Laboratory Investigation of Multigenerational Siloxane Oxidation Chemistry

Abstract:

Volatile methyl siloxanes (VMS) are a class of anthropogenic pollutants of emerging concern that have been detected globally, including in remote arctic environments. Additionally, oxidized VMS have been detected in newly formed atmospheric particles, potentially affecting air quality. Oxidation by OH is the main removal process of VMS and elucidating the oxidation mechanisms and products of VMS is important for understanding the environmental impacts of VMS. The peroxy radical formed from OH oxidation undergoes reactions to create two classes of observed oxidation products: silanols and silyl esters. Recent work has suggested that rapid autooxidation can occur to create very low volatility silanol products. However, this work does not explain observations of the silyl ester and fails to account for previously observed humidity dependences on the formation of silanols and silyl esters. Here, we use laboratory experiments to investigate the fate of long-lived VMS peroxy radicals and multigenerational siloxane chemistry to better constrain oxidized VMS chemistry. Modeling of these chamber experiments informs a potential mechanism of silanol and silyl ester formation as well as the environmental fates of these products.