Oxidation and sources of atmospheric NOx during winter in Beijing based on δ18O-δ15N space of particulate nitrate

The determination of both stable nitrogen (delta N-15-NO3-) and stable oxygen (delta O-18-NO3-) isotopic signatures of nitrate in PM2.5 has shown potential for an approach of assessing the sources and oxidation pathways of atmospheric NOx (NO+NO2). In the present study, daily PM2.5 samples were collected in the megacity of Beijing, China during the winter of 2017-2018, and this new approach was used to reveal the origin and oxidation pathways of atmospheric NOx. Specifically, the potential of field delta N-15-NO3- signatures for determining the NOx oxidation chemistry was explored. Positive correlations between delta O-18-NO3- and delta N-15-NO3- were observed (with R-2 between 0.51 and 0.66, p < 0.01), and the underlying environmental significance was discussed. The results showed that the pathway-specific contributions to NO3- formation were approximately 45.3% from the OH pathway, 46.5% from N2O5 hydrolysis, and 8.2% from the NO3+HC channel based on the delta O-18-delta N-15 space of NO3-. The overall nitrogen isotopic fractionation factor (epsilon N) from NOx to NO3- on a daily scale, under winter conditions, was approximately +16.1 parts per thousand +/- 1.8 parts per thousand (consistent with previous reports). Two independent approaches were used to simulate the daily and monthly ambient NOx mixtures (delta N-15-NOx), respectively. Results indicated that the monthly mean values of delta N-15-NOx compared well based on the two approaches, with values of -5.5 parts per thousand +/- 2.6 parts per thousand, -2.7 parts per thousand +/- 1.9 parts per thousand, and -3.2 parts per thousand +/- 2.2 parts per thousand for November, December, and January (2017-2018), respectively. The uncertainty was in the order of 5%, 5 parts per thousand and 5.2 parts per thousand for the pathway-specific contributions, the epsilon N, and delta N-15-NOx, respectively. Results also indicated that vehicular exhaust was the key contributor to the wintertime atmospheric NOx in Beijing (2017-2018). Our advanced isotopic perspective will support the future assessment of the origin and oxidation of urban atmospheric NOx. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study used nitrogen and oxygen isotopes to analyze nitrate in PM2.5 in Beijing during the winter of 2017-2018, revealing the sources and oxidation pathways of atmospheric NOx. Results indicated that vehicular exhaust was the key contributor to wintertime atmospheric NOx in Beijing (2017-2018).