Dinitrogen pentoxide (N₂O₅) is a key reactive nitrogen species that plays a significant role in atmospheric nitrogen cycling. While N₂O₅ is thought to form primarily through nocturnal NO₂ and NO₃ reactions, our study observed persistent and unusual diurnal patterns of N₂O₅ concentrations with distinct daytime peaks in urban Xiamen, southeastern China. Linear regression analysis showed that the unknown daytime N₂O₅ 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 N₂O₄ and photocatalytic oxidation of NO₂ by TiO₂ on surfaces were key factors explaining the unknown N₂O₅ sources. A chemical box model indicated that these heterogeneous reactions significantly contributed to daytime N₂O₅ formation and led to a 12.6% increase in net O₃ production. This study highlights the significance of heterogeneous chemical processes in urban N₂O₅ formation, and underscores their non-negligible impacts on photochemical pollution.