Revisiting the reaction of OH + SO2: a crucial reaction in the chemistry of our atmosphere

Of the many millions of reactions that occur in the Earth’s atmosphere, the reaction of OH + SO2 is of singular interest to atmospheric chemists and climate scientists, since it leads to the formation of sulfuric acid, which acts as a nucleus upon which aerosol particles can grow. Once formed, these tiny particles are responsible for many of the worst health impacts of air pollution, and are estimated to be responsible for millions of deaths globally each year. These particles are also able to perturb the radiative balance of our atmosphere and are thought to have an overall cooling effect upon the Earth’s climate. Accordingly, this reaction is well-known and has been studied many times previously. However, major disagreement remains about how rapid this reaction is, especially under conditions that are directly applicable to those found in Earth’s atmosphere. The reasons for these discrepancies are primarily related to the experimental difficulties associated with this reaction.

In this study, we make an extensive series of careful measurements under experimental conditions that have never been accessed before. Where our results can be compared with previous measurements, we find good general agreement with the literature, although some inconsistency with a less direct experimental determination is noted. The reasons for this disagreement are unclear, however, our use of new methodologies, our coverage of new experimental conditions, and our consistency with the large majority of previous measurements indicates that we have now constrained this reaction rate with a new level of certainty.

These results will be of interest to atmospheric chemistry and climate modellers, whose calculations depend on this reaction to assess the effects on SO2 on air pollution and the global climate.