School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou, 510640, China.
School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou, 510640, China; Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou, 510006, China.
Chemosphere. 2020 Oct;256:126997. doi: 10.1016/j.chemosphere.2020.126997. Epub 2020 May 22.
Syringic acid (Syr) is an abundant component in aerosol particles. Multiphase photo-oxidation in aerosol phase provides an important oxidation pathway for Syr in the atmosphere. In this work, we studied the multiphase degradation of Syr by measuring rate coefficients of its reactions with potential radical oxidants (OH and SO) in aqueous solutions and by theoretical calculations, and degradation mechanisms by identifying the (intermediate) products. Rate coefficients, in 10 M s, were obtained as 32 ± 2 (pH 3) and 25 ± 2 (pH 6) for reactions with OH radical, and 1.7 ± 0.1 (pH 3) and 0.9 ± 0.02 (pH 6) for reactions with SO. Reactions of Syr with OH and SO were all in diffusion-control limit. Rate coefficients' difference under pH 6 and pH 3 in SO reaction was caused by Coulomb's force between negatively charged species. Theoretical calculations showed that the reaction of Syr with OH starts mainly by hydrogen atom transfer (HAT) from phenolic groups and secondly by OH addition to the aromatic ring. No product was identified in the reaction of Syr and OH radical at pH 3 due probably to the rapid mineralization of phenoxy radical formed from HAT, while products after OH additions were identified for a reaction at pH 6. On the other hand, reaction of Syr with SO starts by single-electron transfer (SET), forming Syr, which can undergo hydrolysis, sulfation, and dimerization with Syr and other aromatic intermediates, etc. Dimerization products from the phenoxy-type radical were not found here.
丁香酸（Syr）是气溶胶颗粒中丰富的成分。气溶胶相中多相光氧化为大气中 Syr 的氧化提供了重要途径。在这项工作中，我们通过测量其与潜在自由基氧化剂（OH 和 SO）在水溶液中的反应速率系数，并通过理论计算来研究 Syr 的多相降解，通过鉴定（中间）产物来确定降解机制。在 pH 3 时，与 OH 自由基的反应速率系数为 32±2，在 pH 6 时为 25±2；在 pH 3 时与 SO 的反应速率系数为 1.7±0.1，在 pH 6 时为 0.9±0.02。Syr 与 OH 和 SO 的反应均处于扩散控制极限。SO 反应在 pH 6 和 pH 3 下的速率系数差异是由于带负电荷物质之间的库仑力引起的。理论计算表明，Syr 与 OH 的反应主要通过酚基的氢原子转移（HAT）开始，其次是 OH 加成到芳环上。在 pH 3 下，由于 HAT 形成的苯氧自由基迅速矿化，因此在 Syr 和 OH 自由基的反应中没有鉴定出产物，而在 pH 6 的反应中鉴定出了 OH 加成产物。另一方面，Syr 与 SO 的反应通过单电子转移（SET）开始，形成 Syr，它可以进行水解、磺化和与 Syr 及其他芳族中间体的二聚化等反应。这里没有发现来自酚型自由基的二聚化产物。
