Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Emissions of ozone (O<sub>3</sub>) precursors in the United States have decreased in recent decades, and near-surface O<sub>3</sub> concentrations showed a significant decrease in summer but an increase in winter. In this study, an O<sub>3</sub> source tagging technique is utilized in a chemistry-climate model to investigate the source contributions to O<sub>3</sub> concentrations in the U.S. from various emitting sectors and regions of nitrogen oxides (NO<sub>x</sub>) and reactive carbon species during 1995–2019. We show that domestic emission reductions from energy and surface transportation are primarily responsible for the decrease in summertime O<sub>3</sub> during 1995–2019. However, in winter the emission control also weakens the NO<sub>x</sub> titration process, resulting in considerable increases in O<sub>3</sub> levels from natural sources. Additionally, increases in aviation and shipping activities and transpacific transport of O<sub>3</sub> from Asia largely contribute to the winter O<sub>3</sub> increase. Changes in large-scale circulation also explain 15 % of the O<sub>3</sub> increasing trend.
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