Highly-resolved nitrogen load to surface water from farmland ammonia emissions in Southeastern China

Abstract

Ammonia (NH₃) emissions from agriculture significantly impact aquatic ecosystems through the deposition of ammonia nitrogen (NHx-N). Models established on a gridded map provide a useful tool to investigate NH3 dispersion and deposition. However, coarse model grid cells in major atmospheric chemistry models often lead to large bias in modeled N burden due to NHx-N deposition in those small household farms, which are typical in Southeastern China with dense farmland water networks. Here, we firstly simulated atmospheric concentrations of NH3 and NH4+ in 2020 in Changshu, a representative area with developed agriculture and dense river and farmland water network attached to the Taihu Lake basin in Southeast China, using WRF-CMAQ model. To address the bias caused by coarse model grids, we employed a large leaf model to calculate dry deposition velocities for NH₃ and NH₄⁺ at a high resolution of 10 meters. This allowed for a more accurate estimation of the nitrogen load to surface waters from re-emitted farmland NH₃. The Results show that coarse particulate NH4+ (CNH4+) exhibited the largest dry deposition velocity, followed by gaseous NH3, and fine particulate NH4+ (FNH4+). The dry deposition velocities of NH3 and NH4+ were significantly influenced by land use type. Our results indicate that approximately 118.0 tons of nitrogen from ammonia reemission from farmlands entered into surface water in 2020 in Changshu via atmospheric NHx-N deposition. The wet deposition of NH4+ was the largest contributor (49.8%), followed by dry deposition of gaseous NH₃ (20.9%), CNH4+ (20.7%), and FNH4+ (8.6%), respectively.