Revisiting oxygen isotope anomalies in NO2 and nitrate in the nocturnal atmosphere: a modeling approach

Abstract

The oxygen isotope anomaly (or mass-independent fractionation signal, Δ¹⁷O) in atmospheric nitrate has been widely used to investigate NO_x oxidation chemistry and serves as a proxy to probe atmospheric oxidation capacity. However, interpretations of Δ¹⁷O(NO₃^-) measurements have heavily relied on several key assumptions that are not fully constrained nor verified by observations. With recent advances in reported Δ¹⁷O(NO₂) observations, we revisit the conventional method for computing atmospheric Δ¹⁷O(NO₃^-) using detailed oxygen isotope box model simulations, focusing on Δ¹⁷O transfer during nighttime NO_x oxidation. Our model results indicate that the assumptions of photochemical steady state and unidirectional Δ¹⁷O transfer during NO₂ oxidation in previous studies are insufficient to predict nighttime Δ¹⁷O(NO₃^-). Importantly, the rapid thermal decomposition of N₂O₅ facilitates oxygen isotope exchange between NO₂ and NO₃, bringing Δ¹⁷O close to equilibrium under typical atmosphere boundary layer conditions, which contrasts with the widely hypothesized unidirectional transfer of Δ¹⁷O from NO₂ to NO₃ and N₂O₅. The variations in Δ¹⁷O(NO₂) are instead controlled by multiple processes that can be categorized into two classes: (i) NO_x emissions and oxidation processes that lower Δ¹⁷O(NO₂), and (ii) the NO₂-NO₃-N₂O₅ thermal cycles that elevate it. Neglecting these factors can lead to severe overestimations of Δ¹⁷O in the produced nitrate. In our specific simulation under typical urban winter nighttime conditions, the simplification resulted in an overestimation of up to 11.6 ‰ in Δ¹⁷O of surface Δ¹⁷O(NO₃^-). The deviation of daytime result was smaller but still notable at 2.5 ‰. Predicting nighttime Δ¹⁷O(NO₃^-) is further complicated by stratification in the nocturnal boundary layer, which produces different Δ¹⁷O(NO₃^-) between the near-surface and residual layers. Our results highlight the importance of detailed Δ¹⁷O transfer modeling for explaining atmospheric Δ¹⁷O(NO₃^-) measurements, and future studies are expected to incorporate transport and turbulent mixing processes into Δ¹⁷O modeling.