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Featured researches published by David M. Rowley.


Geophysical Research Letters | 1999

OIO and the atmospheric cycle of iodine

R. A. Cox; W. J. Bloss; Roger Jones; David M. Rowley

IO and BrO radicals are intermediates in the atmospheric photo-oxidation of iodo- and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss.


Physical Chemistry Chemical Physics | 2005

Kinetics of the gas phase HO2 self-reaction: Effects of temperature, pressure, water and methanol vapours

Daniel Stone; David M. Rowley

The kinetics of the gas phase HO2 self-reaction have been studied using flash photolysis of Cl2/CH3OH/O2/N2 mixtures coupled with time-resolved broadband UV absorption spectroscopy. The HO2 self-reaction rate coefficient (HO2 + HO2 --> H2O2 + O2 (R1)) has been determined as a function of temperature (236 < T < 309 K, at 760 Torr) and pressure (100 < p < 760 Torr, at 296 K). In addition, the effects of water vapour ((0-6.0) x 10(17) molecules cm(-3), 254 < T < 309 K at 760 Torr, 400 < p < 760 Torr at 296 K) and methanol vapour ((0.06-4.7) x 10(17) molecules cm(-3), 254 < T < 309 K, at 760 Torr) on the rate coefficient have been characterised. The observed rate coefficient, k1, was found to exhibit a negative temperature dependence with both pressure dependent and pressure independent components, in agreement with previous studies. Furthermore, the rate coefficient k1 was found to be enhanced in the presence of elevated H2O or CH3OH concentrations, as reported previously. This study reports the most extensive characterisation of the rate coefficient k1 as a function of T, p, [H2O] and [CH3OH]. The present results indicate that k1 is higher at low temperatures, and that enhancement of k1 by H2O is greater, than has been reported previously. The pressure dependence of k1 at ambient temperature is in good agreement with previous studies. The rate enhancement by CH3OH reported here is in good agreement with previous studies at ambient temperatures but is smaller at low temperatures than the most recent previous investigation suggests. The rate coefficient k1 is adequately parameterised by: k1(760 Torr) = {(1.8 +/- 0.8) x 10(-14) exp((1500 +/- 120)/T/K)} x {1 + (2.0 +/- 4.9) x 10(-25) [H2O] exp((4670 +/- 690)/T/K)} x (1 + (0.56 +/- 1.00) x 10(-21) [CH3OH] exp((2550 +/- 500)/T/K)} cm(-3) molecule(-1) s(-1), where [H2O] and [CH3OH] are in molecules cm(-3). Errors are 1 sigma, and statistical only. The atmospheric implications of these results are briefly discussed.


Journal of the Chemical Society, Faraday Transactions | 1994

Mechanism of atmospheric oxidation of 1,1,1,2-tetrafluoroethane (HFC 134a)

Oliver V. Rattigan; David M. Rowley; Oliver Wild; Roderic L. Jones; R. Anthony Cox

The chlorine-initiated photooxidation of hydrofluorocarbon 134a (CF3CH2F) has been studied in the temperature range 235–318 K and at 1 atm total pressure using UV absorption. Trifluoroacetyl fluoride [CF3C(O)F] and formyl fluoride [HC(O)F] were observed as the major products. IR analysis of the reaction mixture also showed carbonyl fluoride [C(O)F2] as a product. By measurement of the yields of [HC(O)F] from the photooxidation as a function of [O2] and temperature, the rate of the unimolecular decomposition of the oxy radical, CF3CHFO, reaction (5), was determined relative to its reaction with O2, reaction (4): CF3CHFO + O2→ CF3C(O)F + HO2(4), CF3CHFO → CF3+ HC(O)F (5) The results were treated using both an arithmetic derivation and numerical integration with a detailed reaction scheme. Inclusion of other recently published kinetic data leads of the following recommended rate expression for reaction (5) at 1 atm k5= 7.4 × 1011 exp[(–4720 ± 220)/T] s–1 The errors are 1σ.The observation of enhanced product yields in the present work is attributed to the reaction of the CF3O radical with HFC 134a leading to further peroxy radical formation. The results have been incorporated into a 2D atmospheric model to assess the environmental implications of HFC 134a release in the troposphere.


