Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with C2H5Br, nC3H7Br, and 1,2C2H4B

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Kinetic and Mechanistic Study of the Reactions of
Atomic Chlorine with C2H5Br, n-C3H7Br, and
1,2-C2H4Br2

• Patrick Laine
• EAS
• Advisor Paul Wine

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Relevance

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

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Ozone 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

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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

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OH Alkyl Bromide
Data retrieved from Nasa Evaluation No. 15
Data retrieved from NIST Database
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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

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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

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Reactions

• Cl CH3CH2Br ? CH2CH2Br ? Br
• Cl CH3CH2Br ? CH3CHBr
• Cl CH3CH2CH2Br ? CH3CHCH2Br ? Br
• Cl CH3CH2CH2Br ? CH2CH2CH2Br
• Cl CH3CH2CH2Br ? CH3CH2CHBr
• Cl CH2BrCH2Br ? BrCHCH2 Br

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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.

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Photon counts vs. Time
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Data 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
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Bimolecular Plot is Derived
Slope k 5.2 x 10 -11 cm3 molec-1 s-1
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Table of Kinetic Results
k in units of 10-12 cm3 molec-1 s-1
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Arrhenius Plots
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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

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Data 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
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Yield Data
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Reactions

• Cl CH3CH2Br ? CH2CH2Br ? Br
• Cl CH3CH2Br ? CH3CHBr
• Cl CH3CH2CH2Br ? CH3CHCH2Br ? Br
• Cl CH3CH2CH2Br ? CH2CH2CH2Br
• Cl CH3CH2CH2Br ? CH3CH2CHBr
• Cl CH2BrCH2Br ? BrCHCH2 Br

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Experiment 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)

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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.

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Acknowledgements

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

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