NO x and O 3 Trends at U.S. Non-Attainment Areas for 1995-2020: Influence of COVID-19 Reductions and Wildland Fires on Policy-Relevant Concentrations.

We analyzed NO 2 and O 3 data from 32 U.S. non-attainment areas (NAAs) for 1995-2020. Since 1995, all regions have shown steady reductions in NO 2 and the weekend-weekday pattern indicates that the O 3 production regime in most NAAs has transitioned to a NO x -limited regime, while a few NAAs remain NO x -saturated. In the eastern U.S., all NAAs have made steady progress toward meeting the current (70 ppb) O 3 standard, but this is less true in midwestern and western NAAs, with most showing little improvement in peak O 3 concentrations since about 2010. Due to COVID-19 restrictions, NO 2 concentrations were substantially reduced in 2020. In the eastern NAAs, we see significant reductions in both NO 2 and peak O 3 concentrations. In the midwestern U.S., results were more variable, with both higher and lower O 3 values in 2020. In the western U.S. (WUS), we see variable reductions in NO 2 but substantial increases in O 3 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 O 3 metric in the WUS, and this is making it more difficult for these regions to meet the O 3 standard.