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

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 SO2 oxidation by center dot OH radicals in mechanisms catalyzed by solar radiation while ignoring other pathways of non-radical SO2 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 CO2 capture technology, can facilitate the conversion of atmospheric SO2 to sulfate in a non-center dot OH-radical oxidation mechanism. The initial process is the MEA-induced SO2 hydrolysis leading to the formation of HOSO-2 center dot MEAH+. The latter entity is thereafter oxidized by ozone (O-3) and nitrogen dioxide (NO2) to form HSO4- center dot MEAH+, which is an identified stabilizing entity in sulfate-based aerosol formation. Results show that the HOSO2- center dot MEAH+ reaction with O-3 is kinetically and thermodynamically more feasible than the reaction with NO2. The presence of an additional water molecule further promotes the HOSO-2 center dot MEAH+ reaction with O-3, which occurs in a barrierless process, while it instead favors HONO formation in the reaction with NO2. The investigated pathway highlights the potential role alkanolamines may play in SO2 oxidation to sulfate, especially under conditions that are not favorable for center dot 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.