Nitrogen isotope fractionation during gas-to-particle conversion of NO_x to NO₃⁻ in the atmosphere – implications for isotope-based NO_x source apportionment

Abstract. Atmospheric fine-particle (PM_2.5) pollution is frequently associated with the formation of particulate nitrate (pNO₃⁻), the end product of the oxidation of NO_x gases (NO + NO₂) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO₃⁻ 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 δ¹⁵N values of fresh pNO₃⁻ (δ¹⁵N–pNO₃⁻) in PM_2.5 at a rural site in northern China, where atmospheric pNO₃⁻ can be attributed exclusively to biomass burning. The observed δ¹⁵N–pNO₃⁻ (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 δ¹⁵N–pNO₃⁻ and δ¹⁵N–NO_x (Δ(δ¹⁵N)) can be reconciled by the net N isotope effect (ε_N) associated with the gas–particle conversion from NO_x to NO₃⁻. For the biomass burning site, a mean ε_N( ≈ Δ(δ¹⁵N)) 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 (NO₂ reaction with OH versus hydrolysis of dinitrogen pentoxide (N₂O₅) with H₂O) 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 δ¹⁵N values (10.93±3.32 ‰; n = 43) of ambient pNO₃⁻ 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 pNO₃⁻ 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 pNO₃⁻ formation, the observed δ¹⁵N–pNO₃⁻ at the study site would erroneously imply that NO_x is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ¹⁵N–NO₃⁻ 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 pNO₃⁻ formation is neglected.