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Absolute rate constants have been measured for the gas-phase reactions of hydroxyl radicals with a series of methyl esters: methyl propionate (k1), methyl butyrate (k2), methyl valerate (k3), and methyl caproate (k4). Experiments were carried out using the pulsed laser photolysis−laser induced fluorescence technique over the temperature range 253−372 K. The obtained kinetic data were used to derive the following Arrhenius expressions: k1 = (1.45 ± 0.42) × 10-12 exp[−(148 ± 86)/T]; k2 = (0.96 ± 0.29) × 10-12 exp[(380 ± 91)/T]; k3 = (1.37 ± 0.64) × 10-12 exp[(401 ± 142)/T]; k4 = (2.46 ± 1.04) × 10-12 exp[(326 ± 130)/T] (in units of cm3 molecule-1 s-1). At room temperature, the rate constants obtained (in units of 10-12 cm3 molecule-1 s-1) were as follows: methyl propionate (0.83 ± 0.09); methyl butyrate (3.30 ± 0.25); methyl valerate (4.83 ± 0.55); methyl caproate (7.15 ± 0.70). Our results are compared with the previous determinations and discussed in terms of structure−activity relationships.

International audience; The yield of 1- and 2-butyl nitrates in the gas-phase reactions of NO with n-C4H9O2 and sec-C4H9O2, obtained from the reaction of F atoms with n-butane in the presence of O2, was determined over the pressure range of 100–600 Torr at 298 K using a high-pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. The yield of butyl nitrates was found to increase linearly with pressure from about 3% at 100 Torr to about 8% at 600 Torr. The results obtained are compared with the available data concerning nitrate formation from NO reaction with other small alkylperoxy radicals. These results are also discussed through the topology of the lowest potential energy surface mainly obtained from DFT(B3LYP/aug-cc-pVDZ) calculations of the RO2 + NO reaction paths. The formation of alkyl nitrates, due essentially to collision processes, is analyzed through a model that points out the pertinent physical parameters of this system.

We have studied the impact of the recently observed reaction NO+HO2→HNO3 on atmospheric chemistry. A pressure and temperature-dependent parameterisation of this minor channel of the NO+HO2→NO2+OH reaction has been included in both a 2-D stratosphere-troposphere model and a 3-D tropospheric chemical transport model (CTM). Significant effects on the nitrogen species and hydroxyl radical concentrations are found throughout the troposphere, with the largest percentage changes occurring in the tropical upper troposphere (UT). Including the reaction leads to a reduction in NOx everywhere in the troposphere, with the largest decrease of 25% in the tropical and Southern Hemisphere UT. The tropical UT also has a corresponding large increase in HNO3 of 25%. OH decreases throughout the troposphere with the largest reduction of over 20% in the tropical UT. The mean global decrease in OH is around 13%, which is very large compared to the impact that typical photochemical revisions have on this modelled quantity. This OH decrease leads to an increase in CH4 lifetime of 5%. Due to the impact of decreased NOx on the OH:HO2 partitioning, modelled HO2 actually increases in the tropical UT on including the new reaction. The impact on tropospheric ozone is a decrease in the range 5 to 12%, with the largest impact in the tropics and Southern Hemisphere. Comparison with observations shows that in the region of largest changes, i.e. the tropical UT, the inclusion of the new reaction tends to degrade the model agreement. Elsewhere the model comparisons are not able to critically assess the impact of including this reaction. Only small changes are calculated in the minor species distributions in the stratosphere.

Depuis environ dix millénaires, l’Homme a appris à gérer et produire ses propres ressources alimentaires. En Ligurie, l’exploitation du milieu marin proche et l’acquisition des techniques agropastorales au moment de la néolithisation (VIemillénaire av. J.-C.) sont mises en évidence par les études archéologiques et paléoenvironnementales. Cependant, connaître la prépondérance de l’utilisation d’un milieu par rapport à un autre et l’évolution de ces choix de subsistance, au cours du Néolithique, n’est pas chose aisée. La présence de restes humains (sépultures) et d’animaux (couches d’habitat) a permis l’utilisation de méthodes isotopiques et notamment l’étude du δ13C et du δ15N du collagène, témoin de l’origine des ressources protéiniques consommées. Cette méthode tente, d’une part, d’appréhender l’importance ou non des protéines d’origine marine dans l’alimentation des premiers agropasteurs et, d’autre part, de cerner les différences alimentaires qui ont pu exister au sein des populations. Deux sites néolithiques côtiers ont fait l’objet de notre étude : Pendimoun (France) et Arene Candide (Italie). Les résultats montrent, entre autres, l’absence de consommation régulière de ressources marines et des différences alimentaires au sein des populations. For the last ten millennia humans have learned how to manage and produce their own food resources. In Liguria, the exploitation of the marine and coastal environment as well as the acquisition and use of agriculture and pastoralism at the time of Neolithisation (i.e. 6th millennia BC) are evidenced by both archaeological and palaeoenvironmental studies. However, it is difficult, using these methods, to determine the relative contributions to human diets made by foods from these difference sources, especially throughout the Neolithic period. The presence of human burials and animal remains from various sites from this region allows the use of isotopic methods, and in particular the study of the δ13C and δ15N values of bone collagen, which provides information on the sources of dietary protein. This method is able to determine the relative importance of marine foods in the diets of the early Neolithic agriculturalists and pastoralists, as well as indicate dietary differences between contemporary populations. Two coastal Neolithic sites were the subject of our study: Pendimoun (France) and Arene Candide (Italy). The results show, contrary to expectations, the absence of regular consumption of marine resources and the food differences within the populations.

