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SUMMARY:The reaction of halogen oxides (BrO and ClO) with CH3O2\; laborato
 ry based temperature dependent kinetic and product studies. - Dr Kimberley
  Leather\, University of Manchester
DTSTART:20121105T151500Z
DTEND:20121105T163000Z
UID:TALK40517@talks.cam.ac.uk
CONTACT:Dr Alex Archibald
DESCRIPTION:Halogens play a major role in catalytic cycles leading to the 
 destruction of ozone not only in the\nstratosphere but also in the troposp
 here and marine environments. In low NO<sub>x</sub> conditions\, indicativ
 e of that in the marine boundary layer\, inclusion of halogen oxides in th
 e methane oxidation cycle not only competes with ozone-producing NO<sub>x<
 /sub> catalysis of peroxy radicals\, but the products formed from the reac
 tion of XO with RO<sub>2</sub> deplete ozone further.\n\nLimited data are 
 available for peroxy radicals reacting with halogen oxides. Temperature de
 pendent\nkinetics and product studies were carried out for the reaction of
  ClO and BrO with CH<sub>3</sub>O<sub>2</sub> using a turbulent flow tube 
 coupled to a chemical ionisation mass spectrometer (CIMS).\n\nAn Arrhenius
  expression was obtained for the overall rate coefficient of CH<sub>3</sub
 >O<sub>2</sub> + ClO reaction:\n\nk(T) = 1.96 (+0.28 −0.24) x 10<sup>-11
 </sup> exp[(-626 ± 35) / T] cm<sup>3</sup> molecule<sup>-1</sup> s<sup>-1
 </sup>. Over a range of pressure (100–\n200 Torr) and temperature (298
 –223 K) no pressure dependence is observed. The smaller rate\ncoefficien
 ts measured at lower temperatures compared with both previous low temperat
 ure\nstudies are believed to arise through the reduction of secondary chem
 istry and greater sensitivity in\nterms of reactant detection (hence much 
 lower initial concentrations were employed). These new\ndata reduce the ef
 fectiveness of ozone loss cycles involving reaction of CH<sub>3</sub>O<sub
 >2</sub> + ClO in the polar stratosphere by around a factor of 1.5 and res
 trict the importance of the reaction to the tropical and extra-tropical cl
 ean marine environments in the troposphere.\n\nFor the reaction of BrO + C
 H3O2 an Arrhenius expression was determined to be k(T) = 2 .42 (+1.02 −0
 .72)x10<sup>\n-14</sup> exp[(1617 ± 94) / T] cm<sup>3</sup> molecule<sup>
 -1</sup> s<sup>-1</sup> over a temperature range of 243 – 296 K. No pres
 sure dependence was observed for this reaction. A more pronounced apparent
  negative activation energy\, double that retrieved in a previous study\, 
 was observed. HOBr was found to be the major product with a yield of 0.8 
 ± 0.1 at 296 K. HOOBr was found to be a minor product with an upper\nlimi
 t of 0.1. According to modelling studies\, the increased apparent negative
  activation energy would\nlead to a significant source of HOx and a non-ne
 gligible source of HC(O)OH in the upper troposphere.
LOCATION:Unilever Lecture Theatre\, Department of Chemistry
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