Formation yields of glyoxal and methylglyoxal from the gas-phase OH radical-initiated reactions of toluene, xylenes, and trimethylbenzenes as a function of NO2 concentration

Article abstract of DOI:10.1021/jp105112h

Aromatic hydrocarbons comprise ～20% of non-methane volatile organic compounds in urban areas and are transformed mainly by atmospheric chemical reactions with OH radicals during daytime. In this work we have measured the formation yields of glyoxal and methylglyoxal from the OH radical-initiated reactions of toluene, xylenes, and trimethylbenzenes over the NO₂ concentration range (0.2-10.3) ×10¹³ molecules cm^-3. For toluene, o-, m-, and p-xylene, and 1,3,5-trimethylbenzene, the yields showed a dependence on NO₂, decreasing with increasing NO₂ concentration and with no evidence for formation of glyoxal or methylglyoxal from the reactions of the OH-aromatic adducts with NO₂. In contrast, for 1,2,3- and 1,2,4-trimethylbenzene the glyoxal and methylglyoxal formation yields were independent of the NO₂ concentration within the experimental uncertainties. Extrapolations of our results to NO₂ concentrations representative of the ambient atmosphere results in the following glyoxal and methylglyoxal yields, respectively: for toluene, 26.0 ± 2.2% and 21.5 ± 2.9%; for o-xylene, 12.7 ± 1.9% and 33.1 ± 6.1%; for m-xylene, 11.4 ± 0.7% and 51.5 ± 8.5%; for p-xylene, 38.9 ± 4.7% and 18.7 ± 2.2%; for 1,2,3-trimethylbenzene, 4.7 ± 2.4% and 15.1 ± 3.3%; for 1,2,4-trimethylbenzene, 8.7 ± 1.6% and 27.2 ± 8.1%; and for 1,3,5-trimethylbenzene, 58.1 ± 5.3% (methylglyoxal).

Full text of DOI:10.1021/jp105112h

1
0140
J. Phys. Chem. A 2010, 114, 10140–10147
Formation Yields of Glyoxal and Methylglyoxal from the Gas-Phase OH Radical-Initiated
Reactions of Toluene, Xylenes, and Trimethylbenzenes as a Function of NO Concentration
2
Noriko Nishino,†,‡ Janet Arey,*,†,§ and Roger Atkinson*,†,§,⊥
Air Pollution Research Center, UniVersity of California, RiVerside, California 92521
ReceiVed: June 3, 2010; ReVised Manuscript ReceiVed: August 3, 2010
Aromatic hydrocarbons comprise ∼20% of non-methane volatile organic compounds in urban areas and are
transformed mainly by atmospheric chemical reactions with OH radicals during daytime. In this work we
have measured the formation yields of glyoxal and methylglyoxal from the OH radical-initiated reactions of
13
-3
toluene, xylenes, and trimethylbenzenes over the NO
For toluene, o-, m-, and p-xylene, and 1,3,5-trimethylbenzene, the yields showed a dependence on NO
decreasing with increasing NO concentration and with no evidence for formation of glyoxal or methylglyoxal
from the reactions of the OH-aromatic adducts with NO . In contrast, for 1,2,3- and 1,2,4-trimethylbenzene
the glyoxal and methylglyoxal formation yields were independent of the NO concentration within the
experimental uncertainties. Extrapolations of our results to NO concentrations representative of the ambient
2
concentration range (0.2-10.3) × 10 molecules cm .
2
,
2
2
2
2
atmosphere results in the following glyoxal and methylglyoxal yields, respectively: for toluene, 26.0 ( 2.2%
and 21.5 ( 2.9%; for o-xylene, 12.7 ( 1.9% and 33.1 ( 6.1%; for m-xylene, 11.4 ( 0.7% and 51.5 ( 8.5%;
for p-xylene, 38.9 ( 4.7% and 18.7 ( 2.2%; for 1,2,3-trimethylbenzene, 4.7 ( 2.4% and 15.1 ( 3.3%; for
1
,2,4-trimethylbenzene, 8.7 ( 1.6% and 27.2 ( 8.1%; and for 1,3,5-trimethylbenzene, 58.1 ( 5.3%
(
methylglyoxal).
Introduction
yields of unsaturated 1,4-diketones and of 1,4-butenedial suggest
that these are formed as co-products to the 1,2-dicarbonyls.
8
Aromatic hydrocarbons, which comprise ∼20% of non-
methane volatile organic compounds in urban areas, are released
into the atmosphere from vaporization of gasoline and diesel
fuel and in vehicle exhaust. In the lower troposphere, reaction
with OH radicals is their dominant loss process, and aromatic
hydrocarbons contribute significantly to the formation of ozone
However, the formation yields of the unsaturated 1,4-dialde-
hydes and 1,4-ketoaldehydes (other than 1,4-butenedial) are
significantly lower than those of their potential co-product 1,2-
1
-3
8
dicarbonyls. Despite numerous studies, a complete accounting
of the carbon balance has not been achieved, and even assuming
that the unsaturated 1,4-dicarbonyls are co-products to the 1,2-
3
and secondary organic aerosol on local scales. Aromatic
dicarbonyls, only ∼60-70% of the overall reaction products
hydrocarbons can react with OH radicals by H-atom abstraction
from the C-H bonds of the alkyl substituent group(s) leading
to the formation of ring-retaining products such as aromatic
have been identified.7
Furthermore, there are significant discrepancies in the reported
formation yields of the 1,2-dicarbonyls glyoxal and methyl-
glyoxal from toluene, the xylenes, and the trimethylbenzenes
3,4
aldehydes and nitrates. However, under atmospheric conditions
90% of the OH radical-initiated reactions of benzene, toluene,
>
(
see, for example, ref 8). This may be due, in part, to the fact
that differing NO concentrations were used in previous studies,
with the glyoxal and methylglyoxal yields being expected to
depend on the NO concentration. Thus, for the o- and p-xylene,
,2,3- and 1,2,4-trimethylbenzene reactions, the yields of
xylenes, and trimethylbenzenes proceed by OH radical addition
to the aromatic ring to form a hydroxycyclohexadienyl-type
2
radical (hereafter termed an “OH-aromatic adduct”), which
2 2
subsequently reacts with O and NO
(see Scheme 1).3
-5
2
1
Previous kinetic and product studies show that for benzene,
toluene, and o- and p-xylene, the NO and the O reactions
become equally important at NO
mixing ratios in air of ∼1.2-5
ppm,5 indicating that the OH-aromatic adducts predominantly
react with O under atmospheric conditions.