Absolute rate coefficients have been determined for the gas-phase reactions of hydroxyl radicals with methyl acrylate (k1), methyl methacrylate (k2) and ethyl acrylate (k3). Experiments were performed using two different techniques, the relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. The kinetic data obtained were used to derive the following Arrhenius expressions in the temperature range 253–374 K (in units of cm3 molecule−1 s−1): k 1 = ( 2.0 ± 0.8 ) × 1 0 - 12 exp [ ( 553 ± 51 ) / T ] , k 2 = ( 2.5 ± 0.8 ) × 1 0 - 12 exp [ ( 821 ± 55 ) / T ] , k 3 = ( 2.3 ± 0.8 ) × 1 0 - 12 exp [ ( 580 ± 65 ) / T ] . At 298 K, the reaction rate constants obtained by the two methods were in good agreement. In addition, the UV absorption spectra for the three unsaturated esters have been determined at (298±2) K and the absorption cross-sections in the wavelength region 215–298 nm were reported. The results are presented, discussed and used to estimate the atmospheric lifetimes for the studied esters.

A relative kinetic technique was used in combination with long-path Fourier transform infrared spectroscopy to study the kinetics of the reactions of N-methylformamide (HC(O)NHCH3), N,N-dimethylformamide (HC(O)N(CH3)2) and N,N-dimethylacetamide (CH3C(O)N(CH3)2) with OH radicals and Cl atoms at 298 ± 2 K and atmospheric pressure 1000 ± 10 mbar of air. The measured OH and Cl reaction rate coefficients (in units of cm3 molecule−1 s−1) were, respectively, (0.86 ± 0.24) × 10−11 and (0.97 ± 0.17) × 10−10 for N-methylformamide, (1.4 ± 0.3) × 10−11 and (1.9 ± 0.3) × 10−10 for N,N-dimethylformamide, (1.9 ± 0.3) × 10−11 and (2.5 ± 0.4) × 10−10 for N,N-dimethylacetamide. The results are presented and discussed in terms of structure–activity relationships and the atmospheric impact of the amides.

A high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer was used to investigate the minor channel (1b) producing nitric acid, HNO3, in the HO2 + NO reaction for which only one channel (1a) is known so far: HO2 + NO --> OH + NO2 (1a), HO2 + NO --> HNO3 (1b). The reaction has been investigated in the temperature range 223-298 K at a pressure of 200 Torr of N2 carrier gas. The influence of water vapor has been studied at 298 K. The branching ratio, k1b/k1a, was found to increase from (0.18(+0.04/-0.06))% at 298 K to (0.87(+0.05/-0.08))% at 223 K, corresponding to k1b = (1.6 +/- 0.5) x 10(-14) and (10.4 +/- 1.7) x 10(-14) cm3 molecule(-1) s(-1), respectively at 298 and 223 K. The data could be fitted by the Arrhenius expression k1b = 6.4 x 10(-17) exp((1644 +/- 76)/T) cm3 molecule(-1) s(-1) at T = 223-298 K. The yield of HNO3 was found to increase in the presence of water vapor (by 90% at about 3 Torr of H2O). Implications of the obtained results for atmospheric radicals chemistry and chemical amplifiers used to measure peroxy radicals are discussed. The results show in particular that reaction 1b can be a significant loss process for the HO(x) (OH, HO2) radicals in the upper troposphere.

Rate coefficients for the gas-phase reactions of hydroxyl radicals with a series of alkoxy esters of structure RC(O)O(CH2)nOR‘, where R = H, CH3, R‘ = CH3, C2H5, and n = 1−2, have been determined with use of relative and absolute rate methods. Relative rate measurements were performed in a Teflon reaction chamber at 298 ± 2 K and atmospheric pressure. Absolute rate measurements were made with pulsed laser photolysis−laser induced fluorescence over the temperature range 263−372 K at pressures of ∼100 Torr. The kinetic data are used to derive Arrhenius expressions for the reactions and tropospheric lifetimes for the alkoxy esters. The reactivity of the alkoxy esters is discussed in light of the current understanding of the atmospheric chemistry of oxygenated organic compounds.

Absorption cross-sections for benzaldehyde, o -tolualdehyde, m -tolualdehyde and p -tolualdehyde have been measured in the wavelength region 252–368 nm using two different systems (D 2 lamp-diode array and D 2 lamp-monochromator). In addition, cross sections at 253.7, 312.2 and 365 nm have been measured using a Hg pen ray lamp and were found in good agreement with those obtained using the D 2 lamp-diode array system. The absorption spectra of the four aromatic aldehydes have been found to exhibit fine structures similarly to aromatic hydrocarbons. The calculated atmospheric photolysis lifetimes suggest that photolysis is a potentially important removal process for these aromatic aldehydes in the troposphere.

