J. Hjorth

Ozonolysis at vegetation surfaces : A source of acetone, 4-oxopentanal, 6-methyl-5-hepten-2-one, and geranyl acetone in the troposphere

Abstract The present study gives a possible explanation for the ubiquitous occurrence of 6-methyl-5-hepten-2-one and acetone in ambient air and reports for the first time on a widespread occurrence of geranyl acetone and 4-oxopentanal. We have conducted a series of laboratory experiments in which it is demonstrated that significant amounts of geranyl acetone, 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), and acetone are formed by the reaction of ozone with foliage of common vegetation in the Mediterranean area (Quercus ilex>Citrus sinensis>Quercus suber>Quercus freinetto>Pinus pinea). In order to rule out biological formation, epicuticular waxes were extracted from the leaves, dispersed on glass wool and allowed to react with a flow of artificial air. Significant amounts of 6-MHO and 4-OPA were formed at ozone concentrations of 50–100xa0ppbv, but not at zero ozone. A number of terpenoids common in vegetation contain the structural element necessary for ozonolytic formation of 6-MHO. Two sesquiterpenes (nerolidol; farnesol), and a triterpene (squalene) selected as representative test compounds were demonstrated to be strong precursors for acetone, 4-OPA, and 6-MHO. Squalene was also a strong precursor for geranyl acetone. The atmospheric lifetime of geranyl acetone and 6-MHO is less than 1xa0h under typical conditions. For the present study, we have synthesized 4-OPA and investigated the kinetics of its gas-phase reaction with OH, NO3, and O3. A tropospheric lifetime longer than 17xa0h under typical conditions was calculated from the measured reaction rate constants, which explains the tropospheric occurrence of 4-OPA. It is concluded that future atmospheric chemistry investigations should included geranyl acetone, 6-MHO, and 4-OPA. In a separate experiment it was demonstrated that human skin lipid which contains squalene as a major component is a strong precursor for the four above-mentioned compounds plus nonanal and decanal. The accidental touching of material which later comes into contact with ozone can lead to strong artifact formation of these carbonyl compounds. Previously published results on these compounds must be seen in this new light.

KINETIC STUDY OF GAS-PHASE REACTIONS OF PINONALDEHYDE AND STRUCTURALLY RELATED COMPOUNDS

Rate constants for the reactions of OH, NO3, and O3 with pinonaldehyde and the structurally related compounds 3-methylbutanal, 3-methylbutan-2-one, cyclobutylmethylketone, and 2,2,3-trimethyl-cyclobutyl-1-ethanone have been measured at using on-line Fourier transform infrared spectroscopy. The rate constants obtained for the reactions with pinonaldehyde were: ,

Measurements of acetone and other gas phase product yields from the OH-initiated oxidation of terpenes by proton-transfer-reaction mass spectrometry (PTR-MS)☆

Abstract The atmospheric oxidation of several terpenes appears to be a potentially relevant source of acetone in the atmosphere. Proton-transfer-reaction mass spectrometry was used as an on-line analytical method in a chamber study to measure acetone and other gas phase products from the oxidation of α- and β-pinene initiated by OH radicals in air and in the presence of NOx. Acetone may be formed promptly, following attack by the OH radical on the terpene, via a series of highly unstable radical intermediates. It can also be formed by slower processes, via degradation of stable non-radical intermediates such as pinonaldehyde and nopinone. Primary acetone and pinonaldehyde molar yields of 11±2% (one σ) and 34±9% (one σ), respectively, were found from the reaction between α-pinene and the OH radical. After all α-pinene had been consumed, an additional formation of acetone due to the degradation of stable non-radical intermediates was observed. The total amount of acetone formed was 15±2% (one σ) of the reacted α-pinene. An upper limit of 12±3% (one σ) for the acetone molar yield from the oxidation of pinonaldehyde was established. From the reaction between β-pinene and the OH radicals, primary acetone and nopinone molar yields of 13±2% (one σ) and 25±3% (one σ), respectively, were observed. Additional amounts of acetone were formed by the further degradation of the primary product, such as the most abundant product nopinone. The total amount of acetone formed was 16±2% (one σ) of the reacted β-pinene. An upper limit of 12±2% (one σ) for the acetone molar yield from the oxidation of nopinone was established. The observed product yields from α- and β-pinene are in good agreement with other studies using mass-spectrometric and gas chromatographic analytical techniques, but differ significantly from previous studies using spectroscopic methods. Possible reasons for the discrepancies are discussed.

