Origin of Low Ozone above Western North America: An Investigation of Sources and Trends

Abstract. While free-tropospheric ozone (O₃) over western North America (WNA) has increased since the mid-1990s, research has primarily focused on the mean. We investigate the lower tail (O₃ < 33^rd percentile) to characterize the evolving remote background state. Because these air masses are minimally affected by episodic extremes, they offer a clearer window into long-term shifts in background O₃, transport, and photochemistry. Using FLEXPART-ERA5 source–receptor relationships (SRRs) from 1994 to 2021, we analyze the transport history of air masses reaching WNA (25–55° N, 130–90° W). Despite no robust SRR trends within the lower and mid-troposphere (0-8 km), changing emission patterns suggest an intensifying remote influence. Specifically, WNA's surface NO_X emissions have decreased while lower-tail O₃ continues to rise, aligning with increasing surface emissions from Southeast Asia and intensified shipping. In contrast, UTLS (8–13 km) SRRs show a clear increase, indicating growing influence from high-altitude sources, including enhanced transport from Southeast Asia and the tropical Pacific, and rising global aircraft emissions. GMI chemical simulations corroborate these findings, revealing that net O₃ production over Southeast Asia increased by 157 % in the lower troposphere and 7 % in the free troposphere between 2007 and 2019. The rise in WNA's low O₃ percentiles is driven by the combined influence of intensified transport from Southeast Asia and the tropical Pacific, along with increasing global aircraft and shipping emissions. Ultimately, these trends reflect both the rapid growth of Southeast Asia emissions and shifting trans-Pacific transport.