Prompt HO2 formation following the reaction of OH with aromatic compounds under atmospheric conditions

Article abstract of DOI:10.1021/jp210946y

The secondary formation of HO₂ radicals following OH + aromatic hydrocarbon reactions in synthetic air under normal pressure and temperature was investigated in the absence of NO after pulsed production of OH radicals. OH and HO_x (=OH + HO₂) decay curves were recorded using laser-induced fluorescence after gas-expansion. The prompt HO₂ yields (HO₂ formed without preceding NO reactions) were determined by comparison to results obtained with CO as a reference compound. This approach was recently introduced and applied to the OH + benzene reaction and was extended here for a number of monocyclic aromatic hydrocarbons. The measured HO₂ formation yields are as follows: toluene, 0.42 ± 0.11; ethylbenzene, 0.53 ± 0.10; o-xylene, 0.41 ± 0.08; m-xylene, 0.27 ± 0.06; p-xylene, 0.40 ± 0.09; 1,2,3-trimethylbenzene, 0.31 ± 0.06; 1,2,4-trimethylbenzene, 0.37 ± 0.09; 1,3,5- trimethylbenzene, 0.29 ± 0.08; hexamethylbenzene, 0.32 ± 0.08; phenol, 0.89 ± 0.29; o-cresol, 0.87 ± 0.29; 2,5-dimethylphenol, 0.72 ± 0.12; 2,4,6-trimethylphenol, 0.45 ± 0.13. For the alkylbenzenes HO₂ is the proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O₂. In most product studies the only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so far. Together with the yields of phenols from other studies, the HO₂ yields determined in this work set an upper limit to the combined yields of epoxides and oxepins that was found to be significant (≥0.3) for all investigated alkylbenzenes except m-xylene. For the hydroxybenzenes the currently proposed HO₂ coproducts are dihydroxybenzenes. For phenol and o-cresol the determined HO₂ yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO ₂ forming reaction channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.

Full text of DOI:10.1021/jp210946y

ARTICLE
pubs.acs.org/JPCA
Prompt HO2 Formation Following the Reaction of OH with Aromatic
Compounds under Atmospheric Conditions
Sascha Nehr, Birger Bohn,* and Andreas Wahner
Institut f€ur Energie- und Klimaforschung IEK-8, Troposph€are, Forschungszentrum J€ulich GmbH, 52425 J€ulich, Germany
ABSTRACT: The secondary formation of HO2 radicals follow-
ing OH + aromatic hydrocarbon reactions in synthetic air under
normal pressure and temperature was investigated in the absence
of NO after pulsed production of OH radicals. OH and HOx
(=OH + HO2) decay curves were recorded using laser-induced
fluorescence after gas-expansion. The prompt HO2 yields (HO2
formed without preceding NO reactions) were determined by
comparison to results obtained with CO as a reference com-
pound. This approach was recently introduced and applied to the
OH + benzene reaction and was extended here for a number of
monocyclic aromatic hydrocarbons. The measured HO2 formation yields are as follows: toluene, 0.42 ( 0.11; ethylbenzene, 0.53 (
0.10; o-xylene, 0.41 ( 0.08; m-xylene, 0.27 ( 0.06; p-xylene, 0.40 ( 0.09; 1,2,3-trimethylbenzene, 0.31 ( 0.06; 1,2,4-
trimethylbenzene, 0.37 ( 0.09; 1,3,5-trimethylbenzene, 0.29 ( 0.08; hexamethylbenzene, 0.32 ( 0.08; phenol, 0.89 ( 0.29;
o-cresol, 0.87 ( 0.29; 2,5-dimethylphenol, 0.72 ( 0.12; 2,4,6-trimethylphenol, 0.45 ( 0.13. For the alkylbenzenes HO2 is the
proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O2. In most product studies the
only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so
far. Together with the yields of phenols from other studies, the HO2 yields determined in this work set an upper limit to the
combined yields of epoxides and oxepins that was found to be significant (e0.3) for all investigated alkylbenzenes except m-xylene.
For the hydroxybenzenes the currently proposed HO2 coproducts are dihydroxybenzenes. For phenol and o-cresol the determined
HO2 yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO2 forming reaction
channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.
’ INTRODUCTION
been widely studied experimentally.16ꢀ23 Under atmospheric
conditions the adduct predominantly reacts with O2 via both
reversible addition to give a peroxy radical (adduct-O2, Figure 1, B)
and irreversible reaction pathways. Proposed major products
are phenols (C),14,24ꢀ27 epoxides (E, F),27ꢀ31 oxepins (I),32 and
a bicyclic peroxy radical (H).31 The bicyclic peroxy radical can
undergo reactions with NO and subsequently with O2, finally
yielding ring fragmentation products (e.g., α-dicarbonyls) and
HO2.33 Recent research showed that organic nitrate formation
in the bicyclic peroxy radical + NO reaction is a minor channel
(<10%).34 Phenols, epoxides, and oxepins are considered to
be the coproducts of prompt HO2; i.e., HO2 formed without
the preceding reaction of peroxy radicals with NO. Formation
of phenols14,24ꢀ27 and epoxides27ꢀ31 was experimentally con-
firmed, whereas the oxepin pathway was shown to be inoperative
at least for the OH + benzene reaction.35 There is no experi-
mental evidence for oxepin formation from alkylbenzenes but
quantum chemical computations support this pathway.32 Among
the currently proposed prompt HO2 coproducts, only phenols
Aromatic hydrocarbons, such as benzene,1 alkylbenzenes,2ꢀ5
and hydroxybenzenes,6,7 comprise a significant fraction of vola-
tile organic compounds observed in urban areas. The emission of
aromatics is mostly linked to anthropogenic activities like
incomplete combustion of fossil fuels, evaporation from indus-
trial production plants, and residential wood burning.1,8 During
the daytime, the atmospheric degradation of aromatic hydro-
carbons is almost exclusively initiated by the reaction with OH
radicals9 and leads to the formation of ozone and secondary
organic aerosol.10,11 Because of their atmospheric lifetimes of up
to several days,12 aromatics impact air quality on a local as well as
on a regional scale.13
The current understanding of the OH-initiated atmospheric
oxidation of alkylbenzenes is depicted in Figure 1 using the
example of o-xylene. From kinetic experiments and product
studies it is known that the OH + alkylbenzene reaction proceeds
via (i) reversible OH-addition to the ring, (ii) H-atom abstrac-
tion from the CꢀH bonds (preferentially from substituent alkyl
groups), and (iii) dealkylation. The bulk of the reaction (g90%)9
proceeds via addition forming an aromaticꢀOH adduct (in the
following briefly referred to as adduct) (Figure 1, A). The H-atom
abstraction channel finally yielding benzaldehydes (Figure 1, K, NO
reaction step required) and the dealkylation pathway14,15 (J) are
of minor importance. The subsequent reactions of the adduct have
Special Issue: A. R. Ravishankara Festschrift
Received: November 14, 2011
Revised:
December 22, 2011
Published: December 23, 2011
r
2011 American Chemical Society
6015
dx.doi.org/10.1021/jp210946y J. Phys. Chem. A 2012, 116, 6015–6026
|

The Journal of Physical Chemistry A
ARTICLE
Figure 1. Postulated reaction pathways of the OH-initiated oxidation of o-xylene.14,61,66 For convenience, different resonance structures and possible
isomers are not shown. HO2 formed without preceding NO reaction steps is indicated in boldface.
were quantified for most of the investigated alkylbenzenes.14,26 In
these cases the measurement of prompt HO2 yields can help set
an upper limit to the combined yield of the epoxides and oxepins.
The OH-initiated atmospheric oxidation of hydroxybenzenes
is displayed in Figure 2 using the example of o-cresol. Reac-
tions proceed via (i) reversible OH-addition to the ring and
(ii) H-atom abstraction from the substituent hydroxyl group
(H-atom abstraction from the alkyl substituent is negligible).36
Comparable to the OH + alkylbenzene reaction, the addition
pathway is predominant (g90%) and the H-atom abstraction
channel finally yielding nitrophenols (Figure 2, IX, NO2 reaction
step required) is of minor importance.37 Under atmospheric
conditions, the subsequent fate of the aromaticꢀOH adduct is
also governed by the reaction with O2. However, the adduct + O2
chemistry is different for hydroxybenzenes and to date no
experimental evidence is available for intermediately formed
peroxy radicals. Several pathways were proposed for the adduct +
O2 reaction depending on the OH-addition site with respect to
the existing hydroxyl substituent.36 Major confirmed products
are dihydroxybenzenes (II) most likely stemming from ortho-
OH-addition followed by direct H-displacement and 1,4-benzo-
quinones (VI).36 Formation of 1,4-benzoquinones was assigned
to ipso-OH-addition, yielding a monocyclic peroxy radical (IV)
upon reaction with O2. The peroxy radical can undergo further
reactions with NO and subsequently with O2, finally yielding 1,4-
benzoquinones. A product study by Olariu et al.36 suggests that
the bulk of the OH + hydroxybenzene reaction (>0.6) proceeds
via ortho-OH-addition whereas ipso-OH-addition is of minor
(<0.1) importance. No experimental evidence was reported for
meta- and/or para-OH-addition. Thus, prompt HO2 is proposed
to result exclusively from ortho-OH-addition. The measurement
of prompt HO2 yields can help to experimentally confirm the
HO2 forming reaction channels by matching the previously
reported36 formation yield of dihydroxybenzenes.