The OH-initiated oxidation of two VOCs directly emitted to the atmosphere through their use as industrial solvents, hexylene glycol (HG, (CH3)2C(OH)CH2CH(OH)CH3) and diacetone alcohol (DA, (CH3)2C(OH)CH2C(O)CH3), has been studied in two photoreactors: a 140 L Teflon bag irradiated by lamps at CNRS-Orleans and the 200 m3 European photoreactor, EUPHORE, irradiated by sunlight. The rate constants for the reactions of HG and DA with OH radicals have been determined at (298 +/- 3) K using a relative rate method: k(HG) = (1.5 +/- 0.4) x 10(-11) and k(DA) = (3.6 +/- 0.6) x 10(-12) cm(3) molecule(-1) s(-1) and have been found in good agreement with estimations from structure-reactivity relationships. The study at Orleans and EUPHORE of the OH-initiated oxidation of hexylene glycol showed the formation of diacetone alcohol, acetone, and PAN as the principal products. The branching ratio of the H-atom abstraction from the > CH- group of HG has been estimated to be (47 +/- 4)% corresponding to the measured formation yield of DA. The formation yields of acetone and PAN lead to the determination of a lower limit of (33 +/- 7)% for the branching ratio of the H-atom abstraction of the -CH2- group of HG. For diacetone alcohol, studies at EUPHORE have shown negligible photolysis under atmospheric conditions (J < 5 x 10(-6) s(-1)) and the formation of acetone, PAN, HCHO, and CO in the OH-initiated oxidation experiments. The molar yield of acetone, close to 100%, corresponds to the branching ratio of the H-atom abstraction from the -CH2- group of DA. The present study has allowed the identification of the nature and the fate of the oxy radicals as intermediates in the oxidation mechanism of both HG and DA. The atmospheric implication of these results, especially the ozone formation potential of HG and DA, is discussed.

We have studied the evolution of the thermoelectric power S(T) with oxygen doping of single-layered Bi2Sr2CuO6+d thin films and ceramics in the overall superconducting (Tc, S290K) phase diagram. While the universal relation between the room-temperature thermopower S290K and the critical temperature is found to hold in the strongly overdoped region (d>0.14), a strong violation is observed in the underdoped part of the phase diagram. The observed behaviour is compared with other cuprates and the different scenarios are discussed.

Rate constants for the OH and Cl reactions with four aromatic aldehydes have been determined at 298 ± 2 K and atmospheric pressure using a relative rate method. The measured rate constants are (in cm3 molecule−1 s−1): kOHkClBenzaldehyde(1.2 ± 0.2) × 10−11(10 ± 1) × 10−11o-Tolualdehyde(1.8 ± 0.2) × 10−11(19 ± 2) × 10−11m-Tolualdehyde(1.7 ± 0.2) × 10−11(17 ± 2) × 10−11p-Tolualdehyde(1.3 ± 0.2) × 10−11(14 ± 2) × 10−11 This work provides the first measurements for the OH and Cl reaction rate constants with tolualdehydes. The results obtained are presented and compared with the previous measurements only available for benzaldehyde. The atmospheric implications are also discussed.

Experiments have been conducted in the laboratory and in the outdoor smog chamber (EUPHORE) to study the photolysis and the OH-initiated oxidation of (1) acrolein (CH2CHCHO) and (2) trans-crotonaldehyde (CH3CHCHCHO). In addition, the UV−visible absorption spectra for these two unsaturated aldehydes have been determined at (298 ± 2) K, and the rate constants for OH reactions have been measured using PLP-LIF technique as function of pressure (20−300) Torr in the temperature range (243−372) K. The obtained rate constant values are k1 = (6.55 ± 1.22) · 10-12 exp[(333 ± 54)/T] and k2 = (5.77 ± 1.14) · 10-12 exp[(533 ± 58)/T] cm3 molecule-1 s-1. From both midday photolysis rates J1 ≤ 2 · 10-6 s-1 for acrolein and J2 ≤ 1.2 · 10-5 s-1 for trans-crotonaldehyde, measured at EUPHORE during summer, and UV−visible absorption cross sections, very low effective quantum yields were derived: Φeff ≤ 0.005 for acrolein, and Φeff ≤ (0.030 ± 0.006) for trans-crotonaldehyde. The major primary products of the OH-initiated oxid...

The reaction rate constants of OH and O3 with ethyl vinyl ether (EVE, C2H5OCHCH2) have been measured in the temperature range 230–372 K and 298 K, respectively. The temperature dependent rate constant for the reaction with OH was found to be well represented by the expression: k1 = (1.55 ± 0.25) × 10−11 exp[(445 ± 13)/T] cm3 molecule−1 s−1, and at 298 K, k1 = (6.8 ± 0.7) × 10−11 cm3 molecule−1 s−1. The reaction rate constant of O3 with EVE was determined to be k2 = (2.0 ± 0.2) × 10−16 cm3 molecule−1 s−1 at 298 K. Using two different simulation chambers, the OH and O3-initiated oxidation of ethyl vinyl ether in air was also investigated. The main products observed from both reactions were formaldehyde and ethyl formate. The results obtained are presented and their atmospheric relevance briefly discussed.

Rate coefficients have been measured for the gas phase reactions of hydroxyl (OH) radicals with three acetals, di-n-propoxymethane (n-C3H7OCH2On-C3H7), di-n-butoxymethane (n-C4H9OCH2On-C4H9) and 1,3-dioxolane. The experiments were carried out using the pulsed laser photolysis - laser induced fluorescence technique over the temperature range 243–372 K and the kinetic data were used to derive the following Arrhenius expressions (in units of cm3 molecule−1 s−1): di-n-propoxymethane, k1=(4.39±1.06)×10−12 exp[(563±74)/T]; di-n-butoxymethane, k2=(5.15±0.66)×10−12 exp[(590±38)/T]; and 1,3-dioxolane, k3=(6.70±0.86)×10−12 exp[(154±36)/T]. At 298 K, the rate coefficients obtained (in units of 10−12 cm3 molecule−1 s−1) are: k1=(27.6±2.3), k2=(37.0±5.2) and k3=(11.1±0.9). The results are discussed in terms of structure–activity relationships and used to derive atmospheric lifetimes for these acetals.

