Mechanisms and SOA Formation for the Atmospheric Degradation of Xylenes

John Wenger

Department of Chemistry and Environmental Research Institute

University College Cork

Ireland

Sources• Automobile fuels, solvents • Aromatics typically contribute 20-25% to anthropogenic VOC emissionsTransformation• Reaction with hydroxyl radical (OH) in troposphere• Formation of ozone, nitrates, secondary organic aerosol (SOA)Impacts• Air pollution and Health effects in urban areasInformation required• Kinetics of reaction, identity & yields of oxidation products• Reactivity of the oxidation products towards OH, NO3, O3, sunlight • Produce a detailed atmospheric degradation mechanism for inclusion in

models (e.g. MCM) used to predict pollution forming ability of VOCs

Sources Transformation Impacts

H abstractionOH addition

.OO

O2

.O

O

+ HO2

O2

NONO

organic nitrate

OH

OO.OH

O2O2

'Phenolic' route

organic nitrate

NO

OH

O

O

OH

O

O

.OO

organic nitrate

NONO

OH

O

O

.O

OH

O.

O

isomerisation

isomerisation

O2

OH

O

O

O

O

O+ HO2

decomposition

O2

'Epoxy-oxy' route

O O

O

O

+ HO2

O O

OO

+ HO2

O O

OO

+ HO2

decompositionO2

'Peroxide-bicyclic' route

(0.625)

(0.10)

(0.155)

[0.18]

[0.18]

[0.64]

(0.12)

D. Johnson et al. Env. Chem, 2005

Dimethylphenols and Tolualdehydes

CHO

CHO

CH3

benzaldehyde

CHO

CH3

o-tolualdehyde

CHO

CH3

m-tolualdehyde p-tolualdehyde

OH

CH3

OHOH

OHOH

OH

CH3

CH3

CH3

CH3

CH3

CH3

CH3CH3

CH3CH3

CH3

2,3-DMP 2,4-DMP 2,5-DMP

2,6-DMP 3,4-DMP 3,5-DMP

Atmospheric Simulation Chamber

• FEP foil (3910 litres)• Dry purified air• Atmospheric P and T• Humidity control

• in situ FTIR spectroscopy • NOx and O3 analysers• GC, GC-MS• Particle Sizer and counter

Kinetic StudiesRelative rate method

k1

ln [aromatic]0 k1 ln [reference]0

[aromatic]t k2 [reference]t

(1) OH + aromatic Products

(2) OH + reference k2 Products

-dln[aromatic] = k1[OH]dt

-dln[reference] = k2[OH]dt

=

OH reactions NO3 reactions

Radical Source CH3ONO + hν→ CH3O + NOCH3O + O2 → CH2O + HO2

HO2 + NO → OH + NO2

NO2 + O3 → NO3 + O2

NO3 + NO2 ↔ N2O5

ReferenceCompound

1,3,5-trimethylbenzene for DMPs1,2,4-trimethylbenzene for aldehydes

o-cresol for DMPsn-propyl ether for aldehydes

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.05 0.1 0.15 0.2

ln([1,3,5-TMB]0/[1,3,5-TMB]t)

ln([

DM

P]0/[

DM

P]t)

- k3t

2,6-Dimethylphenol

3,4-Dimethylphenol

3,5-Dimethylphenol

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

ln([o -cresol]0/[o -cresol]t) - k 6t

ln([

DM

P]0/[

DM

P]t)

- k3t

2,6-Dimethylphenol

3,4-Dimethylphenol

3,5-Dimethylphenol

Kinetics data for dimethylphenols

Compound kOHa kNO3

a τOHb (s) τNO3

c(s)This work Literature

2,3-Dimethylphenol 8.3 ± 1.1 8.0 ± 2.0 2.9 ± 0.3 7502 692,4-Dimethylphenol 7.4 ± 0.8 7.2 ± 1.8 3.1 ± 0.3 8468 652,5-Dimethylphenol 8.8 ± 1.2 8.0 ± 2.3 3.0 ± 0.4 7078 672,6-Dimethylphenol 6.7 ± 0.9 6.6 ± 1.7 4.9 ± 0.5 8635 413,4-Dimethylphenol 8.3 ± 0.9 8.1 ± 2.1 2.5 ± 0.2 7558 803,5-Dimethylphenol 11.4 ± 1.4 11.3 ± 3.0 1.1 ± 0.1 5482 180

a In units of 10-11 cm3 molecule-1 s-1. b τOH = 1/kOH[OH], where 12 hour daily average [OH] = 1.6 × 106 molecule cm-3

c τNO3 = 1/kNO3[NO3], where 12 hour daily average [NO3] = 5 × 108 molecule cm-3

Kinetics results for dimethylphenols

• Both OH and NO3 reactions important• Opposite trends in reactivity for OH and NO3• Different reaction mechanisms

