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Paul Crutzen received his doctorate in meteorology from the University of Stockholm in 1968 and was awarded the Nobel Prize in Chemistry in 1995. In addition to chemistry and atmospheric science, however, the breadth of his accomplishments has also been recognized by biologists, Earth system scientists, and geologists. This tribute provides some insight into Crutzen’s career and how it contributed to so many scientific disciplines. In addition, we offer a road map showing how these diverse contributions were woven together over the course of more than five decades of research. The citation for the 1995 Nobel Prize reads that it was given for “work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone.” The inclusion of the wording “formation … of ozone” applies only to him among the three laureates (Crutzen, Mario Molina, and F. Sherwood Rowland). His research on tropospheric chemistry led to seminal studies of tropical biomass burning, which eventually evolved into the concept later known as “nuclear winter,” a topic in the forefront of far-ranging popular discussions in the 1980s. Last, Crutzen’s proposal for the emergence of the “Anthropocene” as a new geological epoch that would terminate the 11,700-yr-old Holocene is considered by the Earth system science community to be the most pronounced trademark of his remarkable career. Crutzen also received American Meteorological Society’s Battan Award for his coauthorship of Atmosphere, Climate, and Change, recognized by the organization as the best book for general audiences. In the later years of his career, as a member of the Pontifical Academy of Sciences, Crutzen was a key player in the formulation of Laudato Si’, Pope Francis’s encyclical on climate change, which was released in advance of the Conference of Parties (COP 21) meeting that announced the formulation of the Paris Climate Accords in 2015.

The “science-softCon UV/Vis+ Photochemistry Database” (www.photochemistry.org) is a large and comprehensive collection of EUV-VUV-UV–Vis-NIR spectral data and other photochemical information assembled from published peer-reviewed papers. The database contains photochemical data including absorption, fluorescence, photoelectron, and circular and linear dichroism spectra, as well as quantum yields and photolysis related data that are critically needed in many scientific disciplines. This manuscript gives an outline regarding the structure and content of the “science-softCon UV/Vis+ Photochemistry Database”. The accurate and reliable molecular level information provided in this database is fundamental in nature and helps in proceeding further to understand photon, electron and ion induced chemistry of molecules of interest not only in spectroscopy, astrochemistry, astrophysics, Earth and planetary sciences, environmental chemistry, plasma physics, combustion chemistry but also in applied fields such as medical diagnostics, pharmaceutical sciences, biochemistry, agriculture, and catalysis. In order to illustrate this, we illustrate the use of the UV/Vis+ Photochemistry Database in four different fields of scientific endeavor., Journal of Quantitative Spectroscopy and Radiative Transfer, 253, ISSN:0022-4073, ISSN:1879-1352

The hydrocarbon decay method (Rivett et al., 2003) has been used to analyse hydrocarbon data from four contrasting sites in the United Kingdom to estimate night-time levels of NO3 and OH. Remarkably consistent results emerge using alkenes, revealing NO3 and OH levels in the range of 0.01–10 ppt and 1 × 104 − 1 × 106 molecule cm−3, respectively. Weak seasonal cycles are observed where NO3 levels peak in spring and OH in summer. Analysis using alkanes suggests that Cl atom levels of around 2 × 104 molecule cm−3 may be present. How Cl may be formed at night in such high quantities is unknown and may not be the answer to the disparity between the two methods. Copyright © 2008 Royal Meteorological Society

Experimental difficulties sometimes force modellers to use predicted rate coefficients for reactions of oxygenated volatile organic compounds (oVOCs). We examine here methods for making the predictions for reactions of atmospheric initiators of oxidation, NO3, OH, O3 and O(3P), with unsaturated alcohols and ethers. Logarithmic correlations are found between measured rate coefficients and calculated orbital energies, and these correlations may be used directly to estimate rate coefficients for compounds where measurements have not been performed. To provide a shortcut that obviates the need to calculate orbital energies, structure–activity relations (SARs) are developed. Our SARs are tested for predictive power against compounds for which experimental rate coefficients exist, and their accuracy is discussed. Estimated atmospheric lifetimes for oVOCs are presented. The SARs for alkenols successfully predict key rate coefficients, and thus can be used to enhance the scope of atmospheric models incorporating detailed chemistry. SARs for the ethers have more limited applicability, but can still be useful in improving tropospheric models.

Gas-phase rate coefficients for the atmospherically important reactions of NO3, OH and O3 are predicted for 55 α,β-unsaturated esters and ketones. The rate coefficients were calculated using a correlation described previously [Pfrang, C., King, M.D., C. E. Canosa-Mas, C.E., Wayne, R.P., 2006. Atmospheric Environment 40, 1170–1179]. These rate coefficients were used to extend structure–activity relations for predicting the rate coefficients for the reactions of NO3, OH or O3 with alkenes to include α,β-unsaturated esters and ketones. Conjugation of an alkene with an α,β-keto or α,β-ester group will reduce the value of a rate coefficient by a factor of ∼110, ∼2.5 and ∼12 for reaction with NO3, OH or O3, respectively. The actual identity of the alkyl group, R, in −C(O)R or −C(O)OR has only a small influence. An assessment of the reliability of the SAR is given that demonstrates that it is useful for reactions involving NO3 and OH, but less valuable for those of O3 or peroxy nitrate esters.

The gas-pliase rate coefficient for the reaction between OH radicals and CH3SCH2Cl (MCDMS) was determined to be (2.5 +/- 1.3) x 10(-12) cm(3) molecule(-1) s(-1) using the discharge-flow kinetic technique. An estimate of approximate to 10(-10)cm(3) molecule(-1) s(-1) was obtained for the rate coefficient for reaction of Cl with MCDMS. It would appear that the reaction with OH is not the main loss process for CH3SCH2Cl in the marine boundary layer. The possible implications for the MBL of halogen-promoted oxidation of dimethylsulphide are considered.

The kinetics of the reactions of the atoms O(P-3), S(P-3), Se(P-3), and Te((3)p) with a series of alkenes are examined for correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes. These correlations may be employed to predict rate coefficients from the calculated HOMO energy of any other alkene of interest. The rate coefficients obtained from the correlations were used to formulate structure-activity relations (SARs) for reactions of O((3)p), S(P-3), Se (P-3), and Te((3)p) with alkenes. A comparison of the values predicted by both the correlations and the SARs with experimental data where they exist allowed us to assess the reliability of our method. We demonstrate the applicability of perturbation frontier molecular orbital theory to gas-phase reactions of these atoms with alkenes. The correlations are apparently not applicable to reactions of C(P-3), Si(P-3), N(S-4), and Al(P-2) atoms with alkenes, a conclusion that could be explained in terms of a different mechanism for reaction of these atoms.

