The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NOx - NO + NO2). The oxygen-17 excess of nitrate (Delta O-17(NO3-)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Delta O-17 and delta N-15) collected from January to June 2016 in Shanghai China Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 mu g m(-1) with the mean values being (7.6 +/- 4.4 (1SD)), (10.2 = 5.8) and (4.1 = 2.4) Kg m(3) in winter, spring and summer respectively. Delta O-17(NO3-) varied from 20.5 parts per thousand to 31.9 parts per thousand with the mean value being (26.9 +/- 2.8)parts per thousand in winter, followed by (26.6 +/- 1.7)parts per thousand in spring and the lowest (232 +/- 1.6)parts per thousand in summer. Delta O-17(NO3-)-constrained estimates suggest that the conversion of NOx to nitrate is dominated by NO2 + OH and/or NO2 + H2O, with the mean possible contribution of 55-77% in total and even higher (84-92%) in summer. A diurnal variation of Delta O-17(NO3-) featured by high values at daytime (28.6 +/- 1.2 parts per thousand) and low values (25.4 +/- 2.8 parts per thousand) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (tau) and calculations indicate tau is around 15 h during the diurnal sampling period. In terms of delta N-15(NO3-), it changed largely in our observation, from -2.9%. to 18.1 parts per thousand with a mean of (6.4 +/- 4.4)parts per thousand. Correlation analysis implies that the combined effect of NOx emission sources and isotopic fractionation processes are responsible for delta N-15(NO3-) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions. (C) 2020 Elsevier B.V. All rights reserved.