Conversion of Catechol to 4-Nitrocatechol in Aqueous Microdroplets Exposed to O₃ and NO₂

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.