Hydroxyl (OH) radical oxidation of surfactant films formed from woodland aerosol particulate material at the air-water interface

Neutron reflectometry was used to examine the reaction of gas-phase hydroxyl radicals with thin surfactant films at the air–water interface. The films comprised insoluble material extracted from aerosol particulate matter collected from the atmosphere of a broadleaf woodland; sampled above and below the canopy across spring, summer, and winter. The measurements presented here act as a proxy for oxidation reactions at the air–water interface of broadleaf woodland atmospheric aqueous aerosols. The material extracted from the woodland atmosphere formed stable surfactant-like thin films at the air–water interface, with maximum thicknesses of 30 Å and neutron scattering length densities between 0.1 × 10⁻⁶Å ⁻² and 2.5 × 10⁻⁶Å ⁻². Oxidation by hydroxyl radicals reduced the amount of interfacial material, leaving an oxidation-resistant fraction of 20%–60% of the original film. The values of the surface reaction coefficients, determined by KM-SUB, for the reaction of hydroxyl radicals with woodland films were approximately 10⁻⁷ cm² s⁻¹. Film half-lives were estimated to be 1—2 h in typical day-time hydroxyl radical concentrations and 2 days–1 week in night-time concentrations. Thus, organic material extracted from temperate, broadleaf woodland aerosol can form thin, stable surfactant films at the air–water interface that can be partially removed by the gas-phase hydroxyl radical at a significant enough rate to warrant inclusion in atmospheric models.