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

Abstract

Surface ozone (O 3 ) is formed from photochemical reactions of nitrogen oxides (NO x = NO + NO 2 ) and volatile organic compounds (VOCs). O 3 has serious health impacts up to and including premature mortality (Bell et al., 2004;Di et al., 2017). In the U.S., reductions in NO x and VOCs over the past several decades have reduced peak O 3 concentrations considerably (Simon et al., 2015), but at present, there are still more than 100 million Americans living in areas that do not meet the O 3 standard (American Lung Association (ALA), 2020). For O 3 , the U.S. National Ambient Air Quality Standards (NAAQS) is based on the annual fourth highest maximum daily 8-hr average (MDA8) concentration. To be in compliance with the 2015 8-hr O 3 standard, each monitor in a region must have a 3-year average of the fourth highest MDA8 of 70 ppb or less. Using data from 2018 to 2020, the U.S. Environmental Protection Agency (EPA) reports that there are 52 regions in the U.S. that are considered non-attainment areas (NAAs) for the 2015 standard (https://www.epa.gov/air-trends/air-quality-design-values). Most of these NAAs are in large metropolitan areas. However, some areas have relatively low emissions but are downwind of a major metropolitan area. Simon et al. (2015) report downward trends for U.S. anthropogenic NO x and VOCs emissions of 39% and 14%, respectively, for 2002-2011. Hidy and Blanchard (2015) report a near linear relationship between peak

Diverging Ozone Trends Above Western North America: Boundary Layer Decreases Versus Free Tropospheric Increases

“…An attribution study of this free tropospheric enhancement has not yet been conducted. A previous study found strong correlation between enhanced ozone and fires for 2017, 2018, and 2020 in urban areas over WNA (Jaffe et al., 2022), and we suggest that a future attribution study could explore the possibility that wildfire smoke impacted free tropospheric ozone.…”

“…When we limit the study period to a shorter time frame (2004-2021), similar seasonal patterns can still be observed. Consistent positive trends can be observed at MBO, with much stronger enhancements in August-October presumably due to the wildfire impacts (Baylon et al, 2015;Jaffe et al, 2022;Laing et al, 2016). A better agreement of trends between MBO and the free troposphere above WNA/California is observed in April-July.…”

“…Jaffe et al. (2022) analyzed surface data from 22 urban monitoring sites in the U.S. and found that COVID emission reductions significantly reduced peak ozone in the Eastern U.S., but in the western U.S. peak ozone increased in 2020 due to wildfire emissions. While those surface studies focused on urban areas, this study will investigate the COVID‐19 impact on surface ozone trends and variability at rural sites across WNA.…”

“…At the surface, several studies found that ozone increased broadly in many urban areas during the lockdown period (Cooper et al, 2021;Gkatzelis et al, 2021;Keller et al, 2021;Sokhi et al, 2021), likely because reduced NO emissions tend to limit the ozone destruction in highly polluted urban areas (Sillman, 1999). Jaffe et al (2022) analyzed surface data from 22 urban monitoring sites in the U.S. and found that COVID emission reductions significantly reduced peak ozone in the Eastern U.S., but in the western U.S. peak ozone increased in 2020 due to wildfire emissions. While those surface studies focused on urban areas, this study will investigate the COVID-19 impact on surface ozone trends and variability at rural sites across WNA.…”

Investigation of Ozone Formation Chemistry during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA)

“…To meet this standard, the three year average of the annual fourth-highest maximum daily 8 h average (MDA8) O 3 concentration cannot exceed 70 ppb. The successes and challenges that different regions of the U.S. have experienced when trying to comply with the national O 3 standard have been well-documented. − Since O 3 has significant health and regulatory implications at high concentrations, it is essential to understand the chemical and meteorological factors impacting O 3 levels.…”