Chemical Characteristics,Sources And Formation Mechanisms Of Atmospheric Water-Soluble Inorganic Ions At A Background Site Of Yangtze River Delta Region,China

The Yangtze Revier Delta?YRD?is one of the most populated and developed regions in China and has been suffering serious air pollution problem in the past decades.Many field measurements have been conducted in the YRD region,but most of them have focused on the urban environment,and atmospheric behavior of air pollutant on a regional scale has not been given much attention,and thus,sources and formation mechnisms of haze over the YRD region remain unclear.In order to improve our understanding on the pollution characteristics such as sources,and formation mechnisms of airborne particles in the YRD region,PM_2.5 and size-segregated particles were collected at Chongming Island,a background site of YRD,in the winter and summer of 2018.In this thesis,we first analyzed the chemical compositions,diurnal variations,size distributions and sources of water-soluble inorganic ions in the collected samples.Then,we used ISORROPIA-II thermodynamic model to calaulte the acidity of aerosols and investigated the factors influencing the acidity.Based on the PM_2.5.5 loadings,the whole campaign in the two seaons were categorized into clean,transition and polluted periods,among which differences in pollution characteristics were discussed.Afterwards,based on the pollution characteristics,different types of haze events were identified,along with detailed disucssion on their origins,concentraions,compositions and acidities by using a series of statistic methods such as linear fit regression analysis,PCA?principal component analysis?and PMF?positive matrix factorization?methods.Based on the above work,the main conclusions are drawn as follows:?1?The averaged mass concentration of PM_2.5 was 33.33±21.18?g/m³ during the summer campaign,indicating that air quality in Chongming Island is generally better than the first grade standard value?35?g/m³?of China National Ambient Air Quality Standard?GB3095-2012?.PM_2.5.5 mass concentration in winter was about 1.8-fold of that in summer,which is in the moderate pollution range.Concentration of total water-soluble inorganic ions?TWSII?in PM_2.5 during the summer campaign accounted for 42.4%of the PM_2.5 mass,which is similar to that during the winter campaign,indicating that the water-soluble inorganic ions were the main components of PM_2.5 in both seasons.The total mass concentrations of sulfate,nitrate and ammonium?SNA?in the polluted periods in winter and summer were more than 4times of those in the clean periods,indicating that the high level of PM_2.5 at the background site during the haze periods is largely caused by the accumulation of secondary particulate matter.?2?SO₄^2-(43.6%of PM_2.5)was the most abundant ions in summer,followed by NO₃^-?26.7%?and NH₄⁺?18.6%?,while NO₃^-?47.1%?was the most abundant ion in winter,followed by SO₄^2-?22.5%?and NH₄⁺?20.8%?.In summer,the intense photochemical oxidation fovaors the formation of SO₄^2-,which was the highest in the daytime.NO₃^-increased significantly during the pollution period at night,which is in part due to the lower temperature and higher relative humidity?RH?that are favorable for the partitioning of HNO₃?g?into the solid phase.In contrast,the diurnal variation of SNA in winter was not obvious,which can be ascribed to the regional stagnant conditions.?3?The molar ratio of NO₃^-to SO₄^2-showed that except for the increased mobile sources at night in the polluted periods the stationary sources were the dominant in summer,while in winter mobile sources were the dominant in the whole season.The molar ratio of NH₄⁺to SO₄^2-in summer showed that ammonium was poor in the clean periods(24-h PM_2.5<15?g/m³)and rich in the transition(24-h PM_2.5.5 between 15-35?g/m³)and polluted(24-h PM_2.5>35?g/m³)periods,respectively.Ammonium existed as NH₄HSO₄and NH₄NO₃ in the transition periods and as?NH₄?₂SO₄ and NH₄NO₃ in the polluted periods,respectively.While the molar ratio of NH₄⁺to SO₄^2-in winter showed that ammonium was rich during the whole season.Ammonium existed as NH₄HSO₄ and NH₄NO₃ in the daytime of the clean period and as?NH₄?₂SO₄ and NH₄NO₃ in other periods during the cold season.?4?ISORROPIA-II model results showed that the pH value of aerosol in summer was3.16±1.11,which was lower than that in winter?4.52±0.39?.Concentration of liquid water content?LWC?during winter was relatively high,with an average value of 244±439?g/m³,which was 6 times of that in summer,mainly due to the lower temperature and high concentration of hygroscopic inorganic salts?SNA?in the coal season.The higher LWC of aerosols could dilute the concentration of H⁺_air,and thus lead to the higher pH values.LWC showed an exponential growth trend with an increase of RH in both seasons.?5?Based on the PCA and PMF analysis,we found that during the campaign the water-soluble inorganic ions in PM_2.5 were derived from four sources:secondary oxidation of precursors generated from fossil fuel combustion,biomass burning,and emissions from dust and sea salt.The contribution of secondary sources was dominant in summer and winter?above 50%?,followed by biomass combustion sources?22.5%in summer and 34.5%in winter?.The correlation analysis and PSCF results suggested that there were two types of pollution events occurring in summer,which were characterized by local biomass burning emissions?EP1?and regional transport?EP2?,respectively.And there were two types of pollution events in winter,which were characterized by long-distance transport pollution from the north China plain?EP1and EP2,?and short-distance transported pollution from YRD?EP3 and EP4?, respectively.Source apportionment further revealed that secondary formation was the largest contributor during the two events in summer,followed by biomass burning emission.Sea salt was also found to be an important source during the EP2 period.In contrast,during winter secondary formation was also the largest contributor in the three events?EP1,EP2 and EP3?,and followed by biomass burning emission.During the EP4 perod,biomass burning emission was the largest contributor,followed by the secondary formation.?6?Concerning the size-resolved samples,the water-soluble inorganic ions in the fine mode?<2.1?m?during the winter campaign was about 2-fold of that in summer with an average value of 35.56±21.10?g/m³.The coarse mode?>2.1?m?aerosols during the winter campaign was also about 2-fold of that in summer?7.16±1.46?g/m³?.In the fine mode,SO₄^2-?46.4%of TWSII?was the most abundant ion in summer, followed by NO₃^-?21.2%?and NH₄⁺?21.0%?,while NO₃^-?47.0%?was the most abundant ion in winter,followed by SO₄^2-?21.0%?and NH₄⁺?15.4%?.In the coarse mode,NO₃^-was the most abundant ions in the two seasons.?7?In both summer and winter,NH₄⁺showed unimodal size distribution in the fine mode,while SO₄^2-and K⁺showed a bimodal distribution pattern,peaking in the fine mode.Na⁺,Cl^-and Ca²⁺showed a bimodal distribution pattern,peaking in the coarse mode.In comparison with other ions,NO₃^-presented a bimodal pattern,peaking in the fine and coarse mode in summer,respectively,and a unimodal pattern with a peak in the fine mode in winter.Such a difference in size distribution of NO₃^-is mainly affected by the gas-solid phase partitioning of HNO₃?g?.Based on the linear fit regression analysis,it was found that the coarse mode of NO₃^-in the summer clean period were mainly generated by heterogeneous reactions of the precursors on the sea salt surface,which resulted in an average of 65.5%Cl^-depletion for the coarse mode of sea salts.