The major products observed from the reaction of the OH-
aromatic adducts with O are phenolic compounds, 1,2-
biacetyl [CH
3
C(O)C(O)CH
3
]
and/or 3-hexene-2,5-dione
2
2
[
CH
concentration, decreasing with increasing NO
glyoxal [CHO) ] and methylglyoxal [CH
of NO on their yields has not been systematically investigated.
3
C(O)CHdCHC(O)CH
3
] show a dependence on the NO
2
2
6,9
2
.
However, for
,6
2
3
C(O)CHO] the effect
2
2
In this work we have measured the formation yields of glyoxal
and methylglyoxal from the OH radical-initiated reactions of
2
dicarbonyls (i.e., glyoxal, methylglyoxal and biacetyl (2,3-
butanedione) from benzene and the methyl-substituted benzenes)
and unsaturated 1,4-dicarbonyls.
2
toluene, xylenes, and trimethylbenzenes over the NO concen-
13
-3
tration range of (0.2-10.3) × 10 molecules cm .
3
,4,7
The measured formation
*
To whom correspondence should be addressed. R.A.: e-mail ratkins@
Experimental Methods
mail.ucr.edu, phone (951) 827-4191. J.A.: e-mail janet.arey@ucr.edu, phone
Experiments were conducted in an ∼8000 L collapsible
Teflon chamber at 298 ( 2 K and 740 Torr of dry purified air
(
951) 827-3502.
†
Also in the Environmental Toxicology Graduate Program.
Present address: Department of Chemistry, University of California,
‡
or N
2
2
+ O mixtures. The chamber was equipped with a Teflon-
Irvine, CA.
§
coated fan for rapid mixing of chemicals, and two parallel banks
of blacklamps (λ >300 nm) for irradiation. The mixing fan was
Also at the Department of Environmental Sciences.
Also at the Department of Chemistry.
⊥
1
0.1021/jp105112h  2010 American Chemical Society
Published on Web 08/30/2010

Formation Yields of Glyoxal and Methylglyoxal
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10141
a
SCHEME 1: Possible Reaction Scheme, Based on the
initial concentration of each aromatic hydrocarbon in order to
minimize any formation of second-generation glyoxal and
methylglyoxal. The initial reactant concentrations (in molecules
1
9
20
21
Studies of Bohn and Zetzsch, Bohn, Klotz et al.,
Volkamer et al.,17 and Raoult et al.
22
-3
13
cm ) were the following: aromatic, ∼2.4 × 10 , and CH
3
ONO
13
and NO, ∼(0.48-24) × 10 each. For the 1,2,3- and 1,2,4-
trimethylbenzene reactions, experiments were also performed
in ∼80% O
2
+ ∼20% N
2
and ∼5% O
2
+ ∼95 % N
2
mixtures,
ONO, and NO of
with initial concentrations of aromatic, CH
3
1
3
-3
∼
2.4 × 10 molecules cm each.
Aromatic hydrocarbons, glyoxal, and methylglyoxal were
analyzed by gas chromatography with flame ionization detection
GC-FID). Concentrations of the aromatic hydrocarbons were
(
3
measured by collecting 100 cm gas samples from the chamber
onto Tenax-TA adsorbent, with subsequent thermal desorption
at 250 °C onto a 30 m DB-5 megabore column, initially held at
-1
0
°C and then temperature programmed at 8 °C min . Aromatic
hydrocarbons were calibrated by introducing measured amounts
of standard chemicals into the chamber, the volume of which
was measured as described previously.11 Glyoxal and methylg-
lyoxal were sampled by exposing a 65 µm polydimethylsiloxane/
divinylbenzene (PDMS/DVB) solid phase microextraction
(
SPME) fiber pre-coated with O-(2,3,4,5,6-pentafluorobenzyl)-
12
hydroxylamine (PFBHA) to the chamber contents for 10 min.
The samples were then thermally desorbed at 270 °C onto a
3
4
0 m DB-5 megabore column, temperature programmed from
0 to 260 °C at 8 °C min .
-
1
The concentrations of glyoxal and methylglyoxal were
8
determined by measuring their di-oxime derivatives. There are
three di-oxime isomers of glyoxal and four of methylglyoxal,
and while some isomers co-eluted from the GC column
employed, the most abundant peaks of the glyoxal and meth-
ylglyoxal di-oxime isomers were well separated and therefore
used for quantification. While methylglyoxal mainly formed di-
a
The di-unsaturated 1,6-dicarbonyls such as CH
3
C(O)CHd
CHC(CH
3
)dCHCHO appear to be formed only at NO concentrations
1
3
-3 21
>
2.4 × 10 molecules cm
.
See also the Supporting Information.
on during the course of the experiments, except for experiments
to measure aerosol formation (see below). The reactants were
introduced into the chamber by flushing a Pyrex bulb containing
a measured amount of chemical with heated N
The NO and the initial NO concentrations were monitored
during the experiments with a Thermo Environmental Instru-
ments Inc. Model 42 chemiluminescence NO-NO -NO
analyzer. Because chemiluminescence NO-NO -NO analyz-
ers read “NO ” as the sum of NO , CH ONO, and organic
nitrates, the average NO concentrations during the experiments
were calculated assuming ([NO] + [NO ]) ) constant, where
the assumption was derived from computer calculations using
a detailed reaction mechanism for a CH ONO-NO-
2
gas.
2
2
x
2
x
2
2
3
2
2
3
1
0
NO
2
-toluene-air irradiation system.
OH radicals were generated from the photolysis of CH
3
ONO
ONO-
NO-aromatic-air irradiations was conducted at a light intensity
6
,9
in the presence of added NO. A series of CH
3
Figure 1. Representative plots of the measured amounts of glyoxal
(top) and methylglyoxal (bottom) formed, corrected for secondary
reactions with OH radicals (see text), against the amounts of toluene
-3
corresponding to an NO
2
photolysis rate of J(NO
2
) ∼2.3 × 10
-
1
s , with 2 or 3 irradiation periods per experiment and with
analysis of reactants and products during the intervening dark
periods. Total irradiation times were up to 15 min. The
maximum extent of reaction was kept at ∼30% or less of the
2
reacted, at varying average NO concentrations (in units of molecules
-3
cm ). The experimental data were fitted to first-order regression lines
(s). Note replicate analyses of the final data points with a time interval
of ∼50 min.