A combined study of the OH gas phase reaction and uptake on aqueous surfacesof two carbonates, dimethyl and diethyl carbonate has been carried out todetermine the atmospheric lifetimes of these compounds. Rate coefficients havebeen measured for gas phase reactions of OH radicals with dimethyl and diethylcarbonate. The experiments were carried out using pulsed laser photolysis– laser induced fluorescence over the temperature range 263–372K and the kinetic data were used to derive the following Arrhenius expressions(in units of cm3 molecule−1 s−1):for dimethyl carbonate, k1 = (0.83±0.27)×10−12 exp [−(247± 98)/T] and fordiethyl carbonate, k2 = (0.46±0.15)×10−12 exp [(503± 203)/T]. At 298 K, therate coefficients obtained (in units of 10−12 cm3molecule−1 s−1) are: k1 =(0.35± 0.04) and k2 = (2.31± 0.29). The results arediscussed in terms of structure-activity relationships.The uptake coefficients of both carbonates on aqueous surfaces were measuredas a function of temperature and composition of the liquid phase, using thedroplet train technique coupled to a mass spectrometric detection. Dimethyland diethyl carbonate show very similar results. For both carbonates, themeasured uptake kinetics were found to be independent of the aqueous phasecomposition (pure water, NaOH solutions) but dependent on gas-liquid contacttime which characterises a surface saturation effect. The uptake coefficientvalues show a slight negative temperature dependence for both carbonates.These values vary from 1.4×10−2 to0.6×10−2 in the temperature range of 265–279 Kfor dimethyl carbonate, from 2.4×10−2 to0.9×10−2 in the temperature range of 270–279 Kfor diethyl carbonate. From the kinetic data, the following Henry's lawconstants were derived between 279 and 265 K: dimethyl carbonate,H1 = 20–106 M atm−1; and diethyl carbonate,H2 = 30–98 M atm−1. The reported data showthat the OH reaction is the major atmospheric loss process of these twocarbonates with lifetimes of 33 and 5 days, respectively, while the wetdeposition is a negligible process.

The kinetics and mechanisms of the OH- and Cl-initiated oxidation of two unsaturated HFCs, C4F9CHCH2 and C6F13CHCH2, were investigated. The kinetic study was performed as a function of pressure and temperature for the OH reactions and as a function of pressure at 298 K for Cl atom reactions. The rate constants obtained are (in units of cm3 molecule-1 s-1): k(OH + C4F9CHCH2) = (8.5 ± 1.4) × 10-13 exp[(139 ± 48)/T] and k(OH + C6F13CHCH2) = (1.3 ± 0.5) × 10-12 exp[(31 ± 124)/T] in the temperature range 233−372 K; and k(Cl + C4F9CHCH2) = (8.9 ± 1.0) × 10-11 and k(Cl + C6F13CHCH2) = (9.1 ± 1.0) × 10-11 at 298 K. The OH and Cl reactions rate constants were found to be independent of pressure in the range 10−760 and 15−60 Torr, respectively. The mechanistic study was performed in air at atmospheric pressure, in the presence or absence of NOx. CO and COF2 have been identified as the major secondary products of both OH- and Cl-initiated oxidation of the HFCs. However, there is evidence for the formation of differ...

The rate constants for the reactions of the OH radicals with a series of aldehydes have been measured in the temperature range 243–372 K, using the pulsed laser photolysis-pulsed laser induced fluorescence method. The obtained data for propanaldehyde, iso-butyraldehyde, tert-butyraldehyde, and n-pentaldehyde were as follows (in cm3 molecule−1 s−1): (a) in the Arrhenius form: (5.3 ± 0.5) × 10−12 exp[(405 ± 30)/T], (7.3 ± 1.9) × 10−12 exp[(390 ± 78)/T], (4.7 ± 0.8) × 10−12 exp[(564 ± 52)/T], and (9.9 ± 1.9) × 10−12 exp[(306 ± 56)/T]; (b) at 298 K: (2.0 ± 0.3) × 10−11, (2.6 ± 0.4) × 10−11, (2.7 ± 0.4) × 10−11, and (2.8 ± 0.2) × 10−11, respectively. In addition, using the relative rate method and alkanes as the reference compounds, the room-temperature rate constants have been measured for the reactions of chlorine atoms with propanaldehyde, iso-butyraldehyde, tert-butyraldehyde, n-pentaldehyde, acrolein, and crotonaldehyde. The obtained values were (in cm3 molecule−1 s−1): (1.4 ± 0.3) × 10−10, (1.7 ± 0.3)10−10, (1.6 ± 0.3) × 10−10, (2.6 ± 0.3) × 10−10, (2.2 ± 0.3) × 10−10, and (2.6 ± 0.3) × 10−10, respectively. The results are presented and discussed in terms of structure-reactivity relationships and atmospheric importance. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 676–685, 2000

