Twenty‐Five Years of Lower Tropospheric Ozone Observations in Tropical East Asia: The Influence of Emissions and Weather Patterns.

Tropospheric ozone affects the Earth's radiative balance, oxidative capacity, and air quality, yet the long‐term ozone trend in East Asia and its driver(s) remain poorly understood. Here we present ozone measurements obtained during 1994–2018 on China's southern coast. The measurement location intercepts China's outflow most of the time and the inflow of tropical maritime air during summer. We found an overall increase in the ozone level (0.35 ppbv/year), and the increase occurred mainly during the first half of the 25‐year period but appeared to level off in recent years in Chinese outflow. Large ozone increase (~20% per decade) was found in the maritime air. Model simulations show that recent weather conditions have reduced maritime ozone, counteracting the impact of the growing Southeast Asia's emissions. Our results fill the gap in the long‐term ozone trend in Asia and highlight the complex interaction of weather and emissions in driving the ozone change. Plain Language Summary: Tropospheric ozone has great influence on environmental issues ranging from air quality, to ecosystem productivity and to climate warming. Assessing the ozone impacts in fast‐developing Asia has been hindered by lacking long‐term measurements. We present here the longest continuous record of surface ozone in nonurban areas of tropical Asia. Using comprehensive tools, we reveal that the measurements on the south China's coast detected different ozone trends in air masses from eastern China (increasing in early years but stabilizing lately) and from Southeast Asia (continuously rising and at the largest rate). The latter finding is particularly striking, and we show that the recent weather patterns counteracted the expected increase of ozone due to emissions increases from Southeast Asia. Our results fill the gap in the long‐term ozone trends in Asia and highlight the complex interaction of weather and emission in driving the ozone trend. Key Points: We present the first long‐term record of ozone and CO in tropical East Asia, which shows that ozone has increased at the rate of 0.35 ppbv/yearThe ozone increase occurred during the first half of the 25‐year period and appears to have leveled off during recent years in the outflow from ChinaOzone shows the largest increase in maritime air mass, and weather change has counteracted the ozone increase associated with Southeast Asia's growing emissions [ABSTRACT FROM AUTHOR]