Sub-Diurnal Variability of Urban NOx Isotopes: Keeling Plot and δ15N–Oxidative Ratio Analysis Reveal Hidden Combustion Sources

Abstract

Identifying urban nitrogen oxide (NO_x) sources at subdiurnal scales remains challenging because nitrogen isotopic signatures (δ¹⁵N) can be altered by atmospheric processing. We evaluate whether δ¹⁵N(NO_x) variability is source-dominated under atmospheric conditions and whether combining isotopic and stoichiometric constraints enables urban NO_x source apportionment. We conducted three >36 h observation campaigns in Tsukuba, Japan, including rare nighttime δ¹⁵N(NO_x) measurements, alongside routine observations of NO_x, carbon dioxide (CO₂), oxygen (O₂), temperature, and solar radiation. δ¹⁵N(NO_x) and NO_x 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 NO_x, with regression intercepts of −10.3 ± 1.9‰ and −8.1 ± 0.8‰. By integrating δ¹⁵N(NO_x) 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 NO_x source apportionment during periods of minimized advection and highlights the importance of residential combustion for urban air quality management.