Quantification of different processes in the rapid formation of a regional haze episode in north China using an integrated analysis tool coupling source apportionment with process analysis

Abstract North China Plain (NCP) is one of the most of heavily polluted regions in the world, air pollution associated with haze threatens human health. A high PM2.5 concentration event in the NCP from 30 November to 10 December 2017 was simulated and analyzed by using the Weather Research and Forecasting (WRF) model and the Nested Air Quality Prediction Modeling System (NAQPMS) with an integrated analysis tool coupling source apportionment with process analysis. The weather field simulated by the WRF model and the PM2.5 concentration simulated by NAQPMS agreed well with observations, and the correlation coefficient of PM2.5 between the simulation and observation data remained >0.8 during the study period. We found that this high PM2.5 event can be divided into three phases in NCP: The accumulation of PM2.5 in Phase I was slow and dominated by south or weak winds in the stable boundary layer. In Phase II, PM2.5 concentrations kept a high value in the short term under northern winds resulting from the edge of a cold front. The sustainable high value of PM2.5 concentration in the middle and south of the NCP under northly wind has been less reported. An integrated process contribution analysis and source apportionment technology coupled with NAQPMS showed that the PM2.5 in the north and middle of the NCP were transported to the southern area by the horizontal transmission process. This transport kept PM2.5 high in the south of the NCP and even exceeded the contribution of local emissions. In the Phase III, the strong northern winds from main body of the cold front brought clear air masses and caused the PM2.5 decrease in the whole NCP. The study shows that high dense emissions in the middle of NCP are the main cause for the high PM2.5 events in the whole region, and the north wind could transport the pollutants form upstream region to downstream region, causing the continued high PM2.5 in the middle and south of NCP.