Reactions of isoprene and sulphoxy radical-anions – a possible source of atmospheric organosulphites and organosulphates

Abstract. Transformation of isoprene coupled with auto-oxidation of S^IV in aqueous solutions was studied experimentally and by chemical-kinetic modelling over a broad range of solution acidities (pH=3–9) to complement the research on aqueous-phase and heterogeneous transformation of isoprene reported recently by many laboratories. Isoprene significantly slowed down the auto-oxidation in acidic and basic solutions, and accelerated it slightly in neutral solutions. Simultaneously, production of sulphate ions and formation of solution acidity were significantly reduced. Formation of sulphite and sulphate derivatives of isoprene - sulphurous acid mono-(2-methyl-4-oxo-but-2-enyl) ester (m/z=163), sulphurous acid mono-(4-hydroxy-2-methyl-but-2-enyl) ester (m/z=165), sulphuric acid mono-(2-methyl-4-oxo-but-2-enyl) ester (m/z=179), sulphuric acid mono-(4-hydroxy-2-methyl-but-2-enyl) ester (m/z=181), and possible structural isomers of these species – was indicated by electrospray ionisation mass spectrometric analysis of post-reaction mixtures. The experimental results were explained by changes in a subtle quantitative balance of three superimposed processes whose rates depended in different manner on the acidity of reacting solutions – the scavenging of sulphoxy radical-anions by isoprene, the formation of sulphoxy radical-anions during further reactions of isoprene radicals, and the auto-oxidation of S^IV itself. A chemical mechanism based on this idea was explored numerically to show good agreement with experimental data. In basic and neutral solutions, the model overestimated the consumption of isoprene, probably because reactions of primary sulphite and sulphate derivatives of isoprene with sulphoxy radical-anions were not included. Interaction of isoprene with sulphur(IV) species and oxygen can possibly result in formation of new organosulphate and organosulphite components of atmospheric aerosols and waters, and influence the distribution of reactive sulphur and oxygen species in isoprene-emitting organisms exposed to S^IV pollutants.