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

The occurrence of PM pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NO = NO + NO). The oxygen-17 excess of nitrate (ΔO(NO)) 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 (ΔO and δN) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m in winter, spring and summer respectively. ΔO(NO) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. ΔO(NO)-constrained estimates suggest that the conversion of NO to nitrate is dominated by NO + OH and/or NO + HO, with the mean possible contribution of 55-77% in total and even higher (84-92%) in summer. A diurnal variation of ΔO(NO) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δN(NO), it changed largely in our observation, from -2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NO emission sources and isotopic fractionation processes are responsible for δN(NO) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.