Journal of Atmospheric Chemistry | 1999

The UV-visible absorption cross-sections and atmospheric photolysis rate of HOI

David M. Rowley; Juliane C. Mössinger; R. Anthony Cox; Roderic L. Jones

The UV-visible absorption cross-sections of HOI have been recorded over the wavelength range 278-494 nm and at 298 K following generation of HOI in the gas phase using laser flash photolysis. The gas phase reaction of OH with I2 was used to produce HOI, and the absorption spectrum of HOI was calibrated relative to the consumption of I2. The HOI spectrum recorded exhibits 2 broad absorption maxima of σ = 3.99 × 10-19 cm2 and σ = 2.85 × 10-19 cm2, centred at 338.4 nm and 404.8 nm respectively. The spectrum is adequately described by a parameterisation consisting of two semi-logarithmic Gaussian distribution functions. The HOI spectrum is more intense than that recorded in previous work of Jenkin, but is in good agreement with more recent work by Bauer et al. The parameterised HOI absorption spectrum recorded in this work was used in a radiative model to calculate the atmospheric photolysis rate (J-value) of HOI. These results indicate that, under most sunlit conditions, HOI has a lifetime with respect to solar photolysis of the order of minutes. Experiments attempting to generate HOI by the reaction of O atoms with C2H5I led to complex absorption spectra containing a negative contribution to the absorption from the photolytic removal of an unidentified species. In addition, evidence was found for adsorption and desorption of an iodine-containing species in the reaction vessel. This behaviour is rationalised in terms of the disproportionation of HOI to I2O, and an uncalibrated spectrum tentatively attributed to I2O has been recorded.


Journal of the Chemical Society, Faraday Transactions | 1991

UV absorption spectrum and self-reaction of cyclohexylperoxy radicals

David M. Rowley; Phillip D. Lightfoot; Robert Lesclaux; Timothy J. Wallington

The kinetics and mechanism of the self-reaction of cyclohexylperoxy radicals: 2c-C6H11O2→ 2c-C6H11O + O2(1a), → c-C6H10O + c-C6H11OH + O2(1b) have been studied using both time-resolved and end-product analysis techniques. Determination of the product yields from the photooxidation of Cl2–c-C6H12–O2–N2 mixtures using FTIR spectrometry demonstrates that the branching ratio for the radical-producing channel (1a) is 0.29 ± 0.02 at 295 K. Furthermore, the dependence of the product yields on oxygen partial pressure shows that ring-opening of the cyclohexyloxy radical formed in channel (1a): c-C6H11O + M → CH2(CH2)4CHO + M (4) competes with the reaction with oxygen: c-C6H11O + O2→ c-C6H10O + HO2(2) under atmospheric conditions. Flash photolysis–UV absorption experiments were used to obtain the UV spectrum of the cyclohexylperoxy radical and the kinetics of reaction (1). The spectrum of c-C6H11O2 is similar to those of other alkylperoxy radicals, with a maximum cross-section of (4.95 ± 0.51)× 1018 cm2 molecule–1 at 250 nm, measured relative to a value of 4.55 × 10–18 cm2 molecule–1 for CH3O2 at 240 nm. Reaction (1) is slow compared to the self-reactions of primary alkylperoxy radicals, but is significantly faster than that of isopropylperoxy radicals at room temperature. Experiments as a function of temperature from 253 to 373 K give: kobs(2.0 ± 0.4)× 10–13 exp[–(487 ± 64)K/T] cm3 molecule–1 s–1 for reaction (1). The room-temperature branching ratio measurement enables a value of 2.84 × 10–14 cm3 molecule–1 s–1 to be assigned to k1 at 298 K. The above errors are 1σ and represent experimental uncertainty only; assuming a 10% uncertainty in the CH3O2 calibration cross-section, absolute uncertainties in the values of the cyclohexylperoxy cross-sections and kobs are 16% and 17%, respectively.


