Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using oxygen isotopes

Tracking of reactive nitrogen (N) sources is important for the effective mitigation of N emissions. By combining the N and oxygen (O) isotopes of atmospheric NO₃^-, stable isotope mixing models were recently applied to evaluate the relative contributions of major NO_x sources. However, it has long been unresolved how to accurately constrain the δ¹⁵N differences between NO₃^- and corresponding NO_x (ε_(NO2→NO3-) values). Here, we first incorporated the HC oxidation (NO₂ → NO₃^-) pathway by using Δ¹⁷O values to evaluate the ε_(NO2→NO3-) values, performed on NO₃^- in PM_2.5 collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the Δ¹⁷O-based ε values (ε_{17O-based(NO2→NO3-)}) (15.6 ± 7.4‰) differed distinctly from δ¹⁸O-based ε values (ε_{18O-based(NO2→NO3-)}) (33.0 ± 9.5‰) so did not properly incorporate the isotopic effects of the HC oxidation (NO₂ → NO₃^-) pathway. Based on the ε_(NO2→NO3-) values, δ¹⁵N values of NO_x from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO₃^- in PM_2.5, we further quantified the source contributions by using Stable Isotope Analysis in R (the SIAR model). We found that the respective fractional contributions of CC-NO_x and MC-NO_x were underestimated by 64% and were overestimated by 216% by using ε_{18O-based(NO2→NO3-)} values. We concluded that the new ε_{17O-based(NO2→NO3-)} values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO₃^- sources.