Multi-scale model analysis of boundary layer ozone over East Asia

This study employs the regional Community Multiscale Air Quality (CMAQ) model to examine seasonal and diurnal variations of boundary layer ozone (O-3) over East Asia. We evaluate the response of model simulations of boundary layer O-3 to the choice of chemical mechanisms, meteorological fields, boundary conditions, and model resolutions. Data obtained from surface stations, aircraft measurements, and satellites are used to advance understanding of O-3 chemistry and mechanisms over East Asia and evaluate how well the model represents the observed features. Satellite measurements and model simulations of summertime rainfall are used to assess the impact of the Asian monsoon on O-3 production. Our results suggest that summertime O-3 over Central Eastern China is highly sensitive to cloud cover and monsoonal rainfall over this region. Thus, accurate simulation of the East Asia summer monsoon is critical to model analysis of atmospheric chemistry over China. Examination of hourly summertime O-3 mixing ratios from sites in Japan confirms the important role of diurnal boundary layer fluctuations in controlling ground-level O-3. By comparing five different model configurations with observations at six sites, the specific mechanisms responsible for model behavior are identified and discussed. In particular, vertical mixing, urban chemistry, and dry deposition depending on boundary layer height strongly affect model ability to capture observed behavior. Central Eastern China appears to be the most sensitive region in our study to the choice of chemical mechanisms. Evaluation with TRACE-P aircraft measurements reveals that neither the CB4 nor the SAPRC99 mechanisms consistently capture observed behavior of key photochemical oxidants in springtime. However, our analysis finds that SAPRC99 performs somewhat better in simulating mixing ratios of H2O2 (hydrogen peroxide) and PAN (peroxyacetyl nitrate) at flight altitudes below 1 km. The high level of uncertainty associated with O-3 production in Central Eastern China poses a major problem for regional air quality management. This highly polluted, densely populated region would greatly benefit from comprehensive air quality monitoring and the development of model chemical mechanisms appropriate to this unique atmospheric environment.