Source Apportionment And Synergetic Control Of Fine Particulate Matter And Ozone In The Yangtze River Delta Region

Fine particulate matter(PM_2.5)and ozone（O₃）are the two primary pollutants of regional complex air pollution in China over the past years,detrimental to urban air quality,public health,and sustainable socio-economic development.The formation mechanism and sources of air pollution,the influence of synoptic weather,and control policy-related issues have become research hotspots in the field of global atmospheric environment in recent years.The Yangtze River Delta（YRD）region is one of the key urban agglomerations in China with highly industrialization and urbanization,dense population,and severe air pollution.However,research on the source attribution and synergetic control of PM_2.5and O₃pollution is still quite limited.Through both observational analysis and numerical simulations,the spatio-temporal characteristics,source apportionment,and synergetic control methods of PM_2.5and O₃pollution in YRD from 2013 to 2017 were studied systematically in this dissertation.The characteristics and influencing factors of PM_2.5and O₃pollution in YRD were explored based on various observations.The source attribution of wintertime PM_2.5pollution and summertime O₃pollution under predominant synoptic weather patterns（SWPs）was conducted using an objective classification technique（the self-organizing map,SOM）and Particulate matter and Ozone Source Apportionment Technology（PSAT/OSAT）in the atmospheric chemical model（CAMx）.Based on the source-receptor relationships and a mathematical programming model,the atmospheric environmental capacity under the synergetic control of springtime PM_2.5and O₃pollution was estimated.The following are the main results of this study.（1）The characteristics and influencing factors of PM_2.5and O₃pollution in YRD were investigated through multiple observations.From 2013 to 2017,the trends of seasonal daily mean PM_2.5and daily maximum 8-h average（MDA8）O₃concentrations vary within-8.9～-6.6%yr^-1and+1.9～+6.0%yr^-1,respectively.The trends of regional average aerosol optical depth（AOD）and tropospheric column O₃are-4.5%yr^-1and+0.4%yr^-1,respectively.It suggests a significant increasing trend in PM_2.5pollution and a declining trend in O₃pollution over YRD,owing to the substantial reduction in SO₂,NO_x,NH₃and primary PM_2.5emissions,and increase in VOCs emissions since the Action Plan on Prevention and Control of Air Pollution was implemented in 2013.Generally,PM_2.5concentrations are high in the northwest and low in the southeast.The probability of PM_2.5concentrations exceeding the national air quality standard（defined as exceedance probability）is highest in winter,followed by spring.O₃concentrations are high in central areas and low in peripheral areas,with high exceedance probability in spring and summer.The 2016 field campaign in YRD reveals that secondary inorganic and carbonaceous components account for 70–80%of PM_2.5.Nitrate concentration is significantly higher than sulfate in winter,and the opposite in summer.The sulfur oxidation ratio（SOR）in summer（0.31）is higher than winter（0.24）,while nitrogen oxide ratio（NOR）is higher in winter（0.16）than summer（0.09）.The organic carbon（OC）to elemental carbon（EC）ratio（OC/EC）varies within 2.1-5.1 in summer,significantly higher than winter,suggesting the stronger formation of secondary organic carbon in summer.During high PM_2.5episodes in winter,the YRD is remarkably influenced by regional transport of air pollutants from the north.There may exist an explosive increase in PM_2.5concentration with the enhanced secondary conversion of sulfur and nitrogen,while air mass trajectories mainly originate from the northwest.During high O₃episodes in summer,MDA8 O₃concentration can reach above 300μg/m³.Unfavorable weather conditions（high temperature,strong radiation,low ambient humidity,and weak wind）and regional transport are responsible for high O₃levels.