气溶胶稳定硫同位素研究进展

Research progress on stable sulfur isotopes in aerosols

稳定硫同位素可以用来示踪硫酸盐气溶胶的来源、迁移和转化的过程。本文首先介绍了目前气溶胶稳定硫同位素相关的分析测试方法,即EA-IRMS、MC-ICPMS和NanoSIMS。归纳了国内外气溶胶δ^(34)S的时空组成特征,两极和沿海区域气溶胶δ^(34)S值普遍高于内陆区域;冬季燃煤和夏季生物排放作用使得气溶胶δ^(34)S值冬高夏低。同时,综述了硫酸盐气溶胶稳定硫同位素分馏特征,其质量分馏与SO_(2)氧化途径有关,非质量分馏与SO_(2)光化学反应、燃烧过程和矿物粉尘表面的非均相氧化有关。最后,探讨了稳定硫同位素在示踪气溶胶硫酸盐来源方面的应用,分析了人为成因硫和自然成因硫对气溶胶硫酸盐的贡献;并展望了稳定硫同位素在未来大气气溶胶溯源中的应用,以期为控制硫酸盐气溶胶的污染源提供科学依据。

Background,aim,and scope The compositions of stable sulfur isotopes in aerosols can be used to trace the origin,migration,and transformation of sulfate aerosols.The changes in pollution source and SO_(2)oxidation pathway will lead to the changes ofδ^(34)S of sulfate aerosols,which affects the results of sulfur isotope tracing.In addition,some studies have shown that rather than the photochemical oxidation of SO_(2)in the stratosphere,the combustion process and mineral surface oxidation contribute to the sulfur isotopic anomalies.It is necessary to summarize the oxidation mode and fractionation effect of SO_(2)when analyzing the change of sulfur isotope of aerosol sulfate.Materials and methods The review describes the measurement methods,spatial distribution,and temporal variation of stable sulfur isotope(δ^(34)S)in aerosols.In addition,the fractionation characteristics and tracing researches of the stable sulfur isotope in aerosols are analyzed.Future researches on aerosol sulfur isotope are proposed.Results(1)EA-IRMS enables the measurement of multiple sulfur isotopes with high precision,while the MC-ICPMS is suitable for low-sulfur samples.NanoSIMS has a good application prospect in the analysis of stable sulfur isotope of single-particle aerosol.(2)Theδ^(34)S exhibits the following pattern:ocean area>coastal area>inland area,with its seasonal variation being higher in winter and lower in summer.(3)The oxidation of SO_(2)includes homogeneous oxidation,decreasing theδ^(34)S value,and the heterogeneous oxidation process,increasing theδ^(34)S value.(4)Aerosol sulfur includes anthropogenic sources mainly from vehicle exhaust and fossil fuels,and natural sources from biology,sea salt,and volcano.Theδ^(34)S of sulfate differs among sources.Discussion(1)The spatial distribution of aerosol sulfur isotope is affected by pollution sources and long-distance migration,while the seasonal variation of aerosolδ^(34)S is mainly due to the temporal fluctuation of SO_(2)enriched heavy sulfur isotope emitted from coal combustion in winter and light sulfur isotope from terrestrial biological sources in summer.(2)The oxidation process of SO_(2)is accompanied by the equilibrium fractionation of enriched heavy sulfur isotopes and the kinetic fractionation of enriched light isotopes,which contributes to the difference between the sulfur isotopic abundances of SO_(2)and sulfate aerosols.The photochemical reaction,combustion,and mineral surface oxidation of SO_(2)are the main causes of mass-independent fractionation in aerosol.(3)The sulfur isotope composition contains the information of specific sulfur sources,which can be deemed as a fingerprint to identify sulfur sources and to evaluate the relative contribution and impact of different sulfur sources.Conclusions The sulfur isotopic composition of aerosols is affected by the change of pollution sources and SO_(2)oxidation pathway.To better evaluate the relative contribution of atmospheric sulfur,it is necessary to establish a more accurate measurement method to further explore the sulfur isotope fractionation effect in the process of SO_(2)oxidation.Recommendations and perspectives(1)The researches on mass-independent fractionation of sulfur isotopes in aerosol should be strengthened.On the one hand,a better understanding of the photochemical reaction of SO_(2)in the stratosphere is warranted to clarify the sulfur input from the stratosphere to the troposphere;on the other hand,we suggest analyzing the causes of mass-independent fractionation of sulfur isotope in aerosols.(2)The isotope fractionation should be considered when employing the sulfur isotopes to trace the atmospheric chemical process from source to aerosol particles.