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

Pengzhen He; Zhouqing Xie; Xiawei Yu; Longquan Wang; Hui Kang; Fange Yue

doi:10.1016/j.scitotenv.2020.136727

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

Sci Total Environ. 2020 Apr 20:714:136727. doi: 10.1016/j.scitotenv.2020.136727. Epub 2020 Jan 15.

Authors

Pengzhen He¹, Zhouqing Xie², Xiawei Yu³, Longquan Wang³, Hui Kang³, Fange Yue³

Affiliations

¹ Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China; School of Environment and Tourism, West Anhui University, Lu'an, Anhui 237012, China; Anhui Province Key Laboratory of Polar Environment and Global Change, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China.
² Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China; Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China; Key Lab of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China; Anhui Province Key Laboratory of Polar Environment and Global Change, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China. Electronic address: zqxie@ustc.edu.cn.
³ Institute of Polar Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China; Anhui Province Key Laboratory of Polar Environment and Global Change, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China.

PMID: 31981873
DOI: 10.1016/j.scitotenv.2020.136727

Abstract

The occurrence of PM_2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NO_X = NO + NO₂). The oxygen-17 excess of nitrate (Δ¹⁷O(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 (Δ¹⁷O and δ¹⁵N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m^-³ with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m^-³ in winter, spring and summer respectively. Δ¹⁷O(NO₃^-) 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. Δ¹⁷O(NO₃^-)-constrained estimates suggest that the conversion of NO_X 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 Δ¹⁷O(NO₃^-) 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 δ¹⁵N(NO₃^-), 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_X emission sources and isotopic fractionation processes are responsible for δ¹⁵N(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.

Keywords: Atmospheric life; Diurnal variation; Nitrate formation; Nitrogen isotope; Oxygen isotope; PM(2.5).