First measurements of gas phase sulphuric acid in the stratosphere

Abstract

Sulphuric acid is thought to be the most important nucleating agent in the stratosphere and thus has a key role in the formation of the stratospheric aerosol layer (‘Junge layer’)1,2. As the latter at least temporarily influences the Earth's radiation budget and thereby the Earth's climate3–5, sulphuric acid is a very important stratospheric trace gas. The formation of stratospheric sulphuric acid is considered to proceed via a not yet fully determined photoreaction chain, starting from SO2 (ref 6). Sulphur dioxide, in turn, is thought to result from COS and possibly also from CS2 being transported from the troposphere into the stratosphere7. Direct SO2 injection is also possible. We report here the first measurement of gas phase sulphuric acid abundances in the stratosphere. Our method relies on a recent composition measurement of stratospheric negative ions which revealed the presence of species containing sulphuric acid molecules attached to HSO4− core ions8,9. Studies of relevant negative ion reactions at the NOAA-Aeronomy Laboratory10 have confirmed the identifications and reactions of the observed ions, giving a firm basis for determination of stratospheric H2SO4 abundances from negative ion composition measurements.