Sub-Diurnal Variability of Urban NO x Isotopes: Keeling Plot and δ15N–Oxidative Ratio Analysis Reveal Hidden Combustion Sources

Identifying urban nitrogen oxide (NOx) sources at subdiurnal scales remains challenging because nitrogen isotopic signatures (δ15N) can be altered by atmospheric processing. We evaluate whether δ15N(NOx) variability is source-dominated under atmospheric conditions and whether combining isotopic and stoichiometric constraints enables urban NOx source apportionment. We conducted three >36 h observation campaigns in Tsukuba, Japan, including rare nighttime δ15N(NOx) measurements, alongside routine observations of NOx, carbon dioxide (CO2), oxygen (O2), temperature, and solar radiation. δ15N(NOx) and NOx concentrations were significantly correlated with CO2 and O2, but not with temperature or solar radiation, indicating variability associated with source contributions. Estimated isotope effects from daytime nitrogen dioxide + hydroxy radical and nighttime nitrate radical–dinitrogen pentoxide chemistry were ≤1.1‰ over subdiurnal time scales. Keeling plots revealed isotopically depleted source signatures at higher NOx, with regression intercepts of −10.3 ± 1.9‰ and −8.1 ± 0.8‰. By integrating δ15N(NOx) with the oxidative ratio (OR = −ΔO2/ΔCO2) in a Monte Carlo mixing model, we identified a contribution from gaseous-fuel combustion. These results demonstrate that joint δ15N–OR analysis enables subdiurnal NOx source apportionment during periods of minimized advection and highlights the importance of residential combustion for urban air quality management.