Surface and tropospheric ozone over East Asia and Southeast Asia from observations: distributions, trends, and variability

Abstract. High level of ozone throughout the troposphere is an emerging concern over East Asia and Southeast Asia. Here we analyzed available surface ozone measurements in the past two decades (2005–2021) over eight countries, and ten ozonesonde and aircraft measurements within this region. At surface, seasonal mean ozone over 2017–2021 varies from 30 ppb in Southeast Asia to 75 ppb in summer in North China. The metric of seasonal 95th percentile ozone can identify the multiple hotspots of ozone pollution of over 85 ppb in Southeast Asia. The new WHO peak season ozone standard indicates that both East Asia and Southeast Asia face a widespread risk of long-term exposure. The surface ozone increase in South Korea and Southeast Asia from 2005 was leveling off or even decreased in the past decade, while ozone increase in 2000s over China has amplified after 2013. Surface ozone trends in Japan and Mongolia were flat in the past decade. In the troposphere, the available measurements show an overall increasing tendency at different altitudes from a three-decade perspective and its trend in the past decade remains unclear due to data availability. The difference in tropospheric ozone level between East Asia and Southeast Asia is likely due to the high background ozone from stratospheric intrusion over Northeast Asia. In terms of ozone controls, our results suggest that anthropogenic emissions determine the occurrence of high ozone levels but the underappreciated strong ozone climate penalty, particularly over Southeast Asia, will make ozone controls harder under a warmer climate.