Responses of atmospheric inorganic nitrogen deposition to emissions in a polluted region of southern China

Abstract

Human-induced nitrogen (N) emissions have enhanced atmospheric N deposition in many polluted regions. However, the spatiotemporal relations between N emissions and deposition remain poorly characterized, which hampers N emission management and effect evaluation in polluted regions. This study investigated dry and wet inorganic N (IN) deposition at representative urban and suburban sites from June 2018 to May 2019 and analyzed historical N emission and deposition data in a N-polluted region of southern China. Dry, wet, and total IN deposition were 14.1 and 6.3, 36.4 and 29.0, and 50.5 and 35.3 kg N ha⁻¹ yr⁻¹ at the urban and suburban sites, respectively. Gaseous N accounted for 79–87 % of dry IN deposition and dry deposition accounted for 18–28 % of total IN deposition. The inter-annual variation of wet IN deposition exhibits an inverted “V” shape: the turning point for ammonium (NH₄⁺) was in 1991 due to the decline in the ratio of ammonia to sulfur dioxide and nitrogen oxide and ammonia emissions; nitrate (NO₃⁻) peaked in 2010, reflecting the benefits of national controls on nitrogen oxide emissions. Consequently, a shift to approximately equal NH₄⁺ and NO₃⁻ deposition occurred in 2010–2020. Spatially, annual IN deposition increased with human-induced land use and N emissions, and about 30 % of the area exceeded the critical load for N deposition. Current emission controls are reducing N emissions and deposition but further mitigation measures are needed, especially broader regional strategies.