The atmospheric impacts of monoterpene ozonolysis on global stabilised Criegee intermediate budgets and SO<sub>2</sub> oxidation: experiment, theory and modelling

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.

Sulfur Dioxide Modifies Aerosol Particle Formation and Growth by Ozonolysis of Monoterpenes and Isoprene

“…Journal of Geophysical Research: Atmospheres yields proposed by Newland et al (2018) increase in the order α-pinene < limonene < β-pinene, which is in agreement with the amount of nucleation we observe in our experiments following SO 2 addition.…”

“…Particle nucleation was also observed with β-pinene in the presence of SO 2 , and the number concentration was about a factor of 5 higher than that achieved with α-pinene for approximately the same number of molecules oxidized. This suggests that the yield of H 2 SO 4 produced by oxidation of SO 2 during β-pinene ozonolysis is much greater than from α-pinene ozonolysis, which is in line with a reported higher sCI yield from β-pinene than α-pinene (Hatakeyama & Akimoto, 1994;Newland et al, 2018). Due to this substantial new particle formation, the seed particles only contributed less than 10% to the total condensation sink after SO 2 addition.…”

“…Isoprene has been previously reported to give a significantly higher total yield of sCIs from its reaction with ozone relative to α-pinene or limonene . Our results then suggest that only a fraction of the total isoprene sCI yield reacts with SO 2 , with the remainder likely scavenged by water vapor, which once again is likely to occur rapidly even at a fairly low relative humidity of~15% as used in this study (Newland et al, 2018).…”

“…Numerous studies have suggested that the dominant fate of the smallest sCI (CH 2 OO) is reaction with water Chao et al, 2015;Newland et al, 2015), whereas larger sCIs, such as those produced by ozonolysis of monoterpenes, can be less reactive to water, in particular the syn conformers (Sheps et al, 2014;Taatjes et al, 2013). Newland et al (2018) recently studied the loss of SO 2 during ozonolysis of the same three monoterpenes studied in this work as a function of relative humidity. They found that as RH was increased, the SO 2 loss became less sensitive to [H 2 O], which they attributed to there being at least two chemically distinct sCIs produced from each monoterpene (i.e., anti and syn).…”

Particulate organic nitrates in eastern China: variation characteristics and effects of anthropogenic activities

“…More recently, the important contributions of monoterpene ozonolysis reactions to CIs formation and SO 2 oxidation have 315 been revealed by chamber and theoretical studies (Newland et al, 2018). Therefore, we hypothesized that MHN215 can be formed from pinene-derived CIs, because its precursor, pinene, has a double bond on the ring which is in a chemically similar environment to that of the double bond in limonene.…”