1
0142 J. Phys. Chem. A, Vol. 114, No. 37, 2010
Nishino et al.
TABLE 1: Molar Formation Yields of Glyoxal and Methylglyoxal from the OH Radical-Initiated Reactions of Toluene,
Xylenes, and Trimethylbenzenes As a Function of NO
2
Concentration
1
0-13[NO
2 ave
]
aromatic hydrocarbon
toluene
(molecules cm-3)
glyoxal yield (%)a
methylglyoxal yield (%)a
% reaction
9.35
10.1 ( 0.2
14.0 ( 0.9
23.1 ( 0.7
23.5 ( 1.6
23.7 ( 1.1
8.4 ( 0.7
11.1 ( 1.4
12.7 ( 1.4
13.0 ( 1.9
13.4 ( 2.9
10.1 ( 1.1
5.7 ( 0.4
8.3 ( 0.1
10.5 ( 0.1
10.1 ( 0.9
11.3 ( 0.8
11.0 ( 0.9
12.7 ( 1.6
18.5 ( 1.5
27.5 ( 2.8
29.6 ( 2.0
38.5 ( 4.1
5.9 ( 0.3
3.8 ( 0.6
6.4 ( 1.3
5.1 ( 0.7
4.8 ( 1.2
5.3 ( 0.7
3.3 ( 0.9
3.4 ( 1.0
8.4 ( 1.8
7.1 ( 0.6
9.7 ( 0.7
9.4 ( 0.9
8.6 ( 1.0
8.6 ( 0.6
8.4 ( 1.0
9.1 ( 1.2
8.5 ( 0.5
8.5 ( 2.4
9.8 ( 1.6
7.9 ( 0.2
11.6 ( 1.1
19.7 ( 0.8
20.1 ( 1.8
18.6 ( 1.4
17.3 ( 0.8
27.8 ( 2.0
31.8 ( 3.1
33.5 ( 4.3
34.5 ( 8.0
25.6 ( 2.9
24.3 ( 1.4
34.1 ( 3.7
47.0 ( 1.4
48.0 ( 3.6
56.8 ( 6.9
40.5 ( 5.1
10.8 ( 0.4
13.2 ( 1.0
18.7 ( 1.5
16.9 ( 2.5
17.8 ( 4.4
14.9 ( 1.1
13.0 ( 1.3
18.1 ( 1.2
15.5 ( 1.1
14.2 ( 2.3
14.4 ( 1.0
16.6 ( 1.2
14.2 ( 0.7
23.3 ( 1.5
20.8 ( 1.2
30.6 ( 1.5
28.8 ( 1.6
30.1 ( 3.0
29.0 ( 1.0
24.4 ( 3.1
27.5 ( 2.7
34.3 ( 2.5
22.7 ( 3.9
27.2 ( 6.1
43.1 ( 3.0
50.0 ( 2.1
51.5 ( 0.8
55.9 ( 3.4
55.9 ( 1.9
65.2 ( 10.2
7-17
7-20
10-21
6-16
4-8
4
1
0
0
.09
.19
.61
.32
o-xylene
m-xylene
p-xylene
8.49
6-27
7-28
7-27
9-29
7-20
6-12
12-28
12-40
18-36
6-23
4-10
4-10
6-23
4-23
10-30
8-21
4-8
7-30
6-30
6-26
10-28
5-22
9-27
4-10
5-11
7-30
4-19
11-37
6-26
10-34
7-34
9-24
6-24
10-28
4-10
6-11
6-31
4
1
1
0
0
.04
.06
.25
.68
.28
7.84
3
1
0
0
0
.37
.48
.61
.27
.23
8.20
3
1
0
0
.61
.36
.66
.23
1
,2,3-trimethylbenzene
,2,4-trimethylbenzene
8.71
b
3
2
0
0
0
0
0
.58
.82
.96
.72
.32c
.28
.25
1
10.3
9
3
3
3
1
0
0
0
0
0
.40
.75
.37
.02
.15
.73
.59
d
.34e
.32
.26
1
,3,5-trimethylbenzene
9.07
3.05
0.65
0.57
0.28
0.26
9-33
6-26
5-21
2-9
3-9
a
Indicated errors are two standard deviations of the slopes of plots such as those shown in Figure 1, and do not include uncertainties in the
b
13
-3
SPME/GC-FID response factors. Experiment conducted at an NO
95% N ), and the measured NO concentration was converted to that at an atmospheric O
mixture. Experiment conducted at an NO
measured NO concentration was converted to that at an atmospheric O
Experiment conducted at an NO
concentration was converted to that at an atmospheric O
2
concentration of 0.89 × 10 molecules cm in synthetic air (∼5% O
concentration using the NO /O ratio in the N
+ ∼20% N
ratio in the N + O
+ ∼95% N ), and the measured NO
+ O mixture. Experiment conducted
), and the measured NO concentration was
mixture.
2
+
+
∼
2
2
2
2
2
2
c
13
-3
O
2
2
concentration of 1.30 × 10 molecules cm in synthetic air (∼80% O
2
2
), and the
2
2
concentration using the NO /O
2
2
2
2
mixture.
d
13
-3
2
concentration of 0.84 × 10 molecules cm in synthetic air (∼5% O
concentration using the NO /O ratio in the N
+ ∼20% N
+ O
2
2
2
e
2
-3
2
2
2
2
1
3
at an NO
converted to that at an atmospheric O
2
concentration of 1.35 × 10 molecules cm in synthetic air (∼80% O
2
2
2
2
concentration using the NO /O ratio in the N
2
2
2
2
oximes, mono-oximes were also observed (accounting for e37%
of the total methylglyoxal). Therefore, we derived the concen-
trations of methylglyoxal using the most abundant isomer peak
for each of the mono- and di-oximes. The relative response of
the mono- to di-oxime was estimated based on their effective
measurements of glyoxal and methylglyoxal typically agreed
to within <5% and <10%, respectively.
