Characterize Fine Particulate Matter In The Yangtze River Delta:Chemical Composition,spatiotemporal Variation And Source Apportionment

Fine particulate matter(PM2.5)in the atmosphere reduce visibility,affect climate change,and have adverse effects on human health.The characteristic of PM2.5 is closely related to its chemical composition.However,the scientific understanding of these impacts remains low,mainly due to the highly complex and dynamic chemical and microphysical properties of atmospheric PM and their interactions with emission sources,atmospheric processes and meteorological conditions.The Yangtze River Delta is one of the fastest growing and most densely populated regions in the world.The emission sources and air origin related to regional transport in this area are complicated with extremely high humandity.The aerosol loadings are very high with large seasonal differences in physical and chemical properties.This research is based on the long-term observation of the Station for observing Regional Process of the Earth System(SORPES)of Nanjing University in the western Yangtze River Delta.We also conducted intensive field campaign with a mobile platform in Fengxian,Shanghai in the east end of the Yangtze River Delta.Morever,we took advantage of state-of-art technology,tethered ballon Observation and aircraft to investigate the vertical distribution of aerosol.Different models were also combined with our observations.The total PM2.5 mass concentrations and chemical compositions were observed through different online instruments at the SORPES station from 2014 to 2016.The average PM2.5 mass concentrations were 67.8 ?g m-3.Up to 32.7% of days exceed national average daily standard(75 ?g m-3).We found that the mass concentrations of PM2.5 were highest in winter and lowest in summer.Tow peaks of mass concentrations were observed in December and June.PM2.5 were dominated by the secondary components(77% on average).The contribution of secondary aerosol components especially nitrate increased with the increase of PM.Inorganic aerosols all exist in the form of neutral aerosols(mainly ammonium nitrate and ammonium sulfate).The corresponding time of meteorological data were applied to study the effect of different meteorological conditions on PM2.5 concentrations.The results show that high temperature will promote the formation of sulfate and secondary organic aerosol.To calculate the liquid water content of the aerosol,ISORIOPIA II model was conducted on our measurement data.The results showed that the aerosol liquid water content increased with the increase of pollution,and that the high liquid water content promoted the formation of secondary components.To investigated the dual peak pollution of PM in June and December,we conducted field study in Nanjing Xianlin and Shanghai Fengxian in the YRD simultaneously in December 2017 and June 2018.Aerosols at both sites were dominated by secondary components.In both summer and winter,the contribution of nitrate increased with the increase of total PM.The most severe regional pollution episodes in the Yangtze River Delta in winter were caused by the long-term air transport from the North China Plain.During this pollution episodes,the aerosols were relatively aged and dominated by nitrate and low volatile organic aerosol.The rapid growth of aerosols at the Shanghai site is related to the path of the cold front.In summer,air masses passing through the cityclusters of the YRD were responsible for the pollution episodes with elevated contributions of nitrate,Hydrocarbon-like organic aerosol(HOA)and semi-volatile oxidized organic aerosol(SV-OOA)to PM.During this field campaign,we also conducted aerosol vertical observation based on tathered airship.The average aerosol mass concentrations gradually decreased from ground surface to high altitude.Affected by primary emissions,organic aerosol accounted for the highest proportion on the ground layer.By comparing the difference between the average concentrations of aerosol components from clean and polluted conditions,we find that the increase of surface aerosols was mainly due to the contribution of organic matter.However,the increase of aerosols at heights of 200 meters and above was mainly dominated by nitrates.Due to the limitation of height of airship(1300m),we furher conducted aircraft measurement of PM.The results show that higher contribution of sulfate was oberserved above the boundary layer during pollution episode.Our results highlight the importance of nitrate in the YRD.We further investigate the formation pathway of nitrate based on both long term measurements and intensive field campaign.In summer,the thermodynamic equilibrium,thus the volatilization of ammonium nitrate is the main factor affecting the concentrations of particulate nitrate.The photochemical oxidation of NO2 yeilding nitric acid was also very important.In winter,low temperature conditions are favorable for nitate to stay in particle phase,resulting in high nitate level.According to the average daily concentration of nitrate,we selected individual episode nitrate pollution,and found that the obvious nitrate formation process was observed the night before the pollution episode.The calculation results show that the nitrate produced by the hydrolysis of N2O5 accounted for the up to 80% of the observed nitrate growth.Limited by the titration effect of NO at the surface layer,O3 concentration was higher at the upper night boundary layer.The hydrolysis of N2O5 was more important at the hight.The produced nitrate will be mixed in the surface in the morning of next day,resulting in nitrate concentration peak at ground surface.Our results highlight the effective control measures on nitrogen oxides emissions and O3 chemistry,which can reduce nitrate concentrations and thus the PM loadings in the YRD.In addition,regional or subregional emission control is importan to mitigate haze pollution in city clusters,such as the YRD in Eastern China.