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

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Isotopic compositions (δ 15 N, δ 18 O, and Δ 17 O) of nitrogen dioxide (NO 2 ) are crucial proxies for elucidating the sources and transformations of atmospheric NO 2, yet observations remain scarce. Here, we reported high-resolution (2 h interval) measurements of NO 2 isotopic compositions in a megacity in Central China. Δ 17 O(NO 2 ) exhibited a clear diurnal cycle with elevated values during the day versus night. Daytime Δ 17 O(NO 2 ) varying from 20.2 to 40.1‰ can be explained by variations in ozone (O 3 ) versus peroxyl radical oxidation of nitric oxide (NO). The estimated peroxyl radicals were positively correlated with photochemical oxidants (O x = O 3 + NO 2 ), revealing the ability of Δ 17 O(NO 2 ) to reflect changes in the atmospheric oxidation environment. Nighttime Δ 17 O(NO 2 ) variations (12.9 to 29.8‰) can be categorized into three regimes: the first regime when Δ 17 O(NO 2 ) stabilized at approximately 20‰ can be well explained by Δ 17 O transfer from O 3; the increases of Δ 17 O(NO 2 ) above 20‰ were attributed to the vertical mixing of residual layer air with elevated Δ 17 O(NO 2 ) and the exchange process between NO 2 and dinitrogen pentoxide; and as low as 12.9‰ Δ 17 O(NO 2 ) can be explained by primary NO 2 emissions and the exchange process between NO and NO 2 . The NO–NO 2 exchange also elevated δ 15 N(NO 2 ), consistent with the observed negative relationships between Δ 17 O(NO 2 ) and δ 15 N(NO 2 ) at night.