Kinetic and Mechanistic Kinetic and Mechanistic Study of the Reactions of Study of the Reactions of

Atomic Chlorine with CAtomic Chlorine with C22HH55Br, Br, nn-C-C33HH77Br, and 1,2-CBr, and 1,2-C22HH44BrBr22

Patrick LainePatrick Laine

EASEAS

Advisor: Paul WineAdvisor: Paul Wine

RelevanceRelevance

• Lifetimes of many trace gases are Lifetimes of many trace gases are dependent on gas phase dependent on gas phase atmospheric chemistryatmospheric chemistry

• Impact on ozone levels in Impact on ozone levels in troposphere and stratospheretroposphere and stratosphere

Ozone DepletionOzone Depletion• 90% of O90% of O33 is in the Stratosphere and is is in the Stratosphere and is

considered “good” Oconsidered “good” O33

• Mid 1970’s it was discovered that halogen Mid 1970’s it was discovered that halogen source gases lead to ozone depletionsource gases lead to ozone depletion

• Short lived halogen source gases undergo Short lived halogen source gases undergo chemical conversions in troposphere producing chemical conversions in troposphere producing reactive halogen gases and other compoundsreactive halogen gases and other compounds

Why look at Alkyl Why look at Alkyl Bromides?Bromides?

• The alkyl bromides are emitted from The alkyl bromides are emitted from anthropogenic sources i.e., solvents, anthropogenic sources i.e., solvents, fumigantsfumigants

• These alkyl bromides have been identified by These alkyl bromides have been identified by WMO (2006) as short lived source gases with WMO (2006) as short lived source gases with significant ozone depletion potentials (ODP)significant ozone depletion potentials (ODP)

• Br is 40-100x more effective than Cl at Br is 40-100x more effective than Cl at destroying Odestroying O33

OH + Alkyl Bromide OH + Alkyl Bromide

ReactionReaction k, 298 K (cmk, 298 K (cm33 molecmolec-1-1 s s-1-1))

k, 220 K (cmk, 220 K (cm33 molecmolec-1-1 s s-1-1))

*OH + ethyl *OH + ethyl bromidebromide

3.4 x 103.4 x 10-13-13 1.5 x 101.5 x 10-13-13

*OH + propyl *OH + propyl bromidebromide

1.0 x 101.0 x 10-12-12 6.7 x 106.7 x 10-13-13

**OH + 1,2-**OH + 1,2-dibromoethandibromoethanee

2.0 x 102.0 x 10-13-13 4.3 x 104.3 x 10-14-14

*Data retrieved from Nasa Evaluation No. 15; **Data retrieved from NIST Database

Could Cl reactions be Could Cl reactions be important?important?

• Cl reactions could be important Cl reactions could be important depending on the rate constantdepending on the rate constant

• [Cl] varies throughout the atmosphere[Cl] varies throughout the atmosphere

• Measurements of [Cl] have been Measurements of [Cl] have been increasing with improved technologyincreasing with improved technology

Cl + Alkyl BromideCl + Alkyl Bromide

• Cl + ethyl bromide:Cl + ethyl bromide:– product study by Orlando and Tyndall (2002) product study by Orlando and Tyndall (2002) – kinetic study by Donaghy et al., 1993 (300 K)kinetic study by Donaghy et al., 1993 (300 K)

• k = 1.4 x 10k = 1.4 x 10-11-11 cm cm33 molec molec-1-1 s s-1-1

• Cl + n-propyl bromideCl + n-propyl bromide– kinetic study by Donaghy et al., 1993 (300 K)kinetic study by Donaghy et al., 1993 (300 K)

• k = 6 x 10k = 6 x 10-11 -11 cmcm33 molec molec-1-1 s s-1-1

– theoretical study by Francisco, et al., 2008theoretical study by Francisco, et al., 2008

• Cl + 1,2-dibromoethaneCl + 1,2-dibromoethane– No previous study found in literatureNo previous study found in literature

ReactionsReactions

• Cl + CH3CH2Br CH2CH2Br Br

• Cl + CH3CH2Br CH3CHBr

• Cl + CH3CH2CH2Br CH3CHCH2Br Br

• Cl + CH3CH2CH2Br CH2CH2CH2Br

• Cl + CH3CH2CH2Br CH3CH2CHBr

• Cl + CH2BrCH2Br BrCHCH2 + Br

Laboratory ApproachLaboratory Approach

• Flow a mixture of gases with known Flow a mixture of gases with known concentrations through temperature controlled concentrations through temperature controlled reaction cellreaction cell

• Generate Cl atoms via LFP of ClGenerate Cl atoms via LFP of Cl22 or COCl or COCl22

• Couple LFP with time resolved atomic resonance Couple LFP with time resolved atomic resonance fluorescence spectroscopic detection of Brfluorescence spectroscopic detection of Br

• Monitor kinetics of Br as a function of reactant Monitor kinetics of Br as a function of reactant concentration, Temp., and Press.concentration, Temp., and Press.

