Hydroxyl radical (OH) oxidation of toluene produces ring-retaining
products: cresol and benzaldehyde, and ring-opening products: bicyclic
intermediate compounds and epoxides. Here, first- and later-generation OH
oxidation products from cresol and benzaldehyde are identified in laboratory
chamber experiments. For benzaldehyde, first-generation ring-retaining
products are identified, but later-generation products are not detected. For
cresol, low-volatility (saturation mass concentration, C
Aromatic compounds are emitted from both anthropogenic (e.g., solvent use and
motor vehicle exhaust) and natural (e.g., wildfires) processes. Oxidation of
aromatic compounds leads to the formation of ozone (O
Hydroxyl radical (OH) oxidation of toluene takes place via four pathways,
yielding benzaldehyde, cresol, bicyclic intermediates, and epoxides
(Fig.
Benzaldehyde forms as a result of hydrogen abstraction from the methyl group
of toluene. Reported benzaldehyde yields from toluene oxidation are
relatively consistent in the range of 0.053–0.12
Cresol is produced from OH addition to the aromatic ring of toluene with
subsequent O
Toluene photooxidation pathways from the Master Chemical Mechanism (MCM) v3.3.1 including cresol isomer distribution
Chamber experiments were performed to study products from toluene-OH oxidation under both low- and high-NO conditions. In order to explore later-generation chemistry and identify important precursors for SOA, later-generation ring-retaining products were also used as the initial precursor.
All experiments were performed in the 24 m
Description of experiments.
After the addition of the oxidant, the volatile organic compound (VOC) was
injected. Toluene (99.8 % purity) and benzaldehyde (
For high-NO experiments, NO (501 ppm in N
For experiments in which particle-phase sampling was performed, the last step
included atomization of 0.06 M ammonium sulfate through a
Some studies
In experiment 9, all procedures were the same as described in the proceeding paragraphs, but after 1.5 h of photooxidation, lights were turned off. While lights were off, the decay of 3-methyl catechol oxidation products due to wall deposition was measured. In experiment 10, all procedures were the same as described above, but lights were turned on for only 3.2 h. Once an adequate level of oxidation products from 3-methyl catechol oxdiation was generated, the chamber experiment was ended and purified air was sampled by the chemical ionization mass spectrometer (CIMS) to monitor the desorption of 3-methyl catechol oxidation products off the CIMS walls.
Commercial instruments were used to monitor toluene, nitrogen oxides (NO
A CIMS was used to monitor oxidized
organic compounds in the gas-phase. The CIMS uses a custom-modified triple
quadrupole mass analyzer (Varian 1200)
The MS/MS mode was used to confirm the identity of certain products and to
separate isobaric compounds. In the MS/MS mode, only ions formed from the complex
(Reaction R1) will produce a CF
The CIMS was calibrated using
Independent of the calibrations described above, the loss of
Fraction of CIMS signal detected from the complex ion (Reaction R1) or fragment ions (transfer ion – Reaction R2 – and others).
Estimated vapor pressures and saturation mass concentrations for main products detected by the CIMS from toluene-OH oxidation.
As stated above, CF
3-Methyl catechol calibration was attempted using the same FT-IR method as
3-Methyl catechol, which is more acidic than
Because the CF
2,4,6-Trihydroxy toluene seemed to be more stable and a higher signal was
achieved compared to 5-methyl-benzene-1,2,3-triol. Only major signals for
5-methyl-benzene-1,2,3-triol were above the noise and reported. For
2,4,6-trihydroxy toluene in the MS/MS mode,
The CIMS detected many additional signals from these standards, which are
likely caused by impurities, decomposition outside of the CIMS due to
heating, and fragmentation inside the CIMS during chemical ionization. When
the standards were introduced into the pillow bag at different temperatures,
the ratio of these compounds to the
The sensitivity (all unique signals) determined for
During toluene oxidation,
SOA was collected during the final 4 h of experiments at 24 L min
With such a broad spectrum of compounds and the absence of synthetic
standards, only ions with signals well above the background were selected for
analysis. Ions with signals
DART-generated signal intensity for a given compound is proportional to the
product of its vapor pressure, proton affinity, and concentration
Two vapor pressure estimation methods are used here: (1) the estimation of
vapor pressure of organics, accounting for temperature, intramolecular, and
non-additivity effects (EVAPORATION) method
The chamber experiments were simulated with a kinetic model containing all
reactions related to toluene from MCM v3.3.1
The kinetic model was used to evaluate the extent to which chamber conditions
are representative of those in the atmosphere. The two main concerns in
chamber studies performed under high-NO conditions are high NO
Both
Toluene reacts with OH to form both ring-retaining products (cresol and
benzaldehyde) and ring-opening products (bicyclic intermediate compounds and
epoxides) (Fig.
