Regional Transport Effect Of PM_2.5 Heavy Pollution In The Middle Reaches Of The Yangtze River And The Impact Of Synoptic-scale Circulatio

The Yangtze River middle basin（Hubei-Hunan）has been an important air pollution transport hub in central-east China due to its special geographical location,and the influential mechanism of atmospheric thermodynamic and dynamic conditions in different circulations,regional transport,and local emissions to the transport and convergence for the PM_2.5 in the Yangtze River middle basin needs to be further investigated.Based on the analysis of environment-meteorological observation data and muti-source reanalysis data in the Yangtze River middle basin during winter time from 2015 to 2020,and combined with mathematical statistics,atmospheric dynamics analysis,and the simulation and sensitivity test of the atmospheric chemical coupling model WRF-Chem,we comprehensive understood the pollution characteristics and meteorological influential mechanism in different synoptic circulation of the Yangtze River middle basin,and clarified the regional transport is the main reason for the occurring of the PM_2.5 episodes,revealed the two typical types of transport pollution called“rapid transit transport”（RTT）and“stationary accumulation transport”（SAT）,respectively.We also quantitative measured the influential degree of the regional transport to the atmospheric pollution variation.The main research contents are as follows:（1）The heavy PM_2.5 pollution events over the Yangtze River middle basin during2015-2019 were divided into 4 patterns using the obliquely rotated T-mode principal component analysis（PCT）method,namely,PCT1:high-pressure bottom transport pattern(accounting for 55.4%of the total heavy PM_2.5 pollution days),PCT2:low-pressure convergence accumulation pattern（16.2%）,PCT3:high-pressure static stability accumulation pattern（14.9%）,and PCT4:high-pressure rear transport pattern（13.5%）.Regional transport patterns（PCT1 and PCT4）account for 69%of the total heavy PM_2.5 pollution days,which are the major pattern of heavy PM_2.5 pollution in the Yangtze River middle basin.The southward invasion of cold front,easterly air transport,convergence and accumulation of lower layer,and subsidence movement of middle and lower layer played key roles in the different circulation patterns.（2）RTT and SAT events have the same meteorological trigger conditions in the early stage of PM_2.5 episodes,but the evolution of weather circulation in the later stage is completely different,leading to the differences in the evolution of the two types of heavy pollution processes.In the early stage,the northerly wind of the cold air drove the regional transport of air pollutants,and caused the rapid increase of PM_2.5 concentration in the Yangtze River middle basin.The average increase rate of PM_2.5 concentration in RTT event was 12.5μg m^-3h^-1,which was higher than 5.7μg m^-3h^-1 in SAT event.In the later stage of RTT events,the cold front intensified and affected the Yangtze River middle basin.The heavy pollution maintained for a short time,averaging about 27.5 h,due to the continuous strong northerly wind dispersal and wet scavenging of precipitation.At the later stage of SAT event,the southward cold high weakened and controlled the Yangtze River middle basin,resulting in regional transport detention and accumulation of local emissions,which leading a longer pollution process,with an average of 72.8 h.（3）We simulated a typical SAT PM_2.5 pollution episode during December 7 to 9,2020using the atmospheric chemical coupling model WRF-Chem,and evaluated the contribution of regional transport to the local persistent pollution in the later satge.The results show that regional transport is an important reason for the formation of persistent heavy pollution in the Yangtze River middle basin.In the early stage of regional transmission（7th Day）,the transmission contribution of most areas of the Yangtze River middle basin was as high as70%-90%;and in the later stage of static accumulation（8th to 9th day）,the transmission contribution was still reached 60%-80%（40%-60%）on 8th（9th day）.In addition,based on the WRF-Chem emission reduction scenario,we evaluated the contribution of local sources and regional transport in surrounding areas to the reduction of PM_2.5 in Wuhan during the COVID-19 lockdown period（From January 24 to February 29,2020）.PM_2.5 concentration in Wuhan decreased by 22.8μg m^-3（-36%）compared with the previous period.Meteorological conditions contributed 22.2%to the reduction of PM_2.5 and 77.8%to emission reduction control.The contribution of local and other regions are similar.The results reveal the important role of pollutant emission reduction in the upstream source region in improving air quality in the Yangtze River middle basin.