Analysis Of Pollution Synoptic Patterns And Impacts Of Boundary Layer Characteristics On The Distribution Of PM_2.5 Over The Yangtze River Delta In Winter

The objective weather classification method,combining the data of ground pollutant sites and the Lagrangian trajectory diffusion model HYSPLIT,was used to analyze and discuss the characteristics of the objective synoptic weather classification on pollution,the pollution process and the source of PM_2.5 over the Yangtze river delta region from 28 December,2016 to5 January,2017.Then,an integrated filed campaign on the boundary layer atmosphere and environment was carried out at 3 sites over the Yangtze River Delta（YRD）,Shouxian as rural area,northern suburb of Nanjing and Dongshan in Suzhou as residual area.We found that a two layers of temperature inversion in regional scale of YRD and air pollution event occurred simultaneously from 30 to 31 December 2016.In addition,the vertical distribution characteristics of atmospheric boundary layer and PM_2.5 during the three periods（including single-layer inversion at daytime,two-layers inversion at nighttime and no inversion）at the northern suburb of Nanjing site were studied.The result showed that PM2.5 pollution incidents over YRD during the observation,can be divided as sustained growth phase（P1）mainly resulting from the weather type of cold front（Cw）to the type of the bottom of high pressure type（GBn）,static steady continuous phase（P2）resulting from the weather type of the uniform pressure field at the back of high-pressure type（WGh）and rapid elimination phase due to the wet deposition（P3）.The gradual increase of PM_2.5 concentrations over the YRD during P1might be due to the transport of pollution in the beijing-tianjin-hebei region and the local accumulation.During P2,the steady and continuous state of PM_2.5 over the YRD was mainly affected by local pollution sources.The two layers of inversions appeared in Shouxian,Nanjing,and Dongshan in temporal series and from the northwest to the southeast in spatial series,which was related to the sinking and movement of the high pressure and the divergence center in the upper boundary layer and the ground radiative cooling through nighttime.Dominated by agricultural and living biomass burning,the emission sources of Shouxian site were moderately strong,which led to the vertical distribution of the PM_2.5.5 concentrations from the surface to the upper inversion was relatively uniform,and the height of high PM_2.5 was in accordance with the height of the upper inversion.Due to industrial emission,the elevated sources of the northern suburb of Nanjing site were stronger than Shouxian,the concentration of PM_2.5 had a distinct peak above the surface layer,and the height of the peak decreased with the height of the upper inversion.The emission sources of Dongshan site was weak and the surface concentration of PM_2.5 at Dongshan site was the lowest among three sites.The concentration of the PM_2.5 at Dongshan site was higher above 100m of surface at night（in the residual layer of the boundary layer）,probably from the medium-long range transport from the west of YRD.From the early morning to the noon,the concentration of PM_2.5.5 in the surface gradually increased,while the PM_2.5 in the upper layer was dissipated upward.The differences of the 2 layers of inversion and the emission sources of air pollution were the dominant factors of the vertical distribution of PM_2.5 at the three sites.Under the influence of single-layer inversion and elevated source at daytime at Nanjing station,PM_2.5 concentrations in the downwind area were distributed uniformly under the inversion layer due to the fumigation plume.Under the influence of double-layer inversion and elevated sources at night,the coning plume made the peak of PM_2.5 concentrations between two layer inversions in the downwind area.Under the influence of no inversion layer and elevated source,the plume caused the peak of the PM_2.5 concentrations in the vertical direction（100-600 m）in the downwind area.