OHCH3

CH3

*

*

2,3-dimethylphenol

OHCH3

*

2,4-dimethylphenol

CH3

OHCH3

CH3

*

*

2,5-dimethylphenol

OHCH3CH3

*

2,6-dimethylphenol

OH

CH3

CH3

* *

3,4-dimethylphenol

OH

CH3 CH3

* *

*

3,5-dimethylphenol

Sites activated for OH addition

OH OO

HN

O

O

+ NO3 + HNO3

O

Mode of NO3 attack

OHCH3

CH3

*

2,3-dimethylphenol

OHCH3**

2,4-dimethylphenol

CH3

OHCH3

CH3

2,5-dimethylphenol

OHCH3CH3 **

2,6-dimethylphenol

OH

CH3

CH3

*

3,4-dimethylphenol

OH

CH3 CH3

3,5-dimethylphenol

*

Sites activated for NO3 addition

Compound kOHa kNO3

b τOHc (hr) τNO3

d(hr)This work Literature

benzaldehyde 1.4 ± 0.1 1.2 ± 0.2 4.3 ± 1.3 19.8 128.0o-tolualdehyde 2.1 ± 0.2 1.8 ± 0.2 9.8 ± 2.6 13.6 56.7m-tolualdehyde 2.1 ± 0.2 1.7 ± 0.2 9.5 ± 2.6 13.4 58.7p-tolualdehyde 2.1 ± 0.2 1.3 ± 0.2 9.5 ± 3.0 13.6 58.7

a In units of 10-11 cm3 molecule-1 s-1.b In units of 10-15 cm3 molecule-1 s-1

c τOH = 1/kOH[OH], where 12 hour daily average [OH] = 1.6 × 106 molecule cm-3

d τNO3 = 1/kNO3[NO3], where 12 hour daily average [NO3] = 5 × 108 molecule cm-3

Kinetics results for tolualdehydes

• Only OH reaction is important• Tolualdehyde isomers have the same reactivity• Similar trend in reactivity for OH and NO3

UV Absorption Spectra of Tolualdehydes

Sunlight Photolysis of Tolualdehydes

Sunlight Photolysis of Tolualdehydes

0.00

50.00

100.00

150.00

200.00

250.00

300.00

10:48 11:16 11:45 12:14 12:43 13:12 13:40 14:09 14:38

Time of day (hh:mm)

o-to

lual

dehy

de (p

pbv)

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

j (N

O2)

(s-1

)

GC-PIDj (NO2)

Chamber open

Chamber closed

• photolysis lifetime for o-tolualdehyde = 1-2 hr

0.00

50.00

100.00

150.00

200.00

250.00

300.00

08:24 09:36 10:48 12:00 13:12 14:24 15:36Time of day (hh:mm)

m-to

lual

dehy

de, p

pbv

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

J(N

O2)

-1

m-tolualdehyde GC-PIDJ(NO2)

Chamber open

Chamber closed

• m- and p-tolualdehyde are not photolysed

O

O

1,2-benzenedicarboxaldehyde~ 25% yield Aromatic cyclic alcohol

~ 65% yield

FTIR Spectra of Products

HO

ProposedMechanism

CO

CH3 hν

H

CO

CH3

H

COH

CH2

H

HO O

O

65% 25%

O2

isomerisation

cyclicisation

CO

C H3

H

Secondary Organic Aerosol Formation

0

500000

1000000

1500000

2000000

2500000

3000000

0 20 40 60 80 100 120

Diameter (nm)

dW/d

log(

Dp)

09:47

10:02

10:17

10:32

10:47

11:02

11:17

11:32

11:47

12:07

09:42

CO

C H3

H

Secondary Organic Aerosol Formation

0

50

100

150

200

250

300

08:24 09:36 10:48 12:00 13:12 14:24 15:36

Time (hh:mm)

Aer

osol

Vol

ume

Con

cent

ratio

n (μ

m3 /c

m3 )

0

50

100

150

200

250

300

Aer

osol

Mas

s C

once

ntra

tion

(μg/

m3 )

SMPS Aerosol VolumeTEOM Aerosol Mass

y = 1.0908x + 4.1554R2 = 0.9978

0

50

100

150

200

250

300

0 50 100 150 200 250SMPS Aerosol Volume (μm3/cm3)

TEO

M A

eros

ol M

ass

(μg/

m3 )

Open to sunlight

• Not just formed in laboratory studies!!

