Heterogeneous Reactions of alpha-Pinene on Mineral Surfaces: Formation of Organonitrates and alpha-Pinene Oxidation Products

Organonitrates (ON) are important components of secondary organic aerosols (SOAs). alpha-Pinene (C10H16), the most abundant monoterpene in the troposphere, is a precursor for the formation of several of these compounds. ON from alpha-pinene can be produced in the gas phase via photochemical processes and/or following reactions with oxidizers including hydroxyl radical and ozone. Gas-phase nitrogen oxides (NO2, NO3) are N sources for ON formation. Although gas-phase reactions of alpha-pinene that yield ON are fairly well understood, little is known about their formation through heterogeneous and multiphase pathways. In the current study, surface reactions of alpha-pinene with nitrogen oxides on hematite (alpha-Fe2O3) and kaolinite (SiO2Al2O3(OH)(4)) surfaces, common components of mineral dust, have been investigated. alpha-Pinene oxidizes upon adsorption on kaolinite, forming pinonaldehyde, which then dimerizes on the surface. Furthermore, alpha-pinene is shown to react with adsorbed nitrate species on these mineral surfaces producing multiple ON and other oxidation products. Additionally, gas-phase oxidation products of alpha-pinene on mineral surfaces are shown to more strongly adsorb on the surface compared to alpha-pinene. Overall, this study reveals the complexity of reactions of prevalent organic compounds such as alpha-pinene with adsorbed nitrate and nitrogen dioxide, revealing new heterogeneous reaction pathways for SOA formation that is mineralogy specific.