Wet deposition of sulfur and nitrogen at Mt. Emei in the West China Rain Zone, southwestern China: Status, inter-annual changes, and sources

Abstract

The West China Rain Zone (WCRZ) is of ecological importance and thus, anthropogenic impacts on its vitality are of concern. To ascertain if China's SO2 and NOx emissions reductions in the recent years are reflected in reduced impacts, sulfur and nitrogen wet deposition at Mt. Emei was observed during 2017–2019. The source contributions to sulfur and nitrogen wet deposition were estimated using the positive matrix factorization (PMF) and the source-oriented Community Multi-scale Air Quality (CMAQ) model. The annual Volume Weighted Mean (VWM) pH of precipitation increased from 4.01 to 4.75 in the 1980s–2000s to 5.39 in 2017–2019. However, 12% of the samples during 2017–2019 had pH < 4.50. The VWM SO42− concentration decreased from 80.9 to 26.0 μeq L−1 from 2007–2009 to 2017–2019. The VWM NO3− concentration (18.3 μeq L−1) in 2017–2019 was close to that measured in 2007–2009. Although the dry deposition fluxes onto forest canopy were under-estimated, the mean annual total wet and dry deposition flux of nitrogen (24.3 kg N ha−1 yr−1) was higher than the critical load (CL: 10.0–15.0 kg N ha−1 yr−1), while the flux of sulfur (12.1 kg S ha−1 yr−1) was lower than the CL (16.0–32.0 kg S ha−1 yr−1). The annual wet deposition fluxes of SO42− and NO3− were mainly from industries (54% and 43%, respectively) and power plants (21% and 25%, respectively), and that of NH4+ was mostly from agriculture (88%). Emissions within and outside the Sichuan Basin (SCB) were both important sources of the wet deposition fluxes of sulfur (45% and 39%, respectively) and nitrogen (68% and 29%, respectively), and other unidentified sources accounted for 16% and 3% of the sulfur and nitrogen fluxes, respectively. This study suggests that to protect Mt. Emei's ecosystems, efforts are needed to further control sulfur and nitrogen deposition through reducing emissions within and outside the SCB.