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.