Molecular Characterization of First- and Second-Generation Oxidation Products of Decamethylcyclopentasiloxane in Ambient Fine Particulate Matter

JEEWANI MEEPAGE, Josie Welker, Saeideh Mohammadi, Christopher Brunet, Hanalei Lewine, Rachel Marek, Keri Hornbuckle, Eleanor Browne, Charles Stanier, Elizabeth Stone, University of Iowa

     Abstract Number: 654
     Working Group: Source Apportionment

Abstract
Decamethylcyclopentasiloxane (D₅) is a prominent ingredient in personal care products. It can react to form oxidized volatile methyl siloxanes and contribute to secondary organic aerosol (SOA) in the atmosphere and impact air quality. Yet, the quantitative impact of D₅-derived SOA on ambient fine particulate matter (PM_2.5) has not been characterized. The focus of this study is to develop molecular markers to track the occurrence of D₅-derived SOA to aid in source apportionment. SOA samples of D₅ oxidation in a laboratory oxidative flow reactor (OFR) are compared to PM_2.5 collected at a New York City (NYC) field site. Sample extracts are analyzed by reverse-phase liquid chromatography in conjunction with negative electrospray ionization and high-resolution mass spectrometry. A preliminary list of molecular candidates for tracing D₅-derived SOA in the atmosphere has been developed, based on their repeated observation in OFR-generated SOA samples and detection in the NYC field sample by using exact mass, isotopic ratios, product ion spectra, and signal abundance. It includes a homologous series in which a methyl group in D5 (C10H30O5Si5) is replaced by a hydroxyl group: C₉H₂₈O₆Si₅, C₈H₂₆O₇Si₅, C₇H₂₄O₈Si₅, and C₆H₂₂O₉Si₅. Among these, the first two homologs were observed in the NYC field sample, while all four were observed in the OFR sample, indicating the presence of multi-step oxidation products in ambient PM_2.5. Molecular structures for the most abundant homologs are presented, based on their product ion spectra and chromatographic retention time. Because of their specificity to D₅ and demonstrated detectability in ambient PM_2.5, the identified products may be useful molecular tracers for first- and second-generation oxidation products of D₅-derived SOA.