Dudley E. Shallcross

Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2

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. An elusive intermediate implicated in atmospheric oxidation chemistry has been identified in the laboratory. Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via “Criegee intermediates”—carbonyl oxides—that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction of CH2I with O2. This reaction enabled direct laboratory determinations of CH2OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H2O. The CH2OO reactions with SO2 and NO2 proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.

Research frontiers in the chemistry of Criegee intermediates and tropospheric ozonolysis

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.

Development and application of a possible mechanism for the generation of cis-pinic acid from the ozonolysis of α- and β-pinene

Abstract Recent experimental studies have identified cis- pinic acid (a C 9 dicarboxylic acid) as a condensed-phase product of the ozonolysis of both α - and β -pinene, and it is currently believed to be the most likely degradation product leading to the prompt formation of new aerosols by nucleation. The observed timescale of aerosol formation appears to require that cis- pinic acid is a first-generation product, and a possible mechanism for its formation has therefore been developed. The key step in the proposed mechanism requires that the isomerisation of a complex C 9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO 2 and a C 8 organic radical: Thermodynamic and kinetic arguments are presented, on the basis of semi-empirical electronic structure calculations, which support this proposed mechanism, and thereby the competition between the two pathways. The transfer of the labile aldehydic H atom is shown to be especially facile in this case because it occurs though an unstrained transition state; this feature can in turn be attributed to the cis -substitution of the four-membered ring, which enforces the steric proximity of the acyl-oxy and aldehyde groups. The mechanism can explain the formation of cis -pinic acid from both α - and β -pinene, because the acyl-oxy radical is likely to be formed following the decomposition of excited Criegee biradicals formed in both systems. It is also possible that a similar isomerisation reaction of a complex C 10 α -carbonyl oxy radical by a 1,8 H atom shift might explain the very recently observed formation of cis -10-hydroxy-pinonic acid from α -pinene ozonolysis, and this possibility is also explored. An existing detailed scheme describing the degradation of α -pinene (part of the Master Chemical Mechanism, MCM) is updated to include the proposed cis- pinic acid and cis -10-hydroxy-pinonic acid formation mechanisms, and the values of several uncertain parameters are adjusted on the basis of reported yields of a series of organic products from the ozonolysis of α -pinene. The updated degradation scheme is incorporated into a boundary layer box model, and representative ambient concentrations of the organic acids and other oxygenated products are calculated for a range of representative conditions appropriate to the boundary layer over central Europe. The simulated concentrations of the organic acids in general, and cis -pinic acid in particular, are strongly dependent on the level of NO X , and suggest that new aerosol formation from the oxidation of α -pinene is likely to be more favoured at lower NO X levels.

Direct measurement of Criegee intermediate (CH2OO) reactions with acetone, acetaldehyde, and hexafluoroacetone

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 (CF(3)COCF(3)), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH(2)OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH(2)I radicals, which react with O(2) to yield CH(2)OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH(2)OO and the formation of reaction products are monitored. The rate coefficient for CH(2)OO + hexafluoroacetone is k(1) = (3.0 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1), supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k(2) = (9.5 ± 0.7) × 10(-13) cm(3) molecule(-1) s(-1), and with acetone, k(3) = (2.3 ± 0.3) × 10(-13) cm(3) molecule(-1) s(-1), are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH(2)OO 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 CH(2)OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).

Rate Coefficients of C1 and C2 Criegee Intermediate Reactions with Formic and Acetic Acid Near the Collision Limit: Direct Kinetics Measurements and Atmospheric Implications

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.

Modelling terrestrial biogenic isoprene fluxes and their potential impact on global chemical species using a coupled LSM–CTM model

Abstract In this paper we investigate the important role of the biogenic species isoprene on tropospheric chemistry using a land surface model (LSM) and a three-dimensional (3-D) tropospheric chemistry transport model (CTM). An efficient and conservative coupling scheme is used to couple the LSM to the 3-D CTM. Annual integrations of the coupled model have been performed and the results compared with other estimates. The comparison shows that the annual global isoprene flux from terrestrial vegetation is 530 Tg C yr −1 , which is in good agreement with 503 Tg C yr −1 estimated by a high-resolution (0.5°×0.5°) vegetation model of Guenther et al. (1995, Journal of Geophysical Research 100 (D5), 8873–8892). Comparison of the seasonal variations of the surface emission distribution between the coupled model and Guenther et al. (1995) also shows close agreement. The potential impact of isoprene on the levels of tropospheric species is studied by running the same coupled model for the period of June–December but without biogenic isoprene emissions included, and the results are compared with the run which includes biogenic isoprene emissions. Our comparison indicates a significant difference in O 3 and PAN for both hemispheres. The discrepancy between the run with and without isoprene is predominantly governed by the spatial and temporal variations of terrestrial vegetation. The largest difference is seen in the summertime northern hemisphere at locations with extensive terrestrial vegetation (e.g. North America, Europe, east and southeast Asia, South America and equatorial central Africa). For O 3 , there is about a 4 ppbv increase over the oceanic areas and about an 8–12 ppbv increase over the mid-latitude land areas. For PAN, a maximum of about one order of magnitude in difference, which increases from 0.01 ppbv (without isoprene emissions) to 0.1–0.3 ppbv (with isoprene emissions), is seen in areas of extensive terrestrial vegetation.

