We analyzed rainwater collected from Huanjiang, China, between June, 2007 and May, 2008, with the aim of understanding the origin of the acid rain and its potential impacts on CO2 uptake by carbonate weathering in the rural areas of southwest China. The pH of the samples varied between 4.2 and 6.2, with a volume-weighted mean (VWM) of 5.1. The rainwater was characterized by high concentrations of SO42-, Ca2+, NH4, and Mg2+. The Na+ and CI-, Ca2+ and Mg2+, and SO42- and NO3- were mainly derived from sea salt, the Earth's crust, and anthropogenic sources respectively. The delta S-34- of the rain ranged from -5.6 to -11.0 parts per thousand, with a mean value of -8.4 parts per thousand, suggesting that local combustion of coal was the main source of acid rain in the region and that the effect of vehicle emissions could be disregarded. Grouping precipitation events by their air mass back-trajectories showed that anthropogenic sources of SO42- and NO3- might vary depending on air mass origin and passes. The involvement of sulfuric acid in carbonate weathering at the study site resulted in an increase in weathering rates of about 6.5% and a decrease in CO2 consumption rates by 13.8%. Further, we estimated that anthropogenic sulfate in the atmosphere, mainly from coal combustion, can cause a decrease in CO2 consumption rates of up to 9.6% in the study area. The data available at present indicate that even in rural karst regions of southwest China, involvement of anthropogenically derived sulfate can significantly decrease the carbon uptake rate of carbonate weathering.
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