Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products.
Atmospheric Degradation of Cyclic Volatile Methyl Siloxanes: Radical Chemistry and Oxidation Products.
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DOI：
10.1021/acsenvironau.1c00043
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发表时间：
2022-05-18
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影响因子：
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通讯作者：
Browne, Eleanor C
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文献类型：
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作者：
Alton, Mitchell W;Browne, Eleanor C
环状挥发性甲基硅氧烷（cVMS）是一种人类化学品，由于其广泛使用和环境持久性而受到审查。关于这些化学品的环境浓度和持久性的重要数据是存在的，但其氧化机制知之甚少，阻碍了对cVMS的环境归宿和影响的全面了解。我们在环境室中进行了实验，以表征六甲基环三硅氧烷（D3），八甲基环四硅氧烷（D4），和十甲基环五硅氧烷（D5）在不同的过氧自由基命运（单分子反应或双分子反应与NO或HO 2），接近一系列的大气成分的第一代氧化产物。虽然来自D3的氧化产物的特性随过氧自由基命运而变化，但D4和D5氧化产物的特性和产率基本保持恒定。我们将我们的结果与cVMS氧化化学动力学模型的输出进行比较。该模型中使用的反应机制开发使用先前提出的cVMS氧化反应和标准大气氧化自由基化学的组合。我们发现，该模型无法重现我们的测量，特别是在D4和D5的情况下。在模型中表现不佳的产物有助于确定机制中可能的分支点，这需要进一步研究。此外，我们估计的cVMS氧化产物的物理性质，使用结构-活性关系，并发现它们不应该被显着分配到有机或水性气溶胶。结果表明，cVMS的第一代氧化产物也是长期存在于大气中，这些化合物的环境监测是必要的，以了解cVMS的环境化学和负载。
Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals that have come under scrutiny due to their widespread use and environmental persistence. Significant data on environmental concentrations and persistence of these chemicals exists, but their oxidation mechanism is poorly understood, preventing a comprehensive understanding of the environmental fate and impact of cVMS. We performed experiments in an environmental chamber to characterize the first-generation oxidation products of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) under different peroxy radical fates (unimolecular reaction or bimolecular reaction with either NO or HO2) that approximate a range of atmospheric compositions. While the identity of the oxidation products from D3 changed as a function of the peroxy radical fate, the identity and yield of D4 and D5 oxidation products remained largely constant. We compare our results against the output from a kinetic model of cVMS oxidation chemistry. The reaction mechanism used in the model is developed using a combination of previously proposed cVMS oxidation reactions and standard atmospheric oxidation radical chemistry. We find that the model is unable to reproduce our measurements, particularly in the case of D4 and D5. The products that are poorly represented in the model help to identify possible branching points in the mechanism, which require further investigation. Additionally, we estimated the physical properties of the cVMS oxidation products using structure–activity relationships and found that they should not be significantly partitioned to organic or aqueous aerosol. The results suggest that cVMS first-generation oxidation products are also long-lived in the atmosphere and that environmental monitoring of these compounds is necessary to understand the environmental chemistry and loading of cVMS.