Atmospheric deposition of sulfur and nitrogen in the West China rain zone: Fluxes, concentrations, ecological risks, and source apportionment

Abstract

The West China Rain Zone (WCRZ; 25,000 km2) is a broad mountainous ecotone between the urbanized Sichuan Basin (SCB) and the rural Qinghai-Tibetan Plateau (QTP). The WCRZ is rich in biodiversity and may be influenced by the air pollutants transported from the SCB towards the QTP. As air pollution can cause adverse effects on ecosystem health, this study investigated the fluxes, ecological risks, and sources of sulfur and nitrogen deposition by using a source-oriented community multiscale air quality (CMAQ) model for the entire WCRZ and its constituent national nature reserves in 2015. Results indicate that for sulfur (75%) and nitrogen (70%), the majority of the WCRZ area had annual total deposition fluxes higher than their critical loads. In addition, the annual volume-weighted mean (VWM) total inorganic nitrogen (TIN) wet deposition concentrations in almost the entire WCRZ exceeded the standard for total nitrogen of surface water in protected areas (0.2 mg N L−1). This exceedance is detrimental for the headwater watersheds that have precipitation as their sole water source. In each of the national nature reserves (NNRs), the average annual total deposition fluxes ranged between ~12–38 (sulfur) and ~ 15–38 kg S(N) ha−1 a−1 (nitrogen). About 40–100% (sulfur) and 5–85% (nitrogen) of the areas have annual total fluxes higher than the corresponding critical loads. Furthermore, the average VWM TIN concentration of wet deposition in each WCRZ NNR is higher than 0.2 mg N L−1. Therefore, sulfur and nitrogen deposition likely pose severe risks to some important natural ecosystems in the WCRZ. Based on the source apportionment results, to reduce sulfur and nitrogen deposition in the WCRZ, both SCB and non-SCB emissions should be controlled. In addition, the agricultural NH3 emissions and the industrial and power plant SO2 and NOx emissions should be reduced by larger extents than that of other sectors. This study provides a basis for future atmospheric and ecological observations in the WCRZ.