Previous studies generally recommend a
Kinetic model predictions (version 1 solid lines) compared to CIMS
measurements (data points) under low-NO (
The
Gas-phase chemical mechanism for
Gas-phase chemical mechanism for
Proposed decomposition pathways for bicyclic intermediate compounds
formed from OH oxidation of
3-Methyl catechol oxidation under both low- and high-NO conditions leads to
the following products (Fig.
Without authentic standards for trihydroxy toluene and hydroxy methyl
benzoquinone, quantification cannot be achieved (Sect.
Several products from photooxidation of trihydroxy toluene are also detected
by the CIMS in the 3-methyl catechol oxidation experiments, including
tetrahydroxy toluene, dihydroxy methyl benzoquinone, and various
decomposition products from the bicylic intermediate pathway (Fig.
An array of decomposition products presumably from the bicyclic intermediate
oxidation pathway of
Proposed decomposition pathways for bicyclic intermediate compounds formed from OH oxidation of trihydroxytoluene and tetrahydroxytoluene. Blue and red boxed compounds were detected by CIMS and DART-MS, respectively.
Gas-phase chemical mechanism for benzaldehyde photooxidation under low- and high-NO conditions. MCM v3.3.1 pathways are shown in black. Products detected by the CIMS and DART-MS are boxed in blue and red, respectively, with dashed lines indicating only a minor amount forms.
MCM v3.3.1 recommendations for OH oxidation of benzaldehyde are generally in
agreement with the products detected by the CIMS (Fig.
Other first-generation products are also detected, including signals at
The dominant first-generation product detected from benzaldehyde oxidation
under high-NO conditions is nitrophenol (
OH addition to the aromatic ring of benzaldehyde or benzoic acid is expected
to be only a minor pathway. The rate of OH addition to an aromatic ring is
proportional to the electrophilic nature of the substituents around the ring;
unlike methyl and hydroxy groups, carboxy and formyl groups are not
electrophilic
Oxidation of benzaldehyde under high- and low-NO conditions does not yield
many later-generation products detectable by the CF
Products detected in the gas phase are compared to those detected in the particle phase to further understand the mechanism for toluene SOA formation. Filters, collected at the end of each particle-phase experiment, were analyzed using high-resolution direct analysis in real-time mass spectrometry (DART-MS). As expected, a number of compounds (e.g., trihydroxy toluene, tetrahydroxy toluene, and pentahydroxy toluene) measured in the gas-phase were also detected in the particle-phase by the DART-MS.
The intensity of the DART signal for a given compound is proportional to the
product of the proton affinity, vapor pressure, and concentration of the
compound. The proton affinity of each compound is assumed to be similar, due
to shared ionizable functional groups. To compare the relative amounts of
each product detected, the measured intensity is normalized by the
compound's estimated vapor pressure to produce a normalized intensity (
Vapor pressures for the compounds detected in this study have been estimated
using both EVAPORATION and Nannoolal methods
The same
Particle-phase products detected by DART-MS during oxidation of
toluene under low-NO conditions (
As shown in Fig.
Some of the compounds (e.g., tetrahydroxy toluene and pentahydroxy toluene)
are structural isomers of those produced from the epoxide pathway of toluene
oxidation under low-NO conditions; under high-NO conditions, the products
from the epoxide channel largely decompose (Fig. S6). Signals assigned to
tetrahydroxy toluene and pentahydroxy toluene are dominant in the particle
phase from toluene oxidation under both high- and low-NO conditions
(Fig.
