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 climate^3–5, 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 ions^8,9. 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.