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| Titel |
Rate constant for the reaction of OH with methyl iodide, a re-determination by flash photolysis of water vapour and time resolved resonance fluorescence of OH |
| VerfasserIn |
Shaoliang Zhang, Rafal Strekowski, Cornelius Zetzsch |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040490
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| Zusammenfassung |
| Methyl iodide is a major source gas for atmospheric iodine, and it is mainly emitted from the
ocean. Aqueous-phase reactions, such as hydrolysis and exchange reactions with chloride
control its emissions to the atmosphere, where its lifetime is limited to less than
a week, mainly by photolysis. A minor contribution to the loss processes in the
troposphere is the gas-phase reaction with OH radicals, that has been investigated by
several authors. On the other hand, this reaction turned out to be uncertain in spite of
interest in nuclear safety after the International Phebus Fission Product programme,
initiated in 1988. Some of the most important observed phenomena with regard to
the chemistry of iodine were not predicted, clearly showing the need for carrying
out rate constant determinations for the reactions of I2 and CH3I with OH, which
is a major oxidant product from the air radiolysis under accident conditions. We
have measured the rate constant for the reaction OH + CH3I - H2O + CH2I in
He at 260 mbar in the temperature range from 298 to 362 K. OH radicals were
produced by flash photolysis of H2O in the vacuum-UV at wavelengths > 115
nm using a Xe flash lamp with a MgF2 window. Time profiles of OH radicals are
monitored by resonance fluorescence of the A2 Σ - X2 Πtransition at 308 nm,
induced by the emission from a microwave discharge of a flow of He and H2O,
a few Torr each. The signal is monitored by photon counting and multichannel
scaling, collecting the counts from 50 flashes each, obtaind by pulsed photolysis of
various mixtures of H2O and CH3I under slow-flow conditions. Decays of OH in the
presence of CH3I are observed to be exponential, and the decay rates are found to be
linearly dependent on the concentration of CH3I. Rate constants, k ± 2Ï (in units
of 10-14 cm3 s-1) of 4.14±0.20, 6.33±0.68, 7.31±1.18 and 8.24±1.60 at 298,
326, 352 and 362 K, respectively, are obtained from linear regressions and lead to
an Arrhenius expression of k = 1.5Ã10-12 exp (-1067 K / T) cm3 s-1 . These
rate constants are significantly below the present IUPAC recommendation. The
consequence is an even smaller contribution of the OH reaction to tropospheric
degradation.
Acknowledgements: Shaoliang Zhang and Rafal Strekowski wish to thank the Institut de
Radioprotection et de Sûreté Nucléaire (IRSN) for support and Cornelius Zetzsch
acknowledges funding within the research unit HALOPROC of the German Science
Foundation |
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