Source Apportionment And Inter-city Transport Of PM_2.5 In Southern Hebei Cities Using WRF-Chem

The Weather Research and Forecasting model coupled with Chemistry（WRF-Chem）model was applied over southern Hebei（SHB）cities（Shijiazhuang,Baoding,Xingtai,Handan）to simulate the PM_2.5 concentration and meteorological parameters in January 2015.According to the model results,we calculated the PM_2.5fluxes in the boundary of cities to deduce the inter-city transport routes among SHB cities and their path areas.Based on the Brute Force Method,a total of 37 scenarios were designed at the three regional levels（province,city group,and inter-city）,and regional contributions of PM_2.5 to SHB cities were calculated.The results show that the PM_2.5 fluxes of SHB cities had large inflow and outflow fluxes with south and north neighboring cities.However,the fluxes had large differences with west and east neighboring cities,that is,strong in the west and weak in the east.In vertical layers,the PM_2.5 fluxes were weak in lower layer（layer 1-5,0³20 m）and strong in upper layer（layer 6-9,320¹000 m）.In lower layer,Xingtai and Handan were in the pathways of west to east and north to south.In upper layer,Shijiazhuang and Baoding were in the path of northwest to southeast and southwest to northeast,while Xingtai and Handan in the paths of west to east and south to north.The PM_2.5 concentrations of SHB cities were mainly contributed by their local sources,which resulted in the highest contributions of Hebei province and mid-south Hebei cities.In surrounding provinces,Shijiazhuang and Xingtai were influenced by Shanxi province,while Baoding and Handan were affected by Shandong and Henan province,respectively.In the surrounding city groups,southeast Shanxi city group had large contribution to Shijiazhuang and mid-east Hebei city group contributed most to Baoding.Xingtai and Handan were mainly contributed by northern Henan cities.The study also selected two pollution episodes（EP1:January 11-16 2015,EP2:January 22-27 2015）to analyze the causes combined with the PM_2.5 concentration,meteorological parameters,flux transmission routes and path areas contributions.The results show that increasing PM_2.5 concentration always accompanied by weak southerly wind,decreasing planetary boundary layer height,south to north fluxes,and short distance transmission airflows.The local contribution of SHB cities increased and contributions of their southern cities increased at the same time.On the contrary,when the PM_2.5 concentration of SHB cities declined rapidly,there were strong northerly wind,high planetary boundary layer height,north to south fluxes,and airflows from northwest with long distance transmission.The PM_2.5 local contributions increased but regional contributions had obvious decrease.In summary,adverse meteorological conditions,high local emissions and special geographical location of SHB cities played dominate roles on the heavy pollution.