Calibrations of glyoxal were conducted by using the OH
radical-initiated reaction of 3-methyl-2-butenal, which produces
14
glyoxal in 40 ( 4% yield, as an in situ source of glyoxal.
1
3
carbon numbers (ECN). An ECN value is available for the
methoxime group13 and the ECN of the C-F bond was
determined from GC-FID analyses of toluene (calculated ECN
Irradiations of CH ONO-NO-3-methyl-2-butenal-air mixtures
3
were routinely conducted to calibrate the SPME/GC-FID
response of glyoxal as its oxime derivatives. Initial concentra-
1
3
)
9
7.0 ) and 2,3,4,5,6-pentafluorotoluene (measured ECN )
tions of CH
3
ONO, NO, and 3-methyl-2-butenal were ∼2.4 ×
13
-3
.2). The response of the mono-oxime of methylglyoxal relative
10 molecules cm each. For methylglyoxal calibration, the
OH radical-initiated reaction of methacrolein, with a methyl-
glyoxal yield of 8.4 ( 1.6%, was employed, with initial
to its di-oxime was then calculated to be 0.53. No glyoxal mono-
oxime was observed in our GC-FID analyses. Replicate
15

Formation Yields of Glyoxal and Methylglyoxal
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10143
concentrations of CH
3
ONO, NO, and methacrolein being ∼2.4
13
13
-3
×
10 or 4.8 × 10 molecules cm each. The amounts of
3
-methyl-2-butenal and methacrolein reacted were determined
by Tenax/GC-FID analyses. Corrections for reactive losses of
glyoxal and methylglyoxal with OH radicals were made as
1
4
described previously.
Aerosol formation was monitored during the experiments with
a TSI 3936L72 scanning mobility particle sizer. Since the
continuous use of the mixing fan may have caused wall loss of
aerosols, the experiments at the lowest and the highest NO
2
concentrations were repeated for aerosol measurements under
the same conditions but with the mixing fan off. No seed
particles were added in the chamber for these measurements.
Chemicals. Chemicals used and their stated purities were the
following: toluene (99+%), o-xylene (98%), m-xylene (99+%),
p-xylene (99+%), 1,2,3-trimethylbenzene (90%), 1,2,4-tri-
methylbenzene (98%), 1,3,5-trimethylbenzene (98%), 3-methyl-
2
-butenal (97%), methacrolein (95%), 2,3,4,5,6-pentafluoro-
toluene (99%), and O-(2,3,4,5,6-pentafluorobenzyl)hydroxy-
lamine (98+%), Aldrich; and NO (>99%), Matheson Gas
Products. CH
3
ONO was prepared and stored as described
previously.6
,9
Results
Since glyoxal and methylglyoxal react with OH radicals, the
measured concentrations were corrected as described previ-
1
4
4
ously, using the following rate constants for the reactions with
OH radicals (in units of 10- cm molecule s ): toluene,
12
3
-1 -1
5
.63; o-xylene, 13.6; m-xylene, 23.1; p-xylene, 14.3; 1,2,3-
Figure 2. Plots of the glyoxal (O) and methylglyoxal (3) yields
from the OH radical-initiated reaction of toluene and o-, m-, and
p-xylene against the average NO concentration. The error bars are
2
two least-squares standard deviations. The curves are calculated by
using eq I assuming that 92%, 90%, 96%, and 92% of the toluene,
o-, m-, and p-xylene reactions proceed by OH radical addition,
trimethylbenzene, 32.7; 1,2,4-trimethylbenzene, 32.5; 1,3,5-
trimethylbenzene, 56.7; glyoxal, 11; and methylglyoxal, 15. The
multiplicative correction factors to account for secondary
reaction with OH radicals increase with increasing rate constant
ratio k(OH + product)/k(OH + aromatic hydrocarbons) and with
the extent of reaction. In this work, the maximum values of
these correction factors ranged from 1.06 for methylglyoxal from
3
-11
3
respectively. Toluene: (---) calculated using k
2
) 3.6 × 10
cm
-1 -1
-16
3
-1 -1 5
molecule
s
and k
3
) 5.6 × 10
cm molecule
s , leading
to YNO2 ) -0.042 ( 0.048 and Y ) 0.277 ( 0.020 for glyoxal
O2
1,3,5-trimethylbenzene to 1.36 for methylglyoxal from toluene.
and YNO2 ) -0.040 ( 0.063 and YO2 ) 0.231 ( 0.024 for
-
11
3
methylglyoxal; (s) calculated using k
2
) 3.6 × 10
cm
Due to the short irradiation times and low light intensity
employed here, photolysis of glyoxal and methylglyoxal by
blacklamps was negligible.
Representative plots of the measured amounts of glyoxal and
methylglyoxal formed, corrected for secondary reaction with
OH radicals, against the amounts of toluene reacted are shown
in Figure 1. The experimental data were well fitted by straight
lines, indicating that glyoxal and methylglyoxal are first-
generation products, consistent with previous studies.
linear plots were observed from all the aromatic reactions
studied.
