Radical Production from Alkene Ozonolysis

Radical Production from Alkene Ozonolysis

William Bloss1, Salim Alam1, Andrew Rickard2

Marie Camredon1, Kevin Wyche3, Timo Carr3, Paul Monks3

Amalia Munoz4, Mila Rodenas4, Monica Vasquez4

1 School of Geography, Earth & Environmental Sciences, University of Birmingham

2 NCAS, School of Chemistry, University of Leeds

3 Department of Chemistry, University of Leicester

4 EUPHORE Laboratories, Fundaçion CEAM, Valencia

Alkenes...

Alkene + O3 OH + HO2 + Semi-Volatile Organics

Atmospheric Oxidants Aerosol Precursors

• Urban environments: Alkenes comprise ca. 15 % of NMHCs

• Account for substantial fraction of chemical reactivity (total OH sink)

Chelmsford, London, 2003

• O3 + alkenes comprise 29% of net OH source

(43% during heatwave)

Midday flux in units of 105 molec cm-3 s-1

TORCH and PUMA

Chelmsford, London, 2003

• O3 + alkenes comprise 29% of net OH source

(43% during heatwave)

Midday flux in units of 105 molec cm-3 s-1

TORCH and PUMA

Current atmospheric models...

• Based upon laboratory measurements of yields for individual alkenes...



OH



HO2

Wegener et al., 2007:

Mihelcic et al., 1999: Ethene 0.39 (MIESR)

Qi et al., 2006: Ethene 0.38 (PERCA)

Qi et al., 2009: Propene 0.19

Malkin et al., 2009: Isoprene 0.26 (LIF)



RO2

Qi et al., 2006: Ethene 0.45 (HO2+RO2)

Qi et al., 2009: Propene 0.19

TRAPOZ...

Total Radical Production from Alkene Ozonolysis

Aims :

• Determination of the total radical yields (OH, HO2, RO2) from the ozonolysis of a

range of alkenes of biogenic, anthropogenic origin

• Measurement and identification of the gas-phase degradation products from the

ozonolysis of selected alkenes, monoterpenes

• Improve representation of alkene ozonolysis in chemical mechanisms (MCM)

Approach

• Large Simulation Chamber Experiments with direct radical observations...

EUPHORE

TRAPOZ approach

Chamber experiments

• European Photoreactor (EUPHORE), Valencia

• 2 campaigns ~ 60 experiments

• Instruments include: LIF, PERCA, FT-IR,

CIR-TOF-MS and CO,

HCHO, O3 monitors

Types of Experiment

Alkene + O3 - direct OH / HO2 measurements

Alkene + O3 + CO

Alkene + O3 +

• Alkenes investigated:

- Observation of secondary organics

- HO2 and kO3 measurements

- indirect OH measurements

- RO2 measurements

Experimental conditions:

• Dark, no NOx

• Ambient T & p

• [Alkene] ~ 100 - 500 ppb

• [O3] ~ 100 - 500 ppb

• [CO] or [Cyclohexane] ~ 100’s ppm

• Time: 1 hour – 3 hours+ terpenes

Analysis Approach

• All experiments analysed via models

(secondary processes)

• Model based upon MCM v3.1

with updated ozonolysis scheme

• Use Data to constrain fast CI chemistry

O3 + Ethene rate constant

• Excess CO experiment: scavenge OH radicals

• Optimise k(O3 + C2H4) to fit observed O3, ethene decay profiles

Ethene & Ozone

300.0

350.0

400.0

450.0

500.0

550.0

0 10 20 30 40 50 60 70 80 90

Time [mins]

MR

[p

pb

]

Ethene Model

O3 Model

Ethene FTIR

O3 FTIR

k298k = (1.59 0.30) x 10-18 cm3 molecules-1 s-1 (IUPAC)

k298k = (1.45 0.23) x 10-18 cm3 molecules-1 s-1

OH yield ()

[OH]ss = k1 [O3] [Ethene] + k2 [HO2] [O3]

k3 [Alkene] (+ other losses)

• Absolute [OH]ss too low to measure

in this (ethene) system

• Determine OH yield indirectly, using

scavenger approach ...

α = 0.54β

OH yield: cyclohexane scavenger

• OH + cyclohexane cyclohexanol / cyclohexanone

1) Measure yield of c-hexanone from c-hexane + OH

2) Measure amount of c-hexanone in O3 - Alkene system

3) Obtain cumulative OH production and hence derive OH yield

OH yield: cyclohexane scavenger

• OH + cyclohexane cyclohexanol / cyclohexanone

1) Measure yield of c-hexanone from c-hexane + OH

2) Measure amount of c-hexanone in O3 - Alkene system

3) Obtain cumulative OH production and hence derive OH yield

But: c-hexanone yield depends upon co-reactant conditions, concentrations :

Include these processes and fit to known c-hexane, c-hexanone, c-hexOOH...

.

