5.1      Modeling diurnal variation of surface PM2.5 concentration over East China with WRF-Chem: Impacts from boundary mixing and emission.

Du, Qiuyan, Chun Zhao*, Mingshuai Zhang, Xue Dong, Yu Chen, University of Science and Technology of China, Zhen Liu, The Chinese University of Hong Kong, Zhiyuan Hu, Siyu Chen, Lanzhou University, China, and Qiang Zhang, Tsinghua University, China

Diurnal variation of surface PM2.5 concentration (diurnal PM2.5) could dramatically affect aerosol radiative and healthy impact, and can also well reflect the mechanisms of air pollution formation and evolution through chemical and physical processes.  So far, diurnal PM2.5 and its modeling capability over East China have not been investigated, and therefore, are examined in this study. Based on the observations, the normalized diurnal amplitude of surface PM2.5 concentrations (diurnal index) averaged over East China is relatively small and the weakest in winter. The diurnal variation shows the maximum concentration during the night in spring and fall and during the daytime in summer. In spring and fall, the areas with higher surface PM2.5 concentrations tend to have higher diurnal index. The impacts of planetary boundary layer (PBL) mixing and emission characteristics on modeling the diurnal variation of surface PM2.5 concentration with WRF-Chem are then examined. The PBL schemes in WRF-Chem significantly affect the boundary mixing process and then play the dominant role in modeling the diurnal index in summer, spring, and fall.The simulated diurnal index is found also sensitive the PBL vertical layer configurations of WRF-Chem. The PBL mass exchange coefficient instead of PBL height is found the critical factor determining the boundary mixing in WRF-Chem. The standard WRF-Chem simulations largely overestimate the observed diurnal index averaged over East China. The adjustment of PBL mass exchange coefficient can significantly reduce the modeling biases in diurnal index and also the mean concentrations, particularly at the major cities of East China. It can also reduce the modeling sensitivity to the PBL vertical layer configurations. The diurnal variation and injection height of anthropogenic emissions also play roles on simulating diurnal index, but the impact is much smaller than that from the PBL mixing.