Atmospheric isoprene ozonolysis: impacts of stabilised Criegee intermediate reactions with SO₂, H₂O and dimethyl sulfide

Abstract. Isoprene is the dominant global biogenic volatile organic compound (VOC) emission. Reactions of isoprene with ozone are known to form stabilised Criegee intermediates (SCIs), which have recently been shown to be potentially important oxidants for SO₂ and NO₂ in the atmosphere; however the significance of this chemistry for SO₂ processing (affecting sulfate aerosol) and NO₂ processing (affecting NO_x levels) depends critically upon the fate of the SCIs with respect to reaction with water and decomposition. Here, we have investigated the removal of SO₂ in the presence of isoprene and ozone, as a function of humidity, under atmospheric boundary layer conditions. The SO₂ removal displays a clear dependence on relative humidity, confirming a significant reaction for isoprene-derived SCIs with H₂O. Under excess SO₂ conditions, the total isoprene ozonolysis SCI yield was calculated to be 0.56 (±0.03). The observed SO₂ removal kinetics are consistent with a relative rate constant, k(SCI + H₂O) / k(SCI + SO₂), of 3.1 (±0.5) × 10⁻⁵ for isoprene-derived SCIs. The relative rate constant for k(SCI decomposition) / k(SCI+SO₂) is 3.0 (±3.2) × 10¹¹ cm⁻³. Uncertainties are ±2σ and represent combined systematic and precision components. These kinetic parameters are based on the simplification that a single SCI species is formed in isoprene ozonolysis, an approximation which describes the results well across the full range of experimental conditions. Our data indicate that isoprene-derived SCIs are unlikely to make a substantial contribution to gas-phase SO₂ oxidation in the troposphere. We also present results from an analogous set of experiments, which show a clear dependence of SO₂ removal in the isoprene–ozone system as a function of dimethyl sulfide concentration. We propose that this behaviour arises from a rapid reaction between isoprene-derived SCIs and dimethyl sulfide (DMS); the observed SO₂ removal kinetics are consistent with a relative rate constant, k(SCI + DMS) / k(SCI + SO₂), of 3.5 (±1.8). This result suggests that SCIs may contribute to the oxidation of DMS in the atmosphere and that this process could therefore influence new particle formation in regions impacted by emissions of unsaturated hydrocarbons and DMS.