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 (0) isotopes of atmospheric NO3, stable isotope mixing models were recently applied to evaluate the relative contributions of major NOx sources. However, it has long been unresolved how to accurately constrain the 615N differences between NO3 and corresponding NOx (c(NO2_,NO3) values). Here, we first incorporated the HC oxidation (NO2 NO3) pathway by using A170 values to evaluate the E(NO2_,b103) values, performed on NO3 in PM2.5 collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the A170-based E values (P..-170-based(NO2,NO3)) (15.6 7.4%0) differed distinctly from 6180-based E values ( 180_based(NO2_,NO3)) (33.0 9.5%0) so did not properly incorporate the isotopic effects of the HC oxidation (NO2 > NO3) pathway. Based on the E(NO2,b103) values, 615N values of NOx from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO3 in PM2.5, 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-NOR and MC-NOx were underestimated by 64% and were overestimated by 216% by using E180-based(NO2,NO3) values. We concluded that the new P. 170-based(NO2,NO3) values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO3 sources. (C) 2019 Elsevier B.V. All rights reserved.