Were Wildfires Responsible for the Unusually High Surface Ozone in Colorado During 2021?

Ground-level ozone (O-3) was unusually high in northern Colorado in the summer of 2021 with maximum daily 8-hr average (MDA8) concentrations 6 to 8 parts-per-billion by volume (ppbv) higher than in 2019, 2020, or 2022. One or more of the monitors on the Colorado Front Range exceeded the 2015 U.S. National Ambient Air Quality Standard (NAAQS) of 70 ppbv on 66 of the 122 days from 1 June to 30 September, and this record number of exceedances coincided with the near daily presence of dispersed smoke haze from wildfires in Arizona, California, and the Pacific Northwest. In this paper, we use regulatory and non-regulatory surface O-3 and PM2.5 measurements in conjunction with ground-based lidar observations to estimate how much O-3 was associated with the wildfire smoke. Analyses of the surface measurements suggest that pyrogenic O-3 transported to northern Colorado with the smoke increased the surface concentrations in northern Colorado by an average of 8 ppbv in July, 3 ppbv in August, and 2 ppbv in September. Analysis of the lidar measurements showed these contributions to be as large as 12 ppbv on some days. Production of O-3 from reactions of pyrogenic VOCs and locally emitted NOx appears to have been minimal (<3 ppbv) in the Boulder area, but may have been much larger in the suburbs southwest of downtown Denver.