Many works report NO₃ chemistry in inland regions while less target marine regions. We measured N₂O₅ and related species on a typical island and found intensive nighttime chemistry and rapid NO₃ loss. NO contributed significantly to NO₃ loss despite its sub-ppbv level, suggesting nocturnal NO₃ reactions would be largely enhanced once free from NO emissions in the open ocean. This highlights the strong influences of urban outflow on downward marine areas in terms of nighttime chemistry.