Source Variations And Regional Transport Characteristics Of Atmospheric Fine Particulate Matter Under Intervention And Non-Intervention Conditions

With the implementation of the Air Pollution Prevention and Control Action Plan and the Three-Year Action Plan to Win the Blue Sky Defense,PM_2.5 pollution has been effectively alleviated in China.However,haze still occurs frequently in the middle reaches of the Yangtze River due to the influence of meteorological conditions and geographical location.The abnormal increase of atmospheric concentration in Twain-Hu Basin,a new air pollution hotspot,has changed the spatial distribution of PM_2.5 in China.Therefore,to investigate the atmospheric PM_2.5 pollution status,source variations as well as the inter-regional transport characteristics under intervention and non-intervention conditions in typical cities in the middle reaches of the Yangtze River can help us formulate precise control measures,improve the effect of regional joint prevention and control,and reduce control costs,which is significant for air pollution prevention and control.In this study,atmospheric PM_2.5 in Huangshi,Huanggang and Wuhan city in the middle reaches of the Yangtze River were studied.PM_2.5 were simultaneously collected in Huangshi,Huanggang and Wuhan during 2018-2019,a persistent haze in winter and the air quality assurance period of the 7^th World Military Games.Based on the main chemical components in PM_2.5,including water-soluble inorganic ions,organic carbon（OC）,elemental carbon（EC）and elements,the composition characteristics,influencing factors and source variations were investigated by combining multiple methods,such as positive matrix factor analysis,backward trajectory and potential geographical source region models.The inter-regional interaction patterns under different seasons,non-intervention and intervention conditions were revealed.The main conclusions were as follows:（1）Atmospheric PM_2.5 mass concentration in Huangshi,Huanggang and Wuhan decreased significantly in 2018-2019 compared to 2013,and the air quality improved significantly.However,there were about 20%of samples with PM_2.5 concentration exceeding the second grade of 24-hour average ambient air quality standard(75μg m^-3,GB 3095-2012),which indicated that PM_2.5 pollution in the middle reaches of the Yangtze River still needs attention,especially in winter.PM_2.5 chemical composition were similar between Huangshi and Huanggang,but different from that in Wuhan.The top three chemical species in PM_2.5 in Huangshi and Huanggang were in the following order of NO₃^->SO₄^2->organic matter（OM）,while that in Wuhan was NO₃^->OM>SO₄^2-.NO₃^-accounts for the largest proportion in winter and the lowest in summer.SO₄^2-vice versa.Aerosols in Huangshi and Huanggang were acidic,while aerosols in Wuhan were alkaline.Secondary inorganic aerosols and vehicle emissions were the major contributors to PM_2.5in Huangshi,Huanggang and Wuhan,which accounted for 34.5%-73.9%.（2）Under non-intervention conditions,adverse meteorological conditions of low wind speed and high relative humidity were the main external causes of PM_2.5 pollution during the study period.The northwest wind promoted the increase of PM_2.5concentration,while the strong cold air from the northeast was conducive to the removal of pollutants.The concentration of major chemical components(NO₃^-,SO₄^2-,NH₄⁺,OC and EC)in PM_2.5 gradually increased from clean days to moderate pollution to severe pollution,and the proportion of secondary inorganic ions(NO₃^-,SO₄^2-and NH₄⁺,SNA)increased with the increase of pollution level,while the proportion of OC and EC decreased.The contribution of mixed source of secondary inorganic aerosols and vehicle emissions increased significantly during haze.（3）Under the intervention conditions(i.e.during the control period of the 7^th World Military Games),the concentrations of PM_2.5,NO₂,SO₂ and CO decreased significantly,even though the meteorological conditions of low wind speed and high relative humidity were not conducive to the diffusion of pollutants.In addition,the concentrations of SNA,OC,EC and secondary organic carbon（SOC）significantly decreased,while SOC/OC increased,indicating the important role of SOC in relatively clean days.However,the control measures did not significantly affect the chemical compositions of PM_2.5.Nitrogen oxidation ratio（NOR）and sulfur oxidation ratio（SOR）showed no significant difference between control and non-control periods,indicating that NO₂ and SO₂concentrations were not the key factors to limit the secondary inorganic ions formation in the study area.The contributions of secondary inorganic aerosols,vehicle emissions and road dust decreased significantly in the control period,but secondary inorganic aerosols were still the main source of PM_2.5.Continuing to control SO₂ and NO₂ emissions is still an effective way to reduce PM_2.5 concentration in the study areas.In addition,road dust and vehicles emissions should be paid more attentions.（4）The anthropogenic intervention during the 7^th World Military Games significantly changed the transportation characteristics of pollutants among Wuhan,Huangshi and Huanggang.When heavy pollution occurs in Wuhan,in addition to controlling local emission,controlling the pollutants in Huangshi and Huanggang were conducive to the reduction of pollutants in Wuhan,especially for coal combustion and vehicle emissions in Huanggang,and coal combustion and road dust in Huangshi.Meanwhile,it can avoid the outbreak of regional pollution caused by the transmission of pollutants from Wuhan to Huangshi and Huanggang superimposed on local emission.This study explored the formation mechanisms of haze and revealed the characteristics of inter-regional transport by investigating the compositions and sources of atmospheric PM_2.5 in three typical cities in the middle reaches of the Yangtze River during different seasons,haze and air quality assurance period of the 7th World Military Games,which provided scientific evidence for future air pollution prevention and control.