Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry
Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry
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DOI：
10.1073/pnas.1812147115
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发表时间：
2018-11
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影响因子：
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通讯作者：
Yue Zhao;J. Thornton;H. Pye
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文献类型：
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作者：
Yue Zhao;J. Thornton;H. Pye
重要意义森林排放物氧化产生的高含氧性多功能化合物和大分子产物可推动大气颗粒物的形成和生长，这对空气质量恶化和地球能量平衡有很大贡献。然而，我们对这些低挥发性化合物的反应速度和形成机理的了解仍然很少。在这里，我们提供了一大套有机过氧自由基(RO2)和相关产物的直接观察，这些产物来自于臭氧引发的α-Pinene的氧化。我们观察到了室温下快速的RO2自氧化和有效的RO2交叉反应形成气态二聚体(ROOR‘)，并限制了这些过程在二次有机气溶胶质量形成中的作用。我们的发现对控制受森林排放影响地区的颗粒形成和生长的关键单萜类RO2化学提供了重要的启示。有机过氧自由基(RO2)是有机物大气降解和燃料燃烧的关键中间体，但到目前为止，对复杂反应体系中特定RO2的直接研究还很少，导致我们对它们的去向认识上存在很大差距。我们通过对臭氧引发的α-Pinene氧化反应生成的一组RO2和气相二聚体的直接形态测量表明，∼150气态二聚体(C16-20h24-34O4-13)主要是通过RO2交叉反应形成的，其典型的速率常数为0.75-2×10−12 cm~3分子−1 S−1，下限二聚体形成支化率为4%。这些发现表明，气态二聚体的产量与一氧化氮(NO)浓度有很大的不同，在具有低到中等人为影响的林区的典型条件下，至少有0.2-2.5%的摩尔(0.5-6.6%)的摩尔(质量分数)(即≤百万分之50)。鉴于其极低的挥发性，气态C16-20二聚体为初始粒子的形成提供了潜在的重要有机介质，仅此一项就可以解释在与大气相关的粒子质量负载下测量的α-Pinene二次有机气溶胶质量的5-60%。氧化氧、二聚体和高氧多功能化合物(HOM)对α-Pinene反应浓度和NO的响应表明，在3-10 S∼1和α1 S−1，平均有20%的初级≥-Pinene RO2和10%的臭氧氧化−1自氧化，证实了这两种氧化途径都有效地产生HOM，即使在城市地区典型的较高NO浓度下也是如此。因此，气相二聚体的形成和RO2的自氧化是低挥发性有机化合物的普遍来源，能够推动大气颗粒物的形成和生长。
Significance Highly oxygenated multifunctional compounds and large-molecular-mass products from the oxidation of forest emissions can drive the formation and growth of atmospheric particles, which contribute significantly to degraded air quality and Earth’s energy balance. Yet, our knowledge of the reaction rates and formation mechanisms of these low-volatility compounds remains poor. Here we provide direct observations of a large suite of organic peroxy radicals (RO2) and related products from O3-initiated oxidation of α-pinene. We observe rapid RO2 autoxidation at room temperature and efficient RO2 cross-reactions forming gaseous dimers (ROOR′), and constrain the role of these processes in secondary organic aerosol mass formation. Our findings shed important light on key monoterpene-derived RO2 chemistry governing particle formation and growth in regions impacted by forest emissions. Organic peroxy radicals (RO2) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO2 in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO2 and gas-phase dimers from O3-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C16–20H24–34O4–13) are primarily formed through RO2 cross-reactions, with a typical rate constant of 0.75–2 × 10−12 cm3 molecule−1 s−1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2–2.5% by mole (0.5–6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C16–20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5–60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO2, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO2 from OH reaction and 10% from ozonolysis autoxidize at 3–10 s−1 and ≥1 s−1, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO2 autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.