（2）Based on the SOM approach,the influences of typical SWPs on PM_2.5pollution in YRD during the winters of 2013-2017 were explored.The source contributions to PM_2.5during high episodes were quantified using the CAMx-PSAT model.In winter,the YRD is strikingly influenced by the Siberian High and the East Asian trough.The predominant SWPs include four PM_2.5-polluted types（pre-ridge,post-trough,equal-pressure,and mid-trough）and two PM_2.5-clean types（mid-ridge and post-trough）.The mid-trough type features the heaviest regional PM_2.5pollution,with the regional average PM_2.5concentration,exceedance probability,and AOD reaching 83.2μg/m³,45.9%,and 0.65,respectively.Compared to 2013-2016,the occurrence of PM_2.5-polluted SWPs in 2017 has decreased,implying that improved weather conditions have played a critical role in improving PM_2.5pollution.Source region analysis suggests some differences in regional transport of PM_2.5pollution under different SWPs.On average,the contributions of local emissions and regional transport to PM_2.5concentrations（averaged over all the model grids for the corresponding city）in four representative cities（Shanghai,Nanjing,Hangzhou,and Hefei）vary within 22.3-39.6%and 60.4-77.7%,respectively.Among four cities,local emissions account for the largest proportion of PM_2.5sources in Shanghai（32.0-48.4%）,while PM_2.5in other cities is more contributed by intra-regional transport within YRD（32.9-61.3%）.During the periods affected by the pollution transport from northern China（pre-ridge,post-trough,and mid-trough types）,the contributions of inter-regional transport out of YRD（especially from the north）increase significantly.Source category analysis shows that industrial and residential emissions are the two major sources for PM_2.5pollution,with contributions of 32.1-40.7%and 24.3-37.5%,respectively.The contributions of agricultural,power plants,and transportation emissions are comparable（8-15%）.Especially,the contributions of residential emissions increase dramatically when influenced by pollution transport from the north.Comparatively,sulfate and primary PM_2.5are more contributed by local emissions and mainly come from industrial and residential sources.Nitrate is more significantly affected by regional transport,and its major contributors include industrial,power plants,and transportation sources.Agricultural sources contribute more than 80%to ammonium.（3）Based on the SOM approach,the influences of typical SWPs on O₃pollution in YRD during the summers of 2013-2017 were explored.The source contributions to O₃during high episodes were quantified using the CAMx-OSAT model.The predominant SWPs in summer over YRD include four O₃-polluted types（northeasterly,northerly,southwesterly,and anticyclone）and two O₃-clean types（cyclone and the Meiyu front）.The northerly type features the heaviest regional O₃pollution,with the regional average MDA8 O₃concentration and exceedance probability reaching 121.0μg/m³and 19.2%,respectively.The integrated influences of local chemistry and regional transport are important in O₃pollution under different SWPs.Under all SWPs,MDA8 O₃respectively holds significant negative and positive correlations with relative humidity and sunshine duration,suggesting the importance of chemical production in O₃pollution.Source region analysis shows that O₃pollution is more influenced by regional transport compared to PM_2.5pollution.On average,the contributions of local emissions and regional transport to O₃concentrations in Shanghai,Nanjing,Hangzhou,and Hefei（averaged over all the model grids for the corresponding city）vary within 14.6-19.3%and 80.7-85.4%,respectively.Local production accounts for the largest proportion of O₃sources related to stagnant weather conditions under the anticyclone type.High O₃levels under the northerly and northeasterly types are primarily related to regional transport on various spatial scales.Under the southwesterly type,unfavorable weather conditions superimposed on strong regional transport lead to the distinct spatial disparity of O₃concentrations and the occurrence of severe O₃pollution in downwind coastal regions.The impact of super-regional transport is most pronounced（>50%）along the eastern coast under the cyclone type.Source category analysis shows that transportation and industrial emissions are the two major sources for O₃pollution,with contributions of 22.2-29.8%and 25.1-25.9%,respectively.The contributions of industrial and power plants sources are prominent in downwind coastal regions.Biogenic emissions generally provide a background O₃source（5.8-9.7%）.