EC-05. Investigation of the Volatile Methyl Siloxane Oxidation Mechanism in Urban and Remote Atmospheric Conditions

Abstract
Volatile methyl siloxanes (VMS) are solely anthropogenic pollutants of emerging concern that have been detected globally, including in remote arctic environments. These silicon containing chemicals have high vapor pressures and low water solubility, causing them to mainly partition into the atmosphere. Once in the gas phase, VMS can then react with atmospheric oxidants, like the hydroxyl radical, forming products (primarily silanols and silyl esters) that have different physical and chemical properties than the parent VMS which could change the environmental fate of these chemicals and contribute to urban aerosol formation and growth, thus affecting air quality and visibility. However, the mechanism by which this oxidation occurs is unclear. It is accepted that after hydrogen abstraction from the VMS, the resulting radical rapidly reacts with oxygen to form a peroxy radical. However, this peroxy radical can undergo multiple different reactions, which makes understanding the atmospheric oxidation mechanism complicated and heavily dependent on certain atmospheric trace gas concentrations. As previous studies have suggested mechanisms that do not explain all the observed products and do not agree with some previously published laboratory studies, this work investigated the fate of long- and short-lived VMS peroxy radicals to better constrain the oxidation mechanism and predict how the emissions of these chemicals can affect the environment.