Sources and meteorological factors that control seasonal variation of δ34S values in rainwater

Sulfate concentrations and sulfur isotopic compositions were measured in rainwater in Guiyang city in Southwest China between October 2008 and September 2009 to identify sulfur sources and their impacts on sulfur isotopic composition. The δ34S values of 1235 samples collected during this period ranged from − 12.0 to + 9.4‰, with a volume-weighted mean of − 2.8 ± 9.8‰, suggesting that rainwater sulfate in Guiyang was mainly derived from SO2 produced during coal combustion. The δ34S values of rainwater sulfate increased from 1987 to 2009, with an increment of about + 0.16‰ per year, reflecting a gradual reduction in 34S-depleted SO2 emitted during coal combustion. Seasonal variations in δ34S values were pronounced, with higher values in winter than in summer. Long-distance transport of SO2 from coal burned in northern cities (which was higher than the component from southern cities), and more importantly, the aqueous oxidation of SO2 during transport affected the seasonal variation of rainwater δ34S values in Guiyang. Temperature-dependent aqueous oxidation of SO2 suggested that lower rainwater δ34S coincides with higher temperature.