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

• Published in: Physical chemistry chemical physics : PCCP 2019, Vol.21 (5), p.2325-2336
• Main Authors: Moon, D. R, Ingham, T, Whalley, L. K, Seakins, P. W, Baeza-Romero, M. T, Heard, D. E
• Format: Article
• Language: English
• Subjects: Adsorption; Aerosols; Anatase; Chemical equilibrium; Dust; Electrons; Hydrogen peroxide; Luminous intensity; Nanoparticles; Oxidation; Radicals; Surface area; Surface chemistry; Titanium dioxide; Troposphere
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c8cp06889e

The production of gas-phase hydroperoxyl radicals, HO 2 , is observed directly from sub-micron airborne TiO 2 nanoparticles irradiated by 300-400 nm radiation. The rate of HO 2 production as a function of O 2 pressure follows Langmuir isotherm behaviour suggesting O 2 is involved in the production of HO 2 following its adsorption onto the surface of the TiO 2 aerosol. Reduction of adsorbed O 2 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 HO 2 production decreased significantly over the range of relative humidities between 8.7 and 36.9%, suggesting competitive adsorption of water vapour inhibits HO 2 production. From the data, the adsorption equilibrium constants were calculated to be: K O 2 = 0.27 ± 0.02 Pa −1 and K H 2 O = 2.16 ± 0.12 Pa −1 for RH = 8.7%, decreasing to K O 2 = 0.18 ± 0.01 Pa −1 and K H 2 O = 1.33 ± 0.04 Pa −1 at RH = 22.1%. The increased coverage of H 2 O onto the TiO 2 aerosol surface may inhibit HO 2 production by decreasing the effective surface area of the TiO 2 particle and lowering the binding energy of O 2 on the aerosol surface, hence shortening its desorption lifetime. The maximum yield ( i.e. when [O 2 ] is projected to atmospherically relevant levels) for production of gas-phase HO 2 , 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 TiO 2 used here. This yield decreased to as the RH was increased to 22.1%. Using this value, the rate of production of HO 2 from TiO 2 surfaces under atmospheric conditions was estimated to be in the range 5 × 10 4 -1 × 10 6 molecule cm −3 s −1 using observed surface areas of mineral dust at Cape Verde, and assuming a TiO 2 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 HO 2 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 H 2 O 2 at the surface by photogenerated electrons. Production of HO 2 radicals is observed directly following the near-UV irradiation of airborne TiO 2 nanoparticles.