Insights into the temporal and spatial characteristics of PM2.5 transport flux across the district, city and region in the North China Plain

Abstract

The Beijing–Tianjin–Hebei (BTH) region in the North China Plain (NCP) is significantly affected by inter–district, city and region transport, but systematic works on this aspect using flux calculation method remain insufficient. To obtain further insights into the temporal and spatial characteristics of PM2.5 flux, the Weather Research and Forecasting (WRF) model and the Comprehensive Air Quality Model with Extensions (CAMx) have been applied during January, April, July and October 2016. The results demonstrated that the modeling flux calculation method was suitable for investigating the evolutionary trend of PM2.5 flux by comparing cross–district simulations and observations. The total of monthly inflow, outflow and net flux indicated the intensive interactions and prominent temporal and spatial variations of PM2.5 transport though the boundary segment of the district, city and region. The vertical distribution analysis of net flux shows that PM2.5 net flux between target megacity/region and surroundings differed with the altitude, and the altitude at which its maximum intensity was closely related to neighboring areas and occurrence of month. Notably, three main transport pathways during four months have been identified based on the investigation of cross–city transport, namely the northwest–southeast (NW–SE) for January and April, the southeast–northwest (SE–NW) for July and October, the southwest–northeast (SW–NE) for all months, which were further confirmed by cross–region transport. Additionally, summer and winter monsoon have significant influences on the SE–NW and NW–SE pathway, respectively. Furthermore, this research draws upon a typical process of PM2.5 haze episodes from 11 to 17 October in Beijing, Tangshan and Shijiazhuang, with the aim of demonstrating flux intensity variations in different stages of haze episodes. Overall, the evolution of flux intensity at the higher–altitude layer have similar pattern to that of lower–altitude layer, in which the former was approximately 2 times the latter. Furthermore, inter–city transport along the SW–NE pathway played a crucial role before reaching the most severe stage of pollution, while the local emissions were more conductive to forming heavy pollution extremes. Overall, flux intensity calculations provide scientific support to put forward effective joint control measures and acquire a better understanding of evolutionary mechanism of haze episodes in the NCP.