（4）Based on the CAMx-PSAT/OSAT model,the city-scale source-receptor relationships of springtime PM_2.5and O₃in YRD in 2014 were established.The atmospheric environmental capacity was further estimated using a mathematical programming model.From 2014 to 2017,the occurrence frequency of high PM_2.5and high O₃（double high）pollution episodes have decreased.The double high episodes are most common and widespread in May 2014,with the regional average PM_2.5and daily maximum 1-h average（MDA1）O₃concentrations in polluted days reaching 83.4 and 171.0μg/m³,respectively.The contributions of total anthropogenic emissions of all 33 cities in YRD to PM_2.5and MDA1 O₃concentrations of the corresponding 33 cities vary within 34.8-81.7%and 17.7-40.5%,respectively.It suggests that O₃pollution is significantly contributed by regional transport,leading to limited space for emission reduction.The central areas of YRD are under VOCs-limited conditions,and other areas are in a NO_x-limited regime.Aiming at the maximum permissible pollutant emissions,there are great differences in the precursor emission reduction schemes for different cities and source sectors,under the constraints of different air quality goals(PM_2.5and O₃)and the upper limits of emission reduction ratios.The control of SO₂and primary PM_2.5is most strict,and NH₃control is weakest.Due to the synergistic effect of NO_xand VOCs emission abatement on PM_2.5and O₃,strengthening PM_2.5control or weakening O₃control can lead to less VOCs emission abatement（especially in areas under a VOCs-limited regime）,but at the same time,the enhanced regional NO_xemission abatement is required.It indicates the importance of optimal allocation of atmospheric environmental capacity.Under the constraints of the air quality goals(PM_2.5≤50μg/m³,MDA1 O₃≤160μg/m³),145 optimal emission reduction schemes are obtained.The assessment of emission reduction effectiveness demonstrates that all schemes can achieve the air quality goals for PM_2.5in 33 cities,while the rates of reaching air quality goal of MDA1 O₃concentrations are higher than 81%.After emission reduction,the regional average PM_2.5and MDA1 O₃concentrations decrease by77.6-85.3%and 11.3-24.8%,respectively,and present a roughly linearly increasing response to the emission reduction of precursors（except VOCs）,and a relatively significant nonlinear response to VOCs control.Compared to O₃,the linear response of the decrease in PM_2.5concentrations to the precursor emission reduction is more obvious,thus PM_2.5control is more effective.The reduction of precursor emissions for O₃control should conform to a certain scientific proportion（NO_x/VOCs）and strengthen the regional NO_xcontrol.Finally,8 optimal emission control schemes,as well as atmospheric environmental capacity and emissions reductions of primary pollutants were determined with the attainment of air quality goals for PM_2.5and O₃in all cities of YRD,which provides a reference for regional long-term air quality management.For example,the emission reduction ratios of SO₂,NO_x,NH₃,VOCs and primary PM_2.5in May 2014 are respectively 95.6%,75.5%,33.1%,45.1%,and85.5%,while the atmospheric environmental capacities are respectively 8.3,79.4,102.7,186.9,and 13.0 kt/mon.This study shows an in-depth analysis of the spatiotemporal characteristics,impact factors,source attribution,and synergetic control of PM_2.5and O₃pollution during typical seasons in YRD.Results indicate that PM_2.5and O₃pollution has obvious seasonal variation,with significant differences in the contributions of source regions and source categories under predominant SWPs during high-occurrence seasons.The source analysis results are comparable with previous studies.During wintertime haze episodes,PM_2.5pollution mainly comes from industrial and residential sources.Local emissions account for about 20-40%to PM_2.5,while the regional transport from the north could contribute over 20%.During summertime photochemical pollution episodes,O₃pollution is more significantly affected by regional transport（>80%）,and mainly comes from transportation and industrial sources.During springtime double high episodes,the regional exceedance probability of PM_2.5concentrations is generally higher compared to O₃in YRD.However,the controllable proportion of PM_2.5is significantly larger,with a more obvious linear response to the reduction of precursor emissions.Therefore,PM_2.5control is more strict but less difficult than O₃control.This dissertation can provide a scientific basis for the joint prevention and control of regional complex air pollution.