Aerosol formation from oxidation of cyclic volatile methyl siloxane
Date
2017
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Ambient nanoparticles (defined as smaller than 100 nm in diameter) can disproportionally affect climate and human health relative to their mass loading in the atmosphere. To better understand these effects, knowledge of chemical composition is needed. Organic aerosol constitutes a large portion of this matter and most of this contribution is secondary, meaning that it is formed through reaction of gas phase volatile organic compounds (VOC) with oxidants (OH, O3, NO3) to give semi- or non-volatile products. Recently, silicon was reported as a frequent component of ambient nanoparticles. Measurements with Nano Aerosol Mass Spectrometer (NAMS), which provides quantitative elemental composition of particles in the 10-30 nm diameter range, showed that Si was often observed in urban and suburban environments but rarely detected in a remote environment. The location dependence suggests that Si in these particles is associated with human activity. One possible source is atmospheric oxidation of cyclic volatile methylsiloxanes (cVMS), which are commonly used in personal care products. Owing to high vapor pressure, they are easily released into atmosphere where they may react with OH to form semi- or non-volatile products. In this dissertation, the chemical composition and formation mechanisms of secondary aerosol produced from the OH-initiated oxidation of decamethylcyclopentasiloxane is studied by high performance mass spectrometry. ☐ Firstly, high resolution ESI-MS reveals a large number of monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) oxidation products. With the aid of high resolution and MS/MS, it is shown that oxidation leads mainly to the substitution of a CH3 group by OH or CH2OH, and that a single molecule can undergo many CH3 group substitutions. Dimers also exhibit OH and CH2OH substitutions and can be linked by O, CH2 and CH2CH2 groups. ☐ Secondly, aerosol formation mechanisms were studied with and without the presence of ammonium sulfate seed aerosol. For the unseeded experiments, chemical characterization with high performance mass spectrometry showed that the molecular composition changed substantially with aerosol mass loading in the 1-12 g/m3 range. Monomers (5 Si atoms/molecule) and dimers (10 Si atoms/molecule) dominated the mass spectra of aerosols at higher mass loadings while ring opened species (neither 5 nor 10 Si atoms/molecule) dominated the mass spectra of aerosols at lower mass loadings. Molecular signal intensity dependencies on the aerosol volume-to-surface area ratio suggest that nonvolatile ring opened species are formed in the gas phase and assist particle formation through condensation, while dimers are formed by accretion reactions within the particle phase as the particles grow. These conclusions are supported by experiments in the presence of seed aerosol with similar siloxane aerosol mass loading but higher volume to surface area ratio, where ring-opened species are much less prevalent than monomers or dimers and the aerosol yield is higher. ☐ The influence of biogenic secondary organic aerosol on D5 derived aerosol was studied by mixing β-pinene into reactor. The results showed that aerosol chemical composition containing both β-pinene and siloxane components is dominated by β-pinene SOA with slowly increasing siloxane contribution in this mixed system with increasing aerosol mass loading. Additionally, β-pinene SOA was found to serve in a similar manner to ammonium sulfate as a seed aerosol where ring-opened siloxane products are much less prevalent than siloxane monomers or dimers.
Description
Keywords
Pure sciences, Earth sciences, Aerosol, Cyclic, Oxidation, Siloxane, Volatile