Heterogeneous OH Chemistry of Submicron- sized Aqueous Organic Aerosols: Role of Molecular Structure

氣溶膠是懸浮在空氣中的微粒。它們對人類健康，能見度以及全球氣候均能造成重大影響。當暴露於大氣的氣體氧化劑（例如羥基和臭氧），氣溶膠會在表面附近發生多相氧化並改變化學成分和顆粒大小。有機化合物是氣溶膠的其中一個重要分類，當中不同個體的特性（例如揮發性、氧合程度和極性）各異。有機化合物會視乎其擁有功能群的數量、性質和位置有不同的化學改變。為深入了解分子結構對有機化合物與羥基的多相氧化過程的影響，本論文通過兩組相互關聯的實驗以研究兩個常見的功能群（甲基和羥基）如何影響有機化合物的氧化過程和最終反應產物分布。多相氧化實驗在氣霧流管反應器中進行，並使用裝有可在環境氣壓下運作的電離源（DART）的高分辨率質譜儀來獲得氧化氣溶膠的分子成分信息。論文第一部分比較了二甲基乙酸的兩種異構體（2,2-二甲基乙酸和2,3-二甲基乙酸）的氧化過程，以了解兩個支甲基在不同相對位置對多相氧化的速度和化學的影響。實驗結果顯示2,3-二甲基乙酸的氧化速度是2,2-二甲基乙酸的兩倍。這是由於在2,3-二甲基乙酸的初始氧化過程中，羥基的奪氧反應會形成比較穩定的叔烷基。此外，在兩種異構體的實驗中皆觀測到的醇官能化產物有可能是因位阻效應令烷氧基從分子間奪氧而形成，而碎片化產物則源於烷氧基的分解。結果指出烷氧基化學對於這兩個擁有甲基的二羧酸之化學過程的重要性。論文第二部分研究酒石酸的氧化過程以探討羥基如何影響多相過程。氧化過程產生四種產物：一個C4官能化產物(C4H4O6)和三個碎裂化產物(C3H4O4, C3H2O4, and C3H2O5)。其中的C4官能化產物並不是由過氧自由基自反應中產生，而是從α-羥基過氧自由基的單分子超氧化氫消除過程中形成。官能化和碎裂化產物均佔總質量的重要部分。雖然充氧的有機氣溶膠通常有利碎裂化過程，但此觀察顯示官能化過程對於這種高度氧化的有機酸是重要的。兩組實驗的結果清楚展示出有機化合物的分子結構很有程度控制氧化動力學和氧化中的化學作用，故有需要知道有機分子中的功能群的性質和位置以了解功能化和碎裂化的化學過程之間的競爭。Aerosols are suspending particulates in air that can greatly affect human health, visibility as well as the global climate. When exposed to gas phase oxidants (e.g. hydroxyl (OH) radical and ozone), atmospheric aerosols undergo oxidation at or near the aerosol surface that can significantly modify the aerosol size, composition, and properties (termed heterogeneous oxidation). Organic compounds contribute a significant fraction of the total aerosol mass and have a wide range of properties such as volatility, degree of oxygenation, and polarity. Depending on the number, types and positions of the functional groups, organic compounds can chemically evolve via very different mechanisms. To better understand how the molecular structure governs the heterogeneous OH chemistry of organic aerosols, we investigate the role of two functional groups (methyl and hydroxyl groups), which are commonly present in atmospheric organic compounds, in governing the oxidative kinetics and reaction mechanisms of small oxygenated organic compounds. Heterogeneous OH oxidation experiments are carried out using an atmospheric pressure aerosol flow tube reactor and the molecular composition of the aerosols before and after oxidation are obtained by a high resolution mass spectrometer coupled with an atmospheric pressure soft ionization source (referred to as Direct Analysis in Real Time, DART).In the first study, the OH oxidation of two structural isomers of dimethylsuccinic acid (2,2-dimethylsuccinic acid (2,2-DMSA) and 2,3dimethylsuccinic acid (2,3-DMSA)) is investigated to examine the effect of the relative locations of two branched methyl groups on the heterogeneous oxidative kinetics and chemistry. The heterogeneous reaction of OH radicals with 2,3-DMSA is about 2 times faster than that of 2,2-DMSA. This can be explained by the formation of stable tertiary alkyl radicals by the initial hydrogen abstraction of 2,3-DMSA. For the two isomers, the formation of alcohol functionalization products is likely attributed to the intermolecular hydrogen abstraction of alkoxy radicals due to the steric effect of the two methyl groups, while the fragmentation products are originated from the decomposition of alkoxy radicals. These results suggest that the alkoxy radical chemistry plays an important role in the heterogeneous chemistry of these two methyl substituted dicarboxylic acid.In the second study, we investigate the OH radical-initiated oxidation of tartaric acid to examine how the presence of hydroxyl groups controls the heterogeneous reaction mechanisms. Four major reaction products are formed: a single C4 functionalization product (C4H4O6) and three C3 fragmentation products (C3H4O4, C3H2O4, and C3H2O5). The C4 functionalization product does not appear to originate from peroxy radical self-reactions, but likely forms via the unimolecular HO2 elimination from an α-hydroxylperoxy intermediate, enhanced by the proximity of a hydroxyl group. Both functionalization and fragmentation products contribute significantly to the total aerosol mass. Generally, fragmentation processes are expected to be more favorable than functionalization processes when organic aerosols become more oxygenated, but our observations suggest that functionalization processes can be the dominant reaction pathways for this highly oxygenated organic acid.Overall, the results have clearly demonstrated that the molecular structure of an organic molecule largely controls the heterogeneous oxidative kinetics and reaction mechanisms. Knowledge on the nature and positions of the functional groups is important to understand the competition between functionalization and fragmentation processes during the oxidation.Cheng, Chiu Tung.Thesis M.Phil. Chinese University of Hong Kong 2016.Includes bibliographical references (leaves ).Abstracts also in Chinese.Title from PDF title page (viewed on …).Detailed summary in vernacular field only