’ EXPERIMENTAL METHODS
The instrument used in this work was originally developed to
measure total OH reactivities kOH in ambient air by recording
artificial OH decay curves.38,39 In a recent study performed in our
lab,40 we described modifications of the experimental setup that
facilitate the alternating measurement of OH and HOx (=OH +
HO2) decay curves after pulsed formation of OH in the
presence of selected reactants. In brief, the apparatus consists
of the reaction volume under laminar flow conditions and the
OH-detection cell based on the laser-induced fluorescence (LIF)
technique. The tube-shaped reaction volume (length = 80 cm,
diameter = 4 cm) was operated at atmospheric pressure and 298 K.
Highly purified synthetic air entered the reaction volume at a
typical flow rate of 20 L minꢀ1. Traces of O3 (≈2 ꢁ 1012cmꢀ3
,
produced by O2 photolysis) and H2O (≈3 ꢁ 1017cmꢀ3) were
added. A fourth harmonic Nd:YAG laser (Big Sky, CFR200),
6016
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Figure 2. Postulated reaction pathways of the OH-initiated oxidation of o-cresol.36,75ꢀ77 For convenience, different resonance structures and possible
isomers are not shown. HO2 formed without preceding NO reaction steps is indicated in boldface.
longitudinally directed through the reaction volume, with a pulse
duration of 10 ns and a fluence of 1.5 mJ cmꢀ2 was used for the
pulsed photolysis of O3 at 266 nm. Intermediately formed O(1D)
reacted with water vapor to give OH starting concentrations
of e8 ꢁ 109 cmꢀ3 formed virtually instantaneously after the
266 nm flash. The photolysis laser beam was expanded to about
3 cm to maximize the irradiated reaction volume. Within the time
between two photolysis laser shots (2.5 s), the content of the
reaction volume was completely exchanged. Thus, photolysis of
reaction products can be excluded as a potential radical source.
The detection of OH radicals was performed 50 cm down-
stream of the tube inlet by laser-induced fluorescence technique
after gas expansion. The air was sampled from the center of
the reaction volume through a nozzle (0.2 mm diameter) into
a low pressure detection cell operated at 350 Pa. OH fluo-
rescence was induced at 308 nm by use of a tunable frequency-
doubled dye laser pumped by a second harmonic of a high
repetition rate (8.5 kHz) Nd:YAG laser (Spectra physics, Navi-
gator I). The OH fluorescence emitted from the detection zone
was focused onto a gated photomultiplier (Perkin-Elmer, C 1943 P).
Photon counts were recorded by a multichannel scaler over a 1 s
time interval at a resolution of 5 ms. Sixty decay curves were
accumulated to improve the signal-to-noise ratio. Data from the
first 10 ms after the photolysis laser flash were discarded
because the signal in this interval was unstable. When a small
flow of pure NO is added to the expanding gas upstream of
the detection zone, HO2 can be partly converted to OH (HO2 +
NO f OH + NO2) and detected as additional fluorescence
signal (HOx measurement mode). The HO2 conversion effi-
ciency is limited by the reaction time in the detection cell.
Switching between the OH and HOx measurement modes was
possible within a few minutes. OH decay curves were recorded
immediately before and after the HOx decay curves to verify
constant experimental conditions, i.e., similar OH starting
concentrations and detection sensitivities.
k0HO were assigned to diffusion and wall losses. The contribution
2
of the OH + O3 reaction to the OH background decay rate was
minor (≈0.1 sꢀ1). Radicalꢀradical reactions were estimated
unimportant on the basis of typical rate constants of HO2 + RO2
and the HO2 self-reaction.41
Besides the secondary formation following reactions of OH,
HO2 can also be formed by photolysis of aromatics in the
presence of O2. H-atom formation from the 248 nm photolysis
of benzene and toluene and subsequent HO2 formation via
H + O2 has been observed in previous studies.42ꢀ44 We performed
test experiments in the absence of the OH-precursor O3 to check
whether H-atoms (and subsequently HO2 radicals) are formed by
the 266 nm photolysis of the aromatic hydrocarbons used in this
work. For the alkylbenzenes no detectable HO2 formation was
observed. In contrast, the 266 nm photolysis of the hydroxyben-
zenes led to instantaneous HO2 formation. This photolytically
produced HO2 was at significant levels compared to the secondar-
ily formed HO2 and was considered quantitatively in the evaluation
of experiments with hydroxybenzenes as described below. However,
no attempt was made to quantify the HO2 formation in terms of
quantum yields for the different compounds.
As reported recently,40,45 the LIF HO2 detection technique
features cross-sensitivity to specific organic peroxy radicals
(RO2). This cross-sensitivity is caused by the conversion of
RO2 to organic alkoxy radicals (RO) by NO in the LIF detection
cell. In contrast to, for example, CH3O, more complex RO can
undergo fast decomposition into fragments rapidly forming HO2
in the presence of O2. This applies to α-hydroxy RO from OH +
alkene reactions but also to RO from OH + aromatics reactions,
e.g., the products of the bicyclic species H in Figure 1. The
potential for an additional LIF signal caused by conversion of
RO2 to HO2 decreases with decreasing NO concentration in the
LIF detection cell. Therefore, the NO concentration in the
detection cell was varied over a wide range to quantify and finally
widely avoid any RO2 cross-sensitivity.
In humidified synthetic air with traces of O3, a background
decay rate constant for OH of k0OH = 1.5 ( 0.3 sꢀ1 was observed.
’ MATERIALS
The background decay rate constant for HO2 of k0HO = 1.5 (
Synthetic air was made from highly purified (99.9999%) liquid
samples of N2 and O2. To premix water vapor, the gas flow passed
2
0.5 sꢀ1 was measured upon addition of CO. Both, kO0 H and
6017
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
a saturator filled with pure water (Milli-Q). The aromatic
compounds were used as purchased and had stated purities as
follows: Toluene (Merck, 99.9%), ethylbenzene (Fluka, 99.0%),
o-xylene (Fluka, 99.5%), m-xylene (Fluka, 99.5%), p-xylene
(BDH Prolabo, 99.8%), 1,2,3-trimethylbenzene (1,2,3-TMB,
LGC, 91.7%), 1,2,4-TMB (LGC, 99.7%), 1,3,5-TMB (Sigma
Aldrich, 99.0%), hexamethylbenzene (HMB, Alfa Aesar, 99.0%),
phenol (Merck, 99.0%), o-cresol (Sigma Aldrich, 99.0%), 2,5-
dimethylphenol (Merck, 98.0%), and 2,4,6-trimethylphenol
(Acros Organics, 99.0%). Microliter amounts of toluene, ethyl-
benzene, the xylenes, the trimethylbenzenes, and o-cresol were
injected into silcosteel containers and pressurized to 330 kPa
with nitrogen. The gas mixture from the silcosteel container was
then introduced with a mass flow controller to the main gas flow.
Milligram amounts of HMB, phenol, 2,5-dimethylphenol, and
2,4,6-trimethylphenol were stored in temperature-controlled
flasks that were continuously flushed by a small flow of pure
nitrogen. The nitrogen flux was regulated by a mass flow
controller and finally introduced to the main gas flow. The con-
centration of the respective aromatic was estimated from the
measured OH reactivity. A 1% mixture of CO in nitrogen was
used for experiments, when CO was added (Messer Griesheim,
99.997%). A 10% mixture of NO (Linde, 99.5%) in nitrogen,
used for the conversion of HO2 to OH in the detection cell,
passed a cartridge filled with sodium hydroxide coated silicate
(Ascerite, Sigma-Aldrich) to remove impurities.
The OH + aromatic hydrocarbon (AH) reactions are treated
accordingly.
OH þ AH þ O2 f HO2 þ products
ðR2Þ
This approach is justified because also the intermediates formed
in the OH + AH reactions react rapidly with O2 compared to the
time scale of the experiments, i.e., within about 10ꢀ3 s in the case
of toluene and even faster for the other compounds.23 Conse-
quently, we obtain similar expressions for the time dependence of
OH and HO2 for the aromatic hydrocarbon experiments, except
AH
HO2
for a factor ϕ
that denotes the unknown yield of prompt HO2
and a second term that accounts for a potential photolytical
formation of HO2 (see Experimental Methods).
