Oxygen and sulphur isotope ratios in sulphate from atmospheric precipitations

Full text: Observations of δ34S and δ18O of the sulphate dissolved in rain and snow were carried out in 1979. These data, combined with the δ18O and pH values of water, and the SO42- and Cl- concentrations, provided information on the way of SO42- formation in the atmosphere: (1) High and variable sulphate concentrations (from 1 to 19.3 mg/L) and low Cl- content (below 3 mg/L in most cases) point to continental derivation of the sulphate. (2) Nearly constant values of δ34S (+3.7 per mille) represent an average isotopic composition of numerous SO2 emitters distributed over a large area. (3) The δ18O value of the sulphate is significantly correlated with that of water (r = 0.62, n = 32). The following regression line has been derived from the collected data: δ18Osulphate = 16.0 ± 0.9 + (0.38 ± 0.07) δ18Owater. The slope and intercept of this line depend on the season. In winter, the values approach 1/4 and 14, respectively, for slope and intercept, which confirms the industrial origin of SO2, with an initial δ18O value of +23 per mille and the incorporation of one atom of oxygen from water into SO42-. Higher values of the slope and intercept must be due to partial oxygen isotope exchange between water and SO2/SO3. (author)