Criegee chemistry captured

The chemistry of the atmosphere is complicated. Multitudes of chemical species undergo thousands of reactions, each with its own kinetic rate constant that must be known if the system is to be modeled. Among the key players in atmospheric systems is a class of unstable molecules called Criegee intermediates (CIs), the simplest of which is shown in the figure. Qualitative evidence for CIs’ atmospheric role is strong, but until recently, no one had ever detected a gas-phase CI directly, let alone measured its rates of reaction with other molecules. Atmospheric models have had to rely on indirect estimates of those rates. Now, Craig Taatjes and David Osborn (both at Sandia National Laboratories in Livermore, California) and their colleagues have performed direct kinetic measurements on the reactions between the simplest CI and four small atmospheric molecules: SO₂, NO, NO₂, and water. The key to their success was in how they made the CI: After much trial and error, they found that by reacting CH₂I with O₂, they formed the CI without too much excess energy or too many unwanted byproducts. For three of the four reactions they studied, they found rates that differed by orders of magnitude from previous estimates. The reactions with SO₂ and NO₂ were much faster than expected, and the reaction with NO was much slower—in fact, too slow for them to measure. (O. Welz et al., Science 335, 204, 2012.)—Johanna Miller