Physical Chemistry Chemical Physics | 2005

Kinetic studies of the ClO + ClO association reaction as a function of temperature and pressure

Gavin Boakes; W. H. Hindy Mok; David M. Rowley

The kinetics of the association reaction of ClO radicals: ClO + ClO + M --> Cl2O2+ M (1), have been investigated as a function of temperature T between 206.0-298.0 K and pressure p between 25-760 Torr using flash photolysis with time-resolved UV absorption spectroscopy. ClO radicals were generated following the photolysis of Br2/Cl2O mixtures in nitrogen diluent gas. Charge coupled device (CCD) detection of time resolved absorptions was used to monitor ClO radicals over a broad wavelength window covering the ClO (A 2Pi<-- X 2Pi) vibronic absorption bands. The high pass filtered ClO absorption cross sections were calibrated as a function of temperature between T = 206.0-320 K, and exhibit a negative temperature dependence. The ClO association kinetics were found to be more rapid than those reported in previous studies, with limiting low and high pressure rate coefficients, in nitrogen bath gas, k0 = (2.78 +/- 0.82) x 10(-32) x (T/300)(-3.99 +/- 0.94) molecule(-2) cm6 s(-1) and k(infinity) = (3.37 +/- 1.67) x 10(-12) x (T/300)(-1.49 +/- 1.81) molecule(-1) cm3 s(-1), respectively, (obtained with the broadening factor F(c) fixed at 0.6). Errors are 2sigma. The pressure dependent ClO association rate coefficients (falloff curves) exhibited some discrepancies at low pressures, with higher than expected rate coefficients on the basis of extrapolation from high pressures (p > 100 Torr). Reanalysis of data excluding kinetic data recorded below p = 100 Torr gave k0 = (2.79 +/- 0.85) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1) and k(infinity) = (3.44 +/- 1.83)x 10(-12) x (T/300)(-1.73 +/- 1.91) molecule(-1) cm3 s(-1). Potential sources of the low pressure discrepancies are discussed. The expression for k(0) in air bath gas is k0 = (2.62 +/- 0.80) x 10(-32) x (T/300)(-3.78 +/- 0.98) molecule(-2) cm6 s(-1). These results support upward revision of the ClO association rate coefficient recommended for use in stratospheric models, and the stratospheric implications of the results reported here are briefly discussed.


Journal of the Chemical Society, Faraday Transactions | 1992

Ultraviolet absorption spectrum and self-reaction of cyclopentylperoxy radicals

David M. Rowley; Phillip D. Lightfoot; Robert Lesclaux; Timothy J. Wallington

The kinetics and mechanism of the self-reaction of cyclopentylperoxy radicals: 2 c-C5H9O2→ 2 c-C5H9O + O2(1a), → c-C5H9OH + c-C5H8O + O2(1b), have been studied using both time-resolved and end-product-analysis techniques. Determination of the product yields from the photolysis of Cl2–c-C5H10–O2–N2 mixtures using FTIR spectroscopy demonstrates that ring-opening of the cyclopentoxy radical formed in channel (1a): c-C5H9O + M → CH2(CH2)3CHO + M (3) dominates over reaction with oxygen: c-C5H9O + O2→ c-C5H8O + HO2(2), under atmospheric conditions. Flash photolysis-UV absorption experiments were used to obtain the UV spectrum of the cyclopentylperoxy radical and the kinetics of reaction (1). The spectrum of c-C5H9O2 is similar to those of other alkylperoxy radicals, with a maximum cross-section of (5.22 ± 0.20)× 10–18 cm2 molecule–1 at 250 nm, measured relative to a value of 4.55 × 10–18 cm2 molecule–1 for CH3O2 at 240 nm. The observed second-order rate constant, kobs(–d[c-C5H9O2]/dt= 2kobs[c-C5H9O2]2), for removal of cyclopentylperoxy radicals was dependent on the oxygen partial pressure. Experiments as a function of temperature from 243 to 373 K gave limiting minimum and maximum values of kobs at low ( 50 Torr) oxygen partial pressures, respectively: kmin/cm3 molecule–1 s–1=(1.3 ± 0.4)× 10–14 exp[(188 ± 83)K/T] and kmax/cm3 molecule–1 s–1=(2.9 ± 0.8)× 10–13 exp[–(555 ± 77)K/T]. At low oxygen partial pressures, the only effective removal channel for cyclopentylperoxy radicals is the molecular channel (1b) and kmin can be equated to k1b. Simulations suggest that kmax represents an upper limit on k1 and is at most 25% greater. In light of the present results on the cyclopentylperoxy radical, further experiments were performed on the cyclohexylperoxy radical self-reaction: 2 c-C6H11O2→ 2 c-C6H11O + O2(16a), → c-C6H11OH + c-C6H10O + O2(16b) at low oxygen partial pressures, giving k16b/cm3 molecule–1 s–1=(1.3 ± 0.3)× 10–14 exp[(185 ± 15) K/T] and an estimated k16/cm3 molecule–1 s–1= 7.7 × 10–14 exp(–184 K/T). The above errors are 1σ and represent experimental uncertainty only.


