Secondary Inorganic Particle Pollution under Different Weather Conditions over East China in December 2017: A Model Insight

Eastern China is mainly affected by two kinds of synoptic systems in winter. Stable weather condition is under the control of high-pressure, the pollution in Yangtze River Delta (YRD) mainly comes from the emission of local sources at this time. When affected by the cold front, pollutants from the North China Plain (NCP) are transported through long distances to YRD region, resulting in the aggravation of pollution level over YRD. Numerous studies on the transport of pollutants by the cold frontal passage have been conducted in last decades. However, few researches focus on the contribution of chemical formation process during the cold front episode. In this study, we first evaluate and validate the performance of WRF-Chem simulation by observations, then compare the differences in the contribution of chemical production process to the major secondary inorganic aerosols under different weather conditions by using the diagnostic results in the model. The results show that the YRD region is slightly polluted during stable period, while the concentration of primary secondary inorganic aerosols mainly comes from the contribution of gas precursors generated by local sources emission through chemical conversion process. However, under the influence of strong weather system cold front, YRD is heavily polluted, which indicates that the cold fronts may deteriorate air quality in the downstream areas through the transport of pollutants from upstream. In terms of the proportion of relative contribution, the chemical production process under stable weather contributes more to the column concentration than the cold front, while the absolute contribution values are opposite. However, during the pollution episode, the photochemical activities are weakened, so the contribution of gas phase oxidation pathway to aerosol formation is limited. The emission of local sources is fixed, and the heterogeneous chemical processes considered in the model are relatively simple, thus indicating that the cold front carries both aerosols and gas precursors, and the absolute contribution value increases because of the chemical conversion in the transport path. The vertical distribution of aerosols along the transport pathway significantly exceeds the boundary layer height, indicating that the warm and humid polluted air mass ahead of the cold front is forced to lift into the upper atmosphere along the frontal boundary.