We present updated structure–activity relations (SARs) for the prediction of rate coefficients for gas-phase reactions with alkenes of the major atmospheric oxidants NO 3 , OH and O 3 . Such SARs provide one way of incorporating essential information about reactivity into atmospheric models. Rate coefficients obtained from correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes were used to refine the SARs. SARs have an advantage for the user over the direct application of the correlations in that knowledge of the structure of the alkene of interest is sufficient to estimate rate coefficients, and no quantum-mechanical calculations need to be performed. A comparison of the values predicted by the SARs with experimental data where they exist allowed us to assess the reliability of our method.

Methods are developed for predicting rate coefficients for reactions of initiators of tropospheric oxidation with unsaturated compounds that are abundant in the atmosphere; prognostic tools of this kind are essential for atmospheric chemists and modellers. To pursue the aim of exploring such tools, the kinetics of reactions of NO3, OH and O3 with a series of alkenes are examined for correlations relating the logarithms of the rate coefficients to the energies of the highest occupied molecular orbitals (HOMOs) of the alkenes. A comparison of the values predicted by the correlations with experimental data (where the latter exist) allowed us to assess the reliability of our method. We used a series of theoretical methods to calculate the HOMO energies, and found that higher computational effort improves the agreement of the predicted rate coefficients with experimental values, especially for reactions of NO3 with alkenes that possess vinyllic halogen substituents. As a consequence, it is expedient to suggest new correlations to replace those presented by us and others that were based on the lower level of theory. We propose the following correlations for the reactions of NO3, OH and O3 with alkenes: ln( k NO 3 /cm3 molecule−1 s−1)=6.40(EHOMO/eV)+31.69, ln(kOH/cm3 molecule−1 s−1)=1.21(EHOMO/eV)−12.34 and ln( k O 3 /cm3 molecule−1 s−1)=3.28(EHOMO/eV)−6.78. These new correlations have been developed using the larger experimental data sets now available, and the impact of the extended data on the quality of the correlations is examined in the paper. Atmospheric lifetimes have been calculated from both experimental and estimated rate coefficients to provide an overview of removal efficiencies for different classes of alkenes with respect to oxidative processes initiated by NO3, OH and O3. A figure is presented to show the spatial scales over which alkenes may survive transport in competition with attack by NO3, OH and O3. Removal by NO3 or OH is always more important than removal by O3, and reactions with NO3 dominate for scales up to a few hundred metres.

Rate coefficients for reactions of nitrate radicals (NO3) with the anthropogenic emissions 2-methylpent-2-ene, (Z)-3-methylpent-2-ene, ethyl vinyl ether, and the stress-induced plant emission ethyl vinyl ketone (pent-1-en-3-one) were determined to be (9.3±1.1)×10−12, (9.3±3.2)×10−12, (1.7±1.3)×10−12 and (9.4±2.7)×10−17 cm3 molecule−1 s−1. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC–FID analysis. Experiments with ethyl vinyl ether required a modification of our established procedure that might introduce additional uncertainties, and the errors suggested reflect these difficulties. Rate coefficients are discussed in terms of electronic and steric influences. Atmospheric lifetimes with respect to important oxidants in the troposphere were calculated. NO3-initiated oxidation is found to be the strongly dominating degradation route for 2-methylpent-2-ene, (Z)-3-methylpent-2-ene and ethyl vinyl ether. Atmospheric concentrations of the alkenes and their relative contribution to the total NMHC emissions from trucks can be expected to increase if plans for the introduction of particle filters for diesel engines are implemented on a global scale. Thus more kinetic data are required to better evaluate the impact of these emissions.

The relative-rate technique has been used to obtain rate coefficients for the reaction between the NO3 radical and α,β-unsaturated esters and ketones: methyl E-2-butenoate (k1), methyl 3-methyl-2-butenoate (k2), methyl E-2-methyl-2-butenoate (k3), 3-methyl-3-buten-2-one (k4), E-3-penten-2-one (k5), 4-methyl-3-penten-2-one (k6), and E-3-methyl-3-penten-2-one (k7). The rate constants obtained by the relative-rate method at T = 298 ± 2 K and P = 760 ± 5 Torr (N2 as bath gas) are: k1 = (1.85 ± 0.56) × 10−15 cm3 molecule−1 s−1, k2 = (1.41 ± 0.23) × 10−14 cm3 molecule−1 s−1, k3 = (4.91 ± 1.15) × 10−14 cm3 molecule−1 s−1, k4 = (8.27 ± 6.44) × 10−15 cm3 molecule−1 s−1, k5 = (1.03 ± 0.31) × 10−14 cm3 molecule−1 s−1, k6 = (1.44 ± 0.26) × 10−13 cm3 molecule−1 s−1 and k7 = (1.55 ± 0.20) × 10−13 cm3 molecule−1 s−1. The rate constants are used to show the effect of increased substitution at the carbon–carbon double bond. Group-reactivity factors for the ketone and ester groups are also determined. Atmospheric lifetimes of the compounds against night-time attack by NO3 are estimated.

The absorption cross-sections of Cl2O6 and Cl2O4 have been obtained using a fast flow reactor with a diode array spectrometer (DAS) detection system. The absorption cross-sections at the wavelengths of maximum absorption (lambda(max)) determined in this study are those of Cl2O6: (1.47 +/- 0.15) x 10(-17) cm(2) molecule(-1), at lambda(max) = 276 nm and T = 298 K; and Cl2O4: (9.0 +/- 2.0) x 10(-19) cm(2) molecule(-1), at lambda(max) = 234 nm and T = 298 K. Errors quoted are two standard deviations together with estimates of the systematic error. The shapes of the absorption spectra were obtained over the wavelength range 200-450 nm for Cl2O6 and 200-350 nm for Cl2O4, and were normalized to the absolute cross-sections obtained at lambda(max) for each oxide, and are presented at 1 nm intervals. These data are discussed in relation to previous measurements. The reaction of O with OCIO has been investigated with the objective of observing transient spectroscopic absorptions. A transient absorption was seen, and the possibility is explored of identifying the species with the elusive sym-ClO3 or ClO4, both of which have been characterized in matrices, but not in the gas-phase. The photolysis of OCIO was also re-examined, with emphasis being placed on the products of reaction. UV absorptions attributable to one of the isomers of the ClO dimer, chloryl chloride (ClClO2) were observed; some Cl2O4 was also found at long photolysis times, when much of the ClClO2 had itself been photolysed. We suggest that reports of Cl2O6 formation in previous studies could be a consequence of a mistaken identification. At low temperatures, the photolysis of OCIO leads to the formation of Cl2O3 as a result of the addition of the ClO primary product to OCIO. ClClO2 also appears to be one product of the reaction between O-3 and OCIO, especially when the reaction occurs under explosive conditions. We studied the kinetics of the non-explosive process using a stopped-flow technique, and suggest a value for the room-temperature rate coefficient of (4.6 +/- 0.9) x 10(-19) cm(3) molecule(-1) s(-1) (limit quoted is 2sigma random errors). The photochemical and thermal decomposition of Cl2O6 is described in this paper. For photolysis at k = 254 nm, the removal of Cl2O6 is not accompanied by the build up of any other strong absorber. The implications of the results are either that the photolysis of Cl2O6 produces Cl-2 directly, or that the initial photofragments are converted rapidly to Cl-2. In the thermal decomposition of Cl2O6, Cl2O4 was shown to be a product of reaction, although not necessarily the major one. The kinetics of decomposition were investigated using the stopped-flow technique. At relatively high [OCIO] present in the system, the decay kinetics obeyed a first-order law, with a limiting first-order rate coefficient of 0.002 s(-1). (C) 2004 Elsevier B.V. All rights reserved.