The laser photolysis–resonance fluorescence technique has been used to determine the absolute rate coefficient for the Cl atom reaction with a series of ethers, at room temperature (298 ± 2) K and in the pressure range 15–60 Torr. The rate coefficients obtained (in units of cm3 molecule−1 s−1) are dimethyl ether (1.3 ± 0.2) × 10−10, diethyl ether (2.5 ± 0.3) × 10−10, di-n-propyl ether (3.6 ± 0.4) × 10−10, di-n-butyl ether (4.5 ± 0.5) × 10−10, di-isopropyl ether (1.6 ± 0.2) × 10−10, methyl tert-butyl ether (1.4 ± 0.2) × 10−10, and ethyl tert-butyl ether (1.5 ± 0.2) × 10−10. The results are discussed in terms of structure–reactivity relationship. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 105–110, 2000

The pulsed laser photolysis-resonance fluorescence technique has been used to determine the absolute rate coefficient for the Cl atom reaction with a series of ketones, at room temperature (298 ± 2) K and in the pressure range 15–60 Torr. The rate coefficients obtained (in units of cm3 molecule−1 s−1) are: acetone (3.06 ± 0.38) × 10−12, 2-butanone (3.24 ± 0.38) × 10−11, 3-methyl-2-butanone (7.02 ± 0.89) × 10−11, 4-methyl-2-pentanone (9.72 ± 1.2) × 10−11, 5-methyl-2-hexanone (1.06 ± 0.14) × 10−10, chloroacetone (3.50 ± 0.45) × 10−12, 1,1-dichloroacetone (4.16 ± 0.57) × 10−13, and 1,1,3-trichloroacetone (

Rate coefficients have been measured for the gas phasereactions of hydroxyl (OH) radicals and ozone with twounsaturated esters, allyl acetate(CH3C(O)OCH2CH=CH2) and isopropenylacetate (CH3C(O)OC(CH3)=CH2). The OHexperiments were carried out using the pulsed laserphotolysis – laser induced fluorescence technique overthe temperature range 243–372 K and the kinetic dataused to derive the following Arrhenius expressions (inunits of cm3 molecule-1 s-1): allylacetate, k 1 = (2.33 ± 0.27) ×10-12 exp[(732 ± 34)/T]; and isopropenyl acetate,k 2 = (4.52 ± 0.62) × 10-12exp[(809 ± 39)/T]. At 298 K, the rate coefficients obtained (inunits of 10-12 cm3 molecule-1 s-1)are: k 1 = (27.1 ± 3.0) and k 2= (69.6± 9.4). The relative rate technique has been usedto determine rate coefficients for the reaction ofozone with the acetates. Using methyl vinyl ketone asthe reference compound and a value of4.8 × 10-18 cm3 molecule-1s-1 asthe rate coefficient for its reaction with O3,the following rate coefficients were derived at 298 ± 4 K (in units of10-18 cm3molecule-1 s-1): allyl acetate, (2.4 ± 0.7) andisopropenyl acetate (0.7 ± 0.2). Theresults are discussed in terms of structure-activityrelationships and used to derive atmospheric lifetimesfor the acetates.

The kinetics of the disproportionation reactions of OH and OD radicals, OH + OH → O + H2O (1), OD + OD → O + D2O (3), have been studied by the mass spectrometric discharge-flow method at temperatures between 233 and 360 K and at total pressure of 1 Torr of helium. The following Arrhenius expressions were obtained: k1 = (7.1 ± 1.0) × 10-13exp[(210 ± 40)/T] and k3 = (2.5 ± 0.5) × 10-13exp[(170 ± 60)/T] cm3 molecule-1 s-1. Reaction 4 between OH and OD radicals was also investigated and the same rate constant as for reaction 1 was measured. Both the observed temperature dependence of k1 and the measured kinetic isotopic effect (k1/k3) are compatible with the mechanism proposed in a previous theoretical study (Harding, L. B.; Wagner, A. F. 22nd International Symposium on Combustion, 1988). In addition, a temperature-independent rate coefficient of (1.20 ± 0.25) × 10-10 cm3 molecule-1 s-1 was measured for the reaction D + NO2 → OD + NO in the temperature range 230−365 K.

La cinetique et le mecanisme de la reaction CH 3 O 2 + Br → produits (1) ont ete etudies a 298 K a l'aide de la methode du reacteur a ecoulement avec detection des especes par spectrometrie de masse. La constante de vitesse obtenue est: k 1 = (4.4 ± 0.4) x 10 -13 cm 3 molecule -1 s -1 . Le chemin principal, et probablement unique, de la reaction conduit a la formation de BrO. Ces resultats, qui sont les premiers pour cette reaction, sont compares a ceux de la reaction de CH 3 O 2 avec Cl

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBrI+Br2 (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k1=(2.7±0.4)×10−11 cm3 molecule−1s−1. The rate constant of the reverse reaction: I+Br2Br+IBr (−1) has been obtained from the Br2 consumption rate (with an excess of I atoms) and the IBr formation rate: k−1=(1.65±0.2)×10−13 cm3molecule−1s−1. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k1 and k−1 and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br2, is: K1=k1/k−1=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH298°=−(3.6±0.1) kcal mol−1 and the heat of formation of the IBr molecule, ΔHf,298°(IBr)=(9.8±0.1) kcal mol−1. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998

Recent investigations of the NO3-DMS reaction showed that HNO3 is the dominant nitrogen product. It is argued that this finding can have implications on the NOX - HNO3/nitrate partitioning, in particular lowering the NOX (= NO + NO2) concentrations in the remote marine boundary layer. Since there ozone formation is NOX limited, biogenic DMS sources may influence the ozone production on a global scale.

