Production of HO2 and OH radicals from near-UV irradiated airborne TiO2 nanoparticles

Abstract

The production of gas-phase hydroperoxyl radicals, HO₂, is observed directly from sub-micron airborne TiO₂ nanoparticles irradiated by 300–400 nm radiation. The rate of HO₂ production as a function of O₂ pressure follows Langmuir isotherm behaviour suggesting O₂ is involved in the production of HO₂ following its adsorption onto the surface of the TiO₂ aerosol. Reduction of adsorbed O₂ by photogenerated electrons is likely to be the initial step followed by reaction with a proton produced via oxidation of adsorbed water with a photogenerated hole. The rate of HO₂ production decreased significantly over the range of relative humidities between 8.7 and 36.9%, suggesting competitive adsorption of water vapour inhibits HO₂ production. From the data, the adsorption equilibrium constants were calculated to be: K_O2 = 0.27 ± 0.02 Pa⁻¹ and K_H2O = 2.16 ± 0.12 Pa⁻¹ for RH = 8.7%, decreasing to K_O2 = 0.18 ± 0.01 Pa⁻¹ and K_H2O = 1.33 ± 0.04 Pa⁻¹ at RH = 22.1%. The increased coverage of H₂O onto the TiO₂ aerosol surface may inhibit HO₂ production by decreasing the effective surface area of the TiO₂ particle and lowering the binding energy of O₂ on the aerosol surface, hence shortening its desorption lifetime. The maximum yield (i.e. when [O₂] is projected to atmospherically relevant levels) for production of gas-phase HO₂, normalised for surface area and light intensity, was found to be at a RH of 8.7% for the 80% anatase and 20% rutile formulation of TiO₂ used here. This yield decreased to as the RH was increased to 22.1%. Using this value, the rate of production of HO₂ from TiO₂ surfaces under atmospheric conditions was estimated to be in the range 5 × 10⁴–1 × 10⁶ molecule cm⁻³ s⁻¹ using observed surface areas of mineral dust at Cape Verde, and assuming a TiO₂ fraction of 4.5%. For the largest loadings of dust in the troposphere, the rate of this novel heterogeneous production mechanism begins to approach that of HO₂ production from the gas-phase reaction of OH with CO in unpolluted regions. The production of gas-phase OH radicals could only be observed conclusively at high aerosol surface areas, and was attributed to the decomposition of H₂O₂ at the surface by photogenerated electrons.