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO2) and iodine monoxide (IO) radicals using a violet laser diode at lambda = 404.278 nm. A noise-equivalent (1sigma = root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5x10(-8) was demonstrated with a mirror reflectivity of similar to0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8x10(10) molecule cm(-3) and towards the IO radical of 3.3x10(9) molecule cm(-3) were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO2 + NO --> RO + NO2). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO2 in the reaction NO2 + O --> NO + O-2. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3x10(10) molecule cm(-3) in the discharge-flow-tube experiments.

The atmospheric fate of biogenic species such as the alkyl iodides is controlled mainly by photolysis, and by oxidation initiated by attack of Cl atoms and/or OH radicals. In order to assess the contribution made by OH attack on the simpler alkyl iodides, the absolute second-order rate coefficients for the reactions (1) OH + CH 3 I → products, (2) OH + C 2 H 5 I → products, (3) OH + 1-C 3 H 7 I → products, (4) OH + 2-C 3 H 7 I → products. were determined. These reactions were studied at P=1.5 and 5 Torr and at room temperature (298±2 K) using a fast-flow discharge system with resonance-fluorescence detection of OH radicals. The absolute second-order rate coefficients are k1=(0.99±0.20)×10−13 cm3 molecule−1 s−1, k2=(7.7±1.0)×10−13 cm3 molecule−1 s−1, k3=(25.0±3.0)×10−13 cm3 molecule−1 s−1and k4=(16.0±2.0)×10−13 cm3 molecule−1 s−1. The reactions were pressure independent over the range measured. The major atmospheric global degradation pathway is photolysis, with OH-initiated oxidation gaining importance for the propyl iodides. The effects of variable [Cl] on a local scale are discussed.

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10-13 cm3 molecule-1 s-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×109 molecule cm-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the RR(15) absorption of the b1Σg+-X3Σg- (1,0) band of molecular oxygen at \(\overline{\nu}\)=14553.947 cm-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10-5, 6.9×10-7 and 1.9×10-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz.

Relative-rate and absolute techniques have been used to obtain rate coefficients for the reactions between the NO3 radical and 1,4-pentadiene (k1), Z-1,3-pentadiene (k2) and E-1,3-pentadiene (k3). The rate coefficients obtained by the relative-rate method at T = 300 ± 1 K and P = 759 ± 7 Torr are k1 = (2.3 ± 0.3) × 10−14 cm3 molecule−1 s−1, k2 = (2.0 ± 0.3) × 10−12 cm3 molecule−1 s−1 and k3 = (2.4 ± 0.3) × 10−12 cm3 molecule−1 s−1. The rate coefficients determined by the discharge–flow technique at low pressure (P = 1.8–4.5 Torr) and at T = 300 ± 3 K are k1 = (2.3 ± 0.6) × 10−14 cm3 molecule−1 s−1, k2 = (1.4 ± 0.4) × 10−12 cm3 molecule−1 s−1 and k3 = (1.6 ± 0.4) × 10−12 cm3 molecule−1 s−1. The authors suggest the use of the values k1 = (2.3 ± 0.5) × 10−14 cm3 molecule−1 s−1, k2 = (1.7 ± 0.4) × 10−12 cm3 molecule−1 s−1 and k3 = (2.0 ± 0.4) × 10−12 cm3 molecule−1 s−1 for atmospheric calculations.

A discharge-flow tube equipped with resonance-fluorescence detection for I atoms and chemiluminescence detection for O atoms has been used to investigate the kinetics of the reaction IO + IO → products (1) at T = 296 ± 1 K and P = 1.9–2.2 Torr. The rate constant was found to be k1 = (9.3 ± 1.9) × 10−11 cm3 molecule−1 s−1, where −d[IO]/dt = 2k1[IO]2 and the errors are 95% confidence limits of the mean of the experimental rate constants combined with an estimated further 20% error in other parameters. The determination of the rate constant for reaction (1) also required the investigation of the kinetics of the reaction IO + CF3 → products (2). The rate constant was found to be k2 = (1.6 ± 0.7) × 10−11 cm3 molecule−1 s−1, where errors are 95% confidence limits of the slope of the regression line of a second-order plot, combined with systematic errors of 20% for the calibration of [CF3]. I atoms are generated in reactions (1) and (2)IO + IO → 2I + O2→OIO + ICF3 + IO → CF3O + I The yields have been quantified, and indicate that k1a/k1 = 0.56 ± 0.20, with k1b/k1 = (1 − k1a/k1); and k2a/k2 = 0.4 ± 0.1. Our experiments differ from others previously reported in that the results do not rely on a value for the cross section for absorption of IO in the visible region. Atmospheric modelling studies show that the IO self-reaction plays a minor role in ozone depletion in the marine troposphere and that further work is required to quantify the fate of OIO, a product of this reaction.

Methyl salicylate is one of a number of semiochemicals, signal molecules, emitted by herbivore-infested plants. These signal molecules attract predators of the herbivore, and the chemicals thus act indirectly as part of the defence mechanism of the plant. Previous studies have shown that ozone damage to plants can also elicit the emission of signal molecules. The fate of these signal molecules in the atmosphere is not known. Preliminary studies have been undertaken to examine the atmospheric chemistry of methyl salicylate for the first time. Rate coefficients for the reaction of methyl salicylate with atomic chlorine and with ozone have been determined; the values are (2.8±0.3)×10−12 and ∼4×10−21 cm3 molecule−1 s−1. These results suggest that neither reaction with atomic chlorine nor reaction with ozone will provide important loss routes for methyl salicylate in the atmosphere. The possible importance of photolysis of methyl salicylate in the atmosphere is considered.

