Heterogeneous reaction of N₂O₅ with airborne TiO₂ particles and its implication for stratospheric particle injection

Abstract. Injection of aerosol particles (or their precursors) into the stratosphere to scatter solar radiation back into space has been suggested as a solar-radiation management scheme for the mitigation of global warming. TiO₂ has recently been highlighted as a possible candidate particle because of its high refractive index, but its impact on stratospheric chemistry via heterogeneous reactions is as yet unknown. In this work the heterogeneous reaction of airborne sub-micrometre TiO₂ particles with N₂O₅ has been investigated for the first time, at room temperature and different relative humidities (RH), using an atmospheric pressure aerosol flow tube. The uptake coefficient of N₂O₅ onto TiO₂, γ(N₂O₅), was determined to be ~1.0 × 10⁻³ at low RH, increasing to ~3 × 10⁻³ at 60% RH. The uptake of N₂O₅ onto TiO₂ is then included in the UKCA chemistry–climate model to assess the impact of this reaction on stratospheric chemistry. While the impact of TiO₂ on the scattering of solar radiation is chosen to be similar to the aerosol from the Mt Pinatubo eruption, the impact of TiO₂ injection on stratospheric N₂O₅ is much smaller.