• Secondary organic fraction represents up to 70% of the organic mass of fine aerosols (PM2.5)

• Composition of SOA ?

• Formation mechanisms ?

• Main species contributing to SOA ?

Secondary Organic Aerosol

Denuder-Filter Sampling

Particle phase

Gas phase

Air Flow

Denuder tube coated with XAD-4 resin

Filter

Sorbent

Denuder-Filter Sampling at UCC

• 5-channel glass denuder

• Coated with XAD-4 resin

• Doped with pentafluorobenzyl hydroxyl amine (PFBHA)

Denuder Tube(gas collection)

Filter pack (particle collection)

Air flow

Derivatization of oxygenated organics

Yu et al. ES&T 1998, 32, 2357-2370

FF

F

F F

O N

N O

F

F

F

F

F

MW = 448

Fragment mass = 181

Glyoxal derivatizedfragments here

More than one isomer possible- (multiple peaks)

O

Oglyoxal derivatizes

p-xylene photo-oxidation experiment

• Aerosol mass yield=3.84%

0

100

200

300

400

500

600

700

800

-30 70 170 270 370 470

Time (min)

ppbv

0

50

100

150

200

250

300

350

400

450

ugm

-3

Lights OffLights On

NO NO2

Aerosol mass

O3

p-xylene/5

p-xylene photo-oxidation

OH

O

O

O

O

O

O

O

O

O

OH additionH-abstraction

ringcleavage

OH addition

O

O

GC-MS Analysis of p-xylene extracts (filter)

glyoxal

methylglyoxal

C4 dicarbonyl

hexenedione

methylbutenedial

p-tolualdehyde

p-tolualdehyde 2.34%

glyoxal 2.48%

methylglyoxal 2.41%

C4-dicarbonyl 0.25%

hexenedione 1.10%

dimethylbenzoquinone0.03%methylbutenedial 2.10%

Other 89.28%

GC-MS Analysis of p-xylene extracts (filter)

Gas/Particle Partitioning

• Many organic compounds partition between gas and particle phase

VOC

Gas phase products

oxidized

Kp

Organic aerosol

Gas/Particle Partitioning Values

• Kp calculated both theoretically:

• and experimentally:

][exp aerosolCC

Kpgas

particle

×=

610760

×°×××××

=Lomom

omltheoretica PMW

TRfKpγ

(Pankow)

Gas/Particle Partitioning Values

• Glyoxal and methylglyoxal Kp several orders of magnitude higher than expected

Product p-tolualdehyde Hexenedione Glyoxal Methylglyoxal

Kptheoretical 3.93x10-07 2.04x10-06 1.73x10-09 3.69x10-09

Kpexperimental 8.84x10-06 4.58x10-06 5.57x10-05 3.28x10-05

(vapor pressures from SPARC on line calculator- University of Georgia)

Acid-catalyzed Polymerization-uptake of glyoxal to particles

Liggio et al.

O

H

OH

OH

H

OH

OH

H

OH

OH

OH

HOOH

OH

H

HOOH

OH

H

OOH

H2O

H+

H2O

- H+

H

HOOH

+

HO

OH

OH

O

H

OH

OH

OH

- H+

HO

OH

OH

O

OH

OH

OH

H+

Conclusions• Advances in knowledge of the reactivity of xylene

oxidation products – improved mechanisms

• Small dicarbonyl compounds partition to the particle phase much more than expected from vapor pressure calculations

• Suggests further processes occuring on/in particles (oligomerization) -eg. Kalberer et al. Science 2004

Future Directions

• On-tube derivatization of acids/phenols

• Further chamber experiments on aromatic aldehydes and phenols

• Mass balance for xylene photo-oxidation systems still poor (at best 45%). New experimental approaches using state-of-the-art analytical techniques are needed.

Acknowledgements

• Lars Thuener

• Brice Temime-Roussel

• Ger Rea

• Grainne Clifford

• Perla Bardini

• Robert Healy

• Wahid Mellouki (Orleans)

• Amalia Munoz (Valencia)