A low-pressure discharge-flow system employing LIF detection for CH3O, and resonance-fluorescence detection for Cl, Br and I atoms, was used to investigate the kinetics of the reactions of CH3O with X and XO (where X = Cl, Br and I) at T = 298 K. The rate constants obtained are kCl = (7.6 ± 1.4) × 10−11 cm3 molecule−1 s−1, kBr = (5.3 ± 0.9) × 10−11 cm3 molecule−1 s−1, kI = (8.5 ± 2.3) × 10−11 cm3 molecule−1 s−1, kClO = (3.4 ± 0.7) × 10−11 cm3 molecule−1 s−1, kBrO = (5.5 ± 1.0) × 10−11 cm3 molecule−1 s−1 and kIO = (4 ± 2) × 10−11 cm3 molecule−1 s−1. It is likely that the major channel in the reaction of CH3O with X yields HX and CH2O, while, in the reaction with XO, the main products are HOX and CH2O.

This paper presents correlations between the logarithm of the rate constants of the reaction between RO2+NO and RO2+HO2 (and possibly RO2+CH3O2) and the singly occupied molecular orbital (SOMO) energy of the organic peroxy radical, RO2. These reactions are of the utmost importance in the tropospheric oxidation cycle of organic compounds. A correlation such as this may allow the prediction of rate constants for the reactions of organic peroxy radicals, RO2, with species such as NO, HO2 and CH3O2 where no previous experimental determination has been undertaken. The mathematical form of the correlation for the reactions of RO2 with NO and HO2 is −ln k (cm 3 molecule −1 s −1 )=mE SOMO (RO 2 )+c , where m=1.14 and −1.45 eV−1, c=28.80 and 21.16 for NO and HO2, respectively; ESOMO(RO2) is the SOMO energy of the organic RO2 radical in eV. The origin of the correlation is explained in terms of frontier orbital theory. Six previously unmeasured rate constants are predicted for reactions between RO2 and NO, and 20 rate constants are predicted for the reactions between RO2 and HO2. This appears to be the first time that such a correlation has been applied to a series of radical–radical reactions, and the method holds promise for further chemical systems.

This paper describes a study of the products of the Cl-atom-initiated oxidation of three alkyl iodides, RI=CH3I, C2H5I, and 2-C3H7I, carried out in synthetic air at atmospheric pressure and at room temperature. Fourier-transform infrared spectroscopy was used to follow the decay of reactants and subsequent formation of products. The primary step proceeds via two channels, one of which yields HCl and an iodinated alkyl radical, and the other I atoms and an alkyl chloride. Quantitative analysis of the product yields, together with an assessment of the formation of HCl in secondary processes, allowed the fractional branching into the two channels to be calculated. The channel yielding HCl from RI constitutes a fraction 0.59, 0.93, and 0.68 for R=CH3, C2H5, and 2-C3H7. The iodinated alkyl radical forms first a peroxy, and then an alkoxy, radical in the presence of air. The final products CH2O, CH3CHO, and CH3COCH3 were observed as expected for the decomposition of these radicals with RI=CH3I, C2H5I, and 2-C3H7I, and the fractions of the alkoxy radicals fragmenting to the carbonyl compounds were 0.88, 0.57, and 0.86, respectively. Atomic iodine is formed concomitantly with the carbonyl species, so that these fractions also indicate the yield of I atoms in the secondary process. Alternative reaction pathways for the iodinated alkoxy radicals, in particular reaction with O2, are evaluated and discussed. The yields of I atoms in the primary and secondary steps, taken in combination with kinetic data, make it possible to estimate the contribution of the Cl-initiated oxidation of the alkyl halides to I-atom production in the atmosphere (and, making certain assumptions, the analogous contribution from OH-initiated oxidation). Radical-initiated processes might augment the photolytic yield of I atoms from simple alkyl iodides: the maximum enhancements lie between 5% (CH3I) and more than 30% (2-C3H7I).

The kinetics of the reactions were studied at T = 298 K using a fast-flow discharge system with resonance-fluorescence detection of Cl atoms. The rate coefficients were found to be pressure-independent over the range 1.5–12 Torr, and were k1 = (1.51 ± 0.15) × 10−12 cm3 molecule−1 s−1, k2 = (16.0 ± 1.7) × 10−12 cm3 molecule−1 s−1, k3 = (66.5 ± 7.5) × 10−12 cm3 molecule−1 s−1, k4 = (46.8 ± 4.9) × 10−12 cm3 molecule−1 s−1 and k5 = (0.85 ± 0.09) × 10−12 cm3 molecule−1 s−1. The rates of reaction of these alkyl iodides with Cl atoms in the marine boundary layer were compared with the rates of their photolysis and their reaction with OH. For early morning conditions, it is shown that reaction with Cl atoms can compete with photolysis and dominate over reaction with OH, if Cl atom concentrations are about 1 × 105 molecule cm−3. Atomic iodine may be released following reaction of Cl with the alkyl iodides; this atomic I can participate in catalytic destruction of ozone in the marine boundary layer.

An investigation into the reactions between Cl atoms and acrolein (1), methacrolein (2) and methyl vinyl ketone (3) is presented. Values of the rate constants for the reactions have been determined by an absolute technique for the first time. At a pressure of 1.6 Torr, the rate constants obtained were: k1 = 1.1 ± 0.2, k2 = 3.3 ± 0.6 and k3 = 0.99 ± 0.20 in units of 10−10 cm3 molecule−1 s−1. k1 was also determined at atmospheric pressure using a relative-rate technique. The rate constant obtained was (2.2 ± 0.3) × 10−10 cm3 molecule−1 s−1; the larger value compared with that for 1.6 Torr is thought to reflect a true dependence of the reaction rate on pressure. The final products of the reactions performed under an atmosphere of synthetic air were investigated using FTIR spectroscopy. The only chlorinated organic species identified as products of the reactions were chloroacetaldehyde in the case of reaction of Cl atoms with acrolein; chloroacetone with methacrolein; and chloroacetaldehyde with methyl vinyl ketone. Branching ratios for abstraction (the fraction of reactions occurring by abstraction relative to the sum of addition and abstraction processes) were found to be 0.22 ± 0.13 for acrolein, 0.18 ± 0.02 for methacrolein and

