The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and Its Relevance in Solar Geoengineering Models

In recent years solar geoengineering has been proposed as a promising strategy to contrast global warming induced by anthropogenic CO₂ emissions. These technologies design to inject in the stratosphere massive amounts of molecular species, which can act as precursors for aerosol formation able to partially reflect sunlight. SO₂ is one of these species. Since in the atmosphere several natural ionization sources, such as cosmic rays and corona discharge, are active, we have considered that SO₂⁺ ions can be formed in the stratosphere in a significant amount after being injected by balloons or aircrafts. The SO₂⁺ chemistry could play a role in the dynamics of aerosol formation as a cooling agent. We have studied theoretically and experimentally the reaction of SO₂⁺, produced by tunable synchrotron radiation, with H₂ leading to HSO₂⁺ and H, the latter being involved in the ozone depletion by producing O₂ and OH. This is an ionic possible alternative to OH formation during the nighttime, when the common sunlight process of OH generation cannot occur. In order to explain the experimental reactivity we propose a new non-thermal version of the Variational Transition State Theory. We provide analytic expressions for the temperature dependent rate coefficients, which should be tested in atmospheric kinetic models to fully explore the stratospheric solar geoengineering strategies.