AH
½OHꢂ ¼ ½OHꢂ0 ꢁ expðꢀk tÞ
ð4Þ
OH
AH
AH
½OHꢂ0ðk ꢀ k0OHÞϕ
OH
HO2
½HO2ꢂ ¼
ꢁ fexpðꢀk0HO
Þ
AH
0
2
k
ꢀ k
OH
HO2
AH
ꢀ expðꢀk tÞg þ ½HO2ꢂ0 ꢁ expðꢀk0 tÞ ð5Þ
OH
HO2
AH
OH
Again k
is the total OH reactivity in the presence of the
aromatic hydrocarbon:
AH
OH
k
¼ kOHþAH½AHꢂ þ k0OH
ð6Þ
In the OH mode of the instrument, the obtained LIF signal is
proportional to the OH concentration.
’ RESULTS
SOH µ ½OHꢂ
ð7Þ
Data Evaluation. We alternately measured OH and HOx
decay curves in the presence of aromatic hydrocarbons and
extracted prompt HO2 yields by comparison to CO reference
experiments. The determination of HO2 yields was done by
applying analytical solutions and curve fitting procedures. This
approach was recently developed in our study on the OH +
benzene reaction40 and will be reproduced here briefly and
extended to account for additional photolytic HO2 formation.
The OH + CO reaction gives CO2 and H-atoms that are
quantitatively converted to HO2 within less than 10ꢀ6 s under
the experimental conditions. Therefore, the following overall
reaction applies:
In the HOx mode of the instrument the signal is given by the sum
of the OH signal, the signal from HO2 to OH conversion, and
possibly a signal from RO2 to OH conversion.
SHO µ fOHð½OHꢂ þ fHO ð½HO2ꢂ þ αRO ½RO2ꢂÞÞ
ð8Þ
x
2
2
The lower detection sensitivity of OH in the HOx mode of the
instrument was considered by a factor fOH probably caused by a
loss of OH through reaction with NO. Furthermore, the detec-
tion sensitivity toward HO2 was typically lower by a factor fHO
2
compared to that for OH owing to incomplete HO2 conversion
because of the limited reaction time. The last term in eq 8
considers any contribution of interfering RO2 radicals. αRO is
2
the ratio of the detection sensitivities of RO2 to HO2 that
depends on the NO concentration in the LIF detection cell.
The time dependencies for HO2 and RO2 are assumed to be
similar because radicalꢀradical reactions are negligible under the
employed experimental conditions and background losses are
also considered similar. Thus, the RO2 concentration can be
replaced by the ratio of RO2 and HO2 yields; i.e., the interference
OH þ CO þ O2 f HO2 þ CO2
ðR1Þ
All reactants were used in excess over OH, so that pseudo-first-
order conditions hold. Thus, OH is expected to decay exponen-
tially whereas HO2 should exhibit a rise-and-fall type biexponen-
tial behavior:
CO
½OHꢂ ¼ ½OHꢂ0 ꢁ expðꢀk tÞ
ð1Þ
can be accounted for by a simple factor FRO g 1:40
OH
2
AH
SHO µ fOHð½OHꢂ þ fHO ½HO2ꢂð1 þ αRO ϕAH =ϕ ÞÞ
CO
½OHꢂ0ðk ꢀ k0OH
Þ
x
2
2
RO2
HO2
ꢁ fexpðꢀk0HO tÞ
OH
½HO2ꢂ ¼
CO
0
2
k
ꢀ k
¼ fOHð½OHꢂ þ fHO ½HO2ꢂFRO
Þ
ð9Þ
OH
HO2
2
2
ꢀ expðꢀkCOOHtÞg
ð2Þ
ð3Þ
When the NO concentration was decreased in the LIF detection
cell, the RO2 to OH conversion was effectively suppressed and
αRO ideally approached zero whereas FRO approached unity.
kCOOH denotes the total OH reactivity in the presence of CO:
2
2
= ΦAH was the experimental
AH
HO2
CO
The product FRO ꢁ ϕ
k
¼ kOHþCO½COꢂ þ k0OH
observable that was 2finally determined by fitting eqs 1, 2, 4,
and 5 simultaneously to the respective SOH and SHO decay
curves obtained in the presence of CO and the aromaticx hydro-
carbon. The fits were performed using a LevenbergꢀMarquardt
least-squares fitting procedure46 where only kO0 H = 1.5 sꢀ1
OH
k0OH and k0HO are the background loss decay rate constants of
OH and HO22, respectively. Note that an HO2 yield of unity for
the OH + CO reference reaction is presumed in the time
dependence of HO2.
6018
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Table 1. Fit Results of fOH, fHO , kAOHH, and ΦAH from Combined CO/Alkylbenzene Experiments in Synthetic Air at Different NO
2
Concentrations [NO]D in the LIF Detection Cella
AH
OH
reactant
toluene
[NO]D/1014 cmꢀ3
fOH
fHO
k
/sꢀ1
Φ
AH = ϕ FRO
AH
χ2/DOF
HO2
2
2
0.12
0.12
1.2
0.84
0.89
0.85
0.22
0.20
0.63
22.3
0.42 ( 0.07
0.40 ( 0.13
0.45 ( 0.14
0.42 ( 0.11b
0.56 ( 0.11
0.62 ( 0.10
0.77 ( 0.13
0.53 ( 0.10
0.41 ( 0.08
0.27 ( 0.06
0.43 ( 0.06
0.42 ( 0.15
0.36 ( 0.06
0.40 ( 0.09b
0.44 ( 0.09
0.58 ( 0.11
0.67 ( 0.16
0.31 ( 0.06
0.37 ( 0.09
0.34 ( 0.10
0.37 ( 0.06
0.25 ( 0.06
0.21 ( 0.08
0.29 ( 0.08b
0.31 ( 0.07
0.34 ( 0.12
0.52 ( 0.19
0.67 ( 0.15
0.32 ( 0.08
0.32 ( 0.09
0.33 ( 0.06
0.32 ( 0.08b
1.20
1.23
1.16
72.2
43.8
3.9
9.7
0.87
0.89
0.81
0.86
0.97
0.95
0.87
0.85
1.00
0.94
1.30
1.46
0.14
0.15
0.18
0.22
0.19
0.67
20.1
17.0
17.9
17.9
56.4
74.5
25.9
74.1
25.8
1.17
1.21
1.27
1.17
1.28
1.23
1.20
1.28
1.18
15.0
0.12
0.12
0.12
0.12
0.12
1.2
ethylbenzene
o-xylene
m-xylene
p-xylene
3.9
9.7
0.94
0.91
0.83
0.97
0.95
0.83
0.93
0.89
0.82
1.01
1.34
1.43
0.18
0.17
0.19
0.21
0.27
0.69
25.9
25.0
25.7
38.7
53.3
64.3
18.2
50.4
65.2
1.15
1.28
1.31
1.28
1.33
1.17
1.14
1.31
1.19
15.
1,2,3-TMB
1,2,4-TMB
1,3,5-TMB
0.12
0.12
0.12
0.12
0.24
1.2
2.7
3.9
0.85
0.81
0.90
0.85
0.88
0.94
0.99
1.19
1.06
1.31
1.46
0.14
0.15
0.21
42.7
50.8
48.4
28.5
30.0
17.2
18.7
2.07
1.19
1.38
1.28
1.35
1.24
1.25
9.7
15.0
0.12
0.12
0.39
HMB
a Results were obtained by fitting eq 1, 2, 4, and 5 to the SOH and SHO decay curves. Numbers in bold indicate the prompt HO2 yields at FRO ≈ 1.
x 14
2
b Mean values and mean errors of measurements at [NO]D e 1.2 ꢁ 10 cmꢀ3
.
(measured separately) was held fixed as a constraint. The idea
behind this procedure can be rationalized as follows: By switch-
ing quickly from OH to HOx measurement modes and from CO
to AH reactant, we presumed experimental conditions to be
constant; i.e., the proportionality factors in eqs 7 and 8 are the
same and thus treated as a single fit parameter for each pair of OH
Results) are reasonably close to unity and indicate that the errors
of the data points were slightly underestimated.
Error estimates for the fitted ΦAH were also determined by
taking into account the mutual dependencies of the fit param-
eters. Starting with the fitted values, ΦAH was gradually increased
or decreased and held fixed during the fits until the ratio χ2/DOF
increased by a factor (≈ 1.03) taken from a parametrization of
values for the χ2-distribution for the given DOF and a probability
of 0.68 (≈1σ-errors). Mean errors were then calculated from
lower and upper limits.
and HOx decay curves. The SOH and SHO obtained in the
x
presence of CO then determine the factors fOH and fHO whereas
CO
OH
2 AH
OH
the SOH mainly determine the rate constants k and k . The
SHO in the presence of AH finally defines ΦAH, i.e., the yield of
x
prompt HO2. However, because all the fit parameters depend on
each other more or less strongly, fitting all curves simultaneously
ensures that all available experimental information is considered
adequately and consistently by weighting with experimental
errors according to Poisson statistics (photon counting). The
fit quality was assessed on the basis of the weighted sum of
squared residuals χ2 divided by the degrees of freedom (DOF).
