Abstract
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 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 (±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.
Highlights
Atmospheric chemical processes exert a major influence on atmospheric composition
Owing to the quadratic relationship of [(H2O)2] to [H2O], a small difference in the rate constant can have a large effect, especially at higher [H2O]. Possible explanations for this discrepancy are (i) that the kinetics observed for CH2OO as formed from CH2I2 photolysis are not representative of the behaviour of the CH2OO moiety as formed through alkene ozonolysis; (ii) that the fraction of the total isoprene stabilised Criegee intermediates (SCIs) yield that is CH2OO is lower than that predicted by Zhang et al (2002), and the effect of the (H2O)2 reaction overall is reduced – the predicted yield is in good agreement with those determined experimentally, albeit using indirect methods, so it seems unlikely that the actual CH2OO yield is considerably lower; and (iii) that multiple effects are affecting the curvature of the results shown www.atmos-chem-phys.net/15/9521/2015/
As for the isoprene + O3 as a function of water experiments described in Sect. 3, there is potential for the acid products of the isoprene ozonolysis reaction to provide an additional sink for SCIs in the chamber
Summary
Atmospheric chemical processes exert a major influence on atmospheric composition. Identified gas-phase oxidants include the OH radical, ozone, NO3 and under certain circumstances other species such as halogen atoms. Reactions with these oxidants can lead to (for example) chemical removal of primary air pollutants, formation of secondary pollutants (e.g. ozone, harmful to human and environmental health, and a greenhouse gas), and the transformation of gas-phase species to the condensed phase Newland et al.: Atmospheric isoprene ozonolysis tionalised organic compounds leading to secondary aerosol formation, which can influence radiation transfer and climate)
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