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)
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