The lifetimes and potential change in planetary albedo owing to the oxidation of organic films extracted from atmospheric aerosol by hydroyxl (OH) radical oxidation at the air-water interface of aerosol particles

Abstract. Water insoluble organic material extracted from atmospheric aerosol samples collected in urban (Royal Holloway, University of London, UK) and remote (Halley, Antarctica) locations were shown to form stable thin films at an air-water interface, these organic films reacted quickly with gas-phase OH radicals which may impact planetary albedo. The x-ray reflectivity measurements additionally indicate that the film may be consistent with having a structure with increased electron density of film molecules towards the water, suggesting amphiphilic behaviour. Bimolecular rate constants for gas-phase OH radical oxidation of urban or remote aerosol extracts were typically of the order ∼10¹⁰ cm³ molecule⁻¹ s⁻¹, giving atmospheric lifetimes of the film with respect to gas-phase OH radical oxidation of minutes at typical atmospheric OH radical concentrations. Kinetic modelling of core-shell droplet dynamics suggests film lifetime of a few minutes, depending on ambient OH radical mixing ratio. Modelling the oxidation kinetics with KM SUB suggests half-lives of minutes to an hour and values of k_surf of ∼ 2 × 10⁻⁷ and ∼ 5 × 10⁻⁵ cm² s⁻¹ for urban and remote aerosol film extracts respectively. The lifetimes and half-lives calculated at typical OH atmospheric ambient mixing ratios are smaller than the typical residence time of atmospheric aerosols and thus oxidation of organic material should be considered in atmospheric modelling. Thin organic films at the air-water interface of atmospheric aerosol or cloud droplets may alter the light scattering properties of the aerosol. X-ray reflectivity measurements of atmospheric aerosol film material at the air-water interface resulted in calculated film thickness values to be either ∼10 Å or ∼17 Å for remote or urban aerosol extracts respectively and oxidation did not remove the films completely. One dimensional radiative transfer-modelling suggest the oxidation of thin organic films on atmospheric particles by OH radicals may reduce the planetary albedo by a small, but potentially significant amount.