Carbonaceous aerosols and their potential role in Atmospheric Chemistry

This paper considers the nature of carbonaceous surfaces, the means by which they are activated, the nature of some functional groups that they support, and some reaction mechanisms that may be involved. Because of the strong affinity of carbonaceous surfaces for organic species and because of the ease with which compounds in a high oxidation state can oxidize the carbonaceous surface, it is highly likely that carbonaceous aerosols are interacting chemically with a range of organic species in ways that have, as yet, not been fully characterized but may significantly affect the oxidizing capacity of our atmosphere. If HONO is formed on the surface of carbonaceous aerosols then this could be a significant source of HOz as HONO is readily photolyzed to give OH, and it could explain the large values of HONO often observed in the troposphere. In general, the reduction of NOon carbonaceous aerosols is an important consideration, and it is addressed here.

UV–VIS absorption cross-sections and atmospheric lifetimes of CH2Br2, CH2I2 and CH2BrI

The UV–VIS absorption spectra of CH2Br2, CH2I2 and CH2BrI have been measured over the wavelength range 215–390 nm using a dual-beam diode array spectrometer. The spectra consist of broad continuous absorption bands. CH2Br2 exhibits its maximum cross-section of σ=2.71(±0.16)×10-18 cm2 molecule-1 at λ=219 nm. The magnitude of the peak cross-sections for the iodine-containing molecules above λ=210 nm are σ=1.62(±0.10)×10-18 cm2 molecule-1 at λ=248 nm and σ=3.78(±0.23)×10-18 cm2 molecule-1 at λ=288 nm for CH2I2, and σ=5.67(±0.34)×10-18 cm2 molecule-1 at λ=215 nm and σ=2.34(±0.14)×10-18 cm2 molecule-1 at λ=267 nm for CH2BrI. The temperature dependence of the absorption cross-sections was investigated over the temperature range 348–250 K. A decline in the cross-sections with decreasing temperature was observed in the tail of the spectra. At the peaks the opposite effect was observed. All three gases have been found in the atmosphere and the atmospheric photolysis rates of CH2Br2, CH2I2 and CH2BrI were calculated as a function of altitude and solar zenith angle using the measured cross-sections. Model calculations show that, during sunlit hours, CH2I2 and CH2BrI will be photolysed within minutes and hours, respectively. The photolysis of CH2Br2 is much slower and reaction with the OH radical is the dominant atmospheric loss process.

UV absorption cross-sections and atmosphericphotolysis rates of CF3I, CH3I,C2H5I andCH2ICl

The absorption cross-sections of trifluoromethyl iodide (CF 3 I), methyl iodide (CH 3 I), ethyl iodide (C 2 H 5 I) and chloroiodomethane (CH 2 ICl) have been determined over the wavelength range 235–400 nm, with a spectral resolution of 0.6 nm (FWHM), using a diode array spectrometer. The spectra consist of a broad continuous absorption band with maximum cross-sections of (6.0±0.1)×10 -19 cm 2 molecule -1 for CF 3 I at 267 nm, (1.09±0.02)×10 -18 cm 2 molecule -1 for CH 3 I at 258 nm, (1.18±0.04)×10 -18 cm 2 molecule -1 for C 2 H 5 I at 258 nm and (1.21±0.07)×10 -18 for CH 2 ICl at 270 nm. The temperature dependence of the cross-section was investigated over the range 333–243 K. A decline in the cross-section with decreasing temperature at wavelengths longer than ca. 280 nm was observed in all cases, the decrease being most pronounced in the long wavelength tail of the absorption band. At wavelengths shorter than ca. 270 nm the cross-section increased with decreasing temperatures; the effect, however, was significantly smaller (5–10%), being most pronounced around the band maximum. The temperature dependence was parametrised in order to calculate the atmospheric photolysis rate as a function of altitude, latitude and season. Model calculations show that during sunlit hours the iodides will be rapidly photolysed with tropospheric photodissociation lifetimes of ca. 1 day for CF 3 I, several days for CH 3 I and C 2 H 5 I and several hours for CH 2 ICl.

Central role of carbonyl compounds in atmospheric chemistry

With the exception of acetone it is not generally recognized how important atmospheric carbonyls and alkyl radicals are in the lower stratosphere and upper troposphere. Carbonyl compounds are the crucial intermediate species for the autocatalytic production of OH. For example, in the upper troposphere and lower stratosphere it is calculated based on data assimilation analysis of Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) data that CH3 production due to the degradation of carbonyls contributes around 40% to the overall production of CH3, a key initiation step for HOx production, with the contribution due to the photolysis of CH3CHO being comparable to that of acetone. So correctly modeling the alkyl radical concentrations is of central importance and has not be given the attention it deserves to date. The reactions of carbonyls with Br and Cl are also major sources of HBr and HCl. In short, carbonyl compounds play a central role in atmospheric chemistry close to the tropopause, and this is directly relevant to issues such as the assessment of the impact of air traffic, and ozone depletion.