| Aerosols have important effects on air quality,human health,ecosystems,weather,and climate.Sulfate and nitrate are the main components of aerosols.In urban areas,high concentrations of sulfate and nitrate are one of the main driving forces for haze pollution.In the marine boundary layer(MBL),especially in the remote MBL,the lack of atmospheric observation data is one of the important reasons leading to the uncertainties of current climate change assessment.This paper uses stable isotopes(△17O(SO42-),△17O(NO3-),and δ15N(NO3-))to trace the atmospheric chemistry of sulfate and nitrate,to estimate the relative importance of different oxidation mechanisms during the sampling period from October 2014 to January 2015 in Beijing,from January to June 2016 in Shanghai and from cruise over the Arctic MBL in summer 2012.The main results and conclusions obtained are as follows:1.The △17O(SO42-)of PM2.5 during Beijing haze was reported for the first time.The observed △17O(SO42-)ranged from 0.1 ‰ to 1.6 ‰。with the mean of(0.9±0.3)‰.Calculations suggested that both in-cloud reactions and heterogeneous reactions on aerosol can dominate sulfate production during Beijing haze,but in most cases,the heterogeneous reaction plays a dominant role.Calculations based on the constraint of△17O shows that zero-△17O pathways such as S(1V)+ NO2 and S(IV)+ O2 dominated heterogeneous sulfate production,with the mean fractional contribution of 66%to 73%.Other heterogeneous reactions,such as S(IV)+ O3 and S(IV)+ H2O2,contribute the rest 27-34%of heterogeneous sulfate production.However,as the aerosol pH calculated under stable state assumptions and metastable state assumptions varied largely(7.6±0.1 and 4.7± 1.1,respectively)and the oxidation of SO2 was strongly dependent on aerosol pH,we are unable to quantitatively estimate the relative importance of these two reactions so far.Under the stable state assumption,S(IV)+NO2 may dominate heterogeneous sulfate production,while S(IV)oxidation by O2 can be the dominant pathway providing that highly acidic aerosols(pH ≤ 3)exist.2,The isotopic compositions of atmospheric nitrate(△17O and δ15N)during Beijing haze were reported for the first time.The observed △17O(NO3-)ranged from 27.5 ‰ to 33.9 9‰,with a mean of(30.6±1.8)‰,while δ15N(NO3-)ranged from-2.5 ‰to 19.2 ‰ with an average of(7.4±6.8)‰.There was a good positive correlation between △17O(NO3-)and NO3-concentrations when NO3-<50 μg m-3(r = 0.81,p<0.01).Calculations based on △17O(NO3-)indicate that nocturnal reactions(N2O5 +H2O/Cl-and NO3 + HC)dominated nitrate formation when PM2 5 ≥ 75 μg m-3,with possible fractional contribution of 56-97%.△17O(NO3-)and chemical kinetic calculations show that O3 dominates the oxidation of NO during Beijing haze.The analysis of atmospheric δ15N(NO3-)data shows that the relative importance of NOx emissions from coal combustion and the isotope exchange between NO and NO2 are most likely to explain the observed variations of δ15N(NO3-).3.The isotopic compositions of atmospheric nitrate(△17O and δ15N)in Shanghai in different seasons were reported for the first time.The observed △17O(NO3-)ranged from 20.5 ‰ to 31.9 ‰,with an average of(26.2±2.5)‰,while δ15N(NO3-)varied from-2.9 ‰to 18.1 ‰with an average of(6.4±4.4)‰.During the observational period,△17O(NO3-)generally increased with the increase of PM2.5 concentrations,whileδ15N(NO3-)generally decreased with the increase of PM2.5 concentrations.Seasonally,△17O(NO3-)was the lowest in summer,with a mean of(23.2±1.6)‰,while δ15N(NO3-)was the lowest in spring,with an average of(4.2±2.1)‰.During the diurnal sampling period,△117O(NO3-)was characterized by high values at daytime and low values at night,with the mean of(28.6±1.2)‰and(25.4±2.8)‰,respectively.While there was no significant diurnal variations in δ15N(NO30).Calculations based on △17O(NO3-)indicate that NO2 + OH/H2O dominates the formation of atmospheric nitrate in more than half of the sampling period,which reflects the importance of NO2 + OH/H2O in nitrate production in Shanghai.The estimate based on the △17O(NO3-)continuity equation showed that the lifetime of atmospheric nitrate in Shanghai was about 15 hours.The analysis of δ15N(NO3-)shows that the dominant factors affecting △15N(NO3-)variations are different at different time periods.But in general,the most important factors are the isotope exchange between NO and NO2 and NOx emission sources.4.The isotopic compositions of atmospheric nitrate △17O(NO3-)and δ15N(NO3-))and sulfate(△17O(SO42-))over the cruise of Arctic MBL during the Chinese Arctic Expedition were reported for the first time.After removing samples that may be contaminated by the icebreaker,△17O(nss-SO42-)ranged from 0.2 ‰ to 1.0 ‰,with an average of(0.4±0.2)‰.δ15PN(NO3-)ranged from-7.5 ‰ to-3.5 ‰,with the mean of(-5.6±1.7)‰,and △17O(NO3-)ranged from 21.7 ‰ to 28.8 ‰ with the mean of(26.0±3.0)%。.Calculation based on △17O(nss-SO42-)shows that in the summer Arctic MBL,the possible fractional contribution of S(Ⅳ)+ O3 is low(0-7%),which suggests the formation of nss-SO42-is dominated by H2O2 oxidation and/or zero-△17O pathways.The measured δ15N(NO3-)is inversely related to air temperature(r =-0.96,p<0.01),which is consistent with the observed phenomena in Beijing and Shanghai.Calculations based on measured △17O(NO3-)indicate that O3 oxidation accounts for 87%for all NO oxidation pathways in the summer Arctic MBL,and NO2 + OH contributes 37%to 68%for nitrate formation.The possible fractional contribution from nocturnal reactions(N2O5 + H2O and NO3 + HC)is the rest 32%to 63%. |
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