Tropospheric ozone trends and attributions over East and Southeast Asia in 1995-2019: An integrated assessment using statistical methods, machine learning models, and multiple chemical transport models

We apply a statistical model, two machine learning models, and three chemical transport models to attribute the observed ozone increases over East and Southeast Asia (ESEA) to changes in anthropogenic emissions and climate. Despite variations in model capabilities and emission inventories, all chemical transport models agree that increases in anthropogenic emission are a primary driver of ozone increases in 1995-2019. The models attribute 53 %-59 % of the increase in tropospheric ozone burden over ESEA to changes in anthropogenic emissions, with emission within ESEA contributing by 66 %-77 %. South Asia has increasing contribution to ozone increases over ESEA. At the surface, the models attribute 69 %-75 % of the ozone increase in 1995-2019 to changes in anthropogenic emissions. Climate change also contributes substantially to the increase in summertime tropospheric (41 %-47 %) and surface ozone (25 %-31 %). We find that emission reductions in China since 2013 have led to contrasting responses in ozone levels in the troposphere (decrease) and at the surface (increase). From 2013 to 2019, the ensemble mean derived from multiple models estimate that 66 % and 56 % of the summertime surface ozone enhancement in the North China Plain and the Yangtze River Delta could be attributed to changes in anthropogenic emissions, respectively, with the remaining attributed to meteorological factors. In contrast, changes in anthropogenic emissions dominate summertime ozone increase in the Pearl River Delta and Sichuan Basin (91 %-95 %). Our study underscores the need for long-Term observational data, improved emission inventories, and advanced modeling frameworks to better understand the mechanisms of ozone increases in ESEA.