Atmospheric degradation and global warming potentials of three perfluoroalkenes

The vapour phase reactions of perfluoropropene, CF3aCF ¼ CF2, and perfluorobuta-1,3-diene, CF2 ¼ CFaCF ¼ CF2, with OH, NO3 and O3 were studied at 29874 K and 74075 Torr using long-path FT-IR detection. The reactions with ozone are very slow, kCF3CFCF2þO3 ¼ð 6:271:5Þ� 10 @22 and kCF2CFCFCF2þO3 ¼ ð6:570:2Þ� 10 @21 cm 3 molecules @1 s @1 , and upper limits of 3 � 10 @15 cm 3 molecules @1 s @1 are reported for the NO3 reaction rate coefficients. The OH reaction rate coefficients were determined as kCF3CFCF2þOH ¼ð 2:670:7Þ� 10 @12 and kCF2CFCFCF2þOH ¼ð 1:170:3Þ� 10 @11 cm 3 molecules @1 s @1 ; perfluoropropene gave a nearly quantitative yield of CF3CFO and CF2O as organic products, while perfluorobuta-1,3-diene gave from 130% to 170% of CF2O. A chemistry transport model was applied to calculate the atmospheric distributions and lifetimes of the perfluoroalkenes; the global and yearly averaged lifetimes were calculated as 1.9 day for C2F4 and C4F6 and 6 days for C3F6. Quantitative infrared cross-sections of perfluoroethene, perfluoropropene, and perfluorobuta-1,3-diene have been obtained at 298 K in the region 100–2600 cm @1 . Radiative forcing calculations have been performed for these gases assuming either constant vertical profiles or the distribution derived from the chemistry transport model. The results show that the Global Warming Potentials are totally negligible for these compounds. r 2001 Elsevier Science Ltd. All rights reserved.

Products and mechanisms of the gas phase reactions of NO3 with CH3SCH3, CD3SCD3, CH3SH and CH3SSCH3

Products and mechanisms of the reaction between the nitrate radical (NO3) and three of the most abundant reduced organic sulphur compounds in the atmosphere (CH3SCH3, CH3SH and CH3SSCH3), have been studied in a 480 L reaction chamber using in situ FT-IR and ion chromatography as analytical techniques. In the three reactions, methanesulphonic acid was found to be the most abundant sulphur containing product. In addition the stable products SO2, H2SO4, CH2O, and CH3ONO2 were identified and quantified and thionitric acid-S-methyl ester (CH3SNO2) was observed in the i.r. spectrum from all of the three reactions. Deuterated dimethylsulphide (CD3SCD3) showed an isotope effect on the reaction Deuterated dimethylsulphide (CD3SCD3) showed an isotope effect on the reaction rate constant (kH/kD) of 3.8±0.6, indicating that hydrogen abstraction is the first step in the NO3+CH3SCH3 reaction, probably after the formation of an inital adduct.Based on the products and intermediates identified, reaction mechanisms are proposed for the three reactions.

