Seasonal Cycles, Interannual Variations, and Recent Trends of Surface Ozone over East Asia: 7-years Observations and Regional Chemical Transport Model Analysis

The climatology of surface ozone was investigated over East Asia at a wide range of latitudes by integrating continuous measurements from Japanese monitoring networks. Although a spring maximum and a summer minimum were observed at all seven remote stations, significant latitudinal differences in the seasonal cycles were found, particularly in spring. At low latitudes (20-30 degN) the spring maximum appears in March, while it appears in April at high latitudes (40-50 degN), and in May at middle latitudes (30-40 degN). A regional-scale chemical transport model (RAMS/CMAQ) nested by a GCTM (CHASER) reproduces the latitudinal gradient and the overall seasonal variations well, and suggests that transport patterns of Asian continental outflow coupled with photochemistry result in latitudinal inhomogeneity over East Asia. There are sizable interannual variations associated with El Nino/La Nina events, suggesting that changes in regional meteorology (e.g., transport paths, patterns, and efficiency) and/or enhanced precursor emissions from biomass burning may contribute to the observed variability. Trends for 7 years are not visible at boundary layer sites, but substantial at mountainous sites during continental outflow seasons, possibly due to increasing NOx emissions from East Asia.