Investigation On Source Appointment And Transformation Of Atmospheric PM_2.5 Based On Sulfur And Carbon Isotopic Compositions

The haze pollution is getting more and more attention in Nanjing.Generally,haze pollution in the atmosphere is mainly caused by the high concentration of PM_2.5.Therefore,it is necessary to figure out the sources and formation mechanisms of PM_2.5,which is favorable to provide scientific basis for the accurate haze control in Nanjing.At present,the source of atmospheric PM_2.5is mainly studied by monitoring its chemical composition together with statistical models.However,this method is susceptible to the environmental influences during the measurement and analysis,and is difficult to reveal the formation pathways of atmospheric PM_2.5.In addition,sulfate in PM_2.5 is the main driving constituent for the formation of haze and the concentration of sulphate will explode during the haze,but its formation mechanism remains unclear,and has become a hot scientific issue that needs to be solved urgently.Therefore,there is an urgent need to find a new method to effectively identify the sources of atmospheric PM_2.5and its formation process.Stable isotopes have specific compositions in different pollution sources and remain relatively stable during migration and transformation,thus providing a new idea for studying the source and transformation process of PM_2.5by isotope techniques.It should be pointed out that this technique is premised on a database with the data of isotopic compositions from different sources in Nanjing region.However,current studies of isotopic compositions of different PM_2.5 potential pollution source are seldom carried out,and the basic data of sulfur and carbon isotopic compositions is lacking.Therefore,this thesis firstly studied the sulfur and carbon isotopic compositions of different potential sources in Nanjing area.The sources and formation process of the pollutants in different seasons were further investigated by determining the sulfur and carbon isotopic compositions in atmospheric PM_2.5.The main research contents and conclusions are as follows:（1）The sulfur and carbon isotopic compositions of vehicle exhaust,coal combustion,biomass burning and dust of potential sources in Nanjing region were studied In addtion,the sulfur isotopic fractionation during coal combustion and carbon isotopic fractionation during biomass burning were explored.The results showed that there are some differences in sulfur and carbon isotopic composition of different pollution sources.δ³⁴S value of soot from corn straw burning was the highest（9.9‰）,while the lowestδ³⁴S value was found in coal ash.The highest and lowestδ¹³C values were observed in dust and C₃ plants burning with the values of-13.45‰and-29.55‰,respectively.In addition,the properties and combustion conditions of coal are important factors leading to sulfur isotope fractionation.The high temperature is beneficial to enrich the light sulfur isotope in coal slag.Meanwhile,the types of biomass and combustion temperature are important factors affecting carbon isotopic fractionation.The combustion process could promote the pyrolysis of biomass to form carbon isotopic fractionation.（2）The δ³⁴S value of atmospheric PM_2.5 samples in different seasons of Nanjing northern suburbs were measured to study the main sulfur sources.The oxygen isotopic compositions of sulfate in PM_2.5(δ¹⁸O)samples were also analyzed to discuss the formation mechanisms of sulfate.The results showed that theδ³⁴S values of sulfate in winter and summer were in the range of 2.7‰to 6.4‰and 1.7‰to 4.8‰,respectively.The main sources of sulfate in winter were found to be the direct emission from coal combustion and vehicle exhaust.In summer,the bio-sulfur sources with lowδ³⁴S value might exist besides the coal combustion.Theδ¹⁸O value of sulfate was in the range of 7.5‰to 16.1‰,indicating that the secondary sulfate was dominant in PM_2.5.The proportion of primary sulfate in winter was higher than that in summer.The main mechanism of homogeneous oxidation in winter and summer is the same,both O₃oxidation and NO₂ oxidation under discharge.However,the main processes of heterogeneous oxidation of SO₂ in winter and summer are different:the oxidation of H₂O₂ and HCl are dominant in winter,and Fe³⁺-catalyzed oxidation under excessive O₂ is dominant in summer.（3）The values of δ¹³C of PM_2.5 samples during different seasons in the northern suburbs of Nanjing were measured to study their carbon sources.The results showed that theδ¹³C values of PM_2.5in winter and summer were in the range of-26.15‰to-24.72‰and-27.11‰to-25.31‰,respectively.The fractions of Secondary Organic Carbon accounts for a small protion of OC,which were 36.99%in winter and 27.37%in summer.The main carbon pollution sources in winter during the sampling period were the vehicle exhaust and coal combustion.Besides,biomass combustion and dust had a minor contribution.The major carbonaceous pollution sources in summer were biomass combustion and vehicle exhaust.In addition,theδ¹³C value of PM_2.5in spring was in the range of-26.56‰to-23.75‰.On the basis of three-phase cluster analysis of water-soluble ions in PM_2.5,it was concluded that the atmospheric PM_2.5 was mainly derived from coal combustion and vehicle exhaust,and are also affected by geological sources and biomass combustion.（4）Based on the heavy haze pollution event in the northern suburbs of Nanjing between December 27,2015 to January 6,2016,water-soluble ions,values ofδ³⁴S andδ¹³C in PM_2.5as well as major pollutants in atmospheric PM_2.5 were determined to systemically study the sources of PM_2.5.The results showed that this haze event was mainly due to the secondary pollution and the mobile source was dominant.Theδ³⁴S value of atmospheric PM_2.5 was in the range from 4.4‰to 6.8‰indicating that the main sources of sulphate in PM_2.5 were vehicle exhaust and coal combustion.NO₂ significantly promotes the reaction rate of SO₂under alkaline conditions,resulting in faster formation of the secondary sulfate and a significant increase in sulfate concentration.Theδ¹³C values in PM_2.5 ranged from-28.43‰to-24.94‰with an average of-26.62‰±1.11‰,suggesting that the carbon sources of PM_2.5during the haze event were mainly motor vehicle exhaust and coal combustions.Throuhg the joint analysis of sulfur and carbon isotopic compositions,it could be concluded that the pollution sources in the northern suburbs of Nanjing were mainly contributed by gasoline-car exhaust before January 1,2016,but diesel-car exhaust and coal combustion after January 1,2016.In an alkaline environment,NO₂ significantly promotes the oxidation rate of SO2,resulting in faster secondary sulfate formation and...