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

High ammonia (NH3) and nitrogen oxide (NOx) 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 NOx 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 NOx 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 NOx emissions in city areas of the YRD.