DOF corresponds to the number of data points minus the
number of fitted parameters and χ2/DOF should ideally range
around unity. The typical values of 1.3 found in this study (see
OH + Alkylbenzenes. Table 1 shows the fit results of
combined CO/alkylbenzene experiments. Typical decay curves
recorded in the presence of CO and toluene are shown in
Figure 3, panels (a) and (b). As outlined above, no HO2
formation from photolysis of alkylbenzenes at 266 nm was
observed in experiments without OH. Consequently, [HO2]0
in eq 5 was set to zero.
To check the influence of interfering RO2 radicals, the NO
concentration in the LIF detection cell, [NO]D, was varied over a
wide range in experiments with toluene, p-xylene, and 1,3,5-TMB.
6019
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Figure 4. Dependence of fitted ΦAH on [NO]D, the NO concentration
in the LIF detection cell. Symbols show results of combined CO/
alkylbenzene experiments. The solid lines show the simulated [NO]D
dependence of ΦAH based on the reactions in Table 2. The dashed lines
AH
HO2
indicate the presumed contribution of ϕ
to ΦAH following the OH +
alkylbenzene reaction based on MCM47,48 recommendations.
Table 2. Parameters and Rate Constants Used in the
Numerical Model to Simulate OH Formation from HO2 and
RO2 Radicals in the LIF Detection Cell
Figure 3. Normalized SOH (blue points) and SHO (black points) obtained
x
in the presence of CO, toluene, and 2,5-dimethylphenol in synthetic air in the
absence of NO in the reaction volume. The NO concentration in the LIF
detection cell was 0.12 ꢁ 1014 cmꢀ3 in each experiment (see text). The green
points in (c) represent SHO obtained in the absence of the OH-precursor
ozone. Full lines correspondxto fitted decays according to equations eqs 1, 2,
reaction time
total pressure
temperature
250 μsa
350 Pa
4, and 5. The red lines show the fitted contributions of HO2 to SHO . The CO
298 K
x
8.1 ꢁ 10ꢀ12 cm3 sꢀ1 b
experiment in (a) is the corresponding reference experiment for (b). For (c)
a similar reference experiment exists but is not shown.
kHO +NOfOH+NO
2
2
kOH+NOfHNO
5.7 ꢁ 10ꢀ14 cm3 sꢀ1 b
1.4 ꢁ 10ꢀ13 cm3 sꢀ1 b
7.7 ꢁ 10ꢀ12 cm3 sꢀ1 c
1.3 ꢁ 10ꢀ12 cm3 sꢀ1 c
1 ꢁ 106 sꢀ1 c
2
kOH+NO fHNO
2
3
As shown in Figure 4, ΦAH was found to increase with increasing
[NO]D, indicating that RO2 radicals contribute significantly
to the LIF signal in the HOx mode at increased [NO]D.
Characterization experiments have already been performed for
the instrument used in this study40 and it has been shown that
RO2 interferences are sufficiently suppressed using low [NO]D.
We performed numerical simulations by taking into account
the reactions in the LIF detection cell to reproduce the NO
dependencies of the different ΦAH. The full lines in Figure 4
show calculated OH concentrations normalized to a reference
case where only HO2 radicals are entering the LIF detection cell.
Rate constants and experimental conditions used for the numer-
ical simulations are listed in Table 2. The model was initialized
kRO +NOfRO+NO
2
2
kRO +NOfRNO
2
3
kROffragments
kfragments+O fHO
9.1 ꢁ 10ꢀ12 cm3 sꢀ1 b,d
2
2
a Calculated by fitting the increase of fHO with [NO]D.40 b NASA
recommendation.74 c MCM recommendatio2n.47,48 d Rate constant as-
sumed similar to CH2OH + O2 f HCHO + HO2.74
and 1,3,5-TMB, the final prompt HO2 yields were therefore
determined by averaging the results obtained at [NO]D e 1.2 ꢁ
1014 cmꢀ3 (boldface in Table 1). For the other alkylbenzenes, the
full dependence of ΦAH on [NO]D was not investigated and most
experiments were performed at [NO]D = 0.12 ꢁ 1014 cmꢀ3
.
with current recommendations from literature regarding the yields
AH
of prompt HO2 and RO2: ϕAROH /ϕ
HO2
= 0.72/0.28 for toluene and
However, it should benoted thata small, residual RO2 interference
HO2
2
p-xylene and ϕARHO /ϕ = 0.82/0.18 for 1,3,5-TMB.47,48 The dotted
AH
(αRO e 0.1) cannot be excluded even at the lowest possible
2
2
[NO]D concentrations because of an incomplete understanding of
transport processes within the LIF detection cell (e.g., turbulence
induced by the gas-expansion and NO mixing effects).40,45 The
prompt HO2 yields determined in this work should therefore be
considered upper rather than lower limits.
OH + Hydroxybenzenes. The fit results of combined CO/
hydroxybenzene experiments are given in Table 3. The experiments
lines in Figure 4 indicate the recommended prompt HO2 yields and
the calculated ΦAH expectedly approach these limits at decreasing
[NO]D, confirming the vanishing influence from peroxy radicals,
AH
i.e., ΦAH ≈ ϕ
at low [NO]D.
HO2
The model calculations are in good agreement with the
experimental ΦAH that within experimental uncertainties already
leveled out at [NO]D ≈ 1ꢀ2 ꢁ 1014 cmꢀ3. For toluene, p-xylene,
6020
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Table 3. Fit Results of fOH, fHO , kAOHH, and ΦAH from Combined CO/Hydroxybenzene Experiments in Synthetic Air at [NO]D =
2
0.12 ꢁ 1014 cmꢀ3 a
AH
OH
AH
ΦAH ≈ ϕ
HO2
reactant
[HO2]0/[OH]0
fOH
fHO
k
/sꢀ1
χ2/DOF
2
phenol
0.33
0.83
0.37
0.81
0.20
0.55
0.01
0.03
0.88
0.81
0.80
0.98
0.90
1.02
0.93
0.93
0.33
0.32
0.20
0.23
0.20
0.20
0.19
0.23
7.9
18.4
9.2
0.89 ( 0.29
0.87 ( 0.29
0.72 ( 0.12
0.45 ( 0.13
1.36
o-cresol
1.48
1.23
1.28
17.4
15.8
38.6
15.8
81.6
2,5-dimethylphenol
2,4,6-trimethylphenol
a Results were obtained by fitting sets of eqs 1, 2, 4, and 5 to SOH and SHO decay curves in the presence of CO and AH. The simultaneous fits cover
x
experiments with two AH concentrations and in the absence of the OH precursor O3 to eliminate the effect of photolytical HO2 formation. Prompt HO2
yields correspond to the measured ΦAH
.
revealed that, except for 2,4,6-trimethylphenol, the yields of
promptly formed HO2 aremuch greater than forthealkylbenzenes.
In experiments without the OH-precursor ozone, an instanta-
neous photolytical HO2 formation was observed as shown in
Figure 3c for the example 2,5-dimethylphenol. The same figure
also shows SOH and SHO curves recorded in the presence of
On the other hand, the remainder (1 ꢀ ϕHO ) should match the
2
combined yields of reaction channels not associated with prompt
HO2 formation. From OH + alkylbenzenes, these are the bicyclic
peroxy radical channel leading to α-dicarbonyls, the H-atom
abstraction finally forming benzaldehydes, and the dealkylation
(Figure 1):
ozone where SHO is expxected to contain an underlying con-
x
alkylbenzene
HO2
1 ꢀ ϕ
¼ ϕdicarbonyl þ ϕbenzaldehyde þ ϕdealkylation ð12Þ
tribution from photolytically produced HO2. The latter, [HO2]0
in eq 5, is assumed to depend linearly on the concentration of the
aromatic hydrocarbon. In a first approach, measurements were
therefore performed at two different AH concentrations and two
For OH + hydroxybenzenes (1 ꢀ ϕHO ) should match the yield
2
of nitrophenols from the H-atom abstraction channel plus the
yield of 1,4-benzoquinones formed following the proposed ipso-
OH-addition (Figure 2):
SOH and SHO decay curves each were recorded. Two sets of
x
eqs 1, 2, 4, and 5 were then fitted simultaneously by assuming
[HO2]0 µ [AH] treating the proportionality factor as an addi-
tional fit parameter. Although this fitting strategy worked tech-
nically, the obtained error limits were significantly greater than in
the case of the alkylbenzenes and approached 100%. Obviously,
the distinction of photolytically and secondarily formed HO2 is
difficult, which results in a large mutual dependence of the
respective contributions and increased error limits. Therefore, an
extended evaluation procedure was adopted where also the decay
hydroxybenzene
HO2
1 ꢀ ϕ
¼ ϕnitrophenol þ ϕquinone
ð13Þ
OH + Toluene and Ethylbenzene. For the OH + toluene
toluene
HO2
reaction we determined ϕ
= 0.42 ( 0.11. This yield is
significantly greater than the ϕphenol (i.e., cresol) yield of
0.18ꢀ0.28 reported in most product studies14,25,27,31,49,50
(Table 4). Exceptions are considerably greater and smaller values
from two studies51,52 that are supposed not to be representative
for atmospheric conditions because of potential influence of
heterogeneous reactions51 or due to losses of primary oxidation
products upon reaction with OH.52 The result of our study is
consistent with the currently proposed toluene degradation
mechanism where the coproducts of prompt HO2 are cresols
curves SHO obtained in the absence of the OH-precursor ozone
x
were implemented in the fits by setting [OH]0 = 0 in eq 5. This led
to similar results with improved error limits as listed in Table 3.
