Atmospheric fine-particle (PM 2.5 ) pollution is frequently associated with the formation of particulate nitrate ( p NO 3 − ), the end product of the oxidation of NO x gases (NO + NO 2 ) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of p NO 3 − to constrain NO x source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the δ 15 N values of fresh p NO 3 − ( δ 15 N– p NO 3 − ) in PM 2.5 at a rural site in northern China, where atmospheric p NO 3 − can be attributed exclusively to biomass burning. The observed δ 15 N– p NO 3 − (12.17±1.55 ‰; n = 8) was much higher than the N isotopic source signature of NO x from biomass burning (1.04±4.13 ‰). The large difference between δ 15 N– p NO 3 − and δ 15 N–NO x (Δ( δ 15 N)) can be reconciled by the net N isotope effect ( ε N ) associated with the gas–particle conversion from NO x to NO 3 − . For the biomass burning site, a mean ε N ( ≈ Δ( δ 15 N)) of 10.99±0.74 ‰ was assessed through a newly developed computational quantum chemistry (CQC) module. ε N depends on the relative importance of the two dominant N isotope exchange reactions involved (NO 2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N 2 O 5 ) with H 2 O) and varies between regions and on a diurnal basis. A second, slightly higher CQC-based mean value for ε N (15.33±4.90 ‰) was estimated for an urban site with intense traffic in eastern China and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NO x at this site. Based on the δ 15 N values (10.93±3.32 ‰; n = 43) of ambient p NO 3 − determined for the urban site, and considering the location-specific estimate for ε N , our results reveal that the relative contribution of coal combustion and road traffic to urban NO x is 32 % ± 11 % and 68 %± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NO x . Moreover, the variation pattern of OH contribution to ambient p NO 3 − formation calculated by the CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during p NO 3 − formation, the observed δ 15 N– p NO 3 − at the study site would erroneously imply that NO x is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ 15 N–NO 3 − data throughout China and its neighboring areas suggests that NO x emissions from coal combustion may be substantively overestimated (by > 30 %) when the N isotope fractionation during atmospheric p NO 3 − formation is neglected.