Physical Chemistry Chemical Physics | 2002

Ab initio investigations of the potential energy surfaces of the XO + HO2 reaction (X = chlorine or bromine)

Nikolas Kaltsoyannis; David M. Rowley

The potential energy surfaces of the title reactions have been studied computationally using coupled cluster techniques. Many true minimum and transition state structures have been located, which connect the reactants with various product channels. The potential surfaces for the bromine reactions are found to be very similar to those of the chlorine analogues. On the singlet surface it is found that XO and HO2 come together to form chain HOOOX, which is 64 kJ mol(-1) (Cl) and 72 kJ mol(-1) (Br) more stable than the reactants. The energy barrier for HOOOX dissociation to either HOX + O-2 ((1)Delta) or HX + O-3 is calculated to be very high (at least 90 kJ mol(-1) for both the chlorine and bromine surfaces). By contrast, reaction along the triplet surface to form HOX + O-2 ((3)Sigma) is found to be energetically facile, with a small (ca. 10 kJ mol(-1)) negative activation barrier with respect to the reactants. The title reactions are therefore predicted to proceed along the triplet surface, implying that the branching ratio into the minor product channels (HX + O-3) is essentially zero. Extensive comparisons are drawn between the present studies and previous experimental and theoretical work. The present calculations predict a slight negative temperature dependence for the overall rate coefficient of the reactions along the triplet surface, in agreement with the current experimental and theoretical consensus. The atmospheric implications of the present studies are discussed.


Journal of the Chemical Society, Faraday Transactions | 1995

A spectroscopic study of Cl2O3

Matthew H. Harwood; David M. Rowley; Raymond A. Freshwater; R. Anthony Cox; Roderic L. Jones

The UV absorption spectrum of dichlorine trioxide, Cl2O3 has been recorded in the wavelength range 220–340 nm and at 223 K using a recently developed flash photolysis–kinetic UV absorption spectroscopy apparatus. Cl2O3 was generated in the reaction between ClO and OClO following the partial photolysis of OClO. The spectrum obtained is smooth, and shows a peak at 267 nm, where σ= 1.76 × 10–17 cm2 molecule–1(base e). Results are broadly consistent with previous determinations of the spectrum, although we report higher cross-sections in the long-wavelength tail of the absorption. The discrepancies between these and previous measurements are discussed, together with the atmospheric implications of this work.


Physical Chemistry Chemical Physics | 2003

Kinetic studies of the gas phase INO2 self-reaction

Kate S. Gawler; Gavin Boakes; David M. Rowley

Flash photolysis of CF3I/NO2/N2 gas mixtures has demonstrated the presence of the INO2 self-reaction: INO2 + INO2 → I2 + 2NO2The occurrence of reaction (1) has been confirmed through simultaneous monitoring of NO2 and I2 by time-resolved UV-visible absorption spectroscopy. The second order rate coefficient for reaction (1), k1, has been measured and the first temperature dependence study of k1 has been carried out. A positive temperature dependence for k1 was observed between 277.7 and 344.8 K which is described by k1(T) = (4.7 ± 0.55) × 10−13 exp((−1670 ± 340)/T) molecule−1 cm3 s−1. Errors are 2σ. The atmospheric implications of these results are briefly discussed.

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Valerio Ferracci

National Physical Laboratory

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Gavin Boakes

University College London

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R. A. Cox

University of Cambridge

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