Isotope Constraints On The Sources And Transformation Mechanisms Of Atmospheric Oxidized Reactive Nitrogen Over The Tibet Plateau

Nitrogen oxides(NOx=NO+NO2)and nitrate are important components of the atmospheric reactive nitrogen cycle.The photochemical cycling of NOx affects the generation and consumption of ozone and peroxyl radicals,thereby significantly impacting the atmospheric oxidation capacity.The oxidized end product of NOx,nitric acid,is not only an important component of particulate pollution but also has significant ecological impacts through its dry and wet deposition processes.N and O stable isotopes(δ15N,⊿17O)have shown great potential in tracing active nitrogen emissions and exploring the mechanisms of atmospheric reactive nitrogen cycling.However,there is currently limited research on the isotope composition of NO2,the precursor of nitric acid,especially regarding the diurnal variation of the isotope composition of NO2 and nitrate in relatively clean atmospheric regions.Obtaining such information is crucial for using isotope technique to clarify the emissions,transformation,and deposition processes of NOx in clean areas,and has practical implications for further reducing the negative environmental effects of NOx emissions and nitrate deposition in polluted areas.In this study,a sampling device for efficiently collecting atmospheric NO2 was developed.Combining this device and an atmospheric particle sampler,the atmospheric NO2 and nitrate in typical areas of the Tibet Plateau(cities,forests,and background areas)were collected synergistically at diurnal resolution.According to the subsequent N and O isotope analysis results of these samples,the source of NOx and the formation mechanisms of nitrate in each region were analyzed,and the main conclusions are as follows:The coated 3D-printed honeycomb denuder(3DP-HCD)can achieve 100%collection of NO2 under different sampling conditions,including flow rate(1.5 to 70 L/min),NO2 concentration(pptv to ppmv levels),RH(20.0%to 82.3%),and continuous sampling time of up to 24 hours,while also maintaining the δ15N characteristics of the NO2 source signal during sampling,meeting the needs of researchers for atmospheric NO2 sampling for isotope analysis in background areas.Based on this device,this study completed the collection of NO2 in various areas of the Tibet Plateau.The isotopic measurement results show that the ⊿17O(NO2)in Lhasa during the day is significantly higher than that in Lulang.This is because the proportion of O3 oxidizing NO in Lhasa is higher than Lulang.The[RO2]estimated based on the ⊿17O(NO2)observation values in Lulang and Lhasa are(16.03±8.02)pptv and(5.07±3.63)pptv,respectively.The significantly higher[RO2]in Lulang may be due to the emission of BVOCs from Lulang forest sources promoting local RO2 generation.There is a significant daytime high and nighttime low trend in ⊿17O(NO2)in both Lulang and Lhasa,this trend is mainly due to the dilution effect of NO emitted sources on atmospheric ⊿17O(NO2).The regional differences of δ15N(NO2)in Tibet Plateau are manifested as Namco<Lulang<Lhasa suburbs<Lhasa urban area,mainly affected by the proportion of NOx emissions from anthropogenic and natural sources in each region.The regional and diurnal variation of δ15N(NO2)are mainly driven by changes in NOx sources.After considering the N isotopic fractionation effect during NOx cycling,the results of NOx source apportionments using theoretically calculated δ15N(NOx)show that the main NOx emission sources in Namco and Lulang are soil,accounting for(35.3±12.6)%and(29.4±6.7)%,respectively.The urban and suburban areas of Lhasa are mainly affected by NOx emissions from coal-fired power plants,accounting for(47.0±9.0)%and(28.2±13.2)%,respectively.The ⊿17O(NO3-)in Namco is significantly higher than that in Lhasa and Lulang during the same period,while the ⊿17O(NO3-)in Lulang is the lowest among these regions.The theoretical and observed results of ⊿17O(NO2)indicate that ⊿17O(NO2)is an important factor affecting the regional differences in ⊿17O(NO3-).The main pathways for nitrate production in different regions of Tibet are NO2+OH and N2O5 hydrolysis,which account for approximately 82%to 84%of the nitrate production pathways,with NO2+OH being the most important nitrate production pathway in all regions.⊿17O(NO2)is the main reason why the daytime mean ⊿17O(NO3-)values in different regions of Tibet are higher than the nighttime values,and the significant diurnal variation of ⊿17O(NO3-)in Lulang may be due to the its shorter residence time of nitrate in the atmosphere compared to other regions.The δ15N(NO3-)of Namco were significantly lower than those in Lhasa and Lulang during the same year,which was consistent with the regional difference of δ15N(NO2).The higher contribution of NOx source from soil in Namco compared with other regions led to this difference,indicating that the difference of NOx emission is an important factor affecting the regional difference of δ15N(NO3-).The higher contribution of N2O5 hydrolysis pathway in Lulang than in Lhasa results in a slightly higher averageδ15N(NO3-)value in Lulang compared to Lhasa,which is inconsistent with their regional differences in δ15N(NO2).This suggests that the N isotopic fractionation effect during nitrate production is also an important factor affecting the regional differences in δ15N(NO3-).δ15N(NO3-)in various regions of Tibet showed significant diurnal high and nocturnal low variation trends,which were opposite to the diurnal trend ofδ15N(NO2).The diurnal variation of δ15N(NO3-)is mainly governed by the N isotopic fractionation effect during the production of nitric acid in daytime and nighttime modes.The results obtained by using δ15N(NO2)and δ15N(NO3-)for NOx source apportionment have large differences,indicating that when using δ15N technique for NOx source tracing,it is necessary to comprehensively consider the N isotopic equilibrium and kinetic fractionation effect during the NOx conversion process.