1. 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

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

3. Ozone Depletion 90% of O 3 is in the Stratosphere and is considered “good” O 3 90% of O 3 is in the Stratosphere and is considered “good” O 3 Mid 1970’s it was discovered that halogen source gases lead to ozone depletion 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 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 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) 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 O 3 Br is 40-100x more effective than Cl at destroying O 3

5. OH + Alkyl Bromide Reaction k, 298 K (cm 3 molec -1 s -1 ) k, 220 K (cm 3 molec -1 s -1 ) *OH + ethyl bromide 3.4 x 10 -13 1.5 x 10 -13 *OH + propyl bromide 1.0 x 10 -12 6.7 x 10 -13 **OH + 1,2- dibromoethane 2.0 x 10 -13 4.3 x 10 -14 *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 reactions could be important depending on the rate constant [Cl] varies throughout the atmosphere [Cl] varies throughout the atmosphere Measurements of [Cl] have been increasing with improved technology Measurements of [Cl] have been increasing with improved technology

7. Cl + Alkyl Bromide Cl + ethyl 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 cm 3 molec -1 s -1 k = 1.4 x 10 -11 cm 3 molec -1 s -1 Cl + n-propyl bromide Cl + n-propyl bromide –kinetic study by Donaghy et al., 1993 (300 K) k = 6 x 10 -11 cm 3 molec -1 s -1 k = 6 x 10 -11 cm 3 molec -1 s -1 –theoretical study by Francisco, et al., 2008 Cl + 1,2-dibromoethane Cl + 1,2-dibromoethane –No previous study found in literature

8. Reactions Cl + CH 3 CH 2 Br  CH 2 CH 2 Br  Br Cl + CH 3 CH 2 Br  CH 3 CHBr Cl + CH 3 CH 2 CH 2 Br  CH 3 CHCH 2 Br  Br Cl + CH 3 CH 2 CH 2 Br  CH 2 CH 2 CH 2 Br Cl + CH 3 CH 2 CH 2 Br  CH 3 CH 2 CHBr Cl + CH 2 BrCH 2 Br  BrCHCH 2 + Br

9. Laboratory Approach Flow a mixture of gases with known concentrations through temperature controlled reaction cell Flow a mixture of gases with known concentrations through temperature controlled reaction cell Generate Cl atoms via LFP of Cl 2 or COCl 2 Generate Cl atoms via LFP of Cl 2 or COCl 2 Couple LFP with time resolved atomic resonance fluorescence spectroscopic detection of Br 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. Monitor kinetics of Br as a function of reactant concentration, Temp., and Press.

10. Photon counts vs. Time

11. Data Analysis S t /S 0 = (k a /(k d -k a ))*A*(exp(-k a *t)-exp(-k d *t))+B*exp(-k d *t) [Cl] = 5 x 10 11 atoms cm -3 [C 3 H 7 Br] = 1.3 x 10 14 molec cm -3

12. Bimolecular Plot is Derived Slope = k = 5.2 x 10 -11 cm 3 molec -1 s -1

13. Table of Kinetic Results k in units of 10 -12 cm 3 molec -1 s -1 Temp. (K) ethyl bromide n-propyl bromide 1,2- dibromoethane 2989.9(.34)56(1.0)14(.20) 2207.2(.15)55(.67)8.5(.04)

14. Arrhenius Plots

15. Yield Measurements Compare magnitude of rise as a result of the Cl + Br 2 reaction (100% Br yield) with the rise from the Cl + Alkyl Bromide reaction for a given set of conditions Compare magnitude of rise as a result of the Cl + Br 2 reaction (100% Br yield) with the rise from the Cl + Alkyl Bromide reaction for a given set of conditions

16. Data Analysis S t /S 0 = (k a /(k d -k a ))*A*(exp(-k a *t)-exp(-k d *t))+B*exp(-k d *t) [Cl] = 5 x 10 11 atoms cm-3 [C 3 H 7 Br] = 1.3 x 10 14 molec cm -3 Time (s) Signal Counts

17. Yield Data Reaction 220 K Br Yield 435 K Br Yield Cl + Ethyl bromide ~30 % ~30 % ~50 % Cl + propyl bromide ~100 % ~60 % Cl + 1,2- dibromoethane 100 %

18. Reactions Cl + CH 3 CH 2 Br  CH 2 CH 2 Br  Br Cl + CH 3 CH 2 Br  CH 3 CHBr Cl + CH 3 CH 2 CH 2 Br  CH 3 CHCH 2 Br  Br Cl + CH 3 CH 2 CH 2 Br  CH 2 CH 2 CH 2 Br Cl + CH 3 CH 2 CH 2 Br  CH 3 CH 2 CHBr Cl + CH 2 BrCH 2 Br  BrCHCH 2 + Br

19. Experiment vs. Atmosphere In our reaction cell: In our reaction cell: – Cl + CH 3 CH 2 Br  CH 2 CH 2 Br  Br + CH 2 =CH 2 In the atmosphere: In the atmosphere: – Cl + CH 3 CH 2 Br  CH 2 CH 2 Br + O 2  BrCH 2 CH 2 (OO)  Products such as BrCH 2 CHO, BrCH 2 CH 2 OH, and BrCH 2 CH 2 OOH (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. 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 Mike Nicovich Zhijun Zhao Zhijun Zhao Paul Wine Paul Wine Dow Huskey Dow Huskey Funded by: Funded by: –NASA Upper Atmospheric Research Program

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