High atmospheric wet nitrogen deposition and major sources in two cities of Yangtze River Delta: Combustion-related NH and non-fossil fuel NO.

High ammonia (NH) and nitrogen oxide (NO) 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 (NH), nitrate (NO), 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 δN values of precipitation NH and NO 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 yr) than those in Nanjing (developed city, 19.4-20.5 kg N ha yr), and were higher at urban sites (20.4-22.5 kg N ha yr) than those at suburban sites (18.7-20.3 kg N ha yr). δN values of precipitation NH increased with an increase in precipitation pH because ambient acidity affects the equilibrium isotope fractionation between NH and NH and wet scavenging coefficients of NH and particulate NH. For NH, combustion-related NH sources (62%-65% with 5.5-6.4 kg N ha yr, including coal combustion, vehicle exhaust, and biomass burning) contributed more than volatilization NH sources (35%-38% with 2.9-3.9 kg N ha yr, including fertilizer application and waste volatilization). For NO, non-fossil fuel NO sources (50%-63% with 3.4-4.1 kg N ha yr, including biomass burning and microbial N cycle) were comparable to fossil fuel NO sources (37%-50% with 2.4-3.4 kg N ha yr, including coal combustion and vehicle exhaust). This study evidenced high N deposition rates and the importance of combustion-related NH emissions and non-fossil fuel NO emissions in city areas of the YRD.