NOx and O3 Trends at U.S. Non‐Attainment Areas for 1995–2020: Influence of COVID‐19 Reductions and Wildland Fires on Policy‐Relevant Concentrations

We analyzed NO2 and O3 data from 32 U.S. non‐attainment areas (NAAs) for 1995–2020. Since 1995, all regions have shown steady reductions in NO2 and the weekend‐weekday pattern indicates that the O3 production regime in most NAAs has transitioned to a NOx‐limited regime, while a few NAAs remain NOx‐saturated. In the eastern U.S., all NAAs have made steady progress toward meeting the current (70 ppb) O3 standard, but this is less true in midwestern and western NAAs, with most showing little improvement in peak O3 concentrations since about 2010. Due to COVID‐19 restrictions, NO2 concentrations were substantially reduced in 2020. In the eastern NAAs, we see significant reductions in both NO2 and peak O3 concentrations. In the midwestern U.S., results were more variable, with both higher and lower O3 values in 2020. In the western U.S. (WUS), we see variable reductions in NO2 but substantial increases in O3 at most sites, due to the influence from huge wildland fires. The recent pattern over the past decade shows that the large amount of wildland fires has a strong influence on the policy‐relevant O3 metric in the WUS, and this is making it more difficult for these regions to meet the O3 standard. Plain Language Summary O3 exposure has serious health impacts up to and including premature mortality. In the U.S., more than 100 million people live in areas that do not meet the current National Ambient Air Quality Standards (NAAQS) for O3. In this analysis, we used available air quality data from 32 major metropolitan areas that do meet the current O3 standard. We find that in nearly all regions there is a significant long‐term relationship between nitrogen oxides and the policy relevant peak O3 concentrations. In addition, the pattern of weekend‐weekday concentrations indicates that nearly all regions have transitioned to a NOx limited regime. Given the strong reductions in nitrogen oxide emissions in 2020 due to COVID‐19 restrictions, we expected to see a significant reduction in peak O3. This was seen at all sites in the eastern U.S. However, in the western U.S., we found a significant increase in peak O3 associated with the large wildfires that burned in 2020. These results provide important data to show how nitrogen oxides control O3 in most regions, but suggest an ongoing challenge for western regions to meet the O3 standard, given the likelihood of continued climate driven wildfire increases. Key Points We found a strong relationship between long‐term conce