Formation yields of glyoxal and methylglyoxal obtained from
least-squares analyses of the data are listed in Table 1, and are
plotted against the average NO
-
1
-1
molecule
.024 and k
s
and an assumed YNO ) 0, leading to Y
O
) 0.282 (
2
2
-
16
3
-1 -1
0
3
) (4.1 ( 1.6) × 10
cm molecule
s
for glyoxal
6
,16
-16
3
and YO2 ) 0.234 ( 0.031 and k
3
) (4.1 ( 2.4) × 10
cm
-1 -1
molecule
s
for methylglyoxal. Glyoxal and methylglyoxal yields
at [NO ] ) 0 are 0.92 × Y . o-Xylene: (s) calculated using k )
2
O2
2
-
11
3
-1 -1
3.6 × 10
cm molecule
s
and an assumed YNO ) 0, leading
2
-
16
3
to Y
O
) 0.142 ( 0.021 and k
3
) (14.9 ( 18.7) × 10
cm
) (9.8
2
-
1
-1
molecule
( 12.1) × 10
and methylglyoxal yields at [NO
s
for glyoxal and YO2 ) 0.367 ( 0.067 and k
cm molecule
3
-
16
3
-1 -1
17,18
s
for methylglyoxal. Glyoxal
Similar
2
] ) 0 are 0.90 × YO2. m-Xylene:
-11
3
-1 -1
(
)
---) calculated using k
2
) 3.6 × 10
cm molecule
s
and k
3
-16
3
-1 -1 5
18 × 10
cm molecule
s , leading to YNO2 ) -0.124 (
0.044 and YO2 ) 0.117 ( 0.006 for glyoxal and YNO2 ) -0.646 (
0.602 and YO2 ) 0.524 ( 0.073 for methylglyoxal; (s) calculated
-
11
3
-1 -1
using k
2
) 3.6 × 10
cm molecule
s
and an assumed YNO2-
)
2
concentrations in Figures 2 and
16
0
, leading to Y
cm molecule
O
) 0.119 ( 0.008 and k
for glyoxal and Y
3
) (6.3 ( 2.2) × 10
) 0.536 ( 0.089 and k
-1 -1
2
3. The experimental uncertainties in the measured product yields
3
-1 -1
s
O
3
)
2
were more pronounced at lower initial CH
concentrations (and hence at lower average NO
3
ONO and NO
concentrations)
-16
3
(
5.4 ( 5.0) × 10 cm molecule
s
for methylglyoxal. Glyoxal
2
and methylglyoxal yields at [NO
] ) 0 are 0.96 × Y
O
2
. p-Xylene:
2
-
11
3
-1 -1
because of the small amounts of aromatic reacted and of
products formed, especially for methylglyoxal with greater
uncertainties in its SPME measurement (see above). As is
evident from Table 1 and Figures 2 and 3, the glyoxal and
methylglyoxal yields from the toluene, o-, m-, and p-xylene,
and 1,3,5-trimethylbenzene reactions decrease with increasing
NO
conducted in air suggest little or no decrease in the glyoxal and
methylglyoxal yields with increasing NO concentration from
the 1,2,3- and 1,2,4-trimethylbenzene reactions (Table 1 and
(---) calculated using k ) 3.6 × 10
cm molecule
s
and k3
2
-
16
3
-1 -1 5
) 8.8 × 10
cm molecule
s , leading to YNO2 ) -0.281 (
0
0
.219 and YO2 ) 0.381 ( 0.055 for glyoxal and YNO2 ) -0.015 (
.083 and YO2 ) 0.203 ( 0.021 for methylglyoxal; (s) calculated
-
11
3
-1 -1
using k
2
) 3.6 × 10
cm molecule
s
and an assumed YNO2 )
-16
0
, leading to Y
cm molecule
O
) 0.423 ( 0.051 and k
for glyoxal and Y
3
) (2.4 ( 1.1) × 10
2
3
-1 -1
s
O
) 0.203 ( 0.023 and k )
3
2
-
16
3
-1 -1
2
concentration. In contrast, our data from experiments
(7.9 ( 5.2) × 10 cm molecule
s
for methylglyoxal. Glyoxal
and methylglyoxal yields at [NO
2
] ) 0 are 0.92 × YO2. ([) The
yields of 3-hexene-2,5-dione, the suggested co-product of glyoxal,
as a function of NO as measured by Bethel et al.
2
2
9

1
0144 J. Phys. Chem. A, Vol. 114, No. 37, 2010
Nishino et al.
,2,3- and 1,2,4-trimethylbenzene were independent of the NO
1
2
concentration within two least-squares standard deviations, and
the solid lines in Figure 3 for these trimethylbenzenes are the
average formation yields assuming no dependence on NO
concentration.
2
Discussion
On the basis of the current understanding of the mechanism
of the OH radical-initiated reactions of benzene and alkyl
benzenes shown in Scheme 1,5
,19-23
and the kinetic analysis of
the OH-aromatic adduct + O
2
reaction provided in the Sup-
porting Information, the formation of glyoxal and methylglyoxal
from these reactions in the presence of NO can be rationalized
by using the following simplified reaction scheme:
aromatic + OH f OH-aromatic adduct
OH-aromatic adduct + NO f
(1)
2
YNO2(glyoxal or methylglyoxal) + other products (2)
Figure 3. Plots of glyoxal (O, b) and methylglyoxal (3, 1) yields
from the OH radical-initiated reaction of 1,2,3-, 1,2,4-, and 1,3,5-
OH-aromatic adduct + O f
2
Y (glyoxal or methylglyoxal) + other products (3)
O
trimethylbenzene against the average NO
2
concentrations (the error bars
2
are two least-squares standard deviations): (O, 3) experiments con-
ducted in air; (b, 1) experiments conducted in synthetic air (∼80%
Hence,
O
2
+ ∼20% N
2
or ∼5% O
2
+ ∼95% N
2 3
) at initial CH ONO and NO
1
3
-3
concentrations of ∼2.4 × 10 molecules cm , where the calculated
NO concentration was converted to that at an atmospheric O
concentration using the NO /O ratio in the N + O mixture (e.g., the
data points at low NO
concentration being carried out in ∼80% O
20% N ). 1,2,3-Trimethylbenzene: The solid lines are the average
formation yields of glyoxal and methylglyoxal of 0.047 ( 0.024 and
.151 ( 0.033, respectively. 1,2,4-Trimethylbenzene: The solid lines
are the average formation yields of glyoxal and methylglyoxal of 0.087
0.016 and 0.272 ( 0.081, respectively; ([) the yields of 3-hexene-
,5-dione, a potential co-product of methylglyoxal, as a function of
2
2
glyoxal or methylglyoxal yield )
2
2
2
2
Y
k [NO ] + Y k [O ]
2 2 O 3 2
2
NO2
2
2
+
(
I)
∼
2
k [NO ] + k [O ]
2
2
3
2
0
where YNO2 and YO2 are the glyoxal or methylglyoxal yields from
the reactions of the OH-aromatic adduct with NO and O
respectively, and k , k , and k are the rate constants for reactions
, 2, and 3, respectively.
