Sulfur, oxygen, and hydrogen isotope compositions of precipitation in Seoul, South Korea

抄録

To evaluate the source of sulfur and the extent of seasonal and local characteristics in Seoul’s precipitation chemistry, we measured sulfur, oxygen, and hydrogen isotopic ratios of precipitation. The pH of precipitation ranges from 4.6 to 7.0 in Seoul. Precipitation shows positive correlations between ions in the Ca²⁺-Mg²⁺-SO₄^2--NO₃^- system, indicating that the dissolution of Ca and Mg particles by H₂SO₄ and HNO₃ from the combustion of fossil fuels is a major process controlling the chemical composition of snow and rain. The values of oxygen and hydrogen isotope composition of rain range -15.7~-3.2‰ and -114.7~-18.0‰, respectively. The H and O isotope values in the summer are plotted near the global meteoric water line by Craig (1961). The d-excess values in winter are higher than 10, which indicate that the source of rain during winter is a dry air mass from continental China. The sulfur isotope ratio of precipitation in Seoul ranges from +3.0 to +7.3‰, which is similar to typical metropolitan air pollution levels. The data suggests that the main source of sulfur is SO₂ produced by the combustion of fossil fuels. The pollutant SO₄ is estimated to have a δ³⁴S_nss range from +1.0 to +6.2‰ in the Seoul area. The δ³⁴S values of precipitation range from +5.1 to +7.3‰ (mean +6.4‰) in winter and from +3.0 to 4.8‰ (mean +4.0‰) in summer. The higher δ³⁴S values in winter (December and February) seem to be correlated to the air mass from northern China, of which the δ³⁴S values of oil or coal is higher than that of southern China. The lower sulfur isotopic values in summer (June to August) are correlated to the air mass moving from southern China. Isotopic composition and chemical concentrations of SO₄ depend on wind provenance, thus supporting the idea that a seasonally transported source for the pollutant sulfur is from China.