Seasonal Variation,Size Distribution And Formation Mechanism Of Dicarboxylic Acids And Related Soa In Chongming Island

Since the Chinese Government promulgated the strict Air Pollution Prevention and Control Action Plan in 2013,air pollution in China has significantly been mitigated,such as the effectively declining PM_2.5concentrations in many Chinese cities.However,due to the rising number of vehicles and the lack of effective controls on volatile organic compounds（VOCs）and NH₃,concentrations of VOCs,NO₃^-and NH₃are still very high,along with an increase in ozone concentrations in many regions of China,demonstrating that atmospheric oxidation capacity has been enhancing.Such changes in concentrations and compositions of air pollutants could significantly alter the physicochemical and optical properties of the atmospheric aerosols in the country especially secondary organic aerosols（SOA）.To improve our understanding on the current air pollution status of Yangtze River Delta（YRD）region,a comprehensive field observation on atmospheric aerosols and its precursors was performed during the summer and winter of 2018 in Chongming Island,a background site of the YRD.In this study,seasonal variations in molecular composition,concentration,size distribution and formation mechanism of dicarboxylic acids and related SOA in Chongming Island were characterized by using a combining GC-MS and GC method.Impact of meteorological conditions such as temperature,relative humidity（RH）on the physicochemical properties of the aerosol aqueous phase in Chongming Island were investigated by using the thermodynamic model ISSOROPIA-Ⅱ.Moreover,a series of statistic methods including backward trajectory clustering analysis,PSCF model,linear regression fitting and organic tracer method were also used for revealing the sources and formation mechanisms of dicarboxylic acids and related SOA in Chongming Isand.The main conclusions are summarized as follows:（1）Concentration of summertime PM_2.5during the sampling period was 35±22μg m^-3.Concentration of total detected dicarboxylic acids in PM_2.5was 359±277 ng m^-3,among which oxalic acid（C₂）was the highest,followed by succinic（C₄）,malonic（C₃）,adipic（C₆）and phthalic（Ph）acids.Concentrations of ketocarboxylic acids andα-dicarbonyls in PM_2.5were 31±28 ng m^-3and 19±18 ng m^-3,respectively.PM_2.5concentration in winter was 59±41μg m^-3and concentration of the total dicarboxylic acid was 360±233 ng m^-3.The winter abundance of the total diacids were similar to those in summer,but the winter concentration of C₂was lower than that in summer,because the atmospheric photochemical oxidation capacity in winter was weaker than that in summer due to the lower temperature and weaker solar radiation.Being different from the case in summer,the second highest diacid in winter in Chongming Island was Ph,which is about 50%higher than that in summer.Such a difference in molecular composition of diacids was mainly due to the increased emissions caused by coal burning and biofuel combustion for house heating in winter in North China Plain,of which the smoke emissions are enriched with Ph precursors such as polycyclic aromatic hydrocarbons,and can be transported into the YRD region.（2）During the summer campaign two haze events（Event Ⅰ and Event Ⅱ）occurred in Chongming Island,which lasted for more than 5 days.The ratio of TDCs/PM_2.5in Event Ⅱ was much higher than in Event Ⅰ,suggesting that PM_2.5particles during Event Ⅱ were more enriched with SOA.Compared to the two pollution episodes,the higher ratios of C₃/C₄and OC/EC during the summer clean period indicated that the ambient aerosols in Chongming Island experienced a stronger aging process.During the winter campaign,four haze events occurred in Chongming Island,among which concentration of PM_2.5in Event Ⅰ was the lowest but was the highest in Event ⅠV.We found that the aerosol aging process in Event Ⅱ period was the weakest,and WSOC in this period was mainly derived from primary sources rather than secondary oxidation.The ratios of C₆/C₉and Ph/C₉in Event ⅡI were similar to those in Event ⅠV,indicating that these two haze periods was probably affected by similar anthropogenic pollution sources.（3）The analysis results of air mass trajectory and the ratio of molecular tracer showed that Event Ⅰ in summer was caused by the local biomass burning around Chongming Island,and the main formation pathway of C₂was aqueous phase oxidation,while Event Ⅱ was mainly affected by the long-distance transport of VOCs emitted from the southern areas such as Shanghai,and Fujian and Zhejiang provinces,and the main formation pathway of C₂was gaseous phase oxidation.The photochemical oxidation of exhaust emissions from ships in the Northwest Pacific Ocean was the main source of C₂and other SOA in Chongming Island during the clean period in summer.During the winter campaign the air pollutants in Event Ⅰ and Event Ⅱ came from the long-distance transport of coal and biofuel combustion for house heating in North China Plain.In contrast,the wintertime Event ⅡI and Event ⅠV were affected by emissions derived from distant biomass combustion from Anhui,Jiangsu,Hubei and other provinces.（4）Size distribution characterization results showed that in summer C₂mainly existed in the fine mode（<2.1μm）during the pollution and clean periods,while the other dicarboxylic acids,such as C₃,C₄,C₉,Gly,m Gly,ωC₂and Pyr,showed a bimodal size distribution pattern,peaking at the fine and coarse（>2.1μm）modes,respectively.In contrast,during winter all the diacids and related SOA were enriched in the fine mode.Diacids in both Event ⅠV and Event Ⅱ were mainly concentrated in fine particles.In Event Ⅱ,the correlation between each diacids were strong（R²>0.8）,and the distribution of diacids were absolutely dominated in the fine particle size segment of 1.1-2.1μm compared with other particle size segments.（5）During the summer clean period,we the first time found that C₂were enriched in the coarse mode of particles,which is due to the heterogeneous reaction of C₂with sea salt particles and the subsequent release of HCl into the gas phase,resulting in 65.5%of Cl^-depletion in the sea salt.Since HCl in atmosphere could be further photochemically decomposed as chlorine radicals by the ultraviolet irradiation,thus the above mentioned heterogeneous reaction could alter the oxidizing capacity of the offshore atmosphere.