| Atmospheric wet and dry deposition,including gas-phase dry deposition,particle-phase dry deposition and wet deposition(rain,snow,fog,hail,etc.),is not only an important link of "Biology-Earth-Chemistry" cycle of active nitrogen,but also a main mechanism of removing atmospheric active nitrogen.With the rapid development of the national economy,the concentration of atmospheric active nitrogen continues to increase,and the composition changes in a variety of ways.The negative ecological effects caused by nitrogen deposition are highlighted.The Three Gorges Reservoir(TGR)has unique topography and meteorology as it covers large cities,rural areas and reservoirs.The ecological environment of the TGR area is fragile.The concentration,load,characterization and impact of nitrogen deposition in this region are widely concerned by the public.Quantitative study of spatial distribution,source and deposition flux of atmospheric nitrogen deposition is the key to assess its ecological and environmental effects,and is one of the important scientific bases for the environmental evolution of regional ecosystems,such as rivers,farmlands,forests and fluctuating zoneTaking "Wanzhou Station for Atmosphere Observation of Chongqing Institute of Green Intelligent Technology of Chinese Academy of Sciences" as the main observation site,this work established six stations along Xiaojiang River Valley in TGR area,including suburban,agricultural and wetland protection areas.Using atmospheric dry and wet deposition sampling equipment,847 precipitation samples were collected from January 2016 to December 2017.By employing multi-functional air pollutant sampler,a total of 480 dry deposition samples were collected,including PM2.5,HNO3,NH3 and NO2.The main active nitrogen and inorganic chemical components in the dry and wet deposition samples were analyzed.On this basis,the spatial and temporal distributions of active nitrogen were discussed,the main sources were analyzed,and the deposition fluxes of active nitrogen were estimated.The results of this study are as follows(1)The average concentrations of NO3--N,NH4+-N,TIN,TON and TN in regional wet deposition were 0.47±0.39 mg L-1,1.07±0.88 mg L-1,1.52±1.20 mg L-1,0.28±0.76 mg L-1 and 1.80±1.50 mg L-1,respectively.These levels were lower than those of typical urban areas,bays and watersheds at home and abroad.The nitrogen concentration in wet deposition was the highest in winter,followed by spring and autumn,and the lowest in summer.Rainfall and source intensity have significant effects on wet deposition nitrogen concentration,of which source intensity is the dominant factor.(2)The sources of active nitrogen in each functional area were quite different.The inorganic nitrogen in urban and suburban sites was mostly contributed by industrial and biomass combustion sources,while that in the agricultural areas was mostly from biomass combustion sources and crustal sources.The sources of TON were quite complex,in the urban area TON mostly came from secondary active nitrogen,which was transformed from primary nitrogen source,while in the suburbs TON were co-discharged by industry,agriculture,aquaculture and biomass combustion.TON in most of the agricultural areas were originated from human and animal manure.For wetland nature reserves,the sources of TIN and TON were similar,which was a mix of industrial and biomass combustion sources.Quantitative source analysis using SIAR isotope model showed that the main sources of NO3--N in the region were coal-burning,transportation,biomass combustion and soil,with the contribution of 26%,27%,25%and 22%respectively.The sources of NH4+-N were biomass combustion(accounted for 47%)>traffic source(19%)>coal combustion(16%)>pig manure(7%)>human manure(6%)>chemical fertilizer(5%)(3)In Wanzhou,the annual average concentrations of NH4+-N,NO3-N,NH3-N,HNO3-N and NO2-N in dry deposition samples were 2.87±3.24 μg N m-3,0.92±0.99μg N m-3,11.92±4.88 μg N m-3,0.25±0.12 μg Nm-3,and 11.73±4.88 μg N m-3,respectively.The levels were lower than those of most typical urban and agricultural areas.The active nitrogen concentration varied significantly with seasons,and showed obvious chemical transformation.Besides seasonal emission intensity,temperature,wind speed and relative humidity were also the key factors affecting the characteristics of active nitrogen concentration and chemical conversion.Reductive active nitrogen played an important role in reducing regional acid deposition.