While reaction 2 may be an elementary reaction, reaction 3
is complex and involves, in part, reversible addition of O to
the OH-aromatic adduct to form a peroxy radical that then reacts
(
2
2
,
2
NO
9
1
2
3
2
as measured by Bethel et al. 1,3,5-Trimethylbenzene: The fitted
1
curve is calculated using eq I assuming that 97% of the 1,3,5-
3
trimethylbenzene reaction proceeds by OH addition, with k
2
) 3.6 ×
-
11
3
-1 -1
1
0
0
cm molecule
s
and an assumed YNO ) 0, leading to Y
O
)
2
2
2
-
16
3
-1 -1
.599 ( 0.054 and k
3
) (16.9 ( 15.1) × 10
cm molecule
s ,
with the methylglyoxal yield at [NO
2
] ) 0 being 0.97 × YO2
.
5,19-23
to form products, including the observed dicarbonyls
Scheme 1 and the Supporting Information). The rate constant
is an effective rate constant that refers to the irreversible
(see
Figure 3). For these experiments conducted in air, the extents
of reaction were smaller at lower NO concentrations (Table 1)
and hence the experimental uncertainties were larger at low NO
and NO ) concentrations. Because the OH-aromatic adducts
react with either O or NO (see Scheme 1), the glyoxal and
methylglyoxal yields are expected to actually depend on the
NO ]/[O ] ratio, rather than only on the NO concentration (see
k
3
5,19,20
x
reaction of the OH-aromatic adducts with O
discussed in the Supporting Information is expected to increase
at high NO concentrations. At 300-317 K, rate constants k
have been reported by Koch et al. for the reactions of NO
with the OH-benzene, OH-toluene and OH-p-xylene adducts,
with those for the OH-toluene and OH-p-xylene adducts being
2
,
and as
x
(
2
2
2
5
2
2
[
2
2
2
5
-11
-11
3
-1
also the Supporting Information). Hence, two additional experi-
ments were carried out for 1,2,3-trimethylbenzene and for 1,2,4-
(3.6 ( 0.5) × 10 and (3.5 ( 0.5) × 10 cm molecule
-1 5
s , respectively. The dashed curves in Figure 2 for the
formation of glyoxal and methylglyoxal from the toluene and
m- and p-xylene reactions are calculated by using a common
trimethylbenzene at an intermediate CH
3
ONO and NO concen-
tration (∼2.4 × 10 molecules cm each) in synthetic air with
different O ). This varied
compositions (∼5% and ∼80% O
the [NO ]/[O ] ratio while allowing a higher extent of reaction
to be achieved than was possible at low NO concentrations
13
-3
-
11
3
-1 -1
2
2
rate constant of k
the rate constant of Koch et al. for the OH-toluene adduct
2
) 3.6 × 10 cm molecule s , based on
5
2
2
5
x
reaction, and the rate constants k
of the OH-toluene, OH-m-xylene, and OH-p-xylene adducts with
(see the caption for Figure 2 for details). However, the fitted
3
of Koch et al. for the reactions
when using air as the diluent gas. The glyoxal and methylglyoxal
formation yields from these additional experiments are listed
in Table 1 and plotted in Figure 3 as the solid symbols. Based
solely on these two experiments using differing percentages of
O
2
curves lead to values of YNO2 < 0, suggesting that glyoxal and
methylglyoxal formation from the reactions of these OH-
O
2
, glyoxal and methylglyoxal from 1,2,3-trimethylbenzene and
methylglyoxal from 1,2,4-trimethylbenzene decrease slightly
with increasing NO concentration. However, using the entire
data set the formation yields of glyoxal and methylglyoxal from
aromatic adducts with NO
observations of Klotz et al.21 that glyoxal is not formed from
the OH-benzene adduct + NO reaction. Therefore, our glyoxal
and methylglyoxal formation yield data from the toluene and
2
is negligible, consistent with the
2
2

Formation Yields of Glyoxal and Methylglyoxal
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10145
m- and p-xylene reactions, as well as from the o-xylene and
For the p-xylene reaction, our present observation of a
(
methylglyoxal only) 1,3,5-trimethylbenzene reactions, were
2
decreasing glyoxal formation yield with increasing NO con-
-
11
3
9
fitted assuming YNO2 ) 0 and using k
2
) 3.6 × 10
cm
centration is consistent with the data of Bethel et al. for
formation of the potential co-product 3-hexene-2,5-dione, and
as shown in Figure 2 our glyoxal yields agree reasonably well
molecule- s , and these calculated fits are shown as the solid
lines in Figures 2 and 3. For toluene and m-xylene the two
curves are indistinguishable, with similar product yields from
1
-1
9
with the 3-hexene-2,5-dione yields measured by Bethel et al.,
the O
and 1,2,4-trimethylbenzene reactions did not show any signifi-
cant dependence on the NO concentration, the product yields
2
reaction being obtained (see Figure 2). Since the 1,2,3-
2
especially at the lower NO concentrations. For the o-xylene
6
reaction, our present and previous work show that all three
6
1,2-dicarbonyls (glyoxal, methylglyoxal, and biacetyl ) decrease
2
were derived as the average of the data, leading to glyoxal and
methylglyoxal yields of 4.7 ( 2.4% and 15.1 ( 3.3% for the
with increasing NO
o-, m-, and p-xylene, and 1,3,5-trimethylbenzene reactions, no
obvious effect of NO on the glyoxal and methylglyoxal
2
concentration. In contrast to the toluene,
1,2,3-trimethylbenzene reaction, and 8.7 ( 1.6% and 27.2 (
8.1% for the 1,2,4-trimethylbenzene reaction, respectively.
