Simulated seasonal variations in nitrogen wet deposition over East Asia

The regional air quality modeling system Regional Atmospheric Modeling System–Community Multiscale Air Quality was applied to estimate the spatial distribution and seasonal variation in nitrogen wet deposition over East Asia in 2010. The simulated results were evaluated by comparing modeled precipitation rates and ion concentrations, such as ammonium（NH_4~＋）, nitrate（NO_3^-）, and sulfate, in rainwater, against observations obtained from Acid Deposition Monitoring Network in East Asia and meteorological stations in China. Comparison of simulated and observed precipitation showed that the modeling system can reproduce seasonal precipitation patterns reasonably well. For major ion species, the simulated results in most cases were in good agreement with those observed. Analysis of the modeled wet deposition distributions indicated that China experiences noticeable variation in wet deposition patterns throughout the year. Nitrogen wet deposition（NH_4~＋ ＋ NO_3^-） during summer and spring accounted for 71% of the annual total（3.9 Tg N yr^（-1））, including 42.7% in summer. Precipitation plays a larger role in the seasonal variation of wet deposition; whereas, aerosol concentrations affect its distribution patterns. In China, the amount of annual nitrogen wet deposition ranged from 1 to 18 kg N ha^（-1）. Nitrogen in wet deposition was mainly in the form of NH_4~＋, accounting for 65.76% of the total amount, and the molar ratio of NH_4~＋∕NO_3^- was mostly more than 1, indicating a relatively larger effect from agricultural activities.

Atmospheric nitrogen deposition in Yangtze River Delta:insights gained from the nitrogen content and isotopic composition of the moss Haplocladium microphyllum

Simple and inexpensive estimation of the rates and sources of atmospheric nitrogen(N)deposition is critical for its effective mitigation in a region with different land-use types.In this study,the N content and N isotopic composition(δ15N)of moss(Haplocladium microphyllum)tissues and precipitation at six sites with three land-use types(urban,suburban,and rural)were measured in the Yangtze River Delta.A significant linear relationship between moss N content and wet N deposition,and a consistent decrease trend for moss N content and wet N deposition from urban to suburban to rural areas were observed.More negativeδ15N of suburban and rural mosses indicated N mainly released from agriculture and effluent,while the less negativeδ15N of urban mosses were mainly influenced by fossil fuel combustion and traffic emissions.Although the negative mossδ15N indicates that reduced N dominates wet N deposition,there was no significant correlation between mossδ15N and the ratio of ammonium to nitrate(NH4+/NO3−).These results reveal that the moss N content andδ15N can be used as a complementary tool for estimating the rates and sources of wet N deposition in a region with different land-use types.

Atmospheric deposition of inorganic nitrogen in a semi-arid grassland of Inner Mongolia, China

Due to increasing global demand for crop production and energy use, more and more reactive nitrogen（Nr） has been generated and emitted to the environment. As a result, global atmospheric nitrogen（N） deposition has tripled since the industrial revolution and the ecological environment and human health have been harmed. In this study, we measured dry and wet/bulk N deposition from July 2013 to December 2015 in a semi-arid grassland of Duolun County, Inner Mongolia, China. The samples of dry and wet/bulk N deposition were collected monthly with a DELTA（DEnuder for Long Term Atmospheric sampling） system and with Gradko passive samplers and a precipitation gauge. The measured results show that the annual mean concentrations of NH_3, NO_2, HNO_3, particulate NH_4~＋（pNH_4~＋） and particulate NO_3^-（pNO_3^-） in atmosphere were 2.33, 1.90, 0.18, 1.42 and 0.42 μg N/m3, respectively, and that the annual mean volume-weighted concentrations of NH_4~＋-N and NO_3^--N in precipitation were 2.71 and 1.99 mg N/L, respectively. The concentrations of Nr components（including NH_3, NO_2, HNO_3, p NH_4~＋, pNO_3^-, NH_4~＋-N and NO_3^--N） exhibited different seasonal variations. Specifically, NO_2 and HNO_3 exhibited higher concentrations in autumn than in summer, while the other Nr components（NH_3, pNH_4~＋, pNO_3^-, NH_4~＋-N and NO_3^--N） showed the highest values in summer. Based on measured concentrations of Nr components and their deposition velocities estimated using the GEOS-Chem global atmospheric chemical transport model, the calculated annual mean dry deposition fluxes were 3.17, 1.13, 0.63, 0.91 and 0.36 kg N/（hm^2·a） for NH_3, NO_2, HNO_3, p NH_4~＋ and pNO_3^-, respectively, and the calculated annual mean wet/bulk deposition fluxes were 5.37 and 3.15 kg N/（hm^2·a） for NH_4~＋-N and NO_3^--N, respectively. The estimated annual N deposition（including dry N deposition and wet/bulk N deposition） reached 14.7 kg N/（hm^2·a） in grassland of Duolun County, approaching to the upper limit of the N critical load（10–15 kg N/（hm^2·a））. Dry and wet/bulk deposition fluxes of all Nr components（with an exception of HNO_3） showed similar seasonal variations with the maximum deposition flux in summer and the minimum in winter. Reduced Nr components（e.g., gaseous NH_3 and p NH_4~＋ in atmosphere and NH_4~＋-N in precipitation） dominated the total N deposition at the sampling site（accounted for 64% of the total N deposition）, suggesting that the deposited atmospheric Nr mainly originated from agricultural activities. Considering the projected future increases in crop and livestock production in Inner Mongolia, the ecological and human risks to the negative effects of increased N deposition could be increased if no mitigation measures are taken.