High-Resolution Measurements of Multi-Isotopic Signatures (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) of Winter NO₂ in a Megacity in...

Isotopic compositions (δ¹⁵N, δ¹⁸O, and Δ¹⁷O) of nitrogen dioxide (NO₂) are crucial proxies for elucidating the sources and transformations of atmospheric NO₂, yet observations remain scarce. Here, we reported high-resolution (2 h interval) measurements of NO₂ isotopic compositions in a megacity in Central China. Δ¹⁷O(NO₂) exhibited a clear diurnal cycle with elevated values during the day versus night. Daytime Δ¹⁷O(NO₂) varying from 20.2 to 40.1‰ can be explained by variations in ozone (O₃) versus peroxyl radical oxidation of nitric oxide (NO). The estimated peroxyl radicals were positively correlated with photochemical oxidants (O = O₃ + NO₂), revealing the ability of Δ¹⁷O(NO₂) to reflect changes in the atmospheric oxidation environment. Nighttime Δ¹⁷O(NO₂) variations (12.9 to 29.8‰) can be categorized into three regimes: the first regime when Δ¹⁷O(NO₂) stabilized at approximately 20‰ can be well explained by Δ¹⁷O transfer from O₃; the increases of Δ¹⁷O(NO₂) above 20‰ were attributed to the vertical mixing of residual layer air with elevated Δ¹⁷O(NO₂) and the exchange process between NO₂ and dinitrogen pentoxide; and as low as 12.9‰ Δ¹⁷O(NO₂) can be explained by primary NO₂ emissions and the exchange process between NO and NO₂. The NO-NO₂ exchange also elevated δ¹⁵N(NO₂), consistent with the observed negative relationships between Δ¹⁷O(NO₂) and δ¹⁵N(NO₂) at night.