Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere
Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere
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甲基丙烯酰过氧硝酸酯（MPAN）的羟基自由基氧化机理及其在大气中形成二次有机气溶胶的途径
DOI：
10.1039/c5cp02001h
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
Seinfeld, John H.
中科院分区：
文献类型：
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作者：
Nguyen, Tran B.;Bates, Kelvin H.;Crounse, John D.;Schwantes, Rebecca H.;Zhang, Xuan;Kjaergaard, Henrik G.;Surratt, Jason D.;Lin, Peng;Laskin, Alexander;Seinfeld, John H.
甲基丙烯酰过氧硝酸酯（MPAN），甲基丙烯醛的酰基过氧硝酸酯，已被认为是一个重要的二次有机气溶胶（SOA）的前体异戊二烯氧化。然而，MPAN通过与羟基自由基（OH）反应产生SOA的机制尚不清楚。我们系统地评估了三个建议的机制在控制室实验，并提供了第一个实验支持的理论预测的内酯形成途径从MPAN + OH反应，产生羟甲基-甲基-α-内酯（HMML）。MPAN-OH加合物的分解产生HMML + NO3（约75%）和羟基丙酮+ CO + NO3（约25%），与大气中的氧气竞争。其他提出的SOA前体的生产，例如，来自MPAN和甲基丙烯醛的甲基丙烯酸环氧化物（MAE）可忽略不计（<2%）。此外，我们表明，β-烯基部分的MPAN是关键的内酯形成。通过OH的H-提取冷形成的烷基自由基不分解成HMML，即使它们在结构上与MPAN-OH加合物相同。SOA的形成从HMML，从聚合加成的内酯的有机化合物在颗粒界面处或在凝聚相，是接近于统一的干燥条件下。然而，SOA产率对颗粒、液态水和溶剂化离子敏感。在水合无机颗粒中，HMML主要与H2O反应生成单体2-甲基甘油酸（2 MGA），或与硫酸盐和硝酸盐水溶液反应分别生成相关的有机硫酸盐和有机硝酸盐。2 MGA，异戊二烯SOA的示踪剂，是半挥发性的，其在气溶胶中的水的住宿随着pH值的降低而降低。条件，提高生产的中性2 MGA抑制SOA质量从HMML通道。考虑到液态水含量和周围颗粒的pH值范围，2 MGA将主要以气态化合物的形式存在于大气的某些部分。
Methacryloyl peroxynitrate (MPAN), the acyl peroxynitrate of methacrolein, has been suggested to be an important secondary organic aerosol (SOA) precursor from isoprene oxidation. Yet, the mechanism by which MPAN produces SOA through reaction with the hydroxyl radical (OH) is unclear. We systematically evaluate three proposed mechanisms in controlled chamber experiments and provide the first experimental support for the theoretically-predicted lactone formation pathway from the MPAN + OH reaction, producing hydroxymethyl-methyl-α-lactone (HMML). The decomposition of the MPAN–OH adduct yields HMML + NO3 (∼75%) and hydroxyacetone + CO + NO3 (∼25%), out-competing its reaction with atmospheric oxygen. The production of other proposed SOA precursors, e.g., methacrylic acid epoxide (MAE), from MPAN and methacrolein are negligible (<2%). Furthermore, we show that the beta-alkenyl moiety of MPAN is critical for lactone formation. Alkyl radicals formed cold via H-abstraction by OH do not decompose to HMML, even if they are structurally identical to the MPAN–OH adduct. The SOA formation from HMML, from polyaddition of the lactone to organic compounds at the particle interface or in the condensed phase, is close to unity under dry conditions. However, the SOA yield is sensitive to particle liquid water and solvated ions. In hydrated inorganic particles, HMML reacts primarily with H2O to produce the monomeric 2-methylglyceric acid (2MGA) or with aqueous sulfate and nitrate to produce the associated organosulfate and organonitrate, respectively. 2MGA, a tracer for isoprene SOA, is semivolatile and its accommodation in aerosol water decreases with decreasing pH. Conditions that enhance the production of neutral 2MGA suppress SOA mass from the HMML channel. Considering the liquid water content and pH ranges of ambient particles, 2MGA will exist largely as a gaseous compound in some parts of the atmosphere.