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

• Published in: Atmospheric chemistry and physics Vol. 15; no. 16; pp. 9521 - 9536
• Main Authors: Newland, M J, Rickard, A R, Vereecken, L, Muñoz, A, Ródenas, M, Bloss, W J
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 26.08.2015; Copernicus Publications
• Subjects: Approximation; Atmosphere; Atmospheric boundary layer; Atmospheric chemistry; Boundary layers; Decomposition; Decomposition reactions; Dependence; Dimethyl; Dimethyl sulfide; Emissions; Experiments; Gases; Hydrocarbons; Intermediates; Isoprene; Kinetics; Mathematical analysis; Nitrogen compounds; Nitrogen dioxide; Organic compounds; Oxidants; Oxidation; Oxides; Oxidizing agents; Ozone; Ozonolysis; Particle formation; Pollutants; Rate constants; Reaction kinetics; Relative humidity; Removal; Sulfates; Sulfides; Sulfur dioxide; Sulphides; Troposphere; Unsaturated hydrocarbons; VOCs; Volatile organic compounds
• ISSN: 1680-7316; 1680-7324
• DOI: 10.5194/acp-15-9521-2015

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 NOx 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 ( plus or minus 0.03). The observed SO2 removal kinetics are consistent with a relative rate constant, k(SCI + H2O) / k(SCI + SO2), of 3.1 ( plus or minus 0.5) 10-5 for isoprene-derived SCIs. The relative rate constant for k(SCI decomposition) / k(SCI+SO2) is 3.0 ( plus or minus 3.2) 1011 cm-3. Uncertainties are plus or minus 2 sigma 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 ( plus or minus 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.