the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Mar 2026
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 < 33rd 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 NOX 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.
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Status: open (until 31 May 2026)
- CC1: 'community comment on Ryoo et al. 2026', Tabish Ansari, 24 Apr 2026 reply
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- 1
(This is a community comment, not a review.)
I enjoyed reading this manuscript by Ryoo et al. Here, they have derived source-receptor relationships using the FLEXPART-ERA5 model and have shown how the most influential source regions change across different percentiles of the ozone distribution in the Western North America. They identify a large influence of South Asia, Southern East Asia, and Southeast Asia in contributing to low ozone percentiles over the Western North America. I would like to point out that these results are in agreement with our recently published findings in ACP (Ansari et al, 2025) where we used explicit NOx and VOC emissions tagging and showed an increasing influence of foreign anthropogenic NOx and to a lesser extent global shipping NOx (see Figures 10, S8, and S10) over NW US, SW US, and Western Canada regions between 2000-2018. The findings become clearer when you focus on the winter months in those figures, which correspond to the low-percentile values analysed here. We have also shown, in Figure 17, that the increasing trend in wintertime and springtime ozone over western US is mainly due to increasing transport of foreign-NOx-produced O3 and not so much due to the reduced wintertime titration due to declining local NOx. Again, these findings are consistent with the results shown in this study.
Another recent study (Li et al., 2023) which used the same explicit dual-tagging technique found increasing contributions of Asian NOx emissions to Western US ozone, particularly in wintertime (see Figure 7 third row, left panel) over the 1995-2019 period.
It would be good to include findings from these studies in the introduction and/or discussion sections to make the study more up-to-date with recent work in the field.
Also, what the authors refer to as "Southeast Asia" here is quite different from the conventional definition of Southeast Asia. This can cause some confusion in the minds of readers-in-a-hurry who only read the abstract or try to cite the study without actually looking at the figures. The dominant source region identified here is in fact (Southern East Asia + South Asia + Southeast Asia).
I wish the authors the best with the rest of the review process.
Tabish Ansari
References of papers mentioned:
Li, P., Yang, Y., Wang, H., Li, S., Li, K., Wang, P., Li, B., and Liao, H.: Source attribution of near-surface ozone trends in the United States during 1995–2019, Atmos. Chem. Phys., 23, 5403–5417, https://doi.org/10.5194/acp-23-5403-2023, 2023.
Ansari, T., Nalam, A., Lupaşcu, A., Hinz, C., Grasse, S., and Butler, T.: Explaining trends and changing seasonal cycles of surface ozone in North America and Europe over the 2000–2018 period: a global modelling study with NOx and VOC tagging, Atmos. Chem. Phys., 25, 16833–16876, https://doi.org/10.5194/acp-25-16833-2025, 2025.