Variability of sulfur and oxygen isotope values within wet precipitation and its correlation with diminished anthropogenic sulfur dioxide (SO2) emission

Abstract

Atmospheric sulfate (SO42−) aerosols have the capacity to scatter solar radiation, thus exerting a cooling effect on the climate. It is widely acknowledged that SO42− deposition significantly contributes to ecosystem effect in aquatic and terrestrial environments worldwide. Over the course of several decades, considerable efforts have been undertaken to mitigate anthropogenic sulfur dioxide (SO2) emissions in China, leading to remarkably low SO2 concentrations. Nevertheless, the implications of reduced anthropogenic SO2 emission on atmospheric sulfate transformation and compositions of isotope values (δ34S-SO42–, δ18O-SO42–) remain inadequately understood. To address this knowledge gap, we conducted multiple rainfall sampling instances between 2010 and 2022 (n = 191), encompassing the period of transitioning anthropogenic SO2 emission from high to low levels. Remarkably, we observed a significant reduction in the volume-weighted mean value (VWMV) of SO42− concentrations, declining from 20.86 mg L−1 in 2010 to 2.41 mg L−1 in 2022, and the VWMV of SO42−/NO3− molar ratios also decreased from 12.90 in 2010 to 0.55 in 2022, reflecting the synchronously decreasing annual SO2 emissions and annual SO2 concentrations from 2010 to 2022. In contrast to the declining trends in VWMV of SO42− concentrations, the VWMV of δ34S-SO42– values exhibited a different pattern from 6.6‰ in 2010 to −0.1‰ in 2017 and slightly increased to 2.5‰ in 2022. Furthermore, the VWMV of δ18O-SO42– values demonstrated an increase from 8.8‰ in 2010 to 14.5‰ in 2018, but subsequently decreased to 5.4‰ in 2022. We attributed the sharply dropping VWMV of δ34S-SO42– values to the reduction in the input of primary sulfate with enriched δ34S-SO42– values. The strict desulfurization reduced SO2 emissions, leading to oxidation of these trace SO2 with negative δ34S values, resulting in low sulfate concentrations and negative δ34S-SO42– values. In addition, the monthly average δ18O-SO42– values exhibited a positive correlation with monthly mean δ18O-H2O values (p < 0.05), strongly indicating the water incorporation during SO2 heterogeneous oxidation. More importantly, during spring rainfall, positive δ18O-SO42– values but relatively negative δ34S-SO42– values were influenced by SO2 heterogeneous oxidation facilitated by air oxygen catalyzed under metal ions in dry weather condition, attributed to the low relative humidity (RH) coupled with high deuterium excess (dexcess) values. Conversely, in summer, negative δ18O-SO42– values but relatively positive δ34S-SO42– values resulted from the SO2 heterogeneous oxidation with H2O2 due to high RH and low dexcess values. Moreover, the VWMV of δ18O-H2O exhibited variations from −3.7‰ in 2019 to −9.7‰ in 2021, and displayed a positive correlation with annual temperature (p < 0.01) and a negative correlation with annual precipitation (p < 0.05), making it a valuable indicator of climate change alongside δ18O-SO42– values. Our findings underscore the importance of continued monitoring of atmospheric wet depositions in response to changes in anthropogenic SO2 emission activities. Furthermore, they highlight the substantial impact of variations in δ34S values of SO2 and primary sulfate on rainwater sulfate concentrations and isotope values.