Analysis Of Chemical Characteristics Of Rainwater And Nitrogen Deposition And Nitrogen Sources In Nanjing For Many Years

Since the industrial revolution,the active nitrogen produced by human activities has increased and returned to the surface ecosystem in the form of dry and wet deposition.The increase of nitrogen deposition will stimulate the frequency of acid rain,which will aggravate soil acidification.Therefore,the systematic study on nitrogen deposition is not only helpful to clarify its main source,but also can provide basic data for environmental monitoring.At present,there are few reports on urban multi-year and multi-point atmospheric nitrogen deposition.In this study,Nanjing,a typical city in the Yangtze River Delta,was taken as an example.Three monitoring points in the urban area(SY),suburban area(XL)and rural area(TS)of Nanjing were selected.From October 2016 to September 2020,an automatic dry and wet settlement collector was used to monitor the wet settlement,and passive sampler combined with diffusion model was used to estimate the dry settlement of NO2 and NH3.The wet nitrogen deposition and dry NO2 and NH3 deposition in Nanjing were quantified.By measuring pH and ion concentration and combining with PMF model,the spatial and temporal distribution of rainwater chemical characteristics and the main ion sources in Nanjing were studied.The main findings are as follows:1.The average annual pH of SY,XL and TS sites varied from 5.83-6.54,5.83-6.46 and 5.52-6.25,respectively.In both SY and XL,the turning point was 2019,with pH rising first and then falling,and TS showed a trend of constant decline.The pH of rainwater has a certain spatial difference,with the 4-year average values of 6.22,6.21 and 6.02 in SY,XL and TS,respectively.The pH value in urban area is equal to that in suburban area,and is significantly higher than that in rural area.2.The main ion concentration order of rainwater in Nanjing is Ca2+>NH4+>SO42->Cl->NO3->Na+>F->Mg2+>K+,NO3-/SO42-and NH4+/Ca2+all show an upward trend,and the acid rain type is nitric acid.Ca2+is the main neutralizing ion,but the contribution of NH4+to neutralization increases gradually.3.The average values of total nitrogen,ammonia nitrogen,nitrate nitrogen and organic nitrogen in Nanjing were 19.27(17.75-21.52),7.97(7.89-8.71),7.45(6.63-8.35),2.57(2.41-2.67)kg N ha-1 yr-1,respectively.The total nitrogen and ammonia nitrogen decreased significantly,the nitrogen removal deposition showed an increasing trend but was not significant,and the organic nitrogen deposition flux had no significant change.The mean values of NO2 and NH3 dry deposition were 6.86(6.34-7.51),2.99(2.15-4.01)kg N ha-1 yr-1,respectively,and NO2 showed a downward trend,but it was not significant.From urban to suburban areas to rural areas,the wet nitrogen deposition fluxes of all forms increased first and then decreased,showing an "n" pattern.The dry deposition of NO2 and NH3 gradually decreased,and only the dry deposition of NH3 reached a significant difference.The total nitrogen deposition flux in Nanjing is 35.8±2.1 kg N ha-1 yr-1,and the ratio of dry and wet deposition is 1:1.4.Principal component analysis(PCA)and Pearson correlation analysis(Pearson correlation)showed that the sources of inorganic nitrogen in each site were consistent and all of them came from industry.The organic nitrogen was different at each site:SY and XLcame from industry,and TS came from coal combustion.PMF model reveals that both NH4+ and NO3-in rainwater of SY and XL are mainly derived from secondary sulfate and nitrate.In TS,NH4+is mainly affected by agricultural activities,while NO3-is mainly affected by traffic exhaust.