(7) HO2 k3

(6b) RO2 k2b

(6a) RO2 k2a

(6c) RO2 k2c

OH

(5a) k1

O2

(5b)

(9) O2

(isomerisation)

OH yield determination from c-hex data

• Fit to observed HO2, chexanone, chexOOH :

• Fit determines [OH] present as f(t)

• Model + known [HO2] also accounts for HO2 + O3 OH + O2 (40 % here)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 50 100 150 200

Time (min)

Mix

ing

Rati

o C

6H

11O

, C

6H

11O

OH

& [

HO

2]/

10

8

HO2

Cyclohexyl

hydroperoxide

Cyclohexanone

OH and HO2 yields

• OH yield = 0.13 0.06

• HO2 data determine an overall

HO2 yield of 0.26 0.07

α = 0.54β = 0.13

γ = 0.07

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 20 40 60 80 100 120 140 160 180 200

Time (min)

[HO

2]

mo

lecu

le.c

m-3

(x

10

8)

Standard MCMv3.1 HO2 yield of 0.13

Radical yields vs literature (ethene)

• OH yield consistent with literature

• Our results, other recent studies indicate HO2 production is substantially higher

than currently assumed

Yield of OH Reference

0.13 (± 0.06) This study

0.13 MCMv3.1

0.16 IUPAC

0.18 (± 0.06) Paulson et al 1999

0.14 Kroll et al., 2001

0.20 (± 0.02) Mihelcic et al 1999

Yield of HO2

0.26 (± 0.07) This study

0.13 MCMv3.1

0.39 (± 0.03) Mihelcic et al., 1999

0.38 (± 0.02) Qi et el., 2006

0.50 (± 0.25) / 0.40 (± 0.20) Wegener et al., 2007

(dry/wet)

0.E+00

2.E+09

4.E+09

6.E+09

8.E+09

1.E+10

0 30 60 90

[HO

2] /m

ole

c/c

m3

Time /mins

0.E+00

1.E+06

2.E+06

3.E+06

4.E+06

5.E+06

6.E+06

0 30 60 90

[OH

] /m

ole

c/c

m3

Time / mins

HOx yields from other alkenes

[OH]ss = k1 [O3] [Ethene] + k2 [HO2] [O3]

k3 [Alkene] (+ other losses)

[ use full model for analysis, not steady state expression ]

• For most systems, k1 and k3 are such that [OH] can be observed directly

[cyclohexanone approach not used]

• Model still required to account for HO2 + O3, other 2 chemistry

OH 200 ppb trans-2-butene / 100 ppb O3 HO2

Repeat process for a range of alkenes...

• ethene, propene, cis- / trans-2-butene, isobutene, 1-butene, TME :

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

YO

H

IUPAC YOH

Atkinson et al '97 (cyclohexane) Marston et al '99 (TMB) This study (LIF)

This study (cyclohexane) Mihelcic et al '99 (Prod Alk+OH) Qi et al '09 (PERCA)

C2H4

C3H6

C2B

1-But T2B

i-But TME


• ethene, propene, cis- / trans-2-butene, isobutene, 1-butene, TME :

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

YO

H

IUPAC YOH

Atkinson et al '97 (cyclohexane) Marston et al '99 (TMB) This study (LIF)

This study (cyclohexane) Mihelcic et al '99 (Prod Alk+OH) Qi et al '09 (PERCA)

C2H4

C3H6

C2B

1-But T2B

i-But TME

OH yields consistent with literature


• ethene, propene, cis- / trans-2-butene, isobutene, 1-butene, TME, isoprene :

0.0

0.5

1.0

1.5

2.0

2.5

3.0

HO

2yie

ld

This study (LIF)

Wegener et al '07

MCM Model

Qi et al '06 / '09 (PERCA)

Mihelcic et al '99

Malkin et al '10


• ethene, propene, cis- / trans-2-butene, isobutene, 1-butene, TME, isoprene :

0.0

0.5

1.0

1.5

2.0

2.5

3.0

HO

2yie

ld

This study (LIF)

Wegener et al '07

MCM Model


Mihelcic et al '99

Malkin et al '10

HO2 yields higher than MCM...

HO2 vs OH

y = 1.9517x - 0.0504R² = 0.8618

0

0.5

1

1.5

2

0 0.25 0.5 0.75 1

HO

2Y

ield

OH Yield

C2H4

C3H6

C2B

1-But

T2B

i-But

TME

C5H8

Atmospheric Implications....

• Compare [HOx] using standard MCMv3.1 and with revised ozonolysis yields.

Mean Nighttime HOx :

• “Small” anthropogenic alkenes only (C2 – C5)

• Model constrained to observations for VOCs, NOx, O3 etc.

Atmospheric Implications....

• Compare [HOx] using standard MCMv3.1 and with revised ozonolysis yields.

Mean Nighttime HOx :

OH HO2 OH HO2

MCMv3.1 4.8104 2.0107 1.2105 4.0107

This Work 6.2104 2.8107 1.30105 5.1107

• Impact upon daytime or diurnal-average HOx is (proportionately) much smaller

Conclusions

• Measured HOx yields for a range of alkenes

• OH yields are consistent with literature

• HO2 yields are larger than MCM currently assumes

• Results in substantially increased night-time HOx, depending upon conditions

Limitations :

• Dependent upon HOx (and other) instrument accuracy...

• Atmospheric implications partially dependent upon mechanism interpretation

• Real atmosphere: H2O, NO, ...

0.0

0.5

1.0

1.5

2.0

2.5

3.0

HO

2yie

ld

This study (LIF)

Wegener et al '07

MCM Model


Mihelcic et al '99

Malkin et al '10

Acknowledgements

• Salim Alam, Marie Camredon* Birmingham (*now at LISA, Paris)

• Andrew Rickard Leeds

• Kevin Wyche, Timo Carr, Paul Monks Leicester

• Amalia Munoz, Mila Rodenas, EUPHORE

Monica Vasquez, Paco Alacreu,

Antonio Caraccosa

• NERC

• Camredon et al., Atmos. Chem. Phys., 10, 2893, 2010

• Alam et al., Phys. Chem. Chem. Phys., submitted, 2010

Documents

Radical Production from Alkene Ozonolysis