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

The high level of ozone throughout the troposphere is an emerging concern for East Asia and Southeast Asia. Here, we analyze available surface ozone measurements from the past two decades (2005-2021) over eight countries, with 10 ozonesonde and aircraft measurements within this region. At the surface, the seasonal mean ozone over 2017-2021 varies from 30 nmol mol-1 (i.e., 30 ppb) in Southeast Asia to 75 nmol mol-1 in summer in northern China. The metric of seasonal 95th percentile ozone can identify the multiple hot spots of ozone pollution of over 85 nmol mol-1 in Southeast Asia. The new World Health Organization (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 decreasing in the past decade, while the ozone increase in the 2000s over China has amplified since 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, although the trend in the past decade remains unclear due to data availability. The difference in the tropospheric ozone level between East Asia and Southeast Asia is likely due to the high background ozone from the 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.