PNNL

Abstract

Emissions of ozone (O3) precursors in the United States have decreased rapidly in recent decades, and near-surface O3 concentrations showed a significant decrease in summer but an increase in winter. In this study, an ozone source tagging technique is utilized in a chemistry-climate model to investigate the source contributions to O3 concentrations in the U.S. from various emitting sectors and regions of nitrogen oxides (NOx) and reactive carbon species during 1995–2019. We show that domestic emission reductions are primarily responsible for the decrease in summertime O3 concentrations in the U.S. during 1995–2019. However, in winter the domestic emission control also weakens the NOx titration process, resulting in considerable increases in O3 levels from natural sources. Additionally, increases in aviation and shipping activities and transpacific transport of O3 from Asia largely contribute to the winter O3 increase. Changes in large-scale circulation also explain 15% of the O3 increasing trend.