Criegee Sighting: Standard mechanistic models for the reaction of ozone with unsaturated hydrocarbons implicate a transient carbonyl oxide compound, termed the “Criegee intermediate,” which has largely eluded detection. Welz et al. (p. 204 ; see the Perspective by Marston ) have now detected the compound by using mass spectrometry, following the low-pressure photolytic reaction of oxygen with diiodomethane, and measured its decay kinetics in the presence of nitric oxide, nitrogen dioxide, and sulfur dioxide. Reaction rates were higher than expected, suggesting that the intermediate may play a more prominent role in atmospheric chemistry than previously assumed.

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.

Eleven instruments for the measurement of ambient concentrations of atmospheric ammonia gas (NH₃), based on eight different measurement methods were inter-compared above an intensively managed agricultural field in late summer 2008 in Southern Scotland. To test the instruments over a wide range of concentrations, the field was fertilised with urea midway through the experiment, leading to an increase in the average concentration from 10 to 100 ppbv. The instruments deployed included three wet-chemistry systems, one with offline analysis (annular rotating batch denuder, RBD) and two with online-analysis (Annular Denuder sampling with online Analysis, AMANDA; AiRRmonia), two Quantum Cascade Laser Absorption Spectrometers (a large-cell dual system; DUAL-QCLAS, and a compact system; c-QCLAS), two photo-acoustic spectrometers (WaSul-Flux; Nitrolux-100), a Cavity Ring Down Spectrosmeter (CRDS), a Chemical Ionisation Mass Spectrometer (CIMS), an ion mobility spectrometer (IMS) and an Open-Path Fourier Transform Infra-Red (OP-FTIR) Spectrometer. The instruments were compared with each other and with the average concentration of all instruments. An overall good agreement of hourly average concentrations between the instruments (R²>0.84), was observed for NH₃ concentrations at the field of up to 120 ppbv with the slopes against the average ranging from 0.67 (DUAL-QCLAS) to 1.13 (AiRRmonia) with intercepts of −0.74 ppbv (RBD) to +2.69 ppbv (CIMS). More variability was found for performance for lower concentrations (<10 ppbv). Here the main factors affecting measurement precision are (a) the inlet design, (b) the state of inlet filters (where applicable), and (c) the quality of gas-phase standards (where applicable). By reference to the fast (1 Hz) instruments deployed during the study, it was possible to characterize the response times of the slower instruments.

Rate coefficients are directly determined for the reactions of the Criegee intermediates (CI) CH₂OO and CH₃CHOO with the two simplest carboxylic acids, formic acid (HCOOH) and acetic acid (CH₃COOH), employing two complementary techniques: multiplexed photoionization mass spectrometry and cavity‐enhanced broadband ultraviolet absorption spectroscopy. The measured rate coefficients are in excess of 1×10⁻¹⁰ cm³ s⁻¹, several orders of magnitude larger than those suggested from many previous alkene ozonolysis experiments and assumed in atmospheric modeling studies. These results suggest that the reaction with carboxylic acids is a substantially more important loss process for CIs than is presently assumed. Implementing these rate coefficients in global atmospheric models shows that reactions between CI and organic acids make a substantial contribution to removal of these acids in terrestrial equatorial areas and in other regions where high CI concentrations occur such as high northern latitudes, and implies that sources of acids in these areas are larger than previously recognized.

The chemistry of carbonyl oxides, known as Criegee intermediates, is central to many aspects of tropospheric chemistry. For decades it has been known that these reactive species, whose electronic structure contains zwitterionic and biradical character, are formed in the ozonolysis of alkenes. However it is only recently that direct measurements of their reaction kinetics have become possible. In this perspective we describe the most recent progress in understanding the reactivity of these historically elusive molecules, explore the atmospheric chemistry implications of new experimental discoveries, and propose important new areas for investigation.