Nitrogen Deposition In Typhoon Precipitation And Tracing Nitrogen Sources Using Isotopes

Nitrogen is one of the essential components of ecosystem circulation,and also a key element of global environmental change.In the past two centuries,especially in the past50 years,human activities have resulted in a large amount of emission of NH₃ and NOx?mainly NO+NO₂?,which continuously increased the concentration of atmospheric active nitrogen and gradually extended atmospheric nitrogen deposition to the global scale.The results show that wet deposition is an essential pattern of nitrogen deposition,which is not only related to nitrogen pollutants in the atmosphere but also affected by rainfall,precipitation types and meteorological factors.As a particular precipitation system,typhoon brings about sudden heavy precipitation,strong intensity and wide influence range,its formation process and meteorological conditions are different from other types of precipitation.At present,there are few studies on the nitrogen deposition of typhoon precipitation,and the samples studied are all single typhoon with low sampling resolution.Thus,it is difficult to reflect the changing characteristics and influencing factors of nitrogen deposition during typhoon precipitation in detail,what impacts will it have on the nitrogen cycle in the ecosystem,so further research is needed.The southeast coast of China is the most significant region affected by typhoons.This study selected Fuzhou as the observation point,high-resolution continuous sampling was conducted for the precipitation of 7 typhoons?Super Typhoon Soudelor,Super Typhoon Dujuan,Typhoon Meranti,Super Typhoon Megi,Typhoon Nesat,Tropical Storm Haitang,Typhoon Maria?,which landed in Fujian province during 2015-2018years,meanwhile,the atmospheric daily precipitation from January to December in 2016year was harvested,and a total of 287 samples were collected.The concentration of different nitrogen forms and the value of nitrate nitrogen and oxygen isotopes in the samples were determined,the conclusions obtained from the analysis are as follows:?1?The concentration of total dissolved nitrogen concentration in the year of 2016atmospheric precipitation was 1.08 mg·L^-1.The content of total dissolved nitrogen by atmospheric deposition was 21.57 kg·hm^-2.NH₄⁺-N dominates the wet deposition of atmospheric nitrogen.The seasonal variation of nitrogen concentration is:spring>autumn>summer>winter.The content of nitrogen by atmospheric deposition are:summer>spring>autumn>winter.The average total dissolved nitrogen concentration in the precipitation of 7 typhoons from 2015 to 2018 year was 0.59mg·L^-1.The precipitation nitrogen of typhoon was mainly NH₄⁺-N.There are great differences in nitrogen wet deposition fluxes of different typhoons.The wet nitrogen deposition fluxes of Super Typhoon Maria was the lowest,and that of super typhoon Megi was the highest.Wet nitrogen deposition is affected by precipitation.The larger the precipitation,the larger the settlement.In general,the nitrogen concentration showed a similar"U"type change characteristic along with the typhoon precipitation process.?2?The ranges of nitrate nitrogen and oxygen isotopes in 2016 precipitation in Fuzhou were respectively-9.28‰⁺2.99‰?mean value:-3.51‰?and 39.73‰⁸3.50‰?mean value:66.94‰?.Higher?¹⁵N-NO₃^-values were observed in the spring and summer than in the autumn and winter?the warm season is slightly higher than the cold season?.Differently,higher?¹⁸N-NO₃^-values were observed in the winter and spring than in the summer and autumn?the cold season is significant than the warm season?.The nitrogen and oxygen isotopes values of nitrate range of the 7 typhoons precipitation from 2015 to2018 year were-14.89‰⁺4.19‰,with the average of-3.80‰and 15.1‰⁶8.29‰,with the average of 44.97‰,respectively.During the precipitation of single typhoon,of which Super Typhoon Megi has the largest variation range.There are obvious variation characteristics of?¹⁵N-NO₃^-and?¹⁸N-NO₃^-,and the various patterns of?¹⁵N-NO₃^-and?¹⁸N-NO₃^-in different typhoon precipitation processes are different.?3?Based on the analysis of N isotopic composition of NO₃^-,it inferred that the sources of nitrate in Fuzhou precipitation mainly include vehicle exhaust emissions,the release of agricultural soils and atmospheric lightning nitrogen fixation.The lightning process and nitrogen isotope fractionation mechanism are the factors that influence the seasonal variation of?¹⁵N-NO₃^-value.The range of typhoon?¹⁵N-NO₃^-value is larger than the annual precipitation in Fuzhou,and the average of?¹⁵N-NO₃^-value is lower than the annual average,which may be caused by the marine vapour source and ozone pollution brought by the typhoon.?4?Based on the analysis of O isotopic composition of NO₃^-,it concluded that during the summer?warm season?,it is likely that an OH radical pathway was most probably dominant in the formation of nitrate,while N₂O₅ was the predominant pathway that controls the formation of nitrate in winter.The reason that the typhoon?¹⁸N-NO₃^-value is lower than the summer precipitation in Fuzhou is that during the typhoon,the oxygen atom is converted more rapidly between O₃ and NO,the photolysis product and OH radical reaction to generate HNO,leading to a lower?¹⁸N-NO^-value.