Study On The Synergistic Patterns And Drivers Of PM_2.5 And Ozone Pollution In Eastern China

In recent years,particulate matter(PM_2.5)and ozone（O₃）have been two important pollutants in China.How to control both PM_2.5 and O₃ has been the focus of China’s air environment management.However,there are complicated effects between them as many studies reported.In this paper,pollution characteristics of PM_2.5 and O₃ and the impact of O₃ on secondary aerosol production in eastern China were firstly analyzed,and then the air quality model（WRF-CMAQ）was used to reveal the synergistic patterns of PM_2.5 and O₃ and their driving factors in some typical cases.Finally,several emission scenarios were simulated for co-control of PM_2.5 and O₃ in the Yangtze River Delta（YRD）region.The pollution characteristics of PM_2.5 and O₃ in the"2+26"cities and the YRD region in eastern China were sorted out,the results showed that the number of days with PM_2.5 and O₃co-pollution,which is defined by daily PM_2.5 concentration higher than75μg/m³ and maximum daily 8-h average（MDA8）O₃ higher than 160μg/m³,is significantly larger in the“2+26”region than that in the YRD region.In 2018 the co-pollution days were rebounded in some cities such as Beijing,Tangshan,Taizhou and Yangzhou,although they generally reduced year by year in both the“2+26”and YRD regions.It is noted that April had the most days of co-pollution from 2017 to 2019.The synergistic phenomenon of PM_2.5 and O₃ pollution under different ozone pollution levels in Beijing,Shanghai and Guangzhou from Apr.to Oct.（high ozone month）in 2017 were investigated based on the observed data.The numbers of hours with consecutive increase of PM_2.5 concentration during time period of at least two hours were counted as the hours of synergistic growth,and their corresponding increased concentration of PM_2.5 during 11a.m.-19 p.m.were estimated as the increased concentration of secondary aerosol.The results showed that Guangzhou had the largest number of hours of synergistic growth（580 h）,followed by Shanghai（543 h）,and Beijing city had the lowest（458 h）.The highest number of hours of synergistic growth occurred in spring（May）and autumn（September）in Guangzhou,in summer（July and August）in Shanghai,and in June and September in Beijing,indicating that the impact of ozone on the formation of secondary aerosol was significant and varied with cities.It is noteworthy that Beijing has the highest increased concentration of secondary aerosol although it has the least hours of synergistic growth.Beside the emission contribution,one of reasons probably is that Beijing has the relative low temperature which can promote the formation of secondary aerosol from the partition between gas and particle.Results indicate that the higher the photochemical activity level the larger the percentage of secondary aerosols in PM_2.5.For these three cities,the increased concentration of secondary aerosol in Beijing is the highest at all photochemical levels valued with ozone concentration.The average increased concentration of secondary aerosol in Beijing（29.1μg/m³）was significantly larger than that in Shanghai（19.5μg/m³）and Guangzhou（15.9μg/m³）at photochemical activity levels with O_3,max≥100μg/m³.Overall,significant synergism between ozone and PM_2.5 was revealed in Beijing,Shanghai and Guangzhou.The air quality simulation was conducted in the Yangtze River Delta region in April,July and October 2018.It was found that O₃ and SOA（secondary organic aerosols）,PM_2.5＿SO₄（sulfate）in Shanghai,Hangzhou,Taizhou and Suzhou were positively correlated,and the correlations between SOA and O₃ were much stronger.However,PM_2.5,PM_2.5＿NO₃（nitrate）,PM_2.5＿NH₄（ammonium）and POA（Primary organic aerosols）all showed the negative correlations with O₃in general.The stronger positive correlations occurred in Shanghai,as a city on the southeast coast,while the northern cities of the Yangtze River Delta,such as Taizhou and Suzhou in Jiangsu,were more likely to have negative correlations.In this part of air quality modeling study,a correlation（R）between SOA and O₃greater than 0.85 for 24 hours per day was used to define the synergistic phenomenon of ozone and PM_2.5 while R value less than 0 was defined as no synergistic phenomenon.It was found that the numbers of days with synergy were significantly greater in April and July than that in October.Furthermore,Shanghai has the most days with synergy while Suzhou has the least maybe due to the higher primary emission intensity of PM_2.5 and much heavier PM_2.5 pollution in Suzhou resulting in the impact of obvious change of radiation on ozone production to some extent.The synergistic phenomena are easy to occur under the condition of the high temperature,the low relative humidity,and the strong radiation and the long sunshine period.In the study of two typical non-synergistic cases in Suzhou with the help of the Integrated Process Rate（IPR）method,it was found that the highest reaction rate of SOA occurred at 7:00 a.m.,and the chemical generation was almost equal to zero in the afternoon while the highest ozone concentration usually occurred in the afternoon.The four synergistic cases in Shanghai,Suzhou and Taizhou can be classified into two types,which were driven by the dominated contribution of local chemical reaction and the dominated transport contribution,respectively.The chemical reaction rates of SOA were highest at 11a.m.-13p.m.（high ozone time）when the synergistic phenomenon occurred.The Integrated Reaction Rate（IRR）analysis revealed similar spatial distribution of Ox PROD,a VOCwith OH and SOA,suggesting that the strong oxidizability,anthropogenic VOC（AVOC）emission and SOA formation are closely related in these synergy cases.The results of emission scenarios simulation indicate that the concentrations of PM_2.5,SOA,and secondary inorganic aerosol（SIA）in the YRD region all decrease with the reduced AVOC emissions.Furthermore,SOA decreases more significantly than SIA.The more the AVOC emission reduction,the greater the concentrations reduction of PM_2.5,SOA,and O₃.From a city perspective,Shanghai has more synergistic phenomena between PM_2.5and O₃,and AVOC emission reduction can all cause the joint reduction of PM_2.5,SOA and O₃ concentrations,and the more the AVOC emission reduction,the greater the reduction of PM_2.5,SOA and O₃concentrations.However,it is more complicated that the relationship between AVOC emission reduction and PM_2.5,SOA and O₃ concentration reduction is more complicated in Suzhou City in the northern YRD compared with Shanghai.Therefore,different cities have different PM_2.5 and O₃interactions,and to achieve the goal of synergistic control of PM_2.5 and O₃concentrations in cities,more targeted emission reduction programs must be proposed based on an in-depth study of the complex relationship between precursors and PM_2.5 and O₃in each city.In this study,the synergistic patterns and driving factors of PM_2.5 and O₃ were investigated through in-depth analysis of observation data and air quality modeling.The results can provide important information for the co-control of PM_2.5 and O₃ pollution in eastern China.