O₃.