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

Author:

Lv Xiaofan1,Lily Makroni1,Tasheh Stanley Numbonui2ORCID,Ghogomu Julius Numbonui23,Du Lin1ORCID,Tsona Tchinda Narcisse1

Affiliation:

1. Environment Research Institute, Shandong University, Qingdao 266237, China

2. Department of Chemistry, Faculty of Science, The University of Bamenda, Bamenda P.O. Box 39, Cameroon

3. Research Unit of Noxious Chemistry and Environmental Engineering, Department of Chemistry, Faculty of Science, University of Dschang, Dschang P.O. Box 67, Cameroon

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 SO2 oxidation by •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–•OH–radical oxidation mechanism. The initial process is the MEA-induced SO2 hydrolysis leading to the formation of HOSO2−•MEAH+. The latter entity is thereafter oxidized by ozone (O3) and nitrogen dioxide (NO2) to form HSO4−•MEAH+, which is an identified stabilizing entity in sulfate-based aerosol formation. Results show that the HOSO2−•MEAH+ reaction with O3 is kinetically and thermodynamically more feasible than the reaction with NO2. The presence of an additional water molecule further promotes the HOSO2−•MEAH+ reaction with O3, 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 •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.

Funder

National Key Research and Development Program of China

National Natural Science Foundation of China

Natural Science Foundation of Shandong Province

Publisher

MDPI AG

Subject

Atmospheric Science,Environmental Science (miscellaneous)

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