[NO]D concentrations were always kept low during the
experiments with hydroxybenzenes. We therefore again assume
ΦAH ≈ ϕ . Because prompt HO2 yields were greater com-
AH
HO2
pared to those of the alkylbenzenes, any residual contribution of
RO2 interferences is expected to be even smaller and negligible.
and an epoxide. By comparison of ϕtHoOluene to the reported cresol
2
yields, formation of 0.14ꢀ0.24 of other HO2 coproducts is
possible. Flowtube studies by Baltaretu et al.27 and Birdsall
et al.31 indeed reported experimental evidence for the epoxide
pathway. The combined yield of cresol and epoxide of 0.35 (
’ DISCUSSION
In the experiments outlined above, we determined HO2 yields
upon the OH + aromatic hydrocarbon reaction in synthetic air in
the absence of NO. These yields are helpful to reduce budget
uncertainties regarding the primary oxidation steps because the
extracted ϕHO should match the combined yields of currently
proposed HO22 coproducts. From OH + alkylbenzenes these
coproducts are phenols, epoxides, and oxepins (Figure 1):
0.0727 is similar to ϕtHoOluene determined in this work. Bloss et al.48
2
estimated a combined yield of 0.28 for cresol and epoxide based
on the data by Volkamer et al.33 to close the budget for the OH-
initiated degradation of toluene. This value is somewhat lower
than the result of our study. Owing to the uncertainties of the
primary product yields, it cannot be ruled out that there are
further reaction channels yielding prompt HO2 like the oxepin
pathway proposed by Klotz et al.53ꢀ55 This reaction channel has
so far only been excluded for OH + benzene.35
alkylbenzene
HO2
ϕ
¼ ϕphenol þ ϕepoxide þ ϕoxepin
ð10Þ
The remainder (1 ꢀ ϕtHoOluene) = 0.58 ( 0.11 is also somewhat
From OH + hydroxybenzenes the coproducts are dihydoxyben-
zenes (Figure 2):
2
greater than the combined yields ϕdicarbonyl + ϕbenzaldehyde deter-
mined in product studies: 0.47 ( 0.03,25 0.39 ( 0.10,33 and
0.35 ( 0.10.27 This discrepancy can be attributed to the great
hydroxybenzene
HO2
ϕ
¼ ϕdihydoxybenzene
ð11Þ
6021
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Table 4. Product Yields of the OH-Initiated Oxidation of Toluene in the Presence of O2 from Literature
prompt HO2 reaction channels
reaction channels not associated with prompt HO2
a
b
c
reference
ϕphenol
ϕepoxide
ϕdicarbonyl
ϕbenzaldehyde
ϕdealkylation
experimental conditions
OH + Toluene
Atkinson et al.49
Seuwen et al.51
Smith et al.25
0.25 ( 0.03
0.53 ( 0.08d
0.18 ( 0.01
0.18 ( 0.03
0.09 ( 0.03
0.07 ( 0.01
0.05 ( 0.01
0.06 ( 0.01
0.06 ( 0.01
0.08 ( 0.01
≈15 ppm NOx
NOx-free
0.10 ( 0.02e
0.41 ( 0.03e
<1 ppm NOx
3ꢀ300 ppb NOx
NOx-free
Klotz et al.50
Moschonas et al.52
Volkamer et al.33
Noda et al.14
Baltaretu et al.27
Birdsall et al.31
Nishino et al.67
this study
0.39 ( 0.10 f
0.30 ( 0.10e,g
0.48 ( 0.03e
<1 ppm NOx
0.01ꢀ0.1 ppm NOx
NOx-free
0.18 ( 0.02
0.28 ( 0.06
≈0.22h
0.05 ( 0.01
0.07 ( 0.03
≈0.05h
0.05 ( 0.02
≈0.07h
≈0.07h
NOx-free
0.08ꢀ4 ppm NOx
NOx-free
ϕHO = 0.42 ( 0.11
(1 ꢀ ϕHO ) = 0.58 ( 0.11
2
2
a Sum of o-, m-, and p-cresol. b 2-Methyl-2,3-epoxy-6-oxo-4-hexenal. c Phenol. d Sum of o- and p-cresol. e Sum of glyoxal and methylglyoxal. f Glyoxal.
g Prediction for α-dicarbonyl formation upon a second OH attack. h Relative product yield.
uncertainty of ϕdicarbonyl and to missing minor (<0.10)14,31
reaction channels like the dealkylation pathway that was experi-
mentally confirmed in flowtube studies by Noda et al.14 and
Birdsall et al.31
formation of epoxides from m-xylene with a yield of ≈0.02 was
reported for the first time. The combined yield of (ϕphenol
+
ϕepoxide) = 0.19 ( 0.0330 corresponds quite well to the prompt
HO2 yield determined in this work. Accordingly, HO2 formation
via the epoxide reaction channel is considered to be of minor
importance for m-xylene. Most of the previous product studies
For the OH + ethylbenzene reaction we determined
ethylbenzene
ϕ
= 0.53 ( 0.10. Only a few experimental studies on
HO2
the photo-oxidation of ethylbenzene are available in literature.56ꢀ59
These studies focused on the formation of secondary organic
aerosol and no quantitative information about gaseous oxidation
products was reported. However, the HO2 coproduct ethylphenol
was found to be a major oxidation product.56,59 Owing to the lack of
absolute product yields, we are not able to draw further conclusions
concerning other reaction channels yielding prompt HO2.
on m-xylene reported combined formation yields of ϕdicarbonyl +
ϕbenzaldehyde ranging between 0.40 and 0.60.26,65ꢀ67 Zhao et al.
reported a considerably lower value of 0.21.30 The authors
attributed this discrepancy mainly to their very low yield of
methylglyoxal (0.15) obtained in a fast turbulent flow reactor.
When the remainder of (1 ꢀ ϕmH‑Oxylene) = 0.73 ( 0.06 is compared
2
to these literature values of ϕdicarbonyl + ϕbenzaldehyde, it becomes
obvious that the carbon balance is not closed. Taking into
account the data by Bandow et al.,65 Smith et al.,26 Arey et al.,66
and Nishino et al.,67 0.10ꢀ0.30 of the primary oxidation products
not associated with prompt HO2 are not identified so far. This
gap can at least partly be closed by the dealkylation pathway
yielding cresol reported by Noda et al.:14 ϕdealkylation = 0.11 (
0.04. Contradictory results were reported by Aschmann et al.15
who determined a cresol yield of <0.02 for m-xylene and
suggested that the ion peaks observed by Noda et al.14 could
correspond to the sum of cresol and methyloxepin (both
resulting from dealkylation and having the same mass-to-
charge-ratio). A corresponding dealkylation mechanism yield-
ing methyloxepin following the OH + m-xylene reaction was
postulated.14,15
OH + Isomeric Xylenes. In the case of o-xylene and p-xylene
we obtained prompt HO2 yields of 0.41 ( 0.08 and 0.40 ( 0.09,
respectively. The formation yields of the corresponding di-
methylphenols were reported to be 0.10ꢀ0.1614,60,61 and
0.12ꢀ0.19,14,26,61ꢀ63 respectively (Table 5). Again, our results
suggest that non-phenol reaction pathways, e.g., epoxide forma-
tion, contribute significantly (<0.30) to the prompt HO2 forma-
tion, which is consistent with the current understanding of the
xylene degradation mechanism. For OH + o-xylene our result is in
line with the estimation by Bloss et al.48 for (ϕphenol + ϕepoxide) =
0.40. For OH + p-xylene the estimation is somewhat lower:
(ϕphenol + ϕepoxide) = 0.28.48 To date, there is no quantitative
information on the formation of epoxide compounds from o- and
p-xylene but species with corresponding molecular weights have
been observed.28,29,64
OH + Isomeric Trimethylbenzenes. The HO2 yields of the
OH + TMB reactions were determined to be 0.31 ( 0.06 (1,2,3-
TMB), 0.37 ( 0.09 (1,2,4-TMB), and 0.29 ( 0.08 (1,3,5-TMB).
Only a few product studies26,63,68,69 reported the formation of
phenols from TMB photo-oxidation. Quantitative information
was only given by Smith et al.26 and Volkamer63 (phenol yields
e0.07, see Table 6). Formation of epoxide compounds has not
yet been observed. Nevertheless, Bloss et al.48 assigned epoxide
yields of 0.21 (1,2,3-TMB), 0.30 (1,2,4-TMB), and 0.14 (1,3,5-
TMB) to close the carbon balance. Our result support the
recommendations given by Bloss et al.48 However, we merely
conclude that non-phenol reaction channels contribute signifi-
cantly to prompt HO2 formation (<0.30).
