Criegee Intermediate–Alcohol Reactions, A Potential Source of Functionalized Hydroperoxides in the Atmosphere

Ozonolysis, the mechanism by which alkenes are oxidized by ozone in the atmosphere, produces a diverse family of oxidants known as Criegee intermediates (CIs). Using a combination of newly acquired laboratory data and global atmospheric chemistry and transport modelling, we find that the reaction of CIs with alcohols – a reaction that was originally employed to trap these reactive species and provide evidence for the ozonolysis mechanism nearly 70 years ago – is occurring in Earth’s atmosphere and may represent a sizeable source of functionalized hydroperoxides therein. Rate coefficients are reported for the reactions of CH₂OO and (CH₃)₂COO + methanol and that of CH₂OO + ethanol. Substitution about the Criegee intermediate is found to have a strong influence over the reaction rate, whereas substitution on the alcohol moiety does not. Although these reactions are not especially rapid, both the precursors to CIs and alcohols have large emissions from the terrestrial biosphere, leading to a high degree of collocation for this chemistry. We estimate that the products of
these reactions, the α-alkoxyalkyl hydroperoxides (AAAHs) have a production rate of ~30 Gg yr^-1. In order to assess the atmospheric lifetime of AAAHs, we used the nuclear ensemble method to construct a UV absorption spectrum from the four lowest energy conformers identified for a representative AAAH, methoxymethyl hydroperoxide. The computed absorption cross section
indicates that these compounds will be lost by solar photolysis, although not so rapidly as to exclude competition from other sinks such as oxidation, thermal decay and aerosol uptake.