Absolute and relative-rate techniques have been used to obtain rate coefficients for the reactions: NO3+CH3C(O)CHCH2→products (1), NO3+CH2C(CH3)CHO→products (2), NO3+CH2CHCHO→ products (3), and NO3+CH2CHC(O)OCH3→products (4). The reaction NO3+CH2C(CH3)C(O)OCH3→ products (5), has been investigated by a relative-rate method only. The rate coefficients obtained by the relative-rate method at T=296±2 K and P=760 Torr are k1=(4.7±1.7)×10-16 cm3 molecule-1 s-1, k2=(3.7±1.0)×10-15 cm3 molecule-1 s-1, k3=(1.1±0.4)×10-15 cm3 molecule-1 s-1, k4=(1.0±0.6)×10-16 cm3 molecule-1 s-1 and k5=(3.6±1.3)×10-15 cm3 molecule-1 s-1. The rate coefficients determined by the discharge-flow technique at low pressure (P=1–10 Torr) and at T=293–303 K are k1=(3.2±0.6)×10-16 cm3 molecule-1 s-1, k2=(9.6±2.0)×10-15 cm3 molecule-1 s-1, k3=(8.9±2.8)×10-15 cm3 molecule-1 s-1, k4=(1.9±0.4)×10-16 cm3 molecule-1 s-1. The discrepancy between the values obtained from the relative-rate technique and the absolute technique are discussed and explained in terms of interference in the absolute study caused by secondary chemistry and fast-reacting impurities. Product studies reveal that methyl glyoxal is a product of reactions (1) and (2) along with peroxymethacryloyl nitrate (MPAN) for reaction (2) in air. A diurnally varying boundary-layer model suggests that reaction (2) is an important loss process for methacrolein and that it can lead to the generation of OH at night.

A discharge fast-flow tube equipped with laser-induced fluorescence detection of NO2, produced from the reaction of CF3O2 with NO, and resonance fluorescence detection for Cl and I atoms, has been employed to determine the rate constants for the reactions CF3O2+Cl→products (1) and CF3O2+I→products (2), at T=296±1 K and P=ca. 2 Torr. The rate constants (in cm3 molecule-1 s-1) have been found to be k1=(4.2±0.8)×10-11 and k2=(3.7±0.8)×10-11. Experimental evidence suggests that ClO is a major product of reaction (1), whereas IO is not produced in reaction (2). A brief discussion of the likely pathways of the reactions is presented.

A low-pressure discharge-flow system equipped with laser-induced fluorescence (LIF) detection of NO2 and resonance-fluorescence detection of OH has been employed to study the self reactions CH2ClO2 + CH2ClO2 products (1) and CHCl2O2 + CHCl2O2 products (2), at T = 298 K and P = 1–3 Torr. Possible secondary reactions involving alkoxy radicals are identified. We report the phenomenological rate constants (kobs) k1obs = (4.1 ± 0.2) × 10−12 cm3 molecule−1 s−1k2obs = (8.6 ± 0.2) × 10−12 cm3 molecule−1 s−1 and the rate constants derived from modelling the decay profiles for both peroxy radical systems, which takes into account the proposed secondary chemistry involving alkoxy radicals k1 = (3.3 ± 0.7) × 10−12 cm3 molecule−1 s−1k2 = (7.0 ± 1.8) × 10−12 cm3 molecule−1 s−1 A possible mechanism for these self reactions is proposed and QRRK calculations are performed for reactions (1), (2) and the self-reaction of CH3O2, CH3O2 + CH3O2 products (3). These calculations, although only semiquantitative, go some way to explaining why both k1 and k2 are a factor of ten larger than k3 and why, as suggested by the products of reaction (1) and (2), it seems that the favored reaction pathway is different from that followed by reaction (3). The atmospheric fate of the chlorinated peroxy species, and hence the impact of their precursors (CH3Cl and CH2Cl2), in the troposphere are briefly discussed. HC(O)Cl is identified as a potentially important reservoir species produced from the photooxidation of these precursors. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 433–444, 1999

A discharge-flow tube coupled with resonance fluorescence and chemiluminescence detection has been used to investigate the reactions IO + HO2 → products (1) and IO + O(3P) → I + O2(2), at T = 296 ± 1 K and P = 1.7 - 2 Torr. The rate constants k-1 and k2 have been found to be (7.1 ± 1.6) × 10-11 cm3 molecule-1 s-1 and (1.35 ± 0.15) × 10-10 cm3 molecule-1 s-1, respectively.

Rate coefficients for the reactions between the NO3 radical and pent-1-ene (k1), hex-1-ene (k2) and hept-1-ene (k3) have been determined at T=296±1 K and P=759±7 Torr by a relative-rate method using the reaction between the NO3 radical and but-1-ene (k4) as the reference reaction. The values of the rate constants measured were k1=(1.51±0.54)×10-14 cm3 molecule-1 s-1, k2=(1.85±0.66)×10-14 cm3 molecule-1 s-1, k3=(2.06±0.62)×10-14 cm3 molecule-1 s-1 and the value of the reference rate constant was k4=(1.35±0.41)×10-14 cm3 molecule-1 s-1.

Two types of correlation relating the value of the energy of the highest occupied molecular orbital (HOMO) of an alkene to the logarithm of its rate constant for reaction with NO3, OH or O3 have been formulated. Both correlations have been shown to be consistent with frontier molecular orbital theory. The correlation can be used to predict the rate constants for the reaction of an alkene with NO3, OH or O3 by calculating the value of the HOMO energy of the alkene. The accuracy of these predictions is quoted as a 48, 40 and 97% minimum probability that the predicted rate constant for reaction of an alkene with NO3, O3 and OH, respectively, will be within a factor of two of the measured rate constant. This probability is increased to a minimum of 73, 80 and approaches 100% for the reactions of NO3, O3 and OH, respectively, with conjugated dienes.

The gas-phase reaction between the NO3 radical and peroxymethacrylic nitric anhydride (MPAN: (CH22C(CH3)C(O)O2NO2) has been studied by a relative-rate method at atmospheric pressure. A value of (1.45±0.45)×10-16 c molecule-1 s-1 was obtained for the rate constant. The atmospheric chemistry of MPAN was considered in a diurnally varying model of the planetary boundary layer.

The rate coefficients for the reaction between atomic chlorine and a number of naturally occurring species have been measured at ambient temperature and atmospheric pressure using the relative rate technique. The values obtained were (4.0 ± 0.8) × 10-10, (2.1 ± 0.5) × 10-10, (3.2 ± 0.5) × 10-10, and (4.9 ± 0.5) × 10-10 cm3 molecule-1 s-1, for reactions with isoprene, methyl vinyl ketone, methacrolein and δ3-carene, respectively. The value obtained for isoprene compares favourably with previously reported values. No values have been reported to date for the rate constants of the other reactions.