Several mutations that cause a decrease of 25 to 65% of the catalytic activity, were introduced at different positions in the phosphofructokinase from Escherichia coli, and the influence of the allosteric activator GDP on these mutants was measured. In the case of the wild-type enzyme, GDP converts the highly cooperative saturation towards fructose-6-phosphate into a hyperbolic saturation with almost no change in the maximum velocity. The mutants Glu148 --> Leu, Leu178 --> Val and Leu178 --> Trp are still cooperative for fructose-6-phosphate, and their cooperativity is also abolished or markedly decreased by GDP. In addition, GDP acts on these mutants as an activator of maximum velocity, and increases their catalytic rate constants by 35 to 65% depending on the mutation. The Leu178 --> Val mutant is even as active as the wild-type enzyme in the presence of GDP. The Thr125 --> Ser mutation decreases the maximum velocity by 60% and also suppresses the cooperativity towards fructose-6-phosphate. Accordingly, the only effect of GDP on the Thr125 --> Ser mutant is on its maximum velocity and not on its affinity for fructose-6-phosphate. However, the maximum velocity of this mutant is not increased by GDP but reduced by 33%. These results show that GDP affects the maximum velocity of these mutants and suggest that the activation by GDP of the wild-type enzyme measured by steady-state kinetics could be partially due to an increase of the maximum velocity, and not exclusively to an increase of the affinity for fructose-6-phosphate.

The formation, abundance and distribution of organic nitrates are relevant for determining the production efficiency and resident mixing ratios of tropospheric ozone (O3) on both regional and global scales. Here we investigate the effect of applying the recently measured direct chemical production of methyl nitrate (CH3ONO2) during NOx recycling involving the methyl-peroxy radical on the global tropospheric distribution of CH3ONO2 and the perturbations introduced towards tropospheric NOx and O3 using the TM5 global chemistry transport model. By comparisons against numerous observations, we show that the global surface distribution of CH3ONO2 can be largely explained by introducing the chemical production mechanism using a branching ratio of 0.3%, when assuming a direct oceanic emission source of ~0.15 Tg N yr−1. On a global scale, the chemical production of CH3ONO2 converts 1 Tg N yr−1 from nitrogen oxide for this branching ratio. The resident mixing ratios of CH3ONO2 are found to be highly sensitive to the dry deposition velocity that is prescribed, where more than 50% of the direct oceanic emission is lost near the source regions, thereby mitigating the subsequent effects due to long-range and convective transport out of the source region. For the higher alkyl nitrates (RONO2) we find improvements in the simulated distribution near the surface in the tropics (10° S–10° N) when introducing direct oceanic emissions equal to ~0.17 Tg N yr−1 . In terms of the vertical profile of CH3ONO2, there are persistent overestimations in the free troposphere and underestimations in the upper troposphere across a wide range of latitudes and longitudes when compared against data from measurement campaigns. This suggests either a missing transport pathway or source/sink term, although measurements show significant variability in resident mixing ratios at high altitudes at global scale. For the vertical profile of RONO2, TM5 performs better at tropical latitudes than at mid-latitudes, with similar features in the comparisons to those for CH3ONO2. Comparisons of CH3ONO2 with a wide range of surface measurements shows that further constraints are necessary regarding the variability in the deposition terms for different land surfaces in order to improve on the comparisons presented here. For total reactive nitrogen (NOy) ~20% originates from alkyl nitrates in the tropics and subtropics, where the introduction of both direct oceanic emissions and the chemical formation mechanism of CH3ONO2 only makes a ~5% contribution to the total alkyl nitrate content in the upper troposphere when compared with aircraft observations. We find that the increases in tropospheric O3 that occur due oxidation of CH3ONO2 originating from direct oceanic emission is negated when accounting for the chemical formation of CH3ONO2, meaning that the impact of such oceanic emissions on atmospheric lifetimes becomes marginal when a branching ratio of 0.3% is adopted.

We have performed both longitudinal and transverse magnetization measurements on a HgBa2CuO4+ 8 single crystal, from which we extract the superconducting parameters (An(0) ~- 1400/~, 611(0) ~ 20 ~). The anisotropy is found (3' ~ 30) to be the lowest of the known values for the HgBa2Ca ._ ~Cu.Ozn+2+8 family. A decrease of the apparent anisotropy is observed close to T c, which we tentatively attribute to a quasi-2D to 3D crossover.

Single crystals of HgBa 2 CuO 4+δ of submillimetric sizes were grown with the same one step, low pressure, gold amalgamation technique used to obtain single crystals of HgBa 2 Ca 2 Cu 3 O 8+δ . Remarkable superconducting properties are displayed by the samples which are optimally doped as grown. The sharpness of the transition profiles of the magnetic susceptibility, its anisotropy dependence and the volume fraction exhibiting the Meissner effect exceed the values obtained with the best crystal samples of Hg-1223. X-rays show that no substitutional defects have been found in the mercury plane, in particular no mixed occupancy of copper at the mercury site. The interstitial oxygen content at ( 1 2 , 1 2 , 0 ) δ = 0.066 ± 0.008 is about one third that observed in optimally doped Hg-1223, resulting in an identical doping level per CuO 2 plane in both compounds.

