Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), Dalian University of Technology, Dalian 116024, China.
Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, United States.
Environ Sci Technol. 2021 Apr 20;55(8):4399-4409. doi: 10.1021/acs.est.0c07925. Epub 2021 Mar 26.
The atmospheric chemistry of isoprene has broad implications for regional air quality and the global climate. Allylic radicals, taking 13-17% yield in the isoprene oxidation by Cl, can contribute as much as 3.6-4.9% to all possible formed intermediates in local regions at daytime. Considering the large quantity of isoprene emission, the chemistry of the allylic radicals is therefore highly desirable. Here, we investigated the atmospheric oxidation mechanism of the allylic radicals using quantum chemical calculations and kinetics modeling. The results indicate that the allylic radicals can barrierlessly combine with O to form peroxy radicals (RO). Under ≤100 ppt NO and ≤50 ppt HO conditions, the formed RO mainly undergo two times "successive cyclization and O addition" to finally form the product fragments 2-alkoxy-acetaldehyde (CHO) and 3-hydroperoxy-2-oxopropanal (CHO). The presented reaction illustrates a novel successive cyclization-driven autoxidation mechanism. The formed 3-hydroperoxy-2-oxopropanal product is a new isomer of the atmospheric CHO family and a potential aqueous-phase secondary organic aerosol precursor. Under >100 ppt NO condition, NO can mediate the cyclization-driven autoxidation process to form CHNO, CHNO, and alkoxy radical-related products. The proposed novel autoxidation mechanism advances our current understanding of the atmospheric chemistry of both isoprene and RO.
异戊二烯的大气化学对区域空气质量和全球气候具有广泛的影响。氯氧化异戊二烯生成的烯丙基自由基，在白天的局部地区，其生成的所有可能中间体的贡献可达 3.6-4.9%。考虑到异戊二烯的大量排放，烯丙基自由基的化学性质非常重要。在这里，我们使用量子化学计算和动力学建模研究了烯丙基自由基的大气氧化机制。结果表明，烯丙基自由基可以无势垒地与 O 结合形成过氧自由基 (RO)。在 ≤100 ppt 的 NO 和 ≤50 ppt 的 HO 条件下，形成的 RO 主要经过两次“连续环化和 O 加成”，最终形成产物碎片 2-烷氧基-乙醛 (CHO) 和 3-过氧-2-氧代丙醛 (CHO)。该反应说明了一种新的连续环化驱动的自动氧化机制。形成的 3-过氧-2-氧代丙醛产物是大气 CHO 族中的一种新异构体，也是潜在的水相二次有机气溶胶前体。在 >100 ppt 的 NO 条件下，NO 可以介导环化驱动的自动氧化过程，形成 CHNO、CHNO 和烷氧基自由基相关产物。所提出的新的自动氧化机制提高了我们对异戊二烯和 RO 大气化学的现有认识。
