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Patrick Laine EAS Advisor: Paul Wine

Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with C 2 H 5 Br, n -C 3 H 7 Br, and 1,2-C 2 H 4 Br 2. Patrick Laine EAS Advisor: Paul Wine. Relevance. Lifetimes of many trace gases are dependent on gas phase atmospheric chemistry

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Patrick Laine EAS Advisor: Paul Wine

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  1. Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with C2H5Br, n-C3H7Br, and 1,2-C2H4Br2 Patrick Laine EAS Advisor: Paul Wine

  2. Relevance • Lifetimes of many trace gases are dependent on gas phase atmospheric chemistry • Impact on ozone levels in troposphere and stratosphere

  3. Ozone Depletion • 90% of O3 is in the Stratosphere and is considered “good” O3 • Mid 1970’s it was discovered that halogen source gases lead to ozone depletion • Short lived halogen source gases undergo chemical conversions in troposphere producing reactive halogen gases and other compounds

  4. Why look at Alkyl Bromides? • The alkyl bromides are emitted from anthropogenic sources i.e., solvents, fumigants • These alkyl bromides have been identified by WMO (2006) as short lived source gases with significant ozone depletion potentials (ODP) • Br is 40-100x more effective than Cl at destroying O3

  5. OH + Alkyl Bromide *Data retrieved from Nasa Evaluation No. 15; **Data retrieved from NIST Database

  6. Could Cl reactions be important? • Cl reactions could be important depending on the rate constant • [Cl] varies throughout the atmosphere • Measurements of [Cl] have been increasing with improved technology

  7. Cl + Alkyl Bromide • Cl + ethyl bromide: • product study by Orlando and Tyndall (2002) • kinetic study by Donaghy et al., 1993 (300 K) • k = 1.4 x 10-11 cm3 molec-1 s-1 • Cl + n-propyl bromide • kinetic study by Donaghy et al., 1993 (300 K) • k = 6 x 10-11 cm3 molec-1 s-1 • theoretical study by Francisco, et al., 2008 • Cl + 1,2-dibromoethane • No previous study found in literature

  8. Reactions • Cl + CH3CH2Br  CH2CH2Br  Br • Cl + CH3CH2Br  CH3CHBr • Cl + CH3CH2CH2Br  CH3CHCH2Br  Br • Cl + CH3CH2CH2Br  CH2CH2CH2Br • Cl + CH3CH2CH2Br  CH3CH2CHBr • Cl + CH2BrCH2Br  BrCHCH2 + Br

  9. Laboratory Approach • Flow a mixture of gases with known concentrations through temperature controlled reaction cell • Generate Cl atoms via LFP of Cl2 or COCl2 • Couple LFP with time resolved atomic resonance fluorescence spectroscopic detection of Br • Monitor kinetics of Br as a function of reactant concentration, Temp., and Press.

  10. Photon counts vs. Time

  11. Data Analysis St/S0 = (ka/(kd-ka))*A*(exp(-ka*t)-exp(-kd*t))+B*exp(-kd*t) [Cl] = 5 x 1011 atoms cm-3 [C3H7Br] = 1.3 x 1014 molec cm-3

  12. Bimolecular Plot is Derived Slope = k = 5.2 x 10 -11 cm3 molec-1 s-1

  13. Table of Kinetic Results k in units of 10-12 cm3 molec-1 s-1

  14. Arrhenius Plots

  15. Yield Measurements • Compare magnitude of rise as a result of the Cl + Br2 reaction (100% Br yield) with the rise from the Cl + Alkyl Bromide reaction for a given set of conditions

  16. Data Analysis St/S0 = (ka/(kd-ka))*A*(exp(-ka*t)-exp(-kd*t))+B*exp(-kd*t) Signal Counts Time (s) [Cl] = 5 x 1011 atoms cm-3 [C3H7Br] = 1.3 x 1014 molec cm-3

  17. Yield Data

  18. Reactions • Cl + CH3CH2Br  CH2CH2Br  Br • Cl + CH3CH2Br  CH3CHBr • Cl + CH3CH2CH2Br  CH3CHCH2Br  Br • Cl + CH3CH2CH2Br  CH2CH2CH2Br • Cl + CH3CH2CH2Br  CH3CH2CHBr • Cl + CH2BrCH2Br  BrCHCH2 + Br

  19. Experiment vs. Atmosphere • In our reaction cell: • Cl + CH3CH2Br  CH2CH2Br  Br + CH2=CH2 • In the atmosphere: • Cl + CH3CH2Br  CH2CH2Br + O2 BrCH2CH2(OO)  Products such as BrCH2CHO, BrCH2CH2OH, and BrCH2CH2OOH (Orlando and Tyndall, 2002)

  20. In the end… Established rate coefficients and branching ratios will be useful for modeling atmospheric processes, may help make predictions for future climate change, and may facilitate a better understanding of ozone depletion.

  21. Acknowledgements • Mike Nicovich • Zhijun Zhao • Paul Wine • Dow Huskey • Funded by: • NASA Upper Atmospheric Research Program

  22. Finlayson-Pitts and Pitts, 2000 (Provided by J.M. Nicovich and P.H. Wine)

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