Variations Of PM_2.5 Pollution Over The Twain-Hu Basin With Interaction Of Atmospheric Boundary Layer In Regional Transport Of Air Pollutants

The Twain-Hu Basin（THB,the lower plain in the provinces of Hubei and Hunan）in Middle Yangtze River reaches,is geographically surrounded by the polluted regions in China.Driven by eastern Asian winter monsoon,the THB became the main receptor of regional transport of air pollutants over central and eastern China（CEC）.The impact of regional transport of air pollutants on PM_2.5 pollution in the receptor region and its spatial variations,the vertical structures of the atmospheric boundary layer（ABL）and aerosols as well as their interactions have been poorly understood in atmospheric environment changes.Based on multi-source observations and model simulations,this study focused on the receptor region of regional PM_2.5transport in the THB to reveal the regional features of interannual variations of PM_2.5 pollution,quantify the relative contribution of anthropogenic emissions and meteorology to the interannual declining trend of PM_2.5,characterize the vertical structures of ABL and aerosols,investigate the interactions of ABL and aerosols during regional PM_2.5 transport and construct the meteorological mechanism of heavy PM_2.5 pollution induced by regional PM_2.5 transport in the THB.Main conclusions are drawn as follows.（1）Quantifying the impact of anthropogenic emissions and meteorology on PM_2.5 changes over the THB in recent yearsPM_2.5 pollution in the THB was second only to Northern China region（NC）over CEC regions and presented the slowest decreasing trend of–1.08μg m^-3 y^-1 and–1.16 d y^-1 in PM_2.5concentrations and heavy pollution days.In this study of 5-year observations and modeling,we targeted the THB to quantify the impact of meteorology and anthropogenic emissions on PM_2.5decline over the THB.The meteorological contribution to PM_2.5 decline presented the distinct spatial pattern over the THB with northern positive rates and southern negative rates.The opposite effects of meteorology on PM_2.5 pollution could accelerate and inhibit the declining trend of PM_2.5 induced by emission reductions in the northern and southern THB,respectively.It is noteworthy that the increasing conversion efficiencies of SO₂ and NO₂ to sulfate and nitrate for secondary PM_2.5 could offset the effect of emission reductions on PM_2.5 decline in the THB during recent years,revealing the enhancing contribution of gaseous precursor conversion to PM_2.5 concentrations under controlling anthropogenic emissions.（2）Spatial variations of ABL structures and PM_2.5 pollution in the receptor region of regional transport of air pollutantsUnder the control of uniform pressure fields,the surface PM_2.5 concentrations and pollution occurrences in the THB were the highest among all types with weak winds,which was a typical synoptic type of local accumulation.The occurrence of northeast-type cold fronts promoted the regional transport of air pollutants from NC to the THB,which was the typical synoptic type of regional transport.During the PM_2.5 pollution induced by local accumulation,the stable ABL accelerated the accumulation of air pollutants near the surface,forming the vertical structure of decreasing aerosol extinction.During the PM_2.5 pollution induced by regional transport,the unstable ABL with strong winds and unstable stratifications in the THB resulted in the unique vertical structure of aerosol extinction peaking at～800 m.As strong wind drove air pollutants southwardly,the near-surface wind was stronger in the upstream region than downstream region,and the regional PM_2.5 transport exerted more significant impact on unstable ABL structures in the upstream region,which revealed the spatial differences in the influence mechanism of ABL structures on PM_2.5 pollution in the receptor region.Regional PM_2.5 transport promoted the vertical mixing of transported PM_2.5 in the upper layer to the near-surface and accelerated the near-surface pollution.The regional PM_2.5 transport played an important role on the spatial distribution of PM_2.5 pollution with more significant impact in upstream receptor region,which was verified by its contribution of 42.3%and 18.1%to PM_2.5 changes in the upstream and downstream THB,respectively.（3）Interaction of ABL and PM_2.5 during a heavy PM_2.5 pollution induced by regional transport in the THBDriven by a cold front with northerly winds,regional PM_2.5 transport was built from upstream NC to downstream THB in January 2019,where the observed ABL structures were unstable throughout the air pollution event.The regional PM_2.5 transport was concentrated in the free troposphere over the downwind receptor region,where the transported PM_2.5 were downwards mixed by enhancing vertical wind shear and weakening capping inversion in the ABL,favoring the vertical PM_2.5 transport to the near-surface.The weakened winds and stabilized stratifications in the ABL favored the sustained PM_2.5 pollution with decreasing vertical structure of PM_2.5 concentrations in the maintenance stage.The removal of PM_2.5 was triggered by increasing winds,activating the dissipation of heavy PM_2.5 pollution.The thermal factor dominated the vertical PM_2.5 changes in growth and maintenance of heavy PM_2.5pollution with enhancing contribution from 61%to 87%,while the mechanical factor controlled the dissipation of heavy PM_2.5 pollution,accounting for 60%.The enhancing inversion on the ABL top induced by the heating effect of aerosols in the free troposphere was unfavorable for the transported PM_2.5 crossing the ABL top to the near-surface and weakened the impact of regional PM_2.5 transport on the near-surface atmospheric environment in the receptor region.Meanwhile,the aerosol radiation forcing caused updrafts in the free troposphere,promoting the vertical mixing and the secondary PM_2.5 formation,which presented a positive feedback on high PM_2.5 concentrations in the free troposphere.