Clay Surface Chemistry of α-Pinene: A Source of Atmospheric Oxidation Products

Resuspended dust particles are important aerosol components that play a key role in air quality. α-Pinene (C10H16), a ubiquitous monoterpene in the troposphere, can react on acidic catalytic surfaces with water and undergo hydration to create various mono- and bicyclic terpene compounds. Although gas–particle reactions between mineral particles and organonitrates and organosulfates have been frequently discussed in the literature, little is known about the heterogeneous reaction types that occur on the surface of these mineral particles when exposed to hydrocarbons. In the current study, surface reactions of α-pinene with five common clays: (i) montmorillonite, (ii) bentonite, (iii) laponite, (iv) hectorite, and (v) Pikes Peak clay, which are components of resuspended mineral dust were investigated under variable humidities. α-Pinene hydrates and isomerizes on the surface of montmorillonite, bentonite, and Pikes Peak clays under both low- and high-humidity conditions, while α-pinene only shows isomerization on laponite and hectorite under high-humidity conditions. It was found that the structure and cation composition of the clays are important for α-pinene isomerization. The open octahedral sites in the dioctahedral structure of montmorillonite, bentonite, and Pikes Peak clays allowed for more isomerization of α-pinene to various terpene compounds, and the clays with more iron cations underwent significant chemical oxidation to α-pinene oxides. Overall, this study reveals the complexity of the reaction processes that occur with prevalent environmental organics, such as α-pinene, providing a new understanding of clay-specific heterogeneous reactions.