Photon counts vs. TimePhoton counts vs. Time

2500

2000

1500

1000

Sig

nal C

ount

s

10x10-3

50

Time (s)

Data AnalysisData Analysis

St/S0 = (ka/(kd-ka))*A*(exp(-ka*t)-exp(-kd*t))+B*exp(-kd*t)

1400

1200

1000

800

600

400

200

543210

x10-3

100500

-50-100

ka = 7355.3 ± 168 kd = 68.34 ± 3.02 A = 1392.3 ± 18.1 B = 69.19 ± 18.9

[Cl] = 5 x 1011 atoms cm-3

[C3H7Br] = 1.3 x 1014 molec cm-3

Bimolecular Plot is DerivedBimolecular Plot is Derived

ka vs [bromopropane] (50Torr - 358K)

0.0

2000.0

4000.0

6000.0

8000.0

10000.0

12000.0

0.0E+00 2.0E+13 4.0E+13 6.0E+13 8.0E+13 1.0E+14 1.2E+14 1.4E+14 1.6E+14 1.8E+14 2.0E+14

[bromopropane] (molecules/cm3)

ka (

s-1

)

Slope = k = 5.2 x 10 -11 cm3 molec-1 s-1

Table of Kinetic ResultsTable of Kinetic Results

k in units of 10k in units of 10-12-12 cm cm33 molec molec-1-1 s s-1-1

Temp. Temp. (K)(K)

ethyl ethyl bromidbromidee

nn-propyl -propyl bromidebromide

1,2-1,2-dibromoethandibromoethanee

2982989.99.9

(.34)(.34)5656

(1.0)(1.0)1414

(.20)(.20)

2202207.27.2

(.15)(.15)5555

(.67)(.67)8.58.5

(.04)(.04)

Arrhenius PlotsArrhenius Plots

810

-12

2

4

6

810

-11

2

4

6

k (c

m3m

olec

ule

-1s-1

)

4.5x10-3

4.03.53.02.52.01.5

1/T (K)

ketBr = 2.1 x 10-11

exp(-225/T)

kdibromoet = 2.8 x 10-17

T2 exp(-92/T)

kn-propylbr = 5.5 x 10-11

Yield MeasurementsYield Measurements

• Compare magnitude of rise as a result of Compare magnitude of rise as a result of the Cl + Brthe Cl + Br22 reaction (100% Br yield) reaction (100% Br yield) with the rise from the Cl + Alkyl Bromide with the rise from the Cl + Alkyl Bromide reaction for a given set of conditionsreaction for a given set of conditions

Data AnalysisData Analysis

SStt/S/S00 = (k = (kaa/(k/(kdd-k-kaa))*A*(exp(-k))*A*(exp(-kaa*t)-exp(-k*t)-exp(-kdd*t))+B*exp(-k*t))+B*exp(-kdd*t)*t)

1400

1200

1000

800

600

400

200

543210

x10-3

100500

-50-100

ka = 7355.3 ± 168 kd = 68.34 ± 3.02 A = 1392.3 ± 18.1 B = 69.19 ± 18.9

[Cl] = 5 x 10[Cl] = 5 x 1011 11 atoms cm-3atoms cm-3[C[C33HH77Br] = 1.3 x 10Br] = 1.3 x 1014 14 molec cmmolec cm-3-3

Time (s)

Sig

nal C

ounts

Yield DataYield Data

ReactionReaction 220 K Br 220 K Br YieldYield

435 K Br 435 K Br YieldYield

Cl + Ethyl Cl + Ethyl bromidebromide

~30 %~30 % ~50 %~50 %

Cl + propyl Cl + propyl bromidebromide

~100 %~100 % ~60 %~60 %

Cl + 1,2-Cl + 1,2-dibromoethadibromoethanene

100 %100 % 100 %100 %

ReactionsReactions

• Cl + CH3CH2Br CH2CH2Br Br

• Cl + CH3CH2Br CH3CHBr

• Cl + CH3CH2CH2Br CH3CHCH2Br Br

• Cl + CH3CH2CH2Br CH2CH2CH2Br

• Cl + CH3CH2CH2Br CH3CH2CHBr

• Cl + CH2BrCH2Br BrCHCH2 + Br

Experiment vs. AtmosphereExperiment vs. Atmosphere

• In our reaction cell:In our reaction cell:– Cl + Cl + CHCH33CHCH22Br Br CH CH22CHCH22Br Br Br + CH Br + CH22=CH=CH22

• In the atmosphere:In the atmosphere:– Cl + Cl + CHCH33CHCH22Br Br CH CH22CHCH22Br + OBr + O22 BrCH BrCH22CHCH22(OO) (OO)

Products such as BrCHProducts such as BrCH22CHO, BrCHCHO, BrCH22CHCH22OH, and OH, and BrCHBrCH22CHCH22OOH (Orlando and Tyndall, 2002)OOH (Orlando and Tyndall, 2002)

In the end…In the end…

Established rate coefficients and Established rate coefficients and branching ratios will be useful for branching ratios will be useful for

modeling atmospheric processes, may modeling atmospheric processes, may help make predictions for future help make predictions for future

climate change, and may facilitate a climate change, and may facilitate a better understanding of ozone better understanding of ozone

depletion.depletion.

AcknowledgementsAcknowledgements

• Mike NicovichMike Nicovich• Zhijun ZhaoZhijun Zhao• Paul WinePaul Wine• Dow HuskeyDow Huskey• Funded by:Funded by:

– NASA Upper Atmospheric Research NASA Upper Atmospheric Research ProgramProgram

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