The remainders of (1 ꢀ ϕoH‑Oxylene) = 0.59 ( 0.08 and (1 ꢀ
2
ϕpH‑Oxylene) = 0.60 ( 0.09 determined in this work are in reasonable
2
agreement with ϕdicarbonyl + ϕbenzaldehyde reported in several
studies: 0.40ꢀ0.6065ꢀ67 for o-xylene and 0.40ꢀ0.7026,62,63,65ꢀ67
for p-xylene (Table 5). The uncertainties of the reported product
yields leave some scope for additional minor reaction pathways like
the dealkylation postulated by Noda et al.: ϕdealkylation
=
0.04ꢀ0.05.14
For m-xylene the situation is different because the gap between
m‑xylene
ϕ
= 0.27 ( 0.06 and ϕphenol (i.e., dimethylphenol) from
HO2
product studies of 0.11ꢀ0.2114,26,30,60,61 is smaller than for o- and
p-xylene. Moreover, in a recent study by Zhao et al.30 the
6022
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
Table 5. Product Yields of the OH-Initiated Oxidation of the Isomeric Xylenes in the Presence of O2 from Literature
prompt HO2 reaction channels
reaction channels not associated with prompt HO2
a
b
c
d
reference
ϕphenol
ϕepoxide
ϕdicarbonyl
ϕbenzaldehyde
ϕdealkylation
experimental conditions
OH + o-Xylene
0.41 ( 0.04e
Bandow et al.65
Gery et al.60
Atkinson et al.61
Arey et al.66
0.05 ( 0.01
0.17 ( 0.07
0.05 ( 0.01
2 ppm NOx
0.10 ( 0.04
0.16 ( 0.02
≈8 ppm NOx
1ꢀ13 ppm NO2
<1 ppm NOx
0.61e
Noda et al.14
Nishino et al.67
this study
0.11 ( 0.05
0.05 ( 0.03
0.01ꢀ0.1 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
0.46 ( 0.06f
ϕHO = 0.41 ( 0.08
(1 ꢀ ϕHO ) = 0.59 ( 0.08
2
2
OH + m-Xylene
Bandow et al.65
Gery et al.60
Atkinson et al.61
Smith et al.26
Zhao et al.30
0.55 ( 0.07f
0.04 ( 0.01
0.13 ( 0.06
0.03 ( 0.01
0.05 ( 0.01
0.06 ( 0.01
2 ppm NOx
0.18 ( 0.07
≈8 ppm NOx
0ꢀ10 ppm NO2
<1 ppm NOx
≈0.3 ppm NOx
<1 ppm NOx
0.01ꢀ0.1 ppm NOx
<20 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
0.21 ( 0.03j
0.11 ( 0.01
0.17 ( 0.03j
0.48 ( 0.02 f
0.15 ( 0.04g
0.46f
0.02 ( 0.01
Arey et al.66
Noda et al.14
Aschmann et al.15
Nishino et al.67
this study
0.14 ( 0.03
0.11 ( 0.04
<0.02
0.63 ( 0.09 f
ϕHO = 0.27 ( 0.06
(1 ꢀ ϕHO ) = 0.73 ( 0.06
2
2
OH + p-Xylene
0.36 ( 0.03f
Bandow et al.65
Atkinson et al.61
Smith et al.26
Bethel et al.62
Volkamer et al.33
Volkamer et al.63
Arey et al.66
0.08 ( 0.01
0.07 ( 0.01
0.10 ( 0.02
0.07 ( 0.01
2 ppm NOx
0.19 ( 0.04
0.13 ( 0.02
0.14 ( 0.02
1ꢀ10 ppm NO2
<1 ppm NOx
0.61 ( 0.11f
(0.32i)
0.40 ( 0.11h
0.8ꢀ3.3 ppm NOx
<1 ppm NOx
0.12 ( 0.03
0.13 ( 0.03
0.08 ( 0.02
<1 ppm NOx
0.49f
<1 ppm NOx
Noda et al.14
Nishino et al.67
0.04 ( 0.03
0.01ꢀ0.1 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
0.58 ( 0.05 f
this study
ϕHO = 0.40 ( 0.09
(1 ꢀ ϕHO ) = 0.60 ( 0.09
2
2
a Primary phenol products: from o-xylene, (2,3 + 3,4)-dimethylphenol; from m-xylene, (2,4 + 2,6 + 3,5)-dimethylphenol; from p-xylene, 2,5-
dimethylphenol. b Sum of 2,4-dimethyl-2,3-epoxy-6-oxo-4-hexenal, 2,6-dimethyl-2,3-epoxy-6-oxo-4-hexenal, and 3,5-dimethyl-2-hydroxyl-3,4-epoxy-5-
c
hexenal. Primary substituted benzaldehyde products: from o-xylene, 2-methylbenzaldehyde; from m-xylene, 3-methylbenzaldehyde; from p-xylene,
4-methylbenzaldehyde. d Cresol. e Sum of glyoxal, methylglyoxal, and dimethylglyoxal. f Sum of glyoxal and methylglyoxal. g Methylglyoxal. h Glyoxal.
i 3-Hexene-2,5-dione; extrapolated to NOx-free conditions. j Sum of 2,4- and 2,6-dimethylphenol.
For the reaction channels not associated with prompt HO2 we
derived (1 ꢀ ϕHO ) = 0.69 ( 0.06 (1,2,3-TMB), 0.63 ( 0.09
(1,2,4-TMB), and 20.71 ( 0.08 (1,3,5-TMB). These results are
similar to (ϕdicarbonyl + ϕbenzaldehyde) = 0.40ꢀ0.70 determined in
previous studies.26,62,66,67,70 Lower values for ϕdicarbonyl of 0.20 (
0.03 (1,2,3-TMB) and 0.36 ( 0.08 (1,2,4-TMB) were deter-
mined by Nishino et al.67 because dimethylglyoxal was not
measured. A single exception is the work by Smith et al.26 who
formation of hexamethyl-2,4-cyclohexadienone following the
OH + HMB reaction in the presence of NO2. Assuming a similar
HMB oxidation mechanism in the presence of O2, hexamethyl-
2,4-cyclohexadienone could be the coproduct of prompt HO2.
However, no experimental evidence for this reaction is available
so far.
OH + Hydroxybenzenes. The prompt HO2 yields extracted
from the OH + hydroxybenzene experiments, ϕpHhOenol = 0.89 (
2
obtained (ϕdicarbonyl + ϕbenzaldehyde) = 0.93 ( 0.25 for the OH +
0.29, ϕHo‑cOresol = 0.87 ( 0.29, and ϕ2H,5O‑dimethylphenol = 0.72 ( 0.12,
2
2
1,3,5‑TMB
1,3,5-TMB reaction. This is somewhat greater than 1 ꢀ ϕ
are considerably greater compared to those from alkylbenzenes
HO2
2,4,6‑trimethylphenol
determined in our study but still in agreement within the combined
errors. The dealkylation pathway may be operative for the OH +
TMB reaction but is probably of minor (<0.10) importance.
OH + Hexamethylbenzene. The OH + HMB reaction gave
ϕHHMOB = 0.32 ( 0.08. No product studies on the photo-oxidation
of H2MB are available. Only a single flowtube study by Berndt and
B€oge71 performed at 295 K and 25 mbar in He reported the
with the exception of ϕ
= 0.45 ( 0.13. Our
HO2
results are consistent with previous product studies reporting
high dihydroxybenzene yields of 0.7ꢀ0.836,72,73 from OH +
phenol and OH + o-cresol (Table 7). This confirms that HO2
is formed as coproduct of dihydoxybenzenes.
