The rapid mass increase of atmospheric nitrate is a critical driving force for the occurrence of fine-particle pollution (referred to as haze hereafter) in Beijing. However, the exact mechanisms for this rapid increase of nitrate mass has been not well constrained from field observations. Here we present the first observations of the oxygen-17 excess of atmospheric nitrate (Δ 17 O(NO 3 − )) collected in Beijing haze to reveal the relative importance of different nitrate formation pathways, and we also present the simultaneously observed δ 15 N(NO 3 − ). During our sampling period, 12 h-averaged mass concentrations of PM 2.5 varied from 16 to 323 μg m −3 with a mean of (141 ± 88 (1σ)) μg m −3 , with nitrate ranging from 0.3 to 106.7 μg m −3 . The observed Δ 17 O(NO 3 − ) ranged from 27.5 ‰ to 33.9 ‰ with a mean of (30.6 ± 1.8) ‰ while δ 15 N(NO 3 − ) ranged from −2.5 ‰ to 19.2 ‰ with a mean of (7.4 ± 6.8) ‰. Δ 17 O(NO 3 − )-constrained calculations suggest nocturnal pathways (N 2 O 5 + H 2 O/Cl − and NO 3 + HC) dominated nitrate production during polluted days (PM 2.5 ≥ 75 μg m −3 ) with the mean possible fraction of 56 − 97 %. For δ 15 N(NO 3 − ), we found that a combined effect of variability in NO X sources and isotopic exchange between NO and NO 2 is likely to be most responsible for its variations. Our results illustrate the potentiality of isotope in tracing NO X sources and nitrate formation pathways, future modelling work with the constraint of isotope data reported here may further improve our understanding of nitrogen cycle during haze.