AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Secondary Organic Aerosol from Gas Phase Methylsiloxane Oxidation: Products and Reaction Mechanisms
YUE WU, Murray Johnston, University of Delaware
Abstract Number: 60 Working Group: Aerosol Chemistry
Abstract Cyclic volatile methylsiloxanes (cVMS), such as octamethylcyclotetrasiloxane (D$_(4)) and decamethylcyclopentasiloxane (D$_(5)), are widely used in consumer products and easily released into the atmosphere owing to their high vapor pressures. Field measurements suggest that ambient nanoparticles contain significant amounts of silicon, and one hypothesis is that cVMS react in the atmosphere to form secondary organic aerosol (SOA) containing silicon. In this work, SOA from cVMS oxidation is characterized by advanced mass spectrometry techniques to elucidate the particle phase products and reaction mechanisms.
During the experiment, the gas vapor of D$_(5) was created by passing clean, dry air over the D$_(5) liquid. This air flow was introduced into a photoreaction chamber (PC) along with an additional flow of ozone that had been bubbled through the deionized water. UV radiation in the PC produced OH from ozone and water, which then reacted with gaseous D$_(5). Particulate matter in the air flow exiting the PC was collected for 20-24 h onto a Teflon coated, glass fiber filter for analysis. The filter was extracted for analysis by direct infusion ESI-MS and thermal decomposition EI-MS. ESI-MS shows the appearance of many ions between 337 and 471 m/z (corresponding to modified D$_(5) monomers), 707 and 857 m/z (corresponding to modified D$_(5) dimers), 1169 and 1213 m/z (corresponding to modified D$_(5) trimers). Each region shows evidence of both functionalization and fragmentation of the D$_(5) precursor. EI-MS confirms two major building blocks of oligomers are D5 and D$_(4)TOH, an OH substituted product of D$_(5). Follow up experiments are being performed with high resolution ESI-MS/MS to determine accurate molecular formulas and confirm structural information. The results will be presented for SOA produced from D$_(5) and other precursors, such as octamethylcyclotetrasiloxane (D$_(4)), hexamethylcyclotrisiloxane (D$_(3)).