Effects Of Different Conditions On Sulfur Isotope Composition During SO₂ Heterogeneous Oxidation

In this paper,the source of SO₄^2-in the atmospheric PM_2.5 in different seasons was traced by analyzing the chemical composition and sulfur isotope components of particulate matter in the atmospheric PM_2.5 in Nanjing.In addition,the heterogeneous oxidation reaction process of atmospheric SO₂ on the mineral surface was also simulated in the laboratory,and the sulfur isotope composition characteristics in the generated sulfate after the reaction were determined.We also studied the effects of different meteorological conditions and mixed gases on the fractionation of the conversion of SO₂ to sulfate sulfur isotope.The results of laboratory simulation were as follows.Under the effect of NO_X,the production of SO₄^2-ranged from 0.016g to 0.065g,with an average of 0.039g;theδ³⁴S value ranged from 1.14‰to 5.93‰,with an average of 3.76‰.Under the effect of NH₃,the production of SO₄^2-ranged from 0.016g to 0.040g,with an average of 0.025g;theδ³⁴S value ranged from 1.46‰to 4.63‰,with an average of 2.82‰.Under the effect of O₃,the production of SO₄^2-ranged from 0.015g to 0.058g,with an average of 0.036g;theδ³⁴S value ranged from1.18‰to 4.95‰,with an average of 3.48‰.The appropriate temperature could accelerate the formation of sulfate,but would not change its reaction mechanism.When NO_X,O₃ and NH₃participated in the reaction,the change of temperature would influence the sulfur isotope composition.Water vapor would enrich the heavy sulfur isotope values.Besides,light and oxygen would also affect the generation of sulfates and different proportions of mixed gases would impact the production of sulfates and the composition of sulfur isotopes.NO_X dominated the fractionation to enrich heavy sulfur.O₃ also dominated the fractionation to enrich heavy sulfur,but lower than NO_X.The atmospheric PM_2.5 concentration in the northern suburbs of Nanjing was lower in summer than that in autumn.The main source of air pollution in summer was secondary pollution of dust and atmospheric aerosol.While in autumn,the main source of air pollution was man-made pollution source such as industrial emission.Theδ³⁴S value of atmospheric PM_2.5 in summer ranged from 1.7‰to 5.9‰,with an average of 3.5±1.0‰.Theδ³⁴S value range in autumn was 2.68‰～4.57‰,and the mean value was 3.61±0.54‰.Theδ³⁴S values in the northern suburbs of Nanjing were positive with little variation,indicating that the source of sulfur in the atmosphere in this area was relatively fixed.The source of sulfur in atmospheric PM_2.5 in the northern suburbs of Nanjing was related to coal combustion,vehicle exhaust,and biomass combustion.The high temperature and high relative humidity in summer might also cause the heterogeneous oxidation reaction of SO₂ and the release of biological sulfur.There were also other sulfur sources with lower δ³⁴S values.