Atmospheric lifetimes, infrared spectra and degradation products of a series of hydrofluoroethers

Abstract The rate constants of the reactions between the OH radical and a series of hydrofluoroethers (HFE) have been measured. The reaction of OH with CHF 2 OCF 2 OCHF 2 (1), CHF 2 OCF 2 CF 2 OCHF 2 (2), CHF 2 OCF 2 CF 2 OCF 2 OCHF 2 (3) and CH 3 OC 4 F 9 (4) were investigated at 295±3xa0K and 740±5xa0Torr total pressure. The following values of the rate constants were determined for the reaction with the OH radical: k 1 =(2.4±0.7)×10 -15 xa0cm 3 xa0molecule -1 xa0s -1 , k 2 =(4.7±1.6)×10 -15 xa0cm 3 xa0molecule -1 xa0s -1 , k 3 =(4.6±1.6)×10 -15 xa0cm 3 xa0molecule -1 xa0s -1 and k 4 =(7.2±1.6)×10 -15 xa0cm 3 molecule -1 s -1 . (All values are given with 2 σ uncertainties). Infrared spectra were obtained for all four HFEs in the range from 600 to 4000xa0cm -1 (3 to 17xa0μm), with the following IBI-values (integrated band intensities): IBI 1 = (5.19±0.23)×10 -16 xa0cmxa0molecule -1 for the 978–1584xa0cm -1 band, IBI 2 =(6.04±0.13)×10 -16 xa0cm molecule -1 for the 930–1501xa0cm -1 band, IBI 3 =(8.49±0.34)×10 -16 xa0cmxa0molecule -1 from the 963–1587xa0cm -1 band and IBI 4 =(4.23±0.14)×10 -16 xa0cm molecule -1 for the 845–1428xa0cm -1 band. Carbonyl fluoride, CF 2 O, was the only fluorine-containing degradation product that was found from the Cl atom-initiated reactions of both CHF 2 OCF 2 OCF 2 CF 2 OCHF 2 and CH 3 OC 4 F 9 , with measured product yields of 60–97% and 20–40% (based on carbon atoms), respectively. Due to the high uptake parameter of CF 2 O to liquid water, its lifetime in the atmosphere is very short (with an upper limit between 15 and 30xa0d). It is rapidly incorporated into raindrops/aerosols, where it eventually degrades to HF and CO 2 . The GWP of CF 2 O is therefore negligible compared to those of CFC-11 and CFC-12.

Products and mechanism of the gas phase reaction between nitrate radical and arenes

SummaryThe reaction between the nitrate radical and ortho- and para-xylene and some deuterated analogues was studied in the gas phase in a 480 l reaction chamber. Sampling of the gas mixture and gas chromatography-mass spectrometry allowed to obtain kinetic isotope effect values in the range of 1.48–1.87. These were similar to the values of 1.25–1.67 observed in the oxidation of these substrates with cerium(IV) ammonium nitrate in acetonitrile. This suggests that the hydrogen abstraction mechanism commonly invoked for these reaction may compete with a single electron transfer and an addition of the nitrate radical to the aromatic compound in the rate-determining first reaction step. Lowering the concentration of dioxygen in the reaction gas mixture leads to an increase of the ratio nitrate ester/aromatic aldehyde.

FTIR studies of reactions between the nitrate radical and haloethenes

Products and mechanisms for the gas-phase reactions of NO3 radicals with CH2=CHCl, CH2=CCl2, CHCl=CCl2,cis-CHCl=CHCl andtrans-CHCl=CHCl in air have been studied. The experiments were carried out at 295±2 K and 740±5 Torr in a 480-L Teflon-coated reaction chamber and at 295±2 K and 760±5 Torr in a 250-L stainless steel reactor. NO3 was generated by the thermal dissociation of N2O5. Experiments with15NO3 and CD2CDCl have also been performed. The initially formed nitrate peroxynitrates decay into carbonyl compounds, nitrates, HCl and ClNO2. In adidtion, there are indications of nitrooxy acid chlorides being produced. The reactions with CH2=CCl2 and CHCl=CCl2 are more complex due to release of chlorine atoms which eventually lead to formation of chloroacid chlorides.A general reaction mechanism is proposed and the observed concentration-time profiles of reactants and products are simulated for each compound. The rate constants for the initial step of NO3 addition to the chloroethenes are determined as: (2.6±0.5, 9.4±0.9, 2.0±0.4 and 1.4±0.4) × 10−16 cm3 molecule−1 s−1 for CH2=CHCl, CH2=CCl2, CHCl=CCl2 andcis-CHCl=CHCl, respectively.