Abstract The missing source of nitrous acid (HONO) is a key uncertainty in understanding atmospheric chemistry in the troposphere. Particulate nitrate photolysis has been proposed as a potential source of daytime HONO, but its contribution remains poorly constrained, with even less attention paid to the impact of nitrate aloft. Here, we investigate the impact of nitrate photolysis on HONO production based on vertical measurements using a tethered airship in Nanjing together with meteorology‐chemistry coupled modeling. We show that nitrate photolysis aloft is about 1.8 times that at the surface and substantially contributes to surface HONO through vertical mixing. Regional simulations further show that nitrate photolysis in the Yangtze River Delta region increases simulated HONO and maximum daily 8‐hr average ozone concentrations by 79.3% and 12.5%, respectively. Our results highlight the importance of nitrate photolysis as a daytime HONO source and its critical implications on atmospheric oxidation capacity in megacities in eastern China.