Conversion of Catechol to 4-Nitrocatechol in Aqueous Microdroplets Exposed to O<sub>3</sub> and NO<sub>2</sub>
Md Sohel Rana, Seth T. Bradley, Marcelo I. Guzmán
Abstract
High Resolution Image Download MS PowerPoint Slide 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 (O 3 ) and hydroxyl radicals (HO), the action of nitrate radicals (NO 3 ) on the surface of aqueous atmospheric particles should become significant at night. This work simulates nighttime interfacial chemistry between hydrated catechol and adsorbed NO 3 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 [NO 2 ] and [O 3 ]. The produced 4-nitrocatechol is quantified by a standard addition in real-time experiments under [NO 2 ]:[O 3 ] 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 NO 3, forming a semiquinone radical, (2) electrophilic NO 3 attack to the ring to yield a cyclohexadienyl radical intermediate, and (3) electrophilic attack to the ring by nitronium ion (NO 2 + ) formed at the interface of water by colliding N 2 O 5 (g) at low pH. Ozonolysis competes strongly with nitration when using [NO 2 ]:[O 3 ] ratios 1:1 or smaller. Instead, nighttime chemistry under higher molar ratios proceeds mainly by nitration with a maximum yield of 0.90 for [NO 2 ]:[O 3 ] = 4:1.