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Catechol is a widespread atmospheric dihydroxybenzene present in vehicle emissions, biomass burning, and combustion pollution plumes. Although the daytime reactivity of catechol is controlled by ozone (O3) and hydroxyl radicals (HO), the action of nitrate radicals (NO3) on the surface of aqueous atmospheric particles should become significant at night. This work simulates nighttime interfacial chemistry between hydrated catechol and adsorbed NO3 to form 4-nitrocatechol during experiments lasting ≤1 μs. Surface-sensitive online electrospray ionization mass spectrometry (OESI-MS) examines the reaction on the water surface under variable ratios of [NO2] and [O3]. The produced 4-nitrocatechol is quantified by a standard addition in real-time experiments under [NO2]:[O3] ratios of 1:1, 2:1, 3:1, and 4:1. Three mechanisms contribute to produce 4-nitrocatechol: (1) electron and proton transfers from catechol to NO3, forming a semiquinone radical, (2) electrophilic NO3 attack to the ring to yield a cyclohexadienyl radical intermediate, and (3) electrophilic attack to the ring by nitronium ion (NO2+) formed at the interface of water by colliding N2O5(g) at low pH. Ozonolysis competes strongly with nitration when using [NO2]:[O3] ratios 1:1 or smaller. Instead, nighttime chemistry under higher molar ratios proceeds mainly by nitration with a maximum yield of 0.90 for [NO2]:[O3] = 4:1.