Analysis of the seasonal variation of ozone in the boundary layer in East Asia using the Community Multi-scale Air Quality model: What controls surface ozone levels over Japan?

Ozone (O3) concentrations in East Asia were simulated using the Community Multi-scale Air Quality model and their reproducibility was confirmed by comparing with observation data at Japanese monitoring sites. The model can reproduce the seasonal variation of surface O3 over Japan. In particular, for the southern parts of Japan, simulated surface O3 concentrations are strongly correlated with observations. Surface O3 distribution over East Asia varies dynamically from season to season according to the meteorological condition. In May and June, 2-month average O3 concentrations in the boundary layer are highest, 55–70 ppbv over East China and Japan at 35–40°N. We estimated the seasonal variation of the contribution of chemically produced O3 by east Asian regional emissions. In the summertime, the contribution of the regional Asian emissions to O3 concentrations is around 40–70% over most of China, and the highest is 60–70% (<35 ppbv) at the mouth area of Yantze River. This suggests that O3 concentrations over East Asia are strongly influenced by the chemical production by regional emissions during summer season. However, the inflow from outside of this model domain also contributes substantial O3 concentration over East Asia and its contribution is predominant in winter and early spring. The seasonal change of O3 concentrations over Japan is characterized by two peaks in spring and autumn and summer minimum. The spring peak event in 2002 consists of split peaks: the first peak in Mar–Apr is mainly influenced by the inflow from outside of this model domain, the second spring peak between May and June is mainly influenced by chemically produced O3 by regional emissions. During the summer season, O3 concentrations are the lowest over year because of the weak Asian outflow and northward penetration of the marine air mass.