Photolysis of CF3COCl in the presence of O2 yields the radical CF3O, which then reacts in the presence of CO and O2 to yield a variety of CF3-containing species initiated by the following reactions: CF 3 O+CO→CF 3 OCO CF 3 OCO+O 2 →CF 3 OC(O)O 2 The reactions of various oxygenated CF3-containing radicals give rise to two different compounds with the general composition CF3OC(O)OyCF3 (y=4 and 3) that show increased thermal stability as the number of oxygen atoms in the peroxidic bonding bridge decreases. A further species of the formula CF3OC(O)O2C(O)OCF3 is also invoked. Parallel reactions lead to the formation of CF3OC(O)O radicals that decompose to give CF3O and CO2, thus providing a way of maintaining the chain reaction that catalytically converts CO into CO2. Experimental evidence for the formation and thermal decay of these intermediates is presented.

The rate coefficients for the reaction of NO 3 with pent-1-ene and hex-1-ene have been measured directly for a range of temperatures between 298 and 433 K using the discharge-flow technique coupled to a laser-induced fluorescence (LIF) detection system to monitor the NO 3 radical. The values obtained for the rate coefficients at room temperature are (6.19 ± 0.38) × 10 −14 and (9.32 ± 0.71) × 10 −14 cm 3 molecule −1 s −1 , respectively. These results are in excellent agreement with the predictions based on the correlation between the rate constant at room temperature and the ionization potential used as an indicator of HOMO energy. The Arrhenius expressions: k(T) = (2.05 ± 0.95) × 10 −11 exp[(−1703 ± 80)/T] and k(T) = (1.24 ± 0.40) × 10 −12 exp[(−816 ± 23)/T] cm 3 molecule −1 s −1 are proposed for the two reactions. The rate constants for the reactions of pent-1-ene and hex-1-ene with NO 3 have been compared with the coefficients for the reactions of these compounds with OH and O( 3 P). The observed trends in reactivity are explained in terms of the presence of electron-withdrawing substituents, which reduce the reactivity of alkenes. Tropospheric half-lives of these compounds, at night and during the day, have been estimated for typical tropospheric NO 3 and OH concentrations, in order to assess the lifetimes of the compounds in the atmosphere.

A discharge fast-flow tube equipped with laser-induced fluorescence detection for NO 2 , produced from the titration of CF 3 O 2 with NO, and chemiluminescence detection for O( 3 P), was employed to determine the rate constant for the reaction CF 3 O 2 +O( 3 P)→products, at T=296±1 K and P=ca. 2 Torr. The rate constant was found to be (6.3±1.0)×10 -11 cm 3 molecule -1 s -1 . Some implications of the results for atmospheric chemistry are discussed.

A discharge-flow system equipped with a laser-induced fluorescence cell to detect the methoxyl radical and a quadrupole mass spectrometer to detect products has been used to determine the kinetics of the reactions: CH 3 ONO + F → products (1), CH 3 O + F → products (2), CH 3 O + CH 3 O → products (3), CH 3 O + n-C 4 H 10 → products (4), CH 3 O + i-C 4 H 10 → products (5), CH 3 O + C 2 H 4 → products (6), at T = 295 K and P = 2–3 Torr. The rate constants determined are: k 1 = (8.0 ± 0.5) × 10 −11 cm 3 molecule −1 s −1 , k 2 = (1.5 ± 0.5) × 10 −10 cm 3 molecule −1 s −1 , k 3 = (1.3 ± 0.4) × 10 −11 cm 3 molecule −1 s −1 , and upper limits for the rate constants k 4 ⩽ 1.0 × 10 −14 cm 3 molecule −1 s −1 , k 5 ⩽ 1.7 × 10 −14 cm 3 molecule −1 s −1 and k 6 ⩽ 1.1 × 10 −15 cm 3 molecule −1 s −1 . In addition, a summary of the sources of CH 3 O developed for use in low-pressure flow systems in this and other groups is presented.

A discharge-flow apparatus equipped with resonance-fluorescence detection for OH radicals has been used to measure rate coefficients for the two reactions over the pressure range 1–20 Torr at T=298 K and ca. 1–3 Torr over the temperature range 300–400 K. The rate constants, k 1 and k 2 , were found to be invariant with pressure, but were found to increase with increasing temperature. In light of these findings, we conclude that at low pressure the reaction mechanism is consistent with an abstraction process. The rate constants derived are k 1 =(4.1±0.8)×10 -13 exp(-604±121/T) cm 3 molecule -1 s -1 and k 2 =(3.30±0.66)×10 -12 exp(-699±140/T) cm 3 molecule -1 s -1 . The existence of pressure-dependent (association) channels for reactions (1) and (2) is briefly discussed. Model calculations are presented that imply that photolysis, and not reaction with the OH radical, is the dominant loss process in the atmosphere for CH 3 ONO 2 and C 2 H 5 ONO 2 . Lifetimes of CH 3 ONO 2 and C 2 H 5 ONO 2 are derived for the lower atmosphere; they vary from a few years to several days depending upon season and location. Possible sources of these alkyl nitrates in the atmosphere are discussed.

Two low-pressure discharge-flow systems, one equipped with laser-induced fluorescence (LIF) detection of NO 2 and the other mass spectrometric detection of NO 2 , have been employed to determine the rate constant for the self reaction CF 3 O 2 +CF 3 O 2 →products, at T=298 K and P=13 Torr. The phenomenological rate constant is k 1 =(2.0±0.1)×10 -12 cm 3 molecule -1 s -1 ; however, after consideration of secondary chemistry, numerical modelling suggests that the true rate constant is k 1 =(1.2±0.3)×10 -12 cm 3 molecule -1 s -1 . In addition, an upper limit has been derived for the reaction CF 3 O 2 +O 3 →products of k 3

The kinetics of the reactions of the nitrate radical, NO3, with 3-chloropropene and 3-bromopropene have been studied in the temperature range 296–428 K, using the discharge-flow technique coupled to detection of NO3 by laser-induced fluorescence. The results obtained for the room-temperature rate constants are (0.49 ± 0.02)× 10–14 and (0.38 ± 0.02)× 10–14 cm3 molecule–1 s–1, respectively, and the proposed Arrhenius expressions are k=(1.92 ± 0.79)× 10–11 exp[(– 2480 ± 143/T)] and k=(1.27 ± 0.37)× 10–12 exp[(– 1737 ± 104)/T] cm3 molecule–1 s–1, respectively (errors quoted as σ).The study of the reaction between the NO3 radical and 3-iodopropene has also been attempted using the same conditions. The decomposition of 3-iodopropene with increasing temperature and the occurrence of some radical–radical reactions makes it difficult to establish the Arrhenius parameters. A room-temperature rate constant of k⩽ 3.91 ± 0.02 × 10–14 cm3 molecule–1 s–1 has been estimated.An estimate of the corresponding rate coefficients for the reactions of these halopropenes with the OH radical has been made. Tropospheric half-lives are calculated at night and during the day for typical NO3 and OH concentrations, and show that both radicals provide an effective tropospheric sink for these compounds.