2
formation yields from 1,2,3- and 1,2,4-trimethylbenzene was
observed (Table 1 and Figure 3). In the previous study of Bethel
3 2
The rate constant ratios k /k derived here for the OH-aromatic
adducts formed in the toluene, and o-, m-, and p-xylene reactions
from the variation of the glyoxal and methylglyoxal yields with
9
et al. of formation of biacetyl from the 1,2,3- and 1,2,4-
trimethylbenzene reactions and of 3-hexene-2,5-dione from the
1,2,4-trimethylbenzene reaction, the measured dicarbonyl yields
2
NO concentration can be compared to that derived similarly
6
decreased slightly with increasing NO concentration, with the
by Atkinson and Aschmann for the OH + o-xylene reaction
2
following extrapolated yields: biacetyl from 1,2,3-trimethyl-
and with the ratios obtained from the individual rate constants
5
5
benzene, YO2 ) 0.52 and YNO2 ≈ 0.30; biacetyl from 1,2,4-
k
k
0
2
and k
2
5
3
reported by Koch et al. The derived values of (10 ×
trimethylbenzene, YO2 ) 0.10 and YNO ≈ 0.05; and 3-hexene-
3
/k ) at room temperature are the following: toluene, 1.6 (
.5, 1.1 ( 0.5 (this work, glyoxal), and 1.1 ( 0.7 (this work,
2
2,5-dione from 1,2,4-trimethylbenzene, YO2 ) 0.31 and Y
NO2
≈
0
.25. As shown in Figure 3, for the 1,2,4-trimethylbenzene
methylglyoxal); o-xylene, ∼1.1 ( 0.5 (derived from the biacetyl
6
reaction our formation yields of methylglyoxal are in reasonably
yields using an analogous analysis to that used here), 4.1 (
9
good agreement with those of Bethel et al. for 3-hexene-2,5-
5
.2 (this work, glyoxal), and 2.7 ( 3.4 (this work, methylgly-
dione, indicating that 3-hexene-2,5-dione is the major co-product
to methylglyoxal. This then suggests that the other potential
co-products to methylglyoxal from the 1,2,4-trimethylbenzene
reaction, namely, CH C(O)C(CH )dCHCHO, HC(O)C(CH )d
oxal); m-xylene, 5.0 ( 1.4 assuming a rate constant of k
2
) 3.6
cm molecule s , 1.7 ( 0.7 (this work, glyoxal),
and 1.5 ( 1.4 (this work, methylglyoxal); and p-xylene, 2.5 (
-11
3
-1 -1 5
×
10
5
3
3
3
0
.8, 0.7 ( 0.4 (this work, glyoxal), and 2.2 ( 1.5 (this work,
C(CH
3
)CHO, and CH
3
C(O)CHdC(CH
3
)CHO, are formed in
methylglyoxal). Considering the assumptions made in deriving
the rate constant ratios in this work, including the expectation
only low yield, and this conclusion is entirely consistent with
the unsaturated 1,4-dicarbonyl formation yields derived by Arey
et al.8
that the effective rate constant k
high NO concentrations (see the Supporting Information), and
that a priori there is no reason why the rate constant ratios k /k
3
may increase somewhat at
Our glyoxal and methylglyoxal formation yields at NO
2
3
2
should be identical for isomeric OH-aromatic adducts, there is
general agreement with the more direct measurements of Koch
concentrations characteristic of ambient atmospheric concentra-
tions (effectively our glyoxal and methylglyoxal yields extrapo-
lated to zero NO ) are listed in Table 2 and compared there to
2
literature data. The only previous studies to measure glyoxal
and methylglyoxal yields from all seven aromatic hydrocarbons
(toluene, the xylenes, and the trimethylbenzenes) are those of
5
et al. If combined with the rate constant ratio of k
3
/k
2
) (4.7
-5
(
4.2) × 10 derived here for the OH-1,3,5-trimethylbenzene
-5
adducts and the ratio of (0.58 ( 0.24) × 10 obtained for the
OH-benzene adduct from the individual rate constants of Koch
5
24
28,30
et al., within the often large experimental uncertainties these
Bandow et al. and Bandow and Washida,
et al.,25 and in some cases our formation yields extrapolated to
low NO concentrations are significantly higher than those
measured in these previous studies.
may be due to the NO concentrations employed in the studies
and Tuazon
rate constant ratios appear to increase approximately linearly
with the number of alkyl substituents n, with a room temperature
2
-
5
24,25,28,30
rate constant ratio of k
3
/k
2
≈ (0.6 + n) × 10 . Note, however,
These differences
that the rate constant measured for k
3
(OH-hexamethylbenzene
2
5
2
24
28,30
adduct) by Koch et al. implies a 10 higher rate constant ratio
than would be predicted from this expression. Furthermore, our
observation of formation yields of glyoxal and methylglyoxal
from 1,2,3- and 1,2,4-trimethylbenzene which are essentially
of Bandow et al., Bandow and Washida,
and Tuazon
et al.25 In this regard, it may be noted that for 1,2,3- and 1,2,4-
trimethylbenzene, where we observe glyoxal and methylglyoxal
yields to be independent of the NO
2
concentration, our glyoxal
and methylglyoxal formation yields are in good agreement with
independent of the NO
glyoxal and methylglyoxal yields from the OH-aromatic adduct
NO reaction 2 are similar to those from the OH-aromatic
adduct + O reaction 3 for these two aromatic hydrocarbons,
or that for these OH-aromatic adducts the ratios k /k are at least
2
concentration suggests either that the
3
0
25
those of Bandow and Washida and Tuazon et al., and this is
also the case for 1,3,5-trimethylbenzene.2
p-xylene, our glyoxal formation yields at low NO
tions are in agreement with those of Volkamer et al.,
5,30
For toluene and
concentra-
+
2
2
2
1
7,27
and
3
2
our glyoxal and methylglyoxal yields from toluene and m- and
p-xylene are in reasonable agreement with the data of Smith
et al.2
an order of magnitude higher than the corresponding rate
constant ratios for benzene, toluene, o-, m-, and p-xylene, and
6,29
5
1
,3,5-trimethylbenzene. Koch et al. measured rate constants k
3
for the OH-hexamethylbenzene adduct + O
2
reaction at 355
Our glyoxal and methylglyoxal formation yields at low NO2
-13
-13
3
and 385 K of (1.8 ( 0.3) × 10 and (1.2 ( 0.2) × 10 cm
concentration, combined with the biacetyl yields from the
molecule- s , respectively, suggesting that a higher rate
constant k , and hence higher rate constant ratio k /k , is the
most likely reason for the essentially NO -independent glyoxal
1
-1
o-xylene
6,28,31
and 1,2,3- and 1,2,4-trimethylbenzene reac-
indicate that under atmospheric conditions the
tions,9
,25,29,30
3
3
2
2
formation of 1,2-dicarbonyls accounts (on a molar basis) for
∼47%, ∼64%, ∼63%, ∼58%, ∼62%, ∼46%, and ∼58% of
the overall products from the toluene, o-, m-, p-xylene, and
and methylglyoxal yields observed from the 1,2,3- and 1,2,4-
trimethylbenzene reactions.

1
0146 J. Phys. Chem. A, Vol. 114, No. 37, 2010
Nishino et al.