(4)The source contributions for NH4+-N were agricultural activities(55.4%)>gasoline combustion(24.2%)>coal combustion(14.2%)>diesel combustion(5.7%)>dust(0.3%)>biomass combustion(0.1%).The source analysis of NO3--N resolved four factors,that is biomass combustion(contributed 50.6%)>gasoline combustion(24.6%)>diesel combustion(17.5%)>agricultural activities(7.3%).Generally,agricultural,transportation,biomass combustion and coal combustion were the main sources of active nitrogen in Wanzhou.Qualitative analysis of probability function of wind speed,direction and conditional concentration at observation station showed that Longdu Industrial area in Wanzhou was the potential source of particulate active nitrogen(NH4+-N and NO3--N),agricultural source on the South Bank of Yangtze River was the main potential source of NH3,and Shenhua Power Plant,Yihua,Suote Salinization and other factories were the potential sources of NO2 and HNO3.A small amount of regional active nitrogen came from the central region of Sichuan Basin(5)The order of regional nitrogen deposition flux was as follow:WZ(Urban Area)>DD(Agricultural Area)>QK(Wetland Reserve)>GY(Suburb)>RH(Suburb)>HB(Agricultural Area),with depositional fluxed of 28.89 kg N ha-1 yr-1,23.19 kg N ha-1 yr-1,21.05 kg N ha-1 yr-1,19.58 kg N ha-1 yr-1,19.24 kg N ha-1 yr-1 and 17.56 kg N ha-1 yr-1,respectively.The average nitrogen depositional flux in all the studied areas was 21.59 kg N ha-1 yr-1,,while was 20.12 kg N ha-1 yr-1 in Xiaojiang watershed.The direct input of nitrogen deposited into Xiaojiang watershed accounted for 2.95%of the total nitrogen input.The depositional fluxes for different nitrogen forms in this region were WetNH4+(43%)>DryNHx(19%)>WetNO3-(15%)>DryNOy(12%)>WetTON(11%).Wet deposition accounted for 69%of total nitrogen deposition,while dry deposition accounted for 31%.Reduced nitrogen and oxidized nitrogen accounted for 70%and 30%of the inorganic nitrogen deposition.Controlling NH3 emission is an effective means to reduce regional nitrogen deposition.(6)In the TGR area,the nitrogen depositional flux was as highest in the tail of the reservoir than in the head of the reservoir and lowest in hinterland,ranging from 9.24 kg N ha-1 yr-1 to 74.88 kg N ha-1 yr-1,with an average of 38.84±20.46 kg N ha-1 yr-1.Nitrogen depositional fluxes in the hinterland of the TGR and the head of the reservoir exceeded critical load of Water ecosystem,and that in the tail of the reservoir exceeded the critical load of forest ecosystem.The nitrogen depositional fluxes in Fuling and Changshou areas exceeded the critical load of farmland ecosystem.Aquatic and forest ecosystems in the TGR area are facing the threat of atmospheric nitrogen deposition,and some farmland ecosystems are also endangered.The amount of nitrogen directly entering the TGR through atmospheric deposition was 4210.26 tons,accounting for 2.7%of the total nitrogen input in the reservoir area,which is close to the percentage of atmospheric deposition input in Xiaojiang River Basin(2.95%)The innovation of this paper lies in:(1)For the first time,the networked observation and systematic study of atmospheric nitrogen deposition in the hinterland of the TGR and typical watershed had been carried out,and a more systematic scientific understanding of the atmospheric nitrogen deposition concentration,source and flux load in the TGR had been obtained.(2)This dissertation combined the characteristic chemical composition statistical method,isotope model,positive matrix factorization(PMF)model,back trajectory model,and conditional probability function to qualitatively identify and quantitatively ananlyze the source of regional nitrogen deposition.The methodology could provide theory for the study of atmospheric deposition,and the results could provide a scientific basis for the control of atmospheric nitrogen emissions in the TGR.(3)Based on the meteorological data and land use types of Wanzhou site,this study localized the deposition rate of gaseous and particulate nitrogen in Wanzhou,which provided reliable parameters for further research on regional nitrogen deposition. |
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