Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes

Abstract

Cyclic volatile methylsiloxanes (cVMS) have been identified as important gas-phase atmospheric contaminants, but knowledge of the molecular composition of secondary aerosol derived from cVMS oxidation is incomplete. Here, the chemical composition of secondary aerosol produced from the OH-initiated oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5) is characterized by high performance mass spectrometry. 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. GC-MS confirms the ESI-MS results. Oxidation of D4 (C8H24O4Si4) exhibits similar substitutions and oligomerizations to D5, though the degree of oxidation is greater under the same conditions and there is direct evidence for the formation of peroxy groups (CH2OOH) in addition to OH and CH2OH.

Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane

“…This implies that at the high loadings observed in this work, aerosol composition is likely dominated by semi-volatile monomer species and nonvolatile dimer species. In the context of available knowledge of aerosol formation for cVMS, Wu and Johnston (2017) observed aerosol size affected aerosol composition, and this work shows the extent of oxidation may also contribute to aerosol chemistry.…”

“…The addition of seed aerosols resulted in an increase in number concentration of 2.6 %, and mass concentration of 44 %. Wu and Johnston (2017) similarly observed positive yield sensitivity to ammonium sulfate seed aerosols.…”

“…Wu and Johnston (2017) reported aerosol yields of 0.08-0.16 at aerosol loadings of 1.2-12 µg m −3 (at constant OH), with yields increasing with reacted D 5 and aerosol loadings. Wu and Johnston (2017) caution that their yields are based on estimated rather than measured D 5 consumption. Our data point for trial 4 (yield of 0.5) is higher than the others, which were in the 0.22-0.3 range.…”

“…While the experimental conditions of trial 4 were unique (high RH, high light intensity, low flow, and long residence time) and consistent with a potentially high yield, we note that we did not perform additional experiments which would replicate or invalidate this finding; we further note that variability (in time) of mass was approximately up to 13 % (by mass) at low flow and 32 % (by mass) at high flow. Wu and Johnston (2017) suggest the aerosol composition is highly dependent on aerosol mass loading with less volatile ring-opened species at low loadings and, as particles grow, composition shifts to dimer formation and semi-volatile monomer species. This implies that at the high loadings observed in this work, aerosol composition is likely dominated by semi-volatile monomer species and nonvolatile dimer species.…”

“…Chamber studies have shown that a range of nonvolatile and semi-volatile oxidation products form upon reaction with OH that can form secondary aerosol (Latimer et al, 1998;Sommerlade et al, 1993;Chandramouli and Kamens, 2001;Johnston, 2016, 2017). Wu and Johnston (2017) studied the molecular composition and formation pathways of aerosol-phase D 5 oxidation products at aerosol loadings of 1-12 µg m −3 and identified three main types of species: monomer (substituted D 5 ), dimer, and ring-opened oxidation products. Janechek et al (2017) atmospheric modeling at 36 km resolution provided the first spatial distribution and potential loadings of the oxidation products, of which some fraction likely form aerosol species.…”