Enhanced Sulfate Formation from Gas-Phase SO2 Oxidation in Non–•OH–Radical Environments

Abstract

Recent research on atmospheric particle formation has shown substantial discrepancies between observed and modeled atmospheric sulfate levels. This is because models mostly consider sulfate originating from (Formula presented.) oxidation by •OH radicals in mechanisms catalyzed by solar radiation while ignoring other pathways of non-radical (Formula presented.) oxidation that would substantially alter atmospheric sulfate levels. Herein, we use high-level quantum chemical calculations based on density functional theory and coupled cluster theory to show that monoethanolamine (MEA), a typical alkanolamine pollutant released from (Formula presented.) capture technology, can facilitate the conversion of atmospheric (Formula presented.) to sulfate in a non (Formula presented.) •OH (Formula presented.) radical oxidation mechanism. The initial process is the MEA-induced (Formula presented.) hydrolysis leading to the formation of (Formula presented.) • (Formula presented.). The latter entity is thereafter oxidized by ozone ((Formula presented.)) and nitrogen dioxide ((Formula presented.)) to form (Formula presented.) • (Formula presented.), which is an identified stabilizing entity in sulfate-based aerosol formation. Results show that the (Formula presented.) • (Formula presented.) reaction with (Formula presented.) is kinetically and thermodynamically more feasible than the reaction with (Formula presented.). The presence of an additional water molecule further promotes the (Formula presented.) • (Formula presented.) reaction with (Formula presented.), which occurs in a barrierless process, while it instead favors HONO formation in the reaction with (Formula presented.). The investigated pathway highlights the potential role alkanolamines may play in (Formula presented.) oxidation to sulfate, especially under conditions that are not favorable for •OH production, thereby providing an alternative sulfate source for aerosol modeling. The studied mechanism is not only relevant to sulfate formation and may effectively compete with reactions with sulfur dioxide and hydroxyl radicals under heavily polluted and highly humid conditions such as haze events, but also an important pathway in MEA removal processes.