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 NO, stable isotope mixing models were recently applied to evaluate the relative contributions of major NO sources. However, it has long been unresolved how to accurately constrain the δN differences between NO and corresponding NO (ε values). Here, we first incorporated the HC oxidation (NO → NO) pathway by using ΔO values to evaluate the ε values, performed on NO in PM collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the ΔO-based ε values (ε) (15.6 ± 7.4‰) differed distinctly from δO-based ε values (ε) (33.0 ± 9.5‰) so did not properly incorporate the isotopic effects of the HC oxidation (NO → NO) pathway. Based on the ε values, δN values of NO from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO in PM, 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 and MC-NO were underestimated by 64% and were overestimated by 216% by using ε values. We concluded that the new ε values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO sources.