More Criegee Sightings: The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H ₂ CO ₂ , was recently detected and monitored in the laboratory. Su et al. (p. 174 ; see the Perspective by Vereecken ) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177 ; see the Perspective by Vereecken ) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms.

Criegee biradicals, i.e., carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF₃COCF₃), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH₂OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH₂I radicals, which react with O₂ to yield CH₂OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH₂OO and the formation of reaction products are monitored. The rate coefficient for CH₂OO + hexafluoroacetone is k₁ = (3.0 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k₂ = (9.5 ± 0.7) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, and with acetone, k₃ = (2.3 ± 0.3) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH₂OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck–Condon-factor calculations. No secondary ozonide was observable in the reaction of CH₂OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).

Carbonyl oxides (“Criegee intermediates”), formed in the ozonolysis of alkenes, are key species in tropospheric oxidation of organic molecules and their decomposition provides a non-photolytic source of OH in the atmosphere (Johnson and Marston, Chem. Soc. Rev., 2008, 37, 699, Harrison et al., Sci. Total Environ., 2006, 360, 5, Gäb et al., Nature, 1985, 316, 535, ). Recently it was shown that small Criegee intermediates, C.I.’s, react far more rapidly with SO₂ than typically represented in tropospheric models, (Welz, Science, 2012, 335, 204, ) which suggested that carbonyl oxides could have a substantial influence on the atmospheric oxidation of SO₂. Oxidation of SO₂ is the main atmospheric source of sulphuric acid (H₂SO₄), which is a critical contributor to aerosol formation, although questions remain about the fundamental nucleation mechanism (Sipilä et al., Science, 2010, 327, 1243, Metzger et al., Proc. Natl. Acad. Sci. U. S. A., 2010 107, 6646, Kirkby et al., Nature, 2011, 476, 429, ). Non-absorbing atmospheric aerosols, by scattering incoming solar radiation and acting as cloud condensation nuclei, have a cooling effect on climate (Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, Cambridge University Press, 2007, ). Here we explore the effect of the Criegees on atmospheric chemistry, and demonstrate that ozonolysis of alkenes via the reaction of Criegee intermediates potentially has a large impact on atmospheric sulphuric acid concentrations and consequently the first steps in aerosol production. Reactions of Criegee intermediates with SO₂ will compete with and in places dominate over the reaction of OH with SO₂ (the only other known gas-phase source of H₂SO₄) in many areas of the Earth's surface. In the case that the products of Criegee intermediate reactions predominantly result in H₂SO₄ formation, modelled particle nucleation rates can be substantially increased by the improved experimentally obtained estimates of the rate coefficients of Criegee intermediate reactions. Using both regional and global scale modelling, we show that this enhancement is likely to be highly variable spatially with local hot-spots in e.g. urban outflows. This conclusion is however contingent on a number of remaining uncertainties in Criegee intermediate chemistry.