Enhanced Sulfate Formation from Gas-Phase SO[sub.2] Oxidation in Non–•OH–Radical Environments

• Published in: Atmosphere Vol. 15; no. 1
• Main Authors: Lv, Xiaofan, Lily, Makroni, Tasheh, Stanley Numbonui, Ghogomu, Julius Numbonui, Du, Lin, Tsona Tchinda, Narcisse
• Format: Journal Article
• Language: English
• Published: MDPI AG 01.01.2024
• Subjects: Aerosols; Atmospheric chemistry; Distribution; Environmental aspects; Solar radiation; Sulfates; China
• ISSN: 2073-4433; 2073-4433
• DOI: 10.3390/atmos15010064

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 SO[sub.2] oxidation by •OH radicals in mechanisms catalyzed by solar radiation while ignoring other pathways of non-radical SO[sub.2] 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 CO[sub.2] capture technology, can facilitate the conversion of atmospheric SO[sub.2] to sulfate in a non–•OH–radical oxidation mechanism. The initial process is the MEA-induced SO[sub.2] hydrolysis leading to the formation of HOSO[sub.2] [sup.−] •MEAH[sup.+] . The latter entity is thereafter oxidized by ozone (O[sub.3] ) and nitrogen dioxide (NO[sub.2] ) to form HSO[sub.4] [sup.−] •MEAH[sup.+] , which is an identified stabilizing entity in sulfate-based aerosol formation. Results show that the HOSO[sub.2] [sup.−] •MEAH[sup.+] reaction with O[sub.3] is kinetically and thermodynamically more feasible than the reaction with NO[sub.2] . The presence of an additional water molecule further promotes the HOSO[sub.2] [sup.−] •MEAH[sup.+] reaction with O[sub.3] , which occurs in a barrierless process, while it instead favors HONO formation in the reaction with NO[sub.2] . The investigated pathway highlights the potential role alkanolamines may play in SO[sub.2] 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.