AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Secondary Organic Aerosol from Gas Phase Methylsiloxane Oxidation
YUE WU, Murray Johnston, University of Delaware
Abstract Number: 47 Working Group: Aerosol Chemistry
Abstract Ambient nanoparticles (<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, secondary organic aerosol (SOA) obtained from siloxane oxidation is studied in detail for the first time. Decamethylcyclopentasiloxane (D$_5, C$_(10)H$_(30)O$_5Si$_5) was chosen as the precursor and D$_5-derived SOA was generated through a Photo-oxidation Chamber (PC) to simulate photo-oxidation in the atmosphere by reaction with hydroxyl radical. Molecular compositions of the products are characterized by Q Exactive TM Plus Hybrid Quadrupole-Orbitrap Mass Spectrometer. The ESI-MS spectra reveal a large number of both monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) products of oxidation. Most of the signal intensity comes from saturated substituted compounds in SOA samples (e.g. C$_9H$_(28)O$_6Si$_5). Mass weighted intensity fraction (MIF) analysis of assigned molecular formulas show that gas phase D$_5 is oxidized to form SOA with an average O/Si ratio of 1.35 and C/Si ratio of 1.75. High resolution ESI-MS/MS is used to give strong evidence for major substitution types along the siloxane ring and the linkages of two siloxane rings to produce dimers. The results show that OH and CH$_2OH (possibly CH$_2OOH) substitutions on the siloxane ring are the dominant types, and dimers are linked by O, CH$_2 and CH$_2CH$_2 groups. High resolution GC-MS is performed as well to confirm that among the substitutions and linkages identified above, that the major building blocks of siloxane-derived SOA are D$_5 (presumably linked in a dimer), D$_5 containing one OH substitution for a methyl group, and dimers linked by either an O or CH$_2 group. Current work is focused on SOA yield measurements in presence and absence of seed aerosol.