College of Safety and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, PR China; Environment Research Institute, Shandong University, Qingdao, 266237, PR China.
Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia.
Chemosphere. 2024 Oct;366:143425. doi: 10.1016/j.chemosphere.2024.143425. Epub 2024 Sep 26.
The oxidation mechanism of methylglyoxal (CHCOCHO) in the aqueous phase plays a crucial role in the formation of secondary organic aerosols (SOA). To date, the investigations of reaction mechanisms of MG in the aqueous phase still needs to be refined, and the oxidation mechanisms of MG in the existence of various oxidants (e.g., HO, O ∙NO, etc.) are in controversy. In this paper, we investigated the hypothesis that small-molecule organic acids are the primary products in cloud water and fog droplets, while large-molecule organic acids and oligomers play crucial roles in wet aerosols. Specifically, the hydration reaction, oxidation mechanism and oligomerization reaction of MG in aqueous phase were investigated on a theoretical basis. It has been indicated that the hydration reaction is a significant initiating reaction of MG in the atmospheric aqueous phase, whose generated hydrated compounds played a critical part in the process of forming oligomers. The aqueous oxidation reaction of MG could form a variety of organic acids, including pyruvic acid, formic acid, acetic acid, and oxalic acid. In the presence of OH radicals, pyruvic acid was the main first-generation production, which undergoes further reactions to form acetic acid, oxalic acid, and mesoxalic acid. Acetic acid was mainly derived from the reaction of OH radicals with pyruvic acid, whereas oxalic and mesoxalic acids were mainly generated by the OH radical reaction for MG and pyruvic acid. Of these, the formation of acetic acid was thermodynamically most favorable. Additionally, the reactions of MG with other oxidants also provided the possible pathways for pyruvic acid production. At 298 K, we calculated the rate constants for the reaction of MGHY with NO, OH, HO radicals, and O to be 4.48 × 10, 2.54 × 10, 1.26 × 10, and 4.38 × 10 M s, with atmospheric aqueous phase lifetimes (τ) of 4.43, 3.12 × 10, 2.21 × 10, and 3.17 × 10 h, respectively. The theoretical results from this work will facilitate the explanation for the MG reaction process in the aqueous phase so as to further correctly estimate the relationship between the aqueous phase chemistry of MG and the formation of SOA.
甲基乙二醛（CHCOCHO）在水相中的氧化机制在二次有机气溶胶（SOA）的形成中起着至关重要的作用。迄今为止，MG 在水相中的反应机制的研究仍需要进一步完善，并且 MG 在各种氧化剂（例如 HO、O ∙NO 等）存在下的氧化机制存在争议。在本文中，我们假设在云水中和雾滴中，小分子有机酸是主要产物，而大分子有机酸和低聚物在湿气溶胶中起着关键作用。具体而言，我们从理论上研究了 MG 在水相中的水合反应、氧化机制和齐聚反应。结果表明，水合反应是 MG 在大气水相中的重要引发反应，其生成的水合化合物在形成低聚物的过程中起着关键作用。MG 的水相氧化反应可以形成各种有机酸，包括丙酮酸、甲酸、乙酸和草酸。在 OH 自由基存在下，丙酮酸是主要的第一代产物，它会进一步反应生成乙酸、草酸和丙二酸。乙酸主要是由 OH 自由基与丙酮酸反应生成的，而草酸和丙二酸主要是由 OH 自由基与 MG 和丙酮酸反应生成的。其中，乙酸的生成热力学上最有利。此外，MG 与其他氧化剂的反应也为丙酮酸的生成提供了可能的途径。在 298 K 下，我们计算了 MGHY 与 NO、OH、HO 自由基和 O 反应的速率常数分别为 4.48×10、2.54×10、1.26×10 和 4.38×10 M s，大气水相寿命（τ）分别为 4.43、3.12×10、2.21×10 和 3.17×10 h。这项工作的理论结果将有助于解释 MG 在水相中的反应过程，从而进一步正确估计 MG 水相化学与 SOA 形成之间的关系。
