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The determination of both stable nitrogen (δ 15 N-NO 3 - ) and stable oxygen (δ 18 O-NO 3 - ) isotopic signatures of nitrate in PM 2.5 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 δ 2 ). In the present study, daily PM 2.5 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 δ 15 N-NO 3 - signatures for determining the NOx oxidation chemistry was explored. Positive correlations between δ 18 O-NO 3 - and δ 15 N-NO 3 - 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 NO 3 - formation were approximately 45.3% from the OH pathway, 46.5% from N 2 O 5 hydrolysis, and 8.2% from the NO 3 +HC channel based on the δ 18 N-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.15 N space of NO 3 - . The overall nitrogen isotopic fractionation factor (εN) from NOx to NO 3 - on a daily scale, under winter conditions, was approximately +16.1‰±1.8‰ (consistent with previous reports). Two independent approaches were used to simulate the daily and monthly ambient NOx mixtures (δ 15 N-NOx), respectively. Results indicated that the monthly mean values of δ 15 N-NOx compared well based on the two approaches, with values of -5.5‰ ± 2.6‰, -2.7‰ ± 1.9‰, and -3.2‰ ± 2.2‰ for November, December, and January (2017-2018), respectively. The uncertainty was in the order of 5%, 5‰ and 5.2‰ for the pathway-specific contributions, the εN, and δ 15 N-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., Competing Interests: Declaration of competing interest The authors declare that they have no conflicts of interest., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

As the most important gas-phase alkaline species, atmospheric ammonia (NH 3 ) contributes considerably to the formation and development of fine-mode particles (PM 2.5 ), which affect air quality and environmental health. Recent satellite-based observations suggest that the North China Plain is the largest agricultural NH 3 emission source in China. However, our isotopic approach shows that the surface NH 3 in the intraregional urban environment of Beijing-Tianjin-Shijiazhuang is contributed primarily by combustion-related processes (i.e., coal combustion, NH 3 slip, and vehicle exhaust). Specifically, the Batch fractionation model was used to describe the partitioning of gaseous NH 3 into particles and to trace the near-ground atmospheric NH 3 sources. With the development of haze pollution, the dynamics of δ 15 N-NH 4 + were generally consistent with the fractionation model. The simulated initial δ 15 N-NH 3 values ranged from -22.6‰ to -2.1‰, suggesting the dominance of combustion-related sources for urban NH 3 . These emission sources contributed significantly (92% on hazy days and 67% on clean days) to the total ambient NH 3 in urban cities, as indicated by a Bayesian mixing model. Based on the Batch fractionation model, we concluded the following: 1) δ 15 N-NH 4 + can be used to model the evolution of fine-mode aerosols and 2) combustion-related sources dominate the near-ground atmospheric NH 3 in urban cities. These findings highlight the need for regulatory controls on gaseous NH 3 emissions transported from local and surrounding industrial sources., (Copyright © 2019 Elsevier Ltd. All rights reserved.)