From torque measurements as a function of the angle and the magnetic field, we extract the magnetization curves and the anisotropy factor for a HgBa 2 Ca 2 Cu 3 O 8+δ single crystal. The anisotropy is found γ = 52 ± 3; it is similar to the one found for the HgBa 2 Ca 3 Cu 4 O 10+δ compound by Zech et al. The analysis of the magnetization close to H c2 ( T ) within the fluctuations theory yields the critical fluctuations parameter τ f = (3.9 ± 0.4) 10 −2 and the in-plane penetration depth λ(0) = 1500 ± 100 A . The superconducting planes separation is found of the order of the unit cell length. The upper critical field is estimaated ( d H c 2 d T ) T c 0 ≈ 14 kOe K −1 .

Absolute rate constants have been measured for the gas phase reactions of hydroxyl radicals with a series of aliphatic acetates: methyl acetate (k1), ethyl acetate (k2), n-propyl acetate (k3), n-butyl acetate (k4), and n-pentyl acetate (k5). Experiments were carried out using the pulsed laser photolysislaser induced fluorescence technique over the temperature range 243−372 K. The obtained kinetic data were used to derive the Arrhenius expressions, and since the Arrhenius plots were slightly curved they were fitted, using a three-parameter expression. The Arrhenius expressions obtained are: k1 = (0.53 ± 0.09) × 10-12 exp[−(128 ± 102)/T] ; k2 = (0.48 ± 0.09) × 10-12 exp[(397 ± 103)/T]; k3 = (1.03 ± 0.12) × 10-12 exp[(370 ± 69)/T] ; k4 = (2.10 ± 0.28) × 10-12 exp[(299 ± 81)/T], and k5 = (2.75 ± 0.46) × 10-12 exp[(302 ± 102)/T] (in units of cm3 molecule-1 s-1). At room temperature, the rate constants obtained (in units of 10-12 cm3 molecule-1 s-1) were as follows: methyl acetate, 0.32 ± 0.03; ethyl acetate...

Absolute rate constants for the reactions of OH radicals with butyl ethyl ether (k1), methyl tert-butyl ether (k2), ethyl tert-butyl ether (k3) tert-amyl methyl ether (k4) and tert-butyl alcohol (k5) have been measured over the temperature range 230–372 K using a pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k1 − k5 when expressed in Arrhenius form gave: k1 = (6.59 ± 0.66) × 10 −12 exp|(362 ± 60)/T|, k2 = (5.03 ± 0.27) × 10−12 exp|&minus(133 ± 30)/T|, k3 = (4.40 ± 0.24) × 10−12 exp|(210 ± 37)/T|,k4 = (4.7 ± 0.7) × 10−12 exp|(82 ± 85)/T|, and k5 = (2.66 ± 0.48) × 10−12 exp| −(270 ± 130)/T|. However, the Arrhenius plots for k1–k5, were slightly curved and are best fitted by the three parameter fits which are given in the article. The room temperature values of k1, k2, k3, k4, and k5 are (2.08 ± 0.23) × 10−11, (3.13 ± 0.36) × 10−12, (8.80 ± 0.50) × 10−12, (6.28 ± 0.45) × 10−12, and (1.08 ± 0.10) × 10−12, respectively, in cm3 molecule−1 s−1. © 1996 John Wiley & Sons, Inc.

The kinetics of C2H5O2 and C2H5O2 radicals with NO have been studied at 298 K using the discharge flow technique coupled to laser induced fluorescence (LIF) and mass spectrometry analysis. The temporal profiles of C2H5O were monitored by LIF. The rate constant for C2H5O + NO Products (2), measured in the presence of helium, has been found to be pressure dependent: k2 = (1.25±0.04) × 10−11, (1.66±0.06) × 10−11, (1.81±0.06) × 10−11 at P (He) = 0.55, 1 and 2 torr, respectively (units are cm3 molecule−1 s−1). The Lindemann-Hinshelwood analysis of these rate constant data and previous high pressure measurements indicates competition between association and disproportionation channels: C2H5O + NO + M C2H5ONO + M (2a), C2H5O + NO CH3CHO + HNO (2b). The following calculated average values were obtained for the low and high pressure limits of k2a and for k2b : k = (2.6±1.0) × 10−28 cm6 molecule−2 s−1, k = (3.1±0.8) × 10−11 cm3 molecule−1 s−1 and k2b ca. 8 × 10−12 cm3 molecule−1 s−1. The present value of k, obtained with He as the third body, is significantly lower than the value (2.0±1.0) × 10−27 cm6 molecule−2 s−1 recommended in air. The rate constant for the reaction C2H5O2 + NO C2H5O + NO2 (3) has been measured at 1 torr of He from the simulation of experimental C2H5O profiles. The value obtained for k3 = (8.2±1.6) × 10−12 cm3 molecule−1 s−1 is in good agreement with previous studies using complementary methods. © 1995 John Wiley & Sons, Inc.

The rate constants for the reactions of OH with dimethyl ether (k1), diethyl ether (k2), di-n-propyl ether (k3), di-isopropyl ether (k4), and di-n-butyl ether (k5) have been measured over the temperature range 230–372 K using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k1,k4, can be expressed in the Arrhenius plots form: k1 = (6.30 ± 0.10) × 10−12 exp[−(234 ± 34)/T] and k4 = (4.13 ± 0.10) × 10−12 exp[(274 ± 26)/T]. The Arrhenius plots for k2,k3, and k5, were curved and they were fitted to the three parameter expressions: k2 = (1.02 ± 0.08) × 10−17T2 exp[(797 ± 24)/T], k3 = (1.84 ± 0.23) × 10−17T2 exp[(767 ± 34)/T], and k5 = (6.29 ± 0.74) × 10−18T2 exp[(1164 ± 34)/T]. The values at 298 K are (2.82 ± 0.21) × 10−12, (1.36 ± 0.11) × 10−11,(2.17 ± 0.16) × 10−11, (1.02 ± 0.10) × 10−11, and (2.69 ± 0.22) × 10−11 for k1, k2, k3, k4, and k5, respectively, (in cm3 molecule−1 s−1). These results are compared to the literature data. © 1995 John Wiley & Sons, Inc.