A discharge-flow system equipped with a laser-induced fluorescence (LIF) cell to detect NO2 and a multi-pass absorption cell to detect NO3 has been used to study the reaction CH3C(O)O2+ NO3→ CH3C(O)O + NO2+ O2(1) at T= 403–443 K and P= 2–2.4 Torr. The rate constant was found to be independent of temperature with a value of k1=(4 ± 1)× 10–12 cm3 molecule–1 s–1. The likely mechanism for the reaction is discussed. The atmospheric implications of reaction (1) are investigated using a range of models and several case studies are presented, comparing model results with actual field measurements. It is concluded that reaction (1) participates in a cycle which can generate OH at night. This reaction cycle (see text) can operate throughout the continental boundary layer, but may even occur in remote regions.

Absolute rate coefficients for the reaction of NO3 with 1,1-dichloropropene, 2,3-dichloropropene and (E)-1,3-dichloropropene have been measured using the discharge-flow technique coupled to an LIF detection system for a range of temperatures from 296 to 430 K. The measured room-temperature rate constants are (1.52 ± 0.76)× 10–14, (1.39 ± 0.30)× 10–14 and (0.95 ± 0.14)× 10–14 cm3 molecule–1 s–1, respectively. The Arrhenius expressions k=(2.85 ± 1.21)× 10–11 exp[–(2274 ± 313)/T], k=(0.16 ± 0.02)× 10–11 exp[–(1420 ± 82)/T], k=(5.86 ± 1.60)× 10–11 exp[(–2575 ± 192)/T] cm3 molecule–1 s–1, are proposed for the three reactions.The reactivity of alkenes containing vinylic halogen atoms is discussed and compared with that of simple alkenes. A dependence of room-temperature rate constants and energy of activation on haloalkene ionization potential, corrected for the strength of the mesomeric interaction between the chlorine atom and the carbon–carbon π bond, is found.Tropospheric half lives of these compounds have been estimated at night and during the day for typical NO3 and OH tropospheric concentrations, in order to assess the lifetime of these compounds in the atmosphere.

The kinetics of the reaction between the radicals NO3 and OClO have been investigated at low pressure and room temperature using three laboratory techniques: discharge stopped-flow, slow-flow, and discharge-flow. In all types of experiment, strong evidence was found to suggest that reaction between the two species occurred on the walls of the reaction cell and led to the release of ClO into the gas phase. No evidence was found for the occurrence of the homogeneous reaction NO3 + OClO → products, and an upper limit for the rate coefficient for this process of 6 × 10-17 cm3 molecule-1 s-1 was determined. These experimental results are in disagreement with those of a previous study and suggest that this gas-phase reaction is unlikely to be of atmospheric importance even in the regions of the nighttime stratosphere where both species coexist.

Electron impact spectroscopy has been used for the first time to obtain energy-loss spectra for chlorine dioxide, OClO, over an energy range 2.5 to 12.5 eV. The differential oscillator strength (DOS) obtained from the energy-loss spectrum is compared with the DOS obtained from optical measurements. Oscillator strengths for several transitions have been calculated from a summation of the DOS and comparisons are also made with previous optical data.

A discharge-flow apparatus was used to determine the branching ratio α for the channel of the reaction of ClO with CH3O2 that leads to the formation of CH3O. The experiments were performed at 2 Torr pressure and at room temperature. ClO was the excess reactant and was measured mass spectrometrically; CH3O was detected by laser-induced fluorescence. The value of α was shown to depend on the fraction β of the reaction of CH3O2 with Cl atoms that leads to the formation of CH3O. For β = 0, 0.5 and 1, the derived values of α are 0.28±0.07, 0.32±0.08 and 0.42±0.09, respectively. Comparison with other published results suggests the possibility of three significant product channels for the reaction of CH3O2 with ClO.

A study of the reactivity of the hydroxyl radical towards a series of hydrocarbons and fluorinated hydrocarbons has been carried out. The activation energy (Ea), the logarithm of the pre-exponential A factor, and the logarithm of the room temperature rate coefficient (k) all correlate strongly with the calculated ionization potential (IP) of the organic reactant. Using these correlations, the accuracy of the experimental database of reaction-rate parameters is examined, and predictions are made of rate parameters for related reactions that have not, as yet, been measured.

