Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States.
Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China.
Environ Sci Technol. 2021 Apr 20;55(8):4430-4439. doi: 10.1021/acs.est.0c08066. Epub 2021 Mar 15.
Large amounts of small α-dicarbonyls (glyoxal and methylglyoxal) are produced in the atmosphere from photochemical oxidation of biogenic isoprene and anthropogenic aromatics, but the fundamental mechanisms leading to secondary organic aerosol (SOA) and brown carbon (BrC) formation remain elusive. Methylglyoxal is commonly believed to be less reactive than glyoxal because of unreactive methyl substitution, and available laboratory measurements showed negligible aerosol growth from methylglyoxal. Herein, we present experimental results to demonstrate striking oligomerization of small α-dicarbonyls leading to SOA and BrC formation on sub-micrometer aerosols. Significantly more efficient growth and browning of aerosols occur upon exposure to methylglyoxal than glyoxal under atmospherically relevant concentrations and in the absence/presence of gas-phase ammonia and formaldehyde, and nonvolatile oligomers and light-absorbing nitrogen-heterocycles are identified as the dominant particle-phase products. The distinct aerosol growth and light absorption are attributed to carbenium ion-mediated nucleophilic addition, interfacial electric field-induced attraction, and synergetic oligomerization involving organic/inorganic species, leading to surface- or volume-limited reactions that are dependent on the reactivity and gaseous concentrations. Our findings resolve an outstanding discrepancy concerning the multiphase chemistry of small α-dicarbonyls and unravel a new avenue for SOA and BrC formation from atmospherically abundant, ubiquitous carbonyls and ammonia/ammonium sulfate.
大量的小α-二羰基化合物（如乙二醛和甲基乙二醛）在大气中通过生物源异戊二烯和人为芳香族化合物的光化学氧化生成，但导致二次有机气溶胶（SOA）和棕色碳（BrC）形成的基本机制仍然难以捉摸。由于甲基的不反应取代，甲基乙二醛通常被认为比乙二醛的反应性差，并且现有的实验室测量表明，甲基乙二醛对气溶胶的增长可忽略不计。在此，我们提供了实验结果，证明了小α-二羰基化合物的显著齐聚作用，导致亚微米气溶胶上形成 SOA 和 BrC。在大气相关浓度下，在不存在/存在气相氨和甲醛的情况下，暴露于甲基乙二醛比暴露于乙二醛更能有效地促进气溶胶的生长和褐变，并且鉴定出非挥发性低聚物和光吸收氮杂环作为主要的颗粒相产物。明显的气溶胶生长和光吸收归因于碳正离子介导的亲核加成、界面电场诱导的吸引以及涉及有机/无机物质的协同齐聚化，导致表面或体积受限反应，这些反应取决于反应性和气体浓度。我们的发现解决了关于小α-二羰基化合物多相化学的一个突出差异，并为从大气中丰富、普遍存在的羰基化合物和氨/硫酸铵形成 SOA 和 BrC 开辟了一条新途径。