The contributions of reaction channels not associated with
prompt HO2 were determined to be (1 ꢀ ϕpHhOenol) = 0.11 ( 0.29
2
6023
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
Table 6. Product Yields of the OH-Initiated Oxidation of the Isomeric Trimethylbenzenes in the Presence of O2 from Literature
ARTICLE
prompt HO2 reaction channels
reaction channels not associated with prompt HO2
a
b
reference
ϕphenol
ϕdicarbonyl
ϕbenzaldehyde
experimental conditions
OH + 1,2,3-TMB
0.70 ( 0.02c
0.52f
0.59c
0.20 ( 0.03d
Bandow et al.70
Bethel et al.62
Arey et al.66
2 ppm NOx
0.8ꢀ3.3 ppm NOx
<1 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
Nishino et al.67
this study
ϕHO = 0.31 ( 0.06
(1 ꢀ ϕHO ) = 0.69 ( 0.06
2
2
OH + 1,2,4-TMB
0.56 ( 0.02c
0.62 ( 0.07c
0.41g
0.50c
0.36 ( 0.08d
Bandow et al.70
Smith et al.26
Bethel et al.62
Arey et al.66
2 ppm NOx
0.02 ( 0.01
0.04 ( 0.01
<1 ppm NOx
0.8ꢀ3.3 ppm NOx
<1 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
Nishino et al.67
this study
ϕHO = 0.37 ( 0.09
(1 ꢀ ϕHO ) = 0.63 ( 0.09
2
2
OH + 1,3,5-TMB
Bandow et al.70
Smith et al.26
Volkamer et al.63
Arey et al.66
0.64 ( 0.03e
2 ppm NOx
0.04 ( 0.01
0.07 ( 0.01
0.90 ( 0.25e
0.03 ( 0.01
0.03 ( 0.01
<1 ppm NOx
<5 ppm NOx
<1 ppm NOx
0.08ꢀ4 ppm NOx
NOx-free
0.60e
0.58 ( 0.05e
Nishino et al.67
this study
ϕHO = 0.29 ( 0.08
(1 ꢀ ϕHO ) = 0.71 ( 0.08
2
2
a Primary phenol products: from 1,2,4-TMB, (2,3,5 + 2,3,6 + 2,4,5)-trimethylphenol; from 1,3,5-TMB, 2,4,6-trimethylphenol. b Primary substituted
benzaldehyde products: from 1,2,4-TMB, (2,4 + 3,4 + 2,5)-dimethylbenzaldehyde; from 1,3,5-TMB, 3,5-dimethylbenzaldehyd. c Sum of glyoxal,
methylglyoxal, and dimethylglyoxal. d Sum of glyoxal and methylglyoxal. e Methylglyoxal. f Dimethylglyoxal; extrapolated to NOx-free conditions g Sum
of dimethylglyoxal and 3-hexene-2,5-dione; extrapolated to NOx-free conditions.
Table 7. Product Yields of the OH-Initiated Oxidation of Phenol and o-Cresol in the Presence of O2 from Literature
prompt HO2 reaction channel
reaction channels not associated with promt HO2
a
b
c
reference
ϕdihydroxbenzene
ϕnitrophenol
ϕquinone
experimental conditions
OH + Phenol
Olariu et al.36
Berndt et al.72
0.80 ( 0.12
0.73 ( 0.04
0.06 ( 0.01
0.04 ( 0.02
0.04 ( 0.01
0.01 ( 0.01
<2 ppn NO
4ꢀ100 ppm NO
NOx-free
This study
ϕHO = 0.89 ( 0.29
(1 ꢀ ϕHO ) = 0.11 ( 0.29
2
2
OH + o-Cresol
0.07 ( 0.02
0.05 ( 0.01
Olariu et al.36
Coeur-Tourneur et al.73
0.73 ( 0.15
0.07 ( 0.02
0.06 ( 0.01
<2 ppm NO
1.3ꢀ1.5 ppm NO
NOx-free
this study
ϕHO = 0.87 ( 0.29
(1 ꢀ ϕHO ) = 0.13 ( 0.29
2
2
a Primary dihydroxybenzene products: from phenol, 1,2-dihydroxybenzene; from o-cresol, 3-methyl-1,2-dihydoxybenzene. b Primary nitrophenol
products: from phenol, 2-nitrophenol; from o-cresol, 6-methyl-2-nitrophenol. c Primary quinone products: from phenol, 1,4-benzoquinone; from
o-cresol, methyl-1,4-benzoquinone.
and (1 ꢀ ϕHo‑Ocresol) = 0.13 ( 0.29. Although the errors are con-
ϕquinone from phenol (0.10 ( 0.02)36 and o-cresol (0.14 (
0.04),36 respectively. No product studies are available in litera-
ture for 2,5-dimethylphenol and 2,4,6-trimethylphenol.
under atmospheric conditions. This experimental approach is
complementary to previous product studies and can help to
reduce budget uncertainties concerning the initial reaction
steps of the OH-initiated atmospheric photo-oxidation of aro-
matics. Our results suggest that for most of the investigated
alkylbenzenes (with the exception of m-xylene) a significant
fraction of prompt HO2 is formed via pathways not forming
phenols (e.g., formation of epoxides or oxepins). The remainder
2
siderable, these yields correspond well to reported ϕnitrophenol
+
’ CONCLUSIONS
We determined yields of promptly formed HO2 (ϕHO
following the reaction of OH with selected aromatic hydrocarbons
)
(1 ꢀ ϕHO ) revealed that, besides the established H-atom abstrac-
2
2
tion and bicyclic peroxy radical pathways, minor non-HO2 forming
6024
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
reaction channels (e.g., dealkylation) remain possible for all
investigated alkylbenzenes. The investigated hydroxybenzenes
(phenol, o-cresol, 2,5-dimethylphenol, 2,4,6-trimethylphenol)
are also forming prompt HO2 with high yields. In the case of
OH + phenol and OH + o-cresol, the results are consistent with
HO2 being exclusively formed as coproduct of dihydroxybenzenes.
(24) Atkinson, R.; Baulch, D.; Cox, R.; Hampson, R.; Kerr, J.; Troe, J.
J. Phys. Chem. Ref. Data 1989, 18, 881–1097.
(25) Smith, D.; McIver, C.; Kleindienst, T. J. Atmos. Chem. 1998,
30, 209–228.
(26) Smith, D.; Kleindienst, T.; McIver, C. J. Atmos. Chem. 1999,
34, 339–364.
(27) Baltaretu, C. O.; Lichtman, E. I.; Hadler, A. B.; Elrod, M. J.
J. Phys. Chem. A 2009, 113, 221–230.
(28) Yu, J. Z.; Jeffries, H. E.; Sexton, K. G. Atmos. Environ. 1997,
31, 2261–2280.
(29) Yu, J. Z.; Jeffries, H. E. Atmos. Environ. 1997, 31, 2281–2287.
(30) Zhao, J.; Zhang, R.; Misawa, K.; Shibuya, K. J. Photochem.
Photobiol. A-Chem. 2005, 176, 199–207.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: b.bohn@fz-juelich.de.
(31) Birdsall, A. W.; Andreoni, J. F.; Elrod, M. J. J. Phys. Chem. A
2010, 114, 10655–10663.
’ ACKNOWLEDGMENT
(32) Cartas-Rosado, R.; Castro, M. J. Phys. Chem. A 2007,
111, 13088–13098.
We thank M. Bachner, H. Fuchs, A. Hofzumahaus, F. Holland,
and S. Lou for useful discussions and technical support. S.N.
thanks the Deutsche Forschungsgemeinschaft for a Ph.D. stu-
dentship under grant BO 1580/3-1.
(33) Volkamer, R.; Platt, U.; Wirtz, K. J. Phys. Chem. A 2001,
105, 7865–7874.
(34) Elrod, M. J. J. Phys. Chem. A 2011, 115, 8125–8130.
(35) Berndt, T.; B€oge, O.; Herrmann, H. Chem. Phys. Lett. 1999,
314, 435–442.
’ REFERENCES
(36) Olariu, R. I.; Klotz, B.; Barnes, I.; Becker, K.-H.; Mocanu, R.
Atmos. Environ. 2002, 36, 3685–3697.
(1) Fortin, T. J.; Howard, B. J.; Parrish, D. D.; Goldan, P. D.; Kuster,
W. C.; Atlas, E. L.; Harley, R. A. Environ. Sci. Technol. 2005, 39, 1403–1408.
(2) Ilgen, E.; Karfich, N.; Levsen, K.; Angerer, J.; Schneider, P.;
Heinrich, J.; Wichmann, H.; Dunemann, L.; Begerow, J. Atmos. Environ.
2001, 35, 1235–1252.
(3) Ilgen, E.; Levsen, K.; Angerer, J.; Schneider, P.; Heinrich, J.;
Wichmann, H. Atmos. Environ. 2001, 35, 1253–1264.
(4) Ilgen, E.; Levsen, K.; Angerer, J.; Schneider, P.; Heinrich, J.;
Wichmann, H. Atmos. Environ. 2001, 35, 1265–1279.
(5) Johnson, M. M.; Williams, R.; Fan, Z.; Lin, L.; Hudgens, E.;
Gallagher, J.; Vette, A.; Neas, L.; Ozkaynak, H. Atmos. Environ. 2010,
44, 4927–4936.
(37) Grosjean, D. Sci. Total Environ. 1991, 100, 367–414.
(38) Hofzumahaus, A.; et al. Science 2009, 324, 1702–1704.
(39) Lou, S.; et al. Atmos. Chem. Phys. 2010, 10, 11243–11260.
(40) Nehr, S.; Bohn, B.; Fuchs, H.; Hofzumahaus, A.; Wahner, A.
Phys. Chem. Chem. Phys. 2011, 13, 10699–10708.
(41) Tyndall, G.; Cox, R.; Granier, C.; Lesclaux, R.; Moortgat, G.;
Pilling, M.; Ravishankara, A.; Wallington, T. J. Geophys. Res. - Atmos.
2001, 106, 12157–12182.
(42) Aluculesei, A.; Tomas, A.; Schoemaecker, C.; Fittschen, C.