The central topic of this review concerns the species XO, where X is F, Cl, Br or I. These molecules are thus the radicals FO, ClO, BrO and IO, but attention is also given to some of their precursors in the laboratory and the atmosphere, as well as to their reservoirs, sinks, and other related species of potential atmospheric importance. Laboratory data on the physics and chemistry of the species and atmospheric determinations of their concentrations are both considered. One aim of the review is to highlight the relationship between the laboratory investigations and the atmospheric studies. The emphasis of the review is on gas-phase processes. After a brief introductory section, the review continues with an examination of laboratory techniques for the study of the halogen-oxide species. This section fast looks at the general properties of the oxides and sources of them for laboratory experiments, then discusses the detection and measurement of the monoxide radicals in the laboratory, and ends with a description of the kinetic tools that have been harnessed in the various studies. The spectroscopy, structure, photochemistry and thermochemistry, of the halogen oxides are discussed in Section III. Both experimental and theoretical aspects are presented. The objectives of the work described are on the one hand to establish the basis for the detection of the radical and the measurement of its concentration in the laboratory and in the atmosphere, and on the other to provide the framework for interpreting pathways, mechanisms and efficiencies of photochemical and thermal reactions. Sections IV, V and VI of the review address the main issues of observed chemistry and its kinetics. Section IV gathers together available kinetic and mechanistic information on gas-phase reactions of FO, ClO, BrO and IO radicals, and the available data are summarized in appropriate tables. Section V reports on the corresponding data available for the gas-phase reactions of certain species containing the XO grouping, which include most of the so-called atmospheric reservoirs of XO radicals. There are three sub-sections, which deal in turn with oxide species, HOX, and XONO2. Heterogeneous processes are introduced in Section VI. Heterogeneous chemistry in the atmosphere is that which occurs on or in ambient condensed phases that are in contact with the gas phase, such as aerosols, clouds, surface waters, and so on. It is becoming increasingly clear that such processes are of importance not only in the stratosphere, but also in the troposphere. Section VII of the review is concerned directly with the atmosphere. The sources and sinks of the compounds, the reaction pathways, temporary and permanent reservoirs, observational evidence, the involvement of the species in atmospheric chemistry, and modelling studies are considered for the troposphere and the stratosphere in turn. The section concludes with a more detailed exposition of the role of modelling of the halogen compounds in the stratosphere. The review concludes with an examination of issues in regard to the halogen oxide species that are unresolved, uncertain, or in need of further research. Further data are required, for example, on the spectroscopy and photochemistry of reservoir compounds, on potential organic sources of atmospheric iodine, and even on the channels for photolysis of compounds such as OClO. Within the field of reaction kinetics, there is a need for further study of the kinetics of dimer formation, and of certain other reactions of the radicals themselves (especially of IO) and some of their reservoirs. A substantial number of problems in heterogeneous chemistry of the species remain to be solved. Not only are some key physical measurements missing, but most of what has been achieved in both chemistry and physics is limited to chlorine-containing species, so that the work needs to be extended to the other halogens. There is also a need for a search for novel reactions occurring on conventional surfaces and for all types of reaction occurring on surfaces that exist within the atmosphere but which have not yet been the subject of laboratory study. So far as the atmosphere itself is concerned, there are important issues to be resolved. They include (i) the involvement of halogen species in episodic tropospherec ozone depletion in the Arctic (and a further question about whether or not such depletion is more widespread); (ii) the role of an active halogen chemistry in the oxidation of VOC; (iii) the significance and detail of stratospheric iodine and iodine-catalysed ozone removal; and (iv) the quantitative description of heterogeneous stratospheric chemistry.

The rate constant for the gas-phase reaction of the hydroxyl radical with trifluoroacetic acid has been determined over the temperature range 283–323 K using a relative rate technique. The results provide a value of k(OH+CF3C(O)(OH) = 2.0 × 10−13 exp (−146±500/T) cm3 molecule−1 s−1 based on k(OH+C2H6)=7.8×10−12 exp(−1020±100/T) cm3 moleculet−1 s−1 for the rate constant of the reference reaction. The OH-radical initiated oxidation of CF3C(O)OH was investigated at 298 K in oxygen at 1 atm total pressure using Fourier-transform infrared spectroscopy. The major carbon-containing products were C(O)F2, CF3O3CF3 and CO2, suggesting that the dominant reaction channel for loss of the CF3C(O)O radical, formed by hydrogen atom abstraction, is carboncarbon bond cleavage. The results indicate that the gas-phase reaction of OH radicals with trifluoroacetic acid may provide a sink for this compound in the troposphere that cannot be neglected.

Previous evaluations of the impact of fluorine chemistry on stratospheric ozone have concluded that the role of fluorine compounds in catalytic ozone removal is negligible. However, recent investigations of the degradation pathways for compounds containing CF3 groups indicates that if the reaction of CF3O with O3 is sufficiently fast, there may be an ozone impact. Some recent measurements indicate that the reaction rate constant of CF3O+O3 is sufficiently low that the ozone impact is likely to be small. However, it is not possible a-priori to rule out significant ozone removal without additional kinetic data on other reactions. We present calculations to illustrate how different key reactions affect the calculated stratospheric concentrations of the CF3X species (CF3, CF3O, CF3O2, CF3OH, CF3OOH, CF3ONO2, CF3O2NO2, CF3OOCl) and their ability to remove stratospheric ozone. We utilize our results to suggest kinetic measurements that could substantially reduce the uncertainties in CF3 chemistry relevant to the determination of ozone depletion potential of CF3-bearing compounds.

This review of the part played by the singlet states of molecular oxygen in the environment deals with atmospheric aspects. There are five bound excited states of molecular oxygen that correlate with two ground state, 3P, oxygen atoms. Of these, three are singlets, although the other two states (triplets) are closely associated with singlet oxygen processes, especially in the mesosphere. A weakly bound quintet state has been invoked, as well, in explaining some aspects of the physical chemistry of the singlet species. Of the three singlet states, the a1Δg is the most familiar. It has a low excitation energy, a long radiative lifetime, and is rather resistant to collisional deactivation in the gas phase. As a consequence, its chemistry has been susceptible to detailed study in the laboratory. These investigations, coupled with estimates of production rates, suggest that O2(a1Δg) is probably not important in initiating much chemical change in the lower atmosphere, at least in the gas phase; excited molecules dissolved in water droplets may promote chemical change under special circumstances. In the stratosphere and mesosphere, each of the bound excited states gives rise to characteristic emission features of the airglow, both by day and by night. The observational data, obtained from the ground, and from balloons, high-flying aircraft, rockets and satellites is surveyed as a background to examining the chemical and photochemical mechanisms by which the different states become excited. These mechanisms clearly differ by day and by night, and they also depend on the altitude from which the emission comes. The most intense feature of the oxygen dayglow, the Infrared Atmospheric Band, comes from O2(a1Δg) that is produced in the photolysis of ozone. Because dayglow measurements are sometimes used to derive ozone concentrations and altitude profiles in the atmosphere, the efficiency of production of the species in the photolysis of ozone is examined critically, and some unexpected laboratory findings are reported. The b1Σ+g state of oxygen is excited during the day largely by resonance scattering, although some is also populated by energy transfer from O(1D) to O2. At night, recombination of O(3P) atoms is the most likely source of excitation of all the states of oxygen. Laboratory experiments that bear on these processes are reviewed, and theoretical estimates of the partitioning of recombination events between the different states are presented. Direct recombination into the a1Δg and b1Σ+g states is unlikely to be efficient enough to produce the observed concentrations of these species, and some indirect process is thus implicated. Laser excitation experiments show that quenching of the three higher excited (ungerade) states of oxygen by O2 and, especially, N2, can generate O2(b1Σ+g) with high efficiency; similar experiments demonstrate explicitly that the quenching of O2(b1Σ+g) by the atmospheric gases yields O2(a1Δg). A consistent excitation scheme for the nightglow emissions is presented; this scheme also pays attention to the “auroral green” line produced by the 1S state of atomic oxygen, the intensities of which in the atmospheres of Earth and Venus provide some clues about the excitation of the molecular states. Finally, the laboratory studies are shown to indicate that the formation of excited molecular oxygen from vibrationally rich hydroxyl (OH) radicals is unlikely to be of major importance in the atmosphere.