Ambient nanoparticles (&lt;100 nm) can influence global climate and human health. To better understand these effects, knowledge of chemical components of nanoparticulate matter is needed. Recently, silicon was reported as a frequent component in ambient nanoparticles and the results showed that Si was often observed in urban and suburban environments but rarely detected in a remote environment, which may indicate that those particles have a man-made source. One possible source is photo-oxidation of airborne siloxanes, organic silicon compounds that are commonly used in personal care products. Owing to high vapor pressure, siloxanes are easily to be released into atmosphere and react with hydroxyl radical in the presence of light to form products that could condense on the existing nanoparticles. In this work, aerosol formation from OH oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5), a cyclic volatile methyl siloxane (cVMS), was studied in a flow-through photo chamber 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 mg/m3 range. Monomers and dimers dominated the mass spectra of aerosols at higher mass loadings while ring opened species 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.

Aerosol formation from OH oxidation of the volatile cyclic methyl siloxane (cVMS) decamethylcyclopentasiloxane

254th National Meeting (2017)

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