High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH3 and non-fossil fuel NOx

Abstract

High ammonia (NH₃) and nitrogen oxide (NO_x) emissions are related to serious air pollution in urban areas and the negative impacts of excessive reactive nitrogen (N) deposition on many ecosystems. However, whether there is a relationship between N deposition rates and their sources with urbanization or not remains unclear in many areas. Here, we investigated the deposition rates of ammonium (NH₄⁺), nitrate (NO₃^-), dissolved organic N, and water-insoluble particular N from July 2017 to June 2018 at two urban and two suburban sites in the Yangtze River Delta (YRD). The δ¹⁵N values of precipitation NH₄⁺ and NO₃^- were measured, and major sources were analyzed using a Bayesian isotope mixing model. Wet N deposition rates were higher in Yangzhou (developing city, 20.3-22.7 kg N ha^-1 yr^-1) than those in Nanjing (developed city, 19.4-20.5 kg N ha^-1 yr^-1), and were higher at urban sites (20.4-22.5 kg N ha^-1 yr^-1) than those at suburban sites (18.7-20.3 kg N ha^-1 yr^-1). δ¹⁵N values of precipitation NH₄⁺ increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH₃ and NH₄⁺ and wet scavenging coefficients of NH₃ and particulate NH₄⁺. For NH₄⁺, combustion-related NH₃ sources (62%-65% with 5.5-6.4 kg N ha^-1 yr^-1, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH₃ sources (35%-38% with 2.9-3.9 kg N ha^-1 yr^-1, including fertilizer application and waste volatilization). For NO₃^-, non-fossil fuel NO_x sources (50%-63% with 3.4-4.1 kg N ha^-1 yr^-1, including biomass burning and microbial N cycle) were comparable to fossil fuel NO_x sources (37%-50% with 2.4-3.4 kg N ha^-1 yr^-1, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH₃ emissions and non-fossil fuel NO_x emissions in city areas of the YRD.

Keywords: Ammonium; Bayesian isotope mixing model; Nitrate; Nitrogen deposition; Stable nitrogen isotopes.