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 (similar to 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.