Chemical characterization and formation of secondary organosiloxane aerosol (SOSiA) from OH oxidation of decamethylcyclopentasiloxane

Abstract

Cyclic volatile methyl siloxanes (cVMS) are anthropogenic chemicals emitted by consumer and industrial products. Evidence from laboratory and ambient studies indicates that cVMS contribute to the formation of secondary organosiloxane aerosol (SOSiA). However, the oxidation products and formation pathway under various atmospheric conditions have yet to be clarified. To address these knowledge gaps, we generated SOSiA from the OH oxidation of decamethylcyclopentasiloxane (D₅-siloxane, ∼200 ppb) in a potential aerosol mass oxidation flow reactor (PAM-OFR, 185 mode) with the photochemical aging from 0.51–7.04 days. The chemical composition of SOSiA was characterized with an online high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and offline electrospray ionization mass spectrometry (ESI-MS). A series of high-molecular-weight SOSiA ions were observed in the AMS mass spectra. Using the ESI-MS analysis, molecular identification of D₅-siloxane oxidation products was provided, allowing us to propose the possible formation pathways of the major products. Our results show the combination of multiple mass spectrometry techniques may further offer new insights into characterizing SOSiA-related species and can be used to identify SOSiA tracers from ambient measurements both indoors and outdoors.