Nitrophenol from benzaldehyde oxidation is also detected in the particle
phase. Part of the signal for C
Considering that many products generated from the cresol pathway in the gas
phase are also detected in the particle phase, the contribution of these
products to toluene SOA is estimated. The experiments conducted here were
optimized to investigate chemistry and not specifically designed to measure
SOA yields, so this estimate is not based on the organic mass produced during
each experiment. SOA yields studies are run differently to account for a
variety of factors including particle and vapor wall losses. Chamber studies
have recently reported toluene SOA mass yields to be between 0.9 and 1.6
Under low-NO conditions the toluene SOA yield with the model corrections for
vapor wall loss (1.6
Gas- and particle-phase measurements by the CIMS and DART-MS confirm that OH
oxidation of dihydroxy toluene leads to low-volatility products that
partition to the particle phase. For example, the following three products,
which form from subsequently adding OH to the aromatic ring, are detected in
the gas and particle phases: trihydroxy toluene (C
The chemical mechanism proposed in Fig.
In the atmosphere, once OH adds to the aromatic ring, O
CIMS signals for trihydroxy toluene
In order to evaluate the products detected by the CIMS, the mechanism outlined in Figs. 3–7 is incorporated into the kinetic model. Version 2 of the kinetic model includes photolysis of hydroxy nitrotoluene and dihydroxy nitrotoluene. Version 3 of the kinetic model includes additional products for 3-methyl catechol and benzaldehyde oxidation (see Sect. S2 for more details).
In order to compare the CIMS results to the kinetic model predictions, all
loss processes need to be constrained for the compounds of interest. This
includes reaction with OH/NO
In experiment 9 (Table
In experiment 10 (Table
For 3-methyl catechol (
The pKa values of compounds similar to trihydroxy toluene demonstrate that
aromatic compounds with an OH group
For simplicity, the wall deposition rate determined for
MCM v3.3.1 does not include the photolysis of many nitro compounds, even
though recent studies have measured fast photolysis rates
Kinetic model predictions (version 1 solid lines, version 2 dashed
lines, version 3 dotted lines) vs. CIMS measurements (data points) for
Under high-NO conditions during 3-methyl catechol oxidation, dihydroxy
nitrotoluene is detected only minimally (
MCM v3.3.1 predictions of nitrophenol, a product of benzaldehyde oxidation,
exceed the CIMS measurements (Fig. S3). An estimated photolysis rate
constant was added to the kinetic model based on that for 2-nitrophenol
measured by
During
In the kinetic model (Version 3), dihydroxy, trihydroxy, and tetrahydroxy
toluene oxidation products are inferred from the products recommended by
MCM v3.3.1
For 3-methyl catechol oxidation, like
When 3-methyl catechol is oxidized under low-NO conditions, trihydroxy
toluene is over-predicted by the kinetic model compared to the CIMS
measurements (Fig.
Under high-NO conditions, the kinetic model over-predicts trihydoxy toluene
formation compared to the CIMS measurements for both
Tetrahydroxy toluene and dihydroxy methyl benzoquinone are both
over-predicted by the kinetic model under low- and high-NO conditions as
compared to experimental results (Fig.
OH addition to an aromatic ring followed by O
In MCM v3.3.1, the bicyclic intermediate peroxy radical reacts either with NO
producing an alkoxy radical that decomposes or with HO
A variety of decomposition products assumed to arise from the bicyclic
intermediate pathway were detected by the CIMS (Figs.
With these additional reactions, the measurements of the bicyclic
intermediate decomposition products formed from 3-methyl catechol oxidation
under low- (Fig. S4) and high-NO conditions are well represented by the
kinetic model. These same products from
The chemistry proposed (Figs.
Additionally, the proposed gas-phase chemical mechanism (Figs.
First- and later-generation cresol and benzaldehyde OH oxidation products are
identified. Evidence suggests that later-generation products from cresol OH
oxidation are particularly important for SOA formation. The following
first-generation OH oxidation products of 3-methyl catechol (i.e.,
second-generation products from
Data used within this work are available upon request. Please email Rebecca Schwantes (rschwant@ucar.edu).
The authors declare that they have no conflict of interest.
This work was supported by National Science Foundation grants AGS-1240604 and AGS-1523500. We thank Hannah Allen and Anke Noelscher for their experimental assistance and Nathan Dalleska and John Crounse for helpful discussions. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Edited by: Y. Rudich Reviewed by: H. Herrmann and T. Mentel