Unexpected Daytime N ₂ O ₅ in Urban Areas: Formation Mechanisms and Environmental Impacts

Abstract Dinitrogen pentoxide (N 2 O 5 ) is a key reactive nitrogen species that plays a significant role in atmospheric nitrogen cycling. While N 2 O 5 is thought to form primarily through nocturnal NO 2 and NO 3 reactions, our study observed persistent and unusual diurnal patterns of N 2 O 5 concentrations with distinct daytime peaks in urban Xiamen, southeastern China. Linear regression analysis showed that the unknown daytime N 2 O 5 production rates were closely correlated with proxies for heterogeneous chemical pathways. An explainable machine learning analysis revealed that the laboratory‐characterized pathways including UV‐induced heterogeneous processes of N 2 O 4 and photocatalytic oxidation of NO 2 by TiO 2 on surfaces were key factors explaining the unknown N 2 O 5 sources. A chemical box model indicated that these heterogeneous reactions significantly contributed to daytime N 2 O 5 formation and led to a 12.6% increase in net O 3 production. This study highlights the significance of heterogeneous chemical processes in urban N 2 O 5 formation, and underscores their non‐negligible impacts on photochemical pollution.