Appl. Phys. B - Lasers Opt. 2008, 92, 379–385.
(43) Kovacs, T.; Blitz, M. A.; Seakins, P. W.; Pilling, M. J. J. Chem.
Phys. 2009, 131, 204304.
(44) Jain, C.; Parker, A. E.; Schoemaecker, C.; Fittschen, C. Chem-
(6) Hawthorne, S.; Miller, D.; Barkley, R.; Krieger, M. Environ. Sci.
Technol. 1988, 22, 1191–1196.
(7) Hawthorne, S.; Miller, D.; Langenfeld, J.; Krieger, M. Environ.
Sci. Technol. 1992, 26, 2251–2262.
PhysChem 2010, 11, 3867–3873.
(45) Fuchs, H.; Bohn, B.; Hofzumahaus, A.; Holland, F.; Lu, K.;
Nehr, S.; Rohrer, F.; Wahner, A. Atmos. Meas. Tech. 2011, 4, 1209–1225.
(46) Craig Markwardt, IDL library, http://cow.physics.wisc.edu/
∼craigm/idl/, 2010.
(8) U.S. Environmental Protection Agency, Our Nation’s Air (EPA-
454/R-09-002), 2010.
(9) Atkinson, R.; Arey, J. Polycyclic Aromatic Compounds 2007,
27, 15–40.
(47) Jenkin, M.; Saunders, S.; Wagner, V.; Pilling, M. Atmos. Chem.
Phys. 2003, 3, 181–193.
(10) Calvert, J. G.; Atkinson, R.; Becker, K. H.; Kamens,
R. M.;Seinfeld, J. H.; Wallington, T. J.; Yarwood, G. Mechanisms of
atmospheric oxidation of aromatic hydrocarbons; Oxford University Press:
Oxford, U.K., 2002.
(11) Atkinson, R.; Arey, J. Chem. Rev. 2003, 103, 4605–4638.
(12) Ravishankara, A.; Wagner, S.; Fischer, S.; Smith, G.; Schiff, R.;
Watson, R.; Tesi, G.; Davis, D. Int. J. Chem. Kinet. 1978, 10, 783–804.
(13) Warneke, C.; et al. J. Geophys. Res.-Atmos. 2004, 109, D10309.
(14) Noda, J.; Volkamer, R.; Molina, M. J. J. Phys. Chem. A 2009,
113, 9658–9666.
(48) Bloss, C.; Wagner, V.; Jenkin, M. E.; Volkamer, R.; Bloss, W. J.;
Lee, J. D.; Heard, D. E.; Wirtz, K.; Martin-Reviejo, M.; Rea, G.; Wenger,
J. C.; Pilling, M. J. Atmos. Chem. Phys. 2005, 5, 641–664.
(49) Atkinson, R.; Aschmann, S.; Arey, J.; Carter, W. Int. J. Chem.
Kinet. 1989, 21, 801–827.
(50) Klotz, B.; Sorensen, S.; Barnes, I.; Becker, K.-H.; Etzkorn, T.;
Volkamer, R.; Platt, U.; Wirtz, K.; Martin-Reviejo, M. J. Phys. Chem. A
1998, 102, 10289–10299.
(51) Seuwen, R.; Warneck, P. Int. J. Chem. Kinet. 1996, 28, 315–332.
(52) Moschonas, N.; Danalatos, D.; Glavas, S. Atmos. Environ. 1999,
33, 111–116.
(53) Klotz, B.; Barnes, I.; Becker, K.-H.; Golding, B. T. J. Chem. Soc.,
Faraday Trans. 1997, 93, 1507–1516.
(54) Klotz, B.; Barnes, I.; Becker, K.-H. Chem. Phys. 1998,
231, 289–301.
(55) Klotz, B.; Barnes, I.; Golding, B. T.; Becker, K.-H. Phys. Chem.
Chem. Phys. 2000, 2, 227–235.
(56) Hoshino, M.; Akimoto, H.; Okuda, M. Bull. Chem. Soc. Jpn.
1978, 51, 718–724.
(57) Forstner, H.; Flagan, R.; Seinfeld, J. Environ. Sci. Technol. 1997,
31, 1345–1358.
(58) Huang, M.; Zhang, W.; Hao, L.; Wang, Z.; Zhao, W.; Gu, X.;
Guo, X.; Liu, X.; Long, B.; Fang, L. J. Atmos. Chem. 2007, 58, 237–
252.
(15) Aschmann, S. M.; Arey, J.; Atkinson, R. Atmos. Environ. 2010,
44, 3970–3975.
(16) Wahner, A.; Zetzsch, C. J. Phys. Chem. 1983, 87, 4945–4951.
(17) Witte, F.; Urbanik, E.; Zetzsch, C. J. Phys. Chem. 1986,
90, 3251–3259.
(18) Knispel, R.; Koch, R.; Siese, M.; Zetzsch, C. Ber. Bunsen-Ges.
1990, 94, 1375–1379.
(19) Bohn, B.; Zetzsch, C. Phys. Chem. Chem. Phys. 1999, 1, 5097–
5107.
(20) Bohn, B. J. Phys. Chem. A 2001, 105, 6092–6101.
(21) Grebenkin, S.; Krasnoperov, L. J. Phys. Chem. A 2004,
108, 1953–1963.
(22) Raoult, S.; Rayez, M. T.; Rayez, J. C.; Lesclaux, R. Phys. Chem.
Chem. Phys. 2004, 6, 2245–2253.
(23) Koch, R.; Knispel, R.; Elend, M.; Siese, M.; Zetzsch, C. Atmos.
Chem. Phys. 2007, 7, 2057–2071.
6025
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026

The Journal of Physical Chemistry A
ARTICLE
(59) Huang, M.; Zhang, W.; Hao, L.; Wang, Z.; Fang, L.; Kong, R.;
Shan, X.; Liu, F.; Sheng, L. J. Environ. Sci. - China 2010, 22, 1570–1575.
(60) Gery, M.; Fox, D.; Kamens, R.; Stockburger, L. Environ. Sci.
Technol. 1987, 21, 339–348.
(61) Atkinson, R.; Aschmann, S.; Arey, J. Int. J. Chem. Kinet. 1991,
23, 77–97.
(62) Bethel, H.; Atkinson, R.; Arey, J. J. Phys. Chem. A 2000,
104, 8922–8929.
(63) Volkamer, R. A DOAS Study on the Oxidation Mechanism of
AromaticHydrocarbons under Simulated Atmospheric Conditions. Ph.
D. thesis, University of Heidelberg, 2001.
(64) Kwok, E. S. C.; Aschmann, S. M.; Atkinson, R.; Arey, J. J. Chem.
Soc., Faraday Trans. 1997, 93, 2847–2854.
(65) Bandow, H.; Washida, N. Bull. Chem. Soc. Jpn. 1985, 58,
2541–2548.
(66) Arey, J.; Obermeyer, G.; Aschmann, S. M.; Chattopadhyay, S.;
Cusick, R. D.; Atkinson, R. Environ. Sci. Technol. 2009, 43, 683–689.
(67) Nishino, N.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2010,
114, 10140–10147.
(68) Wyche, K. P.; Monks, P. S.; Ellis, A. M.; Cordell, R. L.; Parker,
A. E.; Whyte, C.; Metzger, A.; Dommen, J.; Duplissy, J.; Prevot, A. S. H.;
Baltensperger, U.; Rickard, A. R.; Wulfert, F. Atmos. Chem. Phys. 2009,
9, 635–665.
(69) Rickard, A. R.; Wyche, K. P.; Metzger, A.; Monks, P. S.; Ellis,
A. M.; Dommen, J.; Baltensperger, U.; Jenkin, M. E.; Pilling, M. J. Atmos.
Environ. 2010, 44, 5423–5433.
(70) Bandow, H.; Washida, N. Bull. Chem. Soc. Jpn. 1985, 58,
2549–2555.
(71) Berndt, T.; B€oge, O. Int. J. Chem. Kinet. 2001, 33, 124–129.
(72) Berndt, T.; B€oge, O. Phys. Chem. Chem. Phys. 2003, 5, 342–350.
(73) Coeur-Tourneur, C.; Henry, F.; Janquin, M.-A.; Brutier, L. Int.
J. Chem. Kinet. 2006, 38, 553–562.
(74) NASA panel for data evaluation. JPL publication 06-2, evaluation
no. 15; JPL: Pasadena, CA, 2006.
(75) Grosjean, D. Environ. Sci. Technol. 1985, 19, 968–974.
(76) Atkinson, R.; Aschmann, S.; Arey, J. Environ. Sci. Technol. 1992,
26, 1397–1403.
(77) Platz, J.; Nielsen, O.; Wallington, T.; Ball, J.; Hurley, M.;
Straccia, A.; Schneider, W.; Sehested, J. J. Phys. Chem. A 1998, 102,
7964–7974.
6026
dx.doi.org/10.1021/jp210946y |J. Phys. Chem. A 2012, 116, 6015–6026