Topographic effects on nitrogen deposition and critical load exceedance in the Sichuan Basin, China

Abstract

Atmospheric reactive nitrogen (Nr) deposition plays a crucial role in linking air pollution to ecosystem risks. As the primary source region of air pollutants in southwestern China, the Sichuan Basin (SCB) consistently experiences high Nr deposition, while the impact of its complex terrain on Nr deposition remains unclear. This study applied a basin-filling experiment in WRF-CMAQ modeling to investigate the effect of special topography on Nr deposition and exceedance of critical loads in SCB. We find that the basin terrain enhanced Nr deposition in SCB, particularly for oxidized nitrogen (OXN), with its annual dry and wet deposition elevated by 28 % and 25 %, respectively. These topography-induced changes in Nr deposition further deteriorated the ecosystem. The basin topography increased the Nr exceedance of critical load from 0.14 to 0.65 keq ha⁻¹ yr⁻¹. The southwest-to-northeast gradient of Nr deposition in SCB was primarily influenced by topography-induced changes in planetary boundary layer height (PBLH) and accumulated precipitation (PREP). The vertical circulation structure in the lower troposphere and weak winds inhibited the outflow of Nr pollutants, while the leeward eddy from the surface to around 3 km due to the Tibetan Plateau blocking effect promoted the accumulation of near-surface pollutants on the western side. Additionally, the western updrafts transported sufficient moisture to higher, cooler layers, enhancing convective activities and precipitation, and thereby wet deposition. These findings improve the scientific understanding on how topography modulates regional Nr deposition and provide insights for Nr pollution control strategies for areas with complex terrain and abundant air pollutant emissions.