11:30 AM - 11:45 AM
[HTT23-10] Origin of Sulfate Ion in Precipitation of Chugoku District Estimated by Sulfur and Oxygen Isotope Ratios
Keywords:precipitation, Chugoku district, Sulfate ion, S isotope ratio, O isotope ratio
We have sampled monthly precipitations at 6 sampling sites in Tottori and Okayama Prefecture in order to observe seasonal and secular change of cross-border pollution from mainland China. We have measured chemical composition, H-O and Sr isotope ratios of precipitation, and S isotope ratio of sulfate ion. Origins of sulfate and their contribution to sulfate ion in precipitation has been estimated using S isotope ratio. In this study, we measured O isotope ratio of sulfate ion to try to restrict the origin of sulfate ion more accurately.
Oxygen isotope ratio of sulfate ion was measured by TC/EA-IRMS. Barium Sulfate precipitated from precipitation occludes nitrate ion and cannot be used directly oxygen isotope measurement. It is purified by dissolving using chelating agent (DPTA) and re-precipitation. Remaining DPTA is removed by heating at 450°C for 3 hours. Purified barium sulfate is used for oxygen isotope analysis.
Measured isotopic ratios are the mixture of isotopic ratios of seawater sulfate, sulfate resulted from fossil fuel combustion (coal combustion in China and petroleum combustion in Japan) etc. Non-seawater sulfate (nss) oxygen and sulfur isotope ratios of sulfate ion were calculated using chemical composition of precipitation, and oxygen and sulfur isotope ratios of seawater. In a plot of oxygen isotope ratio vs. sulfur isotope ratio, the measured results split into three groups. Group A includes winter high δ34S group (δ18O≈7‰, δ34S≈7‰), which is influenced by Chinese coal combustion. Group B includes summer low δ34S group (δ18O≈7‰, δ34S≈2‰), which is mainly influenced by Japanese petroleum combustion. Group C includes intermediate δ34S and high δ18O group (δ18O≈15‰, δ34S≈4‰) in March. Nss-Sr isotope ratio of group C is highest among samples, indicating the contribution of soluble component of yellow sand. Thus, high δ18O of group C may be due to the soluble sulfate ion of yellow sand. Measurement of δ18O for soluble component of yellow sand is necessary to prove this hypothesis. Unfortunately, oxygen isotope ratio of sulfate is found not to be a good indicator to discriminate between sulfate from Chinese coal combustion and that from Japanese petroleum combustion.