Abstract
Abstract. The gas-phase reaction of alkenes with ozone is known to produce stabilised Criegee intermediates (SCIs). These biradical/zwitterionic species have the potential to act as atmospheric oxidants for trace pollutants such as SO2, enhancing the formation of sulfate aerosol with impacts on air quality and health, radiative transfer and climate. However, the importance of this chemistry is uncertain as a consequence of limited understanding of the abundance and atmospheric fate of SCIs. In this work we apply experimental, theoretical and numerical modelling methods to quantify the atmospheric impacts, abundance and fate of the structurally diverse SCIs derived from the ozonolysis of monoterpenes, the second most abundant group of unsaturated hydrocarbons in the atmosphere. We have investigated the removal of SO2 by SCIs formed from the ozonolysis of three atmospherically important monoterpenes (α-pinene, β-pinene and limonene) in the presence of varying amounts of water vapour in large-scale simulation chamber experiments that are representative of boundary layer conditions. The SO2 removal displays a clear dependence on water vapour concentration, but this dependence is not linear across the range of [H2O] explored. At low [H2O] a strong dependence of SO2 removal on [H2O] is observed, while at higher [H2O] this dependence becomes much weaker. This is interpreted as being caused by the production of a variety of structurally (and hence chemically) different SCIs in each of the systems studied, which displayed different rates of reaction with water and of unimolecular rearrangement or decomposition. The determined rate constants, k(SCI+H2O), for those SCIs that react primarily with H2O range from 4 to 310 × 10−15 cm3 s−1. For those SCIs that predominantly react unimolecularly, determined rates range from 130 to 240 s−1. These values are in line with previous results for the (analogous) stereo-specific SCI system of syn-/anti-CH3CHOO. The experimental results are interpreted through theoretical studies of the SCI unimolecular reactions and bimolecular reactions with H2O, characterised for α-pinene and β-pinene at the M06-2X/aug-cc-pVTZ level of theory. The theoretically derived rates agree with the experimental results within the uncertainties. A global modelling study, applying the experimental results within the GEOS-Chem chemical transport model, suggests that > 97 % of the total monoterpene-derived global SCI burden is comprised of SCIs with a structure that determines that they react slowly with water and that their atmospheric fate is dominated by unimolecular reactions. Seasonally averaged boundary layer concentrations of monoterpene-derived SCIs reach up to 1.4 × 104 cm−3 in regions of elevated monoterpene emissions in the tropics. Reactions of monoterpene-derived SCIs with SO2 account for < 1 % globally but may account for up to 60 % of the gas-phase SO2 removal over areas of tropical forests, with significant localised impacts on the formation of sulfate aerosol and hence the lifetime and distribution of SO2.
Highlights
Chemical oxidation processes in the atmosphere exert a major influence on atmospheric composition, leading to the removal of primary emitted species and the formation of secondary products
Fitting Eq (4) to the data determines values of γ A = 0.22 and γ B = 0.78 (Fig. 5). This is broadly in line with the ratio recommended in the MCMv3.3.1 of 0.27 : 0.73 and with that proposed in Leungsakul et al (2005), who use a Criegee intermediates (CIs)-A : CIB ratio of 0.35 : 0.65, and include some stabilisation of CH2OO and C9-CI from ozone reaction at the exocyclic bond
The Stabilised Criegee intermediates (SCIs) yield is taken to be 0.30, similar to that of (CH3)2COO from 2,3-dimethyl-but-2-ene ozonolysis (Newland et al, 2015a). This may be an underestimation since it has been predicted that stabilisation of small CIs increase with the size of the carbonyl co-product, as this coproduct can take more of the nascent energy of the primary ozonide on decomposition due to a greater number of degrees of freedom available (Nguyen et al, 2009a; Newland et al, 2015b)
Summary
Chemical oxidation processes in the atmosphere exert a major influence on atmospheric composition, leading to the removal of primary emitted species and the formation of secondary products. In many cases either the emitted species or their oxidation products negatively impact air quality and climate (e.g. ozone, which is a potent greenhouse gas). These reactions can transform gas-phase species to the condensed phase, forming secondary aerosol that again can be harmful to health and can both directly and indirectly influence radiative transfer and climate (e.g. SO2 oxidation leading to the formation of sulfate aerosol). We observe the removal of SO2 in the presence of each alkene–ozone system as a function of water vapour concentration This allows us to derive relative SCI kinetics for reaction with H2O, SO2 and unimolecular decomposition. A global modelling study, using the GEOS-Chem global chemical transport model, is performed to assess global and regional impacts of the chemical kinetics of monoterpene SCIs determined in this study
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