Concentrations from This Study
TABLE 2: Comparison of the Formation Yields of Glyoxal and Methylglyoxal at Low NO
2
with Selected Literature Data at Atmospheric Pressure of Air in the Presence of NO
x
molar formation yield (%)
1
0-13[NO
2
]-
3 a
aromatic
toluene
glyoxal
methylglyoxal
(molecules cm
)
ref
15 ( 4
14 ( 4
1.24-3.74
24
25
26
1
2
3
0.5 ( 1.9
3.8 ( 2.5
0.6 ( 6
14.6 ( 0.6
16.7 ( 1.8
∼12 (NO)
0.18-1.03 (NO)
0.10 (NO
0.06 (NO
0
2
)/0.20 (NO)
)/0.28 (NO)
17, 27
3
7 ( 2
37 ( 2
21.5 ( 2.9
23 ( 3
24.6 ( 2.0
33.1 ( 6.1
42 ( 5
31.9 ( 0.9
40 ( 1.8
51.5 ( 8.5
12 ( 2
10.5 ( 3.4
21.7 ( 7.6
2
18
b
2
6.0 ( 2.2
this work
o-xylene
m-xylene
8 ( 1
1.24
28
25
8
.7 ( 1.2
∼12 (NO)
b
1
2.7 ( 1.9
0
this work
13 ( 3
1.24
28
25
29
8
7
.6 ( 1.1
.9 ( 0.4
∼12 (NO)
0.38-1.63 (NO)
b
1
1.4 ( 0.7
24 ( 2
0
this work
p-xylene
1.24
28
25
29
2
3
3
3
2.5 ( 3.9
9.4 ( 11
1.9 ( 5
8.9 ( 4.7
7.2 ( 0.1
∼12 (NO)
0.63-2.66 (NO)
0.10 (NO
0
2
)/0.20 (NO)
17, 27
this work
30
b
18.7 ( 2.2
18 ( 1
15.2 ( 2.5
15.1 ( 3.3
37 ( 1
35.7 ( 1.7
44 ( 7.4
27.2 ( 8.1
64 ( 3
1
1
,2,3-trimethylbenzene
,2,4-trimethylbenzene
1.24
5
4
.8 ( 0.8
.7 ( 2.4
∼12 (NO)
0.25-8.71
1.24
25
c
this work
30
7.8 ( 0.5
4.8 ( 0.5
6.6 ( 1.1
8.7 ( 1.6
∼12 (NO)
25
29
1.66-2.24 (NO)
c
0.26-10.3
this work
30
1
,3,5-trimethylbenzene
1.24
6
0.2 ( 3.3
0 ( 25
8.1 ( 5.3
∼12 (NO)
25
29
9
1.72-1.84 (NO)
b
5
0
this work
a
Only the initial NO and/or initial NO
2
concentrations are given. Unless noted otherwise, the initial NO
2
concentration is listed.
b
concentration. c Glyoxal and methylglyoxal yields were independent of the NO
Extrapolated to zero NO
the averages over the cited NO
2
2
concentration; the yields listed are
2
concentration ranges employed.
1
,2,3-, 1,2,4-, and 1,3,5-trimethylbenzene reactions, respectively.
that the glyoxal and methylglyoxal formed were mostly in the
gas phase and thus their potential loss by conversion to the
particle phase was not important under our experimental
conditions. Since the aerosol concentrations remained constant
or continued to increase after the reaction, no corrections for
wall loss were made. The constant or increasing aerosol
concentrations after the blacklamps were turned off suggest that
heterogeneous reactions were occurring, leading to a net transfer
of material from the gas phase into the aerosol.
Products arising from the H-atom abstraction pathway and
phenolic compounds formed after OH radical addition are also
formed9
∼
,10,29,32,33
and increase the above numbers to ∼72%,
76%, ∼79%, ∼80%, ∼54%, and ∼65% of the overall products
from the toluene, o-, m-, p-xylene, and 1,2,4-, and 1,3,5-
trimethylbenzene reactions, respectively (no formation yields
of H-atom abstraction products or of phenolic products have
been measured to date from the OH + 1,2,3-trimethylbenzene
reaction). Clearly, even assuming that the formation yields of
the unsaturated 1,4-dicarbonyls are equal to those of their
potential co-product 1,2-dicarbonyls, a significant fraction of
the overall reaction pathways and products are presently
unaccounted for, ranging from ∼45% for the 1,2,4-trimethyl-
benzene reaction to ∼20-25% for the xylene isomer reactions.
Aerosol formation was monitored for each aromatic hydro-
Acknowledgment. The authors thank the U.S. Environmental
Protection Agency (Grant No. R833752) for supporting this
research. While this work has been supported by this Agency,
the results and the contents of this publication do not necessarily
reflect the views and the opinions of the Agency. The TSI
3936L72 SMPS instrument was purchased with funds from the
Keck Foundation (Environmental Chamber Research Program
on Air Pollution Grant). N.N. thanks the University of California
Toxic Substances Research & Teaching Program for partial
support, and J.A. thanks the University of California Agricultural
Experiment Station for partial salary support.
carbon at the lowest and the highest NO
aerosol mass concentrations before the reaction were generally
<
m
2
concentrations. The
-3
0.1 µg m , with the maximum concentrations being <0.3 µg
-3
for 1,3,5-trimethylbenzene. An increase in mass concentra-
tion was observed after the reactions at both low and high NO
2
concentrations. Assuming that the density of aerosol is the same
as that of the aromatic hydrocarbon precursor, aerosol formation
yields were derived from the concentrations of aromatic
hydrocarbons reacted and of aerosol formed. Aerosol yields at
Supporting Information Available: A discussion and
kinetic analysis of OH-aromatic adduct + O reactions. This
2
material is available free of charge via the Internet at http://
pubs.acs.org.
low NO
hydrocarbons studied. The experiments at high NO
tions showed higher, but still low, aerosol formation yields of
4% for toluene, 1-3% for xylenes, and <1% for trimethyl-
benzenes. The low yields of aerosol from these reactions indicate
2
concentrations were <0.1% for all the aromatic
2
concentra-
References and Notes
∼
(
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Formation Yields of Glyoxal and Methylglyoxal
J. Phys. Chem. A, Vol. 114, No. 37, 2010 10147
(
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(
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