Heterogeneous reaction of HO₂ with airborne TiO₂ particles and its implication for climate change mitigation strategies

One geoengineering mitigation strategy for global temperature rises resulting from the increased concentrations of greenhouse gases is to inject particles into the stratosphere to scatter solar radiation back to space, with TiO₂ particles emerging as a possible candidate. Uptake coefficients of HO₂, γ(HO₂), onto sub-micrometre TiO₂ particles were measured at room temperature and different relative humidities (RHs) using an atmospheric pressure aerosol flow tube coupled to a sensitive HO₂ detector. Values of γ(HO₂) increased from 0.021 ± 0.001 to 0.036 ± 0.007 as the RH was increased from 11 to 66 %, and the increase in γ(HO₂) correlated with the number of monolayers of water surrounding the TiO₂ particles. The impact of the uptake of HO₂ onto TiO₂ particles on stratospheric concentrations of HO₂ and O₃ was simulated using the TOMCAT three-dimensional chemical transport model. The model showed that, when injecting the amount of TiO₂ required to achieve the same cooling effect as the Mt Pinatubo eruption, heterogeneous reactions between HO₂ and TiO₂ would have a negligible effect on stratospheric concentrations of HO₂ and O₃.