The abatement of acid rain in Guizhou province, southwestern China: Implication from sulfur and oxygen isotopes

The high frequency of acid rain in southern China has captured public and official concern since 1980s. Subsequently, gas emission reduction measures have been implemented to improve the air quality. Variations in SO2 emission intensities can influence the sulfur and oxygen isotopic compositions of sulfate in rainwater, since atmospheric sulfate is mainly formed via the oxidation of sulfur gases from natural and anthropogenic sources. To evaluate the impacts of emission reduction measures on atmospheric sulfate, the seasonal and long-term trends in stable isotopic compositions of sulfate in rainwater in Guizhou province, southwestern China have been investigated based on rainwater samples collected from June 2016 to June 2018 and literature investigation (2000-2010). The results reveal that coal combustion remains a major contributor to sulfate in rainwater, although its SO2 emission has significantly decreased over the past two decades. The delta S-34(sulfate) and delta O-18(sulfate) values in rainwater are negatively correlated and have significant seasonal changes. The seasonality in delta S-34(sulfate) has been interpreted as due to the changes in contributions of dimethyl sulfide and coal combustion, while the seasonal pattern of delta O-18(sulfate) is consistent with that of delta O-18(water) values, indicating sulfate in rainwater is mainly formed by heterogeneous oxidation of SO2. Combined with the data from previous studies (Xiao and Liu, 2002; Liu, 2007; Xiao et al., 2009; Xiao et al., 2014), we found that the volume weighted mean delta S-34 values of sulfate in rainwater in Guizhou province show a marked increase between 2001 and 2018, indicating that the S-34-depleted SO2 emission from coal combustion has declined during this period. Furthermore, the synchronous changes in delta S-34 values, sulfate concentration and pH values of rainwater suggest that the frequency of acid rain in Guizhou province has dropped over the past two decades, which is likely to result from the emission reduction measures taken in Guizhou province. (C) 2020 Elsevier Ltd. All rights reserved.