Size-dependent chemical ageing of oleic acid aerosol under dry and humidified conditions
Size-dependent chemical ageing of oleic acid aerosol under dry and humidified conditions
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DOI：
10.5194/acp-16-15561-2016
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发表时间：
2016-12-16
影响因子：
6.3
通讯作者：
Harrison, Roy M.
中科院分区：
文献类型：
--
作者：
Al-Kindi, Suad S.;Pope, Francis D.;Harrison, Roy M.
本文介绍了一种在干燥和加湿至65%两种相对湿度条件下用臭氧处理油酸气溶胶粒子的化学反应室系统。该装置包括一个气溶胶流管，其中发生臭氧分解，耦合到一个扫描迁移率粒度仪（SMPS）和气溶胶飞行时间质谱仪（ATOFMS），测量不断变化的颗粒大小和组成。在这两种相对湿度条件下，臭氧分解导致颗粒尺寸和质量显著降低，这与从颗粒到气相的挥发性产物的形成一致。源自ATOFMS的质谱显示存在典型观察到的反应产物：壬二酸、壬醛、氧代壬酸和壬酸，以及源自反应中间体与油酸及其氧化产物的反应的一系列较高分子量产物。这些包括辛酸和9-和10-氧代十八烷酸，以及相当高分子量的产物。与ATOFMS的产品产量的定量评价显示了显着的依赖于两个粒度协会（从0.3至2.1 μ m的直径）和相对湿度。在这两种相对湿度条件下，油酸的残留百分比随着粒度的增加而增加，并且主要的较低分子量产物是壬醛和氧代壬酸。在干燥条件下，由于较大颗粒的内部混合较差，较高分子量产物的百分比随着粒度的增加而增加。在加湿条件下，未反应的油酸的百分比更大，除了在最小的颗粒级分中，相对于干燥颗粒几乎没有形成高分子量产物。据推测，水与活性中间体反应，与产生高分子量产物的过程竞争。虽然油酸模型气溶胶系统是有限的相关性，复杂的内部混合的大气气溶胶，本文提出的一般性研究结果提供了有用的见解异质性化学过程的性质。
A chemical reaction chamber system has been developed for the processing of oleic acid aerosol particles with ozone under two relative humidity conditions: dry and humidified to 65 %. The apparatus consists of an aerosol flow tube, in which the ozonolysis occurs, coupled to a scanning mobility particle sizer (SMPS) and an aerosol time-of-flight mass spectrometer (ATOFMS) which measure the evolving particle size and composition. Under both relative humidity conditions, ozonolysis results in a significant decrease in particle size and mass which is consistent with the formation of volatile products that partition from the particle to the gas phase. Mass spectra derived from the ATOFMS reveal the presence of the typically observed reaction products: azelaic acid, nonanal, oxononanoic acid and nonanoic acid, as well as a range of higher molecular weight products deriving from the reactions of reaction intermediates with oleic acid and its oxidation products. These include octanoic acid and 9- and 10-oxooctadecanoic acid, as well as products of considerably higher molecular weight. Quantitative evaluation of product yields with the ATOFMS shows a marked dependence upon both particle size association (from 0.3 to 2.1 mu m diameter) and relative humidity. Under both relative humidity conditions, the percentage residual of oleic acid increases with increasing particle size and the main lower molecular weight products are nonanal and oxononanoic acid. Under dry conditions, the percentage of higher molecular weight products increases with increasing particle size due to the poorer internal mixing of the larger particles. Under humidified conditions, the percentage of unreacted oleic acid is greater, except in the smallest particle fraction, with little formation of high molecular weight products relative to the dry particles. It is postulated that water reacts with reactive intermediates, competing with the processes which produce high molecular weight products. Whilst the oleic acid model aerosol system is of limited relevance to complex internally mixed atmospheric aerosol, the generic findings presented in this paper give useful insights into the nature of heterogeneous chemical processes.