Aqueous Radical Initiated Oxidation of an Organic Monolayer at the Air–Water Interface as a Proxy for Thin Films on Atmospheric Aerosol Studied with Neutron Reflectometry

Neutron reflectometry has been used to study the radical initiated oxidation of a monolayer of the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) at the air-water interface, by aqueous-phase hydroxyl, sulfate and nitrate radicals. The oxidation of organic films at the surface of atmospheric aqueous aerosols can influence the optical properties of aerosol and consequently can impact the Earth’s radiative balance and contribute to modern climate change. The amount of material at the air-water interface was found to decrease on exposure to aqueous-phase radicals and follow a multi-step degradation mechanism. The multi-step degradation mechanism means that lipid molecules in the thin film degrade to form progressively shorter chain surface active products and several reactive steps are required to remove the film from the air-water interface. Calculations to determine the film lifetime indicate that it will take ~4-5 days for the film to degrade to 50 % of its initial amount in the atmosphere and therefore attack by aqueous radicals on organic films is atmospherically important relative to the typical lifetimes of atmospheric aerosol.