Recent field measurements of BrO radical concentrations and estimates of ClO radical concentrations from field measurements of hydrocarbons have been used to calculate the ozone depletion rate in the Arctic boundary layer which occurs during springtime. Considering the ozone cycles involving the ClO/BrO reaction and the BrO self-reaction the calculated ozone depletion rates, as high as 6 ppb/day, are comparable to the observations.

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 semi-empirical quantum mechanical molecular orbital method PM3 has been used to investigate the nature of the transition states for the reactions of hydroxyl radicals with halogenated aldehydes. For each reaction, the geometric and electronic structures of the transition state corresponding to the abstraction of the aldehydic hydrogen atom were determined. The results indicate that the transition states are reactant-like and, from the calculation of electronic charge distributions, it can be concluded that changes in the reactivity along the series are mainly due to inductive effects in the transition states.

During the reaction catalyzed by the phosphofructokinase (EC 2.7.1.11) from Escherichia coli, the phosphoryl group transferred from ATP interacts with Thr-125 [Shirakihara, Y. & Evans, P. R. (1988) J. Mol. Biol. 204, 973-994]. The replacement of Thr-125 by serine changes the saturation by fructose 6-phosphate from cooperative to hyperbolic and abolishes the allosteric inhibition by phosphoenolpyruvate. The same changes, a saturation by fructose 6-phosphate that is no longer cooperative and an activity that is no longer inhibited by phosphoenolpyruvate, are observed with wild-type phosphofructokinase when adenosine 5'-[gamma-thio]triphosphate is used instead of ATP as the phosphoryl donor. These two perturbations of the ATP-Thr-125 interaction lead to the suppression of both the allosteric inhibition by phosphoenolpyruvate and the cooperativity of fructose-6-phosphate saturation, as if replacing the neutral oxygen of ATP by sulfur or removing the methyl group of Thr-125 had "locked" phosphofructokinase in its active conformation. The geometry of this ATP-Thr-125 interaction and/or the presence of the methyl group on the beta-carbon of Thr-125 are crucial for the regulatory properties of phosphofructokinase. This interaction could be a hydrogen bond between the neutral oxygen of the gamma-phosphate of ATP and the hydroxyl group of Thr-125.

The rate parameters for the reaction of the OH radical with CH3Br have been measured using the discharge flow-electron paramagnetic resonance method. The result isk 1=(1.86±0.48)×10−12 exp[−(1230±150)/T] cm3 molecule−1 s−1. This value is compared to earlier data and is found to be in excellent agreement with the most recent results, which greatly increases the accuracy of the ozone depletion potential of CH3Br which can be derived from these kinetic data.

The rate coefficients for the reactions of OH with H2 O2 , C3 H8 and CH4 have been determined using a newly built pulsed laser photolysis-laser induced fluorescence (PLP-LIF) apparatus. While, the reaction of OH with H2 O2 was studied at room temperature, the rate coefficients of OH reactions with CH4 and C3 H8 were measured over a large range of temperature at P = 100 Torr of helium. The measured rate constant for the reaction of OH with H2 O2 was k] = (1.75 ± 0.20) × 10-12 cm3 molecule-1 s-1 . The kinetic data for C3 H8 (k2 ) and CH4 (k3 ) were used to derive the Arrhenius expressions (in cm3 molecule-1 S-1 ) : k2 = (9.81 ± 0.11) × 10-12 exp[-(650 ± 30)/T] between 233 and 363 K, and k3 = (2.56 ± 0.53) × 10-12 exp[-(1765 ± 146)/T] between 233 and 343 K. The results obtained for k1 and k2 provide a validation of the PLP-LIF technique and those for k3 support the low value recently published, which has an important atmospheric impact.

We have used 170Yb3+ Mossbauer spectroscopy on Yb3+ diluted at the rare-earth sites in Pr-xYxBa2Cu3O7 and in PrBa2Cu3O6 to examine the magnetic properties associated with the (hybridized) Pr3+ ions. Magnetic ordering in the non-superconducting compounds PrBa2Cu3O7 (T> = 16 K) and PrBa2Cu3O7 (T = 10 K) is identified by the appearance of a Pr3+-derive d molecular field and by a change in the Yb3+ paramagnetic relaxation rates. Evidence is presented suggesting the ordered Pr3+ magentic moments are aligned well away from the c-axis. The splitting of the three lowest crystal-field levels in PrBa2Cu3O6 is estimated to be about half that (∼ 50 K) in PrBa2Cu3O7. In the superconducting samples of Pr1-xYxBa2Cu3O7, the Yb3+ l ocal probes give evidence for an inhomogeneous behavior linked to statistical variations in the Pr3+/Y3+ occupancies. With increasing x, both the volume of the sample showing local Pr3+ magnetic order and the average ordering temperature decreases. There appears to be an anti-correlation between the superconducting transition temperature and the volume of the sample showing Pr3+ magnetic order, suggesting the local Pr3+ concentration needed to remove superconductivity (in favor of Cu (2)-based magnetic order) is near that needed to support local Pr3+ magnetic order.