Study On Pollution Characteristics And Sources Of Ammonia And Atmospheric Nitrogen Deposition Flux In Xiamen Bay

In recent years,the emissions of anthropogenic sources reactive nitrogen（Nr）mainly including NO.and NH₃ in the atmosphere have increased significantly,resulting in the pollution of atmospheric particulate matter.The excess input of reactive nitrogen to the atmosphere has a bad effect on air quality and ecosystem stability.The larger population density and higher level of urbanization have led to extensive reactive nitrogen released from daily life,fossil fuels combustion and waste treatment around Xiamen Bay.In this study,we focused on inorganic ammonia and ammonium pollution level,source identification and its role in the inorganic nitrogen deposition flux in Xiamen Bay.In the end,the influence of atmospheric nitrogen deposition on nitrogen load and ecological environment effect were evaluated in Xiamen Bay.The detailed results were as follows:（1）Ambient NH₃ concentrations were monitored around Xiamen Bay from June 2015 to May 2018 with the average concentration 7.8±3.8 μg·m-3.Distinct seasonal variations of ambient NH₃ were observed with summer average 1.5 times higher than winter,which agreed well with the seasonal trend of ambient temperatures.Temperature instead of regional transportation dominanted the seasonal variation of ammonia in Xiamen Bay.（2）The average fraction of NH₄⁺in PM_2.5 at the urban and suburban sites of Xiamen was 5.5±3.0%and 5.4±3.2%,respectively,during the sampling periods.Significantly positive correlations between PM_2.5 and NH₄⁺/NO3-/SO42-were observed,suggesting that secondary inorganic ions had a profound impact on PM_2.5 at both sites in Xiamen.Excess ammonia with respect to particulate ammonium was obtained.（3）The values of δ15N-NH₄⁺ were in the range of 5.29～26.13‰ and 1.52-27.03‰at the urban and suburban sites,respectively.It was noted that they were not statistically different based on the paried t-test.At both sites,the seasonal trend for δ15N-NH₄⁺was characterized with slightly heavier in the summer than in other seasons.Moreover,a significant positive correlation between ambient temperatures and δ15N-NH₄⁺ was observed,suggesting the important role of temperatures.Fossil fuel combustion including coal combustion and vehicle emissions dominated NH₄⁺source with the an average contribution 50%at urban site and 63%at suburban based on an Isotope Mixing Model.In order to improve local air quality,regulatory control of transportation and coal combustion in Xiamen Bay and surrounding area may work best.（4）Based on one-year of different nitrogenous compounds at the suburban site,NH₃-N was the dominant species,followed by HNO3-N,NH₄⁺-N and NO3--N.The average molar concentration ratio of reduced nitrogen（NHx）to oxidized nitrogen（NOy）at all seasons was 1.29 suggesting the importance of reduced nitrogen in the nitrogen cycle.（5）The annual wet depositon flux of inorganic was 13.47 kg N·ha-1 in 20]6-20]7 and 5.56 kg N·ha-1 in 2017～2018.The amount of rainfall as well as rain frequency had a great effect on the wet deposition flux.In addition,w-NO3-dominated the wet deposition and the amount of rainfall had a positive correlations with w-NH₄⁺and w-NO3-.（6）The bulk deposition flux was 36.34 kg N·ha-1,and of which the dry deposition flux accounted 85.9%.The highest relative contribution was NH₃,followed by HNO3,p-NO3-,p-NH₄⁺ w-NO3-and W-NH₄⁺In summer and autumn,reduced nitrogen dominated the nitrogen deposition;however,oxidized nitrogen was the dominated species in winter and spring.Fossil fuel combustion including coal combustion and vehicle released into the atmosphere was the main contribution source of nitrogen deposition deposition according to the source apportionment using isotopic mixing model.（7）From 2017 to 2018,the atmospheric nitrogen deposition input to Xiamen Bay was estimated to be 4681 t,accounting for 10.2%to nitrogen load of Xiamen Bay,and it was just lower than the input flux of Jiulong River estuary in Xiamen Bay.The nitrogen deposition could account for 17.2%of the new productivity in Xiamen Bay（based on primary productivity over Taiwan Strait）.Much more attention should be paid to the cumulative effects of long-term nitrogen deposition in the marine ecosystem with high emission density of Nr due to the increasing by the urbanization and industrialization in Xiamen Bay.