Title: Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with C2H5Br, nC3H7Br, and 1,2C2H4B
1Kinetic and Mechanistic Study of the Reactions of
Atomic Chlorine with C2H5Br, n-C3H7Br, and
1,2-C2H4Br2
- Patrick Laine
- EAS
- Advisor Paul Wine
2Relevance
- Lifetimes of many trace gases are dependent on
gas phase atmospheric chemistry - Impact on ozone levels in troposphere and
stratosphere
3Ozone Depletion
- 90 of O3 is in the Stratosphere and is
considered good O3 - Mid 1970s 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
4Why 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
5OH Alkyl Bromide
Data retrieved from Nasa Evaluation No. 15
Data retrieved from NIST Database
6Could 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
7Cl 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
8Reactions
- Cl CH3CH2Br ? CH2CH2Br ? Br
- Cl CH3CH2Br ? CH3CHBr
- Cl CH3CH2CH2Br ? CH3CHCH2Br ? Br
- Cl CH3CH2CH2Br ? CH2CH2CH2Br
- Cl CH3CH2CH2Br ? CH3CH2CHBr
- Cl CH2BrCH2Br ? BrCHCH2 Br
9Laboratory 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.
10Photon counts vs. Time
11Data Analysis
- St/S0 (ka/(kd-ka))A(exp(-kat)-exp(-kdt))Be
xp(-kdt)
Cl 5 x 1011 atoms cm-3 C3H7Br 1.3 x 1014
molec cm-3
12Bimolecular Plot is Derived
Slope k 5.2 x 10 -11 cm3 molec-1 s-1
13Table of Kinetic Results
k in units of 10-12 cm3 molec-1 s-1
14Arrhenius Plots
15Yield 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
16Data Analysis
- St/S0 (ka/(kd-ka))A(exp(-kat)-exp(-kdt))Be
xp(-kdt)
Signal Counts
Time (s)
Cl 5 x 1011 atoms cm-3 C3H7Br 1.3 x 1014
molec cm-3
17Yield Data
18Reactions
- Cl CH3CH2Br ? CH2CH2Br ? Br
- Cl CH3CH2Br ? CH3CHBr
- Cl CH3CH2CH2Br ? CH3CHCH2Br ? Br
- Cl CH3CH2CH2Br ? CH2CH2CH2Br
- Cl CH3CH2CH2Br ? CH3CH2CHBr
- Cl CH2BrCH2Br ? BrCHCH2 Br
19Experiment vs. Atmosphere
- In our reaction cell
- Cl CH3CH2Br ? CH2CH2Br ? Br CH2CH2
- In the atmosphere
- Cl CH3CH2Br ? CH2CH2Br O2 ? BrCH2CH2(OO) ?
Products such as BrCH2CHO, BrCH2CH2OH, and
BrCH2CH2OOH (Orlando and Tyndall, 2002)
20In 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.
21Acknowledgements
- Mike Nicovich
- Zhijun Zhao
- Paul Wine
- Dow Huskey
- Funded by
- NASA Upper Atmospheric Research Program
22Finlayson-Pitts and Pitts, 2000 (Provided by
J.M. Nicovich and P.H. Wine)