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