Rissanen Matti P
Institute for Atmospheric and Earth System Research (INAR), University of Helsinki, Helsinki, Finland.
ACS Earth Space Chem. 2018 Nov 15;2(11):1211-1219. doi: 10.1021/acsearthspacechem.8b00123. Epub 2018 Oct 12.
Atmospheric autoxidation of volatile organic compounds (VOC) leads to prompt formation of highly oxidized multifunctional compounds (HOM) that have been found crucial in forming ambient secondary organic aerosol (SOA). As a radical chain reaction mediated by oxidized peroxy (RO) and alkoxy (RO) radical intermediates, the formation pathways can be intercepted by suitable reaction partners, preventing the production of the highest oxidized reaction products, and thus the formation of the most condensable material. Commonly, NO is expected to have a detrimental effect on RO chemistry, and thus on autoxidation, whereas the influence of NO is mostly neglected. Here it is shown by dedicated flow tube experiments, how high concentration of NO suppresses cyclohexene ozonolysis initiated autoxidation chain reaction. Importantly, the addition of NO ceases covalently bound dimer production, indicating their production involving acylperoxy radical (RC(O)OO•) intermediates. In related experiments NO was also shown to strongly suppress the highly oxidized product formation, but due to possibility for chain propagating reactions (as with RO and HO too), the suppression is not as absolute as with NO. Furthermore, it is shown how NO reactions with oxidized peroxy radicals lead into indistinguishable product compositions, complicating mass spectral assignments in any RO + NO system. The present work was conducted with atmospheric pressure chemical ionization mass spectrometry (CIMS) as the detection method for the highly oxidized end-products and peroxy radical intermediates, under ambient conditions and at short few second reaction times. Specifically, the insight was gained by addition of a large amount of NO (and NO) to the oxidation system, upon which acylperoxy radicals reacted in RC(O)O + NO → RC(O)ONO reaction to form peroxyacylnitrates, consequently shutting down the oxidation sequence.
挥发性有机化合物（VOC）的大气自氧化会迅速形成高度氧化的多功能化合物（HOM），这些化合物在形成环境二次有机气溶胶（SOA）中起着关键作用。作为由氧化过氧（RO）和烷氧（RO）自由基中间体介导的自由基链反应，其形成途径可以被合适的反应伙伴拦截，从而阻止最高氧化态反应产物的产生，进而阻止最可凝物质的形成。通常，人们认为NO会对RO化学产生不利影响，进而影响自氧化过程，然而NO的影响大多被忽视。在此，通过专门的流动管实验表明，高浓度的NO如何抑制环己烯臭氧分解引发的自氧化链反应。重要的是，添加NO会停止共价结合二聚体的产生，这表明它们的产生涉及酰基过氧自由基（RC(O)OO•）中间体。在相关实验中还表明，NO也能强烈抑制高度氧化产物的形成，但由于可能发生链传播反应（RO和HO也会如此），其抑制作用不像NO那样绝对。此外，还展示了NO与氧化过氧自由基的反应如何导致难以区分的产物组成，这使得任何RO + NO系统中的质谱归属变得复杂。本研究以大气压化学电离质谱（CIMS）作为高氧化终产物和过氧自由基中间体的检测方法，在环境条件下且反应时间短至几秒的情况下进行。具体而言，通过向氧化系统中添加大量的NO（和NO）获得了相关见解，在此过程中，酰基过氧自由基在RC(O)O + NO → RC(O)ONO反应中发生反应，形成过氧酰基硝酸盐，从而终止了氧化序列。
