Pressure dependence of two relative ozone formation rate coefficients

Guenther, Juergen; Erbacher, Burkard; Krankowsky, D.; Mauersberger, K.

doi:10.1016/s0009-2614(99)00469-8

Cited by 52 publications

(67 citation statements)

References 20 publications

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“…The results are summarized elsewhere. 1 The experiments involve a ''mass-independent'' isotope effect in ozone formation in scrambled systems and contrastingly different experimental results [25][26][27][28][29] in unscrambled systems. The latter show, instead, dramatic unconventional mass-dependent effects.…”

Section: Introductionmentioning

confidence: 99%

On the theory of the strange and unconventional isotopic effects in ozone formation

Gao

Marcus

2002

The Journal of Chemical Physics

151

186

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The strange mass-independent isotope effect for the enrichment of ozone and the contrastingly unconventional strong mass-dependent effect of individual reaction rate constants are studied using statistical ͑RRKM͒-based theory with a hindered-rotor transition state. Individual rate constant ratios of recombination reactions and enrichments are calculated. The theory assumes ͑1͒ an ''-effect,'' which can be interpreted as a small deviation from the statistical density of states for symmetric isotopomers, compared with the asymmetric isotopomers, and ͑2͒ weak collisions for deactivation of the vibrationally excited ozone molecules. A partitioning effect controls the recombination rate constant ratios. It arises from small differences in zero-point energies of the two exit channels of dissociation of an asymmetric ozone isotopomer, which are magnified into large differences in numbers of states in the two competing exit channel transition states. In enrichment experiments, in contrast, this partitioning factor disappears exactly ͓Hathorn and Marcus, J. Chem.Phys. 112, 9497 ͑2000͔͒, and what remains is the -effect. Both aspects can be regarded as ''symmetry driven'' isotopic effects. The two experiments, enrichments and rate constant ratios, thus reveal markedly different theoretical aspects of the phenomena. The calculated low-pressure ozone enrichments, the low-pressure recombination rate constant ratios, the effects of pressure on the enrichment, on the individual recombination rate constant ratios, and on the recombination rate constant are consistent with the experimental data. The temperature dependence of the enrichment and of the recombination rate constant ratios is discussed and a variety of experimental tests are proposed. O at 130 K and 300 K is used for testing or providing information on the nature of a variationally determined hindered-rotor transition state. The theory is not limited to ozone formation but is intended to apply to other reactions where a symmetrical stable or unstable gas phase molecule may be formed.

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Section: Introductionmentioning

confidence: 99%

On the theory of the strange and unconventional isotopic effects in ozone formation

Gao

Marcus

2002

The Journal of Chemical Physics

151

186

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“…We hypothesize that the high isotope enrichments, reflected in both the δ 18 O and the 17 O values, occur when O 3 oxidation of NO is faster than NO x -O-O 2 isotopic exchange. Isotopic enrichments associated with the formation of O 3 have been extensively studied Guenther et al, 1999;Mauersberger et al, 2003;Thiemens, 1999). At temperature and pressure range of the present experiments, the recombination process should generate O 3 with δ 18 O values between 90-130 ‰ and 17 O values between 30-45 ‰ .…”

Section: Discussion and Model Predictionsmentioning

confidence: 65%

“…The first is the NO 2 -NO equilibrium (Sharma et al, 1970 Guenther et al (1999) and Mauersberger et al (2005). * * * 17 O = 1000 · ln(1+δ 17 O/1000)−0.516 · 1000 · ln(1+δ 18 O/1000) measures the magnitude of mass independent enrichment.…”

Section: Accounting For Isotopes Of Total No X In the Modelmentioning

confidence: 99%

NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation

2014

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Abstract. The oxygen isotope composition of nitrogen oxides (NO x ) in the atmosphere is a useful tool for understanding the oxidation of NO x into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NO x due to NO x -O 2 -O 3 photochemical cycling. At low NO x / O 2 mixing ratios, NO x became progressively and nearly equally enriched in 17 O and 18 O over time until it reached a steady state with 17 O values of 39.3 ± 1.9 ‰ and δ 18 O values of 84.2 ± 4 ‰, relative to the isotopic composition of the initial O 2 gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O 2 , and NO x . A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O 3 formation play a dominant role in controlling NO x isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NO x cycling. The data and model were consistent with previous studies which showed that the NO + O 3 reactions occur mainly via the transfer of the terminal atoms of O 3 . The model predicts that under tropospheric concentrations of NO x and O 3 , the timescale of NO x -O 3 isotopic equilibrium ranges from hours (for ppbv NO x / O 2 mixing ratios) to days (for pptv mixing ratios) and yields steady state 17 O and δ 18 O values of 45 ‰ and 117 ‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O 3 has high concentrations, the equilibrium between NO x and O 3 should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NO x conversion to HNO 3 is faster. The experimentally derived rate coefficients can be used to model the major NO x -O 3 isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.

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“…There exist mainly two types of experimental observations: an unusual "mass-independent" isotopic fractionation ͑MIF͒ of ozone and a large unconventional highly "mass-dependent" isotope effect observed in the various rate constant ratios. [25][26][27][28][29] Rice-RamspergerKassel-Marcus ͑RRKM͒ theory with a master equation approach for the collisional deactivation of the intermediate excited ozone molecules has been applied to study both phenomena. [32][33][34] It was found that the MIF can be explained when an -effect, a non-RRKM effect, [30][31][32][33][34] which reduces dynamically the low-pressure rate constant of the recombination to form symmetric molecules by a factor of compared to that for the asymmetric ones, and a weak collision effect [32][33][34] for the deactivation of excited ozone molecules is also included in the theory.…”

Section: Introductionmentioning

confidence: 99%

An approximate theory of the ozone isotopic effects: Rate constant ratios and pressure dependence

Gao

Marcus

2007

The Journal of Chemical Physics

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The isotopic effects in ozone recombination reactions at low pressures are studied using an approximate theory which yields simple analytic expressions for the individual rate constant ratios, observed under "unscrambled" conditions. It is shown that the rate constant ratio between the two competing channels XYZ→ X + YZ and XYZ→ XY+ Z is mainly determined by the difference of the zero-point energies of diatomic molecules YZ and XY and by the efficiency of the deactivation of the newly formed excited ozone molecules, whereas the mass-independent fractionation depends on a "nonstatistical" symmetry factor and the collisional deactivation efficiency. Formulas for the pressure effects on the enrichment and on the rate constant ratios are obtained, and the calculated results are compared with experiments and more exact calculations. In all cases, ratios of isotope rates and the pressure dependence of enrichments, the agreement is good. While the initial focus was on isotope effects in the formation of O 3 , predictions are made for isotope effects on ratios of rate constants in other reactions such as O + CO → CO 2 , O+NO→ NO 2 , and O + SO → SO 2 .

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Pressure dependence of two relative ozone formation rate coefficients

Cited by 52 publications

References 20 publications

On the theory of the strange and unconventional isotopic effects in ozone formation

On the theory of the strange and unconventional isotopic effects in ozone formation

NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation

An approximate theory of the ozone isotopic effects: Rate constant ratios and pressure dependence

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Pressure dependence of two relative ozone formation rate coefficients

Cited by 52 publications

References 20 publications

On the theory of the strange and unconventional isotopic effects in ozone formation

On the theory of the strange and unconventional isotopic effects in ozone formation

NO&lt;sub&gt;x&lt;/sub&gt; cycle and the tropospheric ozone isotope anomaly: an experimental investigation

An approximate theory of the ozone isotopic effects: Rate constant ratios and pressure dependence

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NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation