Low-molecular weight and oligomeric components in secondary organic aerosol from the photooxidation of p-xylene

Article abstract of DOI:10.1002/jccs.200800068

A laboratory study was performed to investigate the composition of secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon p-xylene. The experiments were conducted by irradiating p-xylene/CH₃ONO/NO/air mixtures in a home-made smog chamber. The aerosol time-of-flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real-time. According to a large number of single aerosol diameters and mass spectra, the size distribution and chemical composition of SOA were determined statistically. Experimental results showed that aerosol created by p-xylene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm(i.e. PM2.5), and aromatic aldehyde, unsaturated dicarbonys, hydroxyl dicarbonys, and organic acid are major product components in the SOA after 2 hours photooxidation. After aging for more than 8 hours, about 10% of the particle mass consists of oligomers with a molecular mass up to 600 daltons. The possible reaction mechanisms leading to these products are also proposed.

Full text of DOI:10.1002/jccs.200800068

456
Journal of the Chinese Chemical Society, 2008, 55, 456-463
Low-Molecular Weight and Oligomeric Components in Secondary Organic
Aerosol from the Photooxidation of p-Xylene
Ming-Qiang Huang* (
Zhen-Ya Wang (
Xue-Jun Gu (
), Wei-Jun Zhang (
), Wen-Wu Zhao (
) and Li Fang (
), Li-Qing Hao (
),
),
)
Laboratory of Environment Spectroscopy, Anhui Institute of Optics & Fine Mechanics,
Chinese Academy of Sciences, Hefei 230031, P. R. China
A laboratory study was performed to investigate the composition of secondary organic aerosol (SOA)
products from photooxidation of the aromatic hydrocarbon p-xylene. The experiments were conducted by
irradiating p-xylene/CH3ONO/NO/air mixtures in a home-made smog chamber. The aerosol time-of-flight
mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual
secondary organic aerosol particles in real-time. According to a large number of single aerosol diameters
and mass spectra, the size distribution and chemical composition of SOA were determined statistically.
Experimental results showed that aerosol created by p-xylene photooxidation is predominantly in the
form of fine particles, which have diameters less than 2.5 mm (i.e. PM2.5), and aromatic aldehyde, unsatu-
rated dicarbonys, hydroxyl dicarbonys, and organic acid are major product components in the SOA after 2
hours photooxidation. After aging for more than 8 hours, about 10% of the particle mass consists of oligo-
mers with a molecular mass up to 600 daltons. The possible reaction mechanisms leading to these products
are also proposed.
Keywords: p-Xylene; Secondary organic aerosol; Oligomers; Smog chamber; Laser
desorption/ionization; Reaction mechanisms.
INTRODUCTION
It is well known that aromatic compounds such as
of toluene.8 The OH-initiated reaction results in minor H-
abstraction from one methyl group to form a methylbenzyl
radical (about 10%) and major OH addition to the aromatic
ring to form a dimethylhydroxycyclohexadienyl radical
(about 90%).9 As shown in Fig. 1, in the presence of O2 and
NO, the subsequent reactions of the methylbenzyl radical
lead to the formation of p-tolualdehyde. Under atmo-
spheric conditions, the dimethylhydroxycyclohexadienyl
radical (the OH-p-xylene adduct) reacts with O2 either by
O2 addition to form primary peroxy radicals or by H-ab-
straction to yield 2,5-dimethylphenol, or by H-abstraction
to form aromatic oxide/oxepin. The fate of the primary
peroxy radicals is governed by competition between reac-
tion with NO to from alkoxy radicals and intramolecular
cyclization to form bicyclic radicals. Theoretical studies
have shown that, instead of reaction with NO, the primary
peroxy radicals from the OH-initiated oxidation of toluene
cyclize, forming bicyclic radicals.10 In the presence of NO,
subsequent reactions of bicyclic radicals resulting from the
OH-initiated of the p-xylene lead to multi-functional or-
benzene, toluene, ethylbenzene, and xylenes (BTEX) are
important constituents of automobile tailpipe exhaust and
evaporative emissions.1-4 In addition to their important role
in photochemical ozone production, oxidation of aromatic
compounds leads to formation of nonvolatile and semi-vol-
atile organic compounds, which are responsible for sec-
ondary organic aerosol (SOA) in urban air.5 Recently, evi-
dence has been found for the formation of oligomers from
photooxidation of aromatic hydrocarbons.6,7 The existence
of oligomers in SOA has significant implications for the
physical behavior of the SOA particles in the atmosphere,
as well as for their behavior in atmospheric models.
p-Xylene is an important constituent of aromatic hy-
drocarbons and is highly reactive with respect to ozone for-
mation. In the atmosphere, the dominant loss process for
p-xylene is the gas-phase reaction with the hydroxyl radi-
cal OH. Its reaction rate with OH radicals is on the order of
10-11 cm3 molecule-1 s-1, nearly four times higher than that
* Corresponding author. E-mail: huangmingqiang@gmail.com

Study on the Components of SOA from p-Xylene Using ATOFMS
J. Chin. Chem. Soc., Vol. 55, No. 2, 2008 457
ganic compounds such as glyoxal, methylglyoxal, and un-
saturated carbonyl compounds. The aromatic oxide/oxepin
channel lead to the formation of unsaturated dicarbonyls,
although it remains uncertain.
tion mechanisms leading to these products are also pro-
posed.
EXPERIMENTAL SECTION
Numerous experiments have been carried out to study
the products from OH-initiated p-xylene oxidation,11-17 and
chemical composition of p-xylene SOA particle formed in
chambers have been studied using off-line techniques,
which have provided important information about the
chemical compositions. However, the off-line techniques
are time-consuming and tend to have sampling artifacts.18
In addition, it is difficult to measure the size and chemical
compositions of the individual SOA particles simulta-
neously and in real-time by using the off-line techniques.
We have demonstrated that aerosol time-of-flight mass
spectrometer (ATOFMS) can be used to measure the size
and chemical composition of toluene SOA particles in
real-time.19 So in this study, photooxidation of p-xylene is
performed using UV-irradiation of p-xylene/CH3ONO/
NO/air mixtures in our home-made smog chamber, and
ATOFMS was employed to detect the particles in different
reaction times. Aromatic aldehyde, saturated dicarbonys,
di-unsaturated 1,6-dicarbonys, hydroxyl dicarbonys, or-
ganic acid, are major products components in the SOAafter
2 hours of photooxidation. After aging for more than 8
hours, about 10% of the particle mass consists of oligomers
with a molecular mass up to 600 daltons. The possible reac-
Photooxidation of p-xylene is performed using UV-
irradiation of p-xylene/CH3ONO/NO/air mixtures in an
850 L sealed collapsible polyethylene smog chamber.20 Its
ratio of surface to volume was 5.8 m-1. Around the cham-
ber, there was equipped with 12 sets of 40-W fluorescent
black lamps symmetrically that provide radiation in the
300-400 nm region. The bag and lamps were housed inside
a highly reflective enclosure to enhance light distribution.
Prior to the start of the experiment, the chamber was con-
tinuously flushed with purified laboratory compressed air
for 40 mins. The compressed air was processed through
three consecutive packed-bed scrubbers, which contain
activated charcoal, silica gel and a Balston DFUÒ filter
(Grade BX) respectively, to remove trace hydrocarbon
compounds, moisture and particles. 2.0 mL/L p-xylene,
20.0 mL/L CH3ONO, 2.0 mL/L NO were injected into the
chamber and mixed with the pre-existing purified air. And
again the chamber was filled with the purified air to 850 L
full volume. Turn on four black lamps and initiate the pho-
tooxidation reaction. Hydroxyl radicals will be generated
by the photolysis of methyl nitrate in air at wavelengths
longer than 300 nm.21 The chemical reactions leading to the
formation of the OH· radical are as follows:
CH3ONO + hn (> 300 nm) ® CH3O + NO
CH3O + O2 ® HCHO + HO2
HO2 + NO ® OH + NO2
After 2 hours and 8 hours phooxidation, the SOAs pro-
duced by the photooxidation were analyzed continuously
using the ATOFMS connected directly to the chamber us-
ing a Teflon line.
RESULTS AND DISCUSSION
According to the design principles on the measuring
system of particle diameter, timing circuit, and laser de-
sorption/ionization setup of ATOFMS, its time-of-flight
mass spectroscopy is only obtained from those particles of
secondary organic aerosol, whose diameter has been mea-
sured. The diameter of an individual particle, number dis-
tribution of SOA particle diameter, and molecular compo-
sition of an SOA particle could be measured using our
Fig. 1. Mechanistic diagram for the OH-initiated oxi-
dation of p-xylene.

458 J. Chin. Chem. Soc., Vol. 55, No. 2, 2008
Huang et al.
ATOFMS.
±0.5.22,23 Based on the typical mass spectra in Fig. 2 and a
large number of other mass spectra, the chemical composi-
tion of SOA particle might be measured statistically. Ac-
cording to the references and possible reaction pathways of
p-xylene photooxidation reaction, we can obtain the chem-
ical composition of SOA. Table 1 lists the m/z and molecu-
lar structures of identified products from photooxidation of
p-xylene, while m/z 27, 30, 38, 46, 50, 62, 66, 84, 118, 128,
134, 146, 161, 179, 183, 192, 205, and 216 represent some
unidentified compounds, most of the chemical composi-
tions of SOA particles in our results are the same as those
reported in references 11-17. However, there are some dif-
ferences between the current study and literature. For ex-
Size and low-molecular-weight components of individ-
ual particle
The chemical composition and size of individual
SOA particles a, b, c, and d produced from the 2 hours
photooxidation of p-xylene are shown in Figs. 2a~2d. It is
shows that each piece of the mass spectrum corresponds to
an aerosol particle, and the diameter and chemical compo-
sition might be different from each other.
When the diameter and chemical composition of the
SOA particle are detected using ATOFMS, their allowable
standard error on the ratio of mass to charge (m/z) is about
Fig. 2. Laser desorption/ionization time-of-flight mass spectra and size of 4 individual p-xylene SOA particles after 2 hours
photooxidation (Aerosol diameter: (a): 1.19 mm, (b): 0.34 mm, (c): 1.03 mm, (d): 0.85 mm).

Study on the Components of SOA from p-Xylene Using ATOFMS
J. Chin. Chem. Soc., Vol. 55, No. 2, 2008 459
Table 1. Molecular structures of speculated products from photooxidation of p-xylene
m/z
Products
Structure
Ref & Comments
58
72
90
92
glyoxal
HCOCOH
11-17
11-17
15
methylglyoxal
oxalic acid
CH3COCOH
HOOC-COOH
O
CH3
2,5-dimethylfuran
14,15
H3C
O
96
2-furaldehyde
p-xylene
13
CHO
106
112
p-xylene precursor
11,14-15
H3C
CH3
H
H
C
3-hexene-2,5-dione
H3C
C
O
O
C
C CH3
O
O
3-methyl-2,5-furandione
13,15
O
CH3
CHO
120
122
p-tolualdehyde
11-17
H3C
CH3
2,5-dimethylphenol
11,13-17
OH
CH3
O
136
138
2,5-dimethyl-1,4-benzoquinone
11,13-15
H3C
CH3
O
p-toluic acid
13
16
H3C
COOH
H
H
C
H
3-methyl-6-oxo-2,4-heptadieneal
C
O
C
C
C CHO
H3C
CH3
CH3 CH3
2,5-dimethyl-2,4-hexadienedial
2-Nitro-p-xylene
16
HC
O
C
H
C
C
H
C CHO
CH3
151
158
14,15
NO2
CH3
CH3
O
2-methyl-4-hydroxy-5-oxo-2-
hexaenoic acid
Newly identified
O
OH
OH
CH3
167
170
p-methylbenzyl nitrate
17
CH2ONO2
H3C
CH3
CH3
O
2-methyl-5-hydroxy-4,6-dioxo-2-
hepteneal
Newly identified
O
O
OH
H
ample, 2-methyl-4-hydroxy-5-oxo-2-hexaenoic acid, 2-
methyl-5-hydroxy-4,6-dioxo-2-hepteneal, and some un-
identified compounds are newly found in our experiments.
It is possibly due to that we detected the particles of SOA
using different sample preparation and analyzing technol-
ogy.
p-Xylene photooxidation mechanisms
Possible mechanisms leading to the p-xylene-OH pho-
tooxidation products identified as SOA components are de-

460 J. Chin. Chem. Soc., Vol. 55, No. 2, 2008
Huang et al.
picted in Figs. 3-5. The species identified in the aerosol
phase are highlighted in the mechanisms by boxes. Car-
bonyl products in the ring-opening route have been sug-
gested as an important radical source and have a large im-
pact on ozone formation in the aromatic hycarbon oxida-
tion.24 The bicyclic route is a major ring opening product
channel for the OH-p-xylene system. Bicyclic radicals re-
sulting from the primary peroxy radical form secondary
peroxy radicals on addition of O2 which then react with
NO to form bicyclic alkoxy radicals (Fig. 3). The bicyclic
alkoxy radicals can further decompose to form unsaturated
dicarbonyl (2-methyl-butenedial and 3-hexene-2,5-dione
in Fig. 3) and corresponding a-hydroxy radicals. Subse-
quent abstraction of a-hydroxy radicals by O2 results in the
formation of glyoxal and methylglyoxal (Fig. 3) respec-
tively. 2-Methyl-butenedial has been postulated to lose a
hydrogen atom from the carbonyl group via OH radical at-
tack, with subsequent O2 addition and rearrangement to
form 3-methyl-2,5-furandione.13 The details of this mecha-
nism are outlined in Fig. 3.
shown in Fig. 4, the outcome of the subsequent ring open-
ing via b-fragmentation can ultimately be the production of
unsaturated dicarbonys, 2,5-dimethyl-2,4-hexadienedial
and 3-methyl-6-oxo-2,4-heptadieneal.
The atmospheric chemistry of toluene oxide/oxepin
has been studied by Klotz et al., and aromatic-oxide/oxepin
is fairly reactive toward OH radicals, with a reaction rate of
2 ´ 10-10 cm3 molecule-1 s-1 for toluene-oxide with OH radi-
cals.26 As shown in Fig. 5, addition of OH to C6 position of
p-xylene-oxide with successive ring cleavage and then
H-abstraction by O2 leads to the formation of unsaturated
1,6-dicarbonyls, 2-methyl-6-oxo-2,4-heptadieneal. In the
presence of OH, O2 and NO, 2-methyl-6-oxo-2,4-hepta-
dieneal can further react to form 2-methyl-5-hydroxy-4,6-
dioxo-2-hepteneal.
Addition of OH to C4 position of p-xylene-oxide
with successive ring cleavage and then H-abstraction by O2
leads to the formation of 1,4-dimethyl-2,3-benzoquinones.
1,4-dimethyl-2,3-benzoquinones can be attacked by OH
radicals, O2, and NO, with successive ring cleavage and
then O abstraction by HO2 leads to the formation of 2-meth-
yl-4-hydroxy-5-oxo-2-hexaenoic acid. The details of this
mechanism are demonstrated in Fig. 5.
In polluted areas the concentration of both aromatics
and nitrogen oxides is significant. Moreover, the ROO+NO
reaction is generally fast.25 With sufficiently high NOx con-
centration, O abstraction by NO from the primary peroxy
radicals leading to form alkoxy radical can play a role. As
Oligomer components of individual particle
Most of the mass spectra of the p-xylene SOA after 2
hours and 8 hours photooxidation appear to have similar
patterns as shown in Fig. 2, implying that the chemical
Fig. 3. Mechanistic diagram of the bicyclic pathway
from OH-initiated oxidation of p-xylene.
Fig. 4. Mechanistic diagram of the alkoxy pathway
from OH-initiated oxidation of p-xylene.

Study on the Components of SOA from p-Xylene Using ATOFMS
J. Chin. Chem. Soc., Vol. 55, No. 2, 2008 461
composition of the products does not markedly change
overall with the irradiation time. However, after aging for
more than 8 hours, about 10% of the particle mass consists
of m/z up to 600 daltons. The typical mass spectra of these
kinds of particles are shown in Fig. 6. We can see that m/z
180, 252, 324, 396, 468, 540, showing the repetitive groups
with m/z = 72, indicative of the oligomeric species.
The traditional view of SOA formation is that gas-
phase oxidation of the parent hydrocarbons leads to the
mutifunctional, low volatility products that partition them-
selves between the gas and aerosol phases.27 It has been as-
sumed that, once in the aerosol phase, the oxidation prod-
ucts did not react further and that the amount of SOAformed
depended entirely on the gas-particle partitioning. How-
ever, recent discoveries show that once the gas-phase polar
oxidation products (e.g. aldehydes and ketones) condense
Fig. 6. Laser desorption/ionization time-of-flight mass
spectra of individual p-xylene SOA particles
after 8 hours photooxidation (Aerosol diame-
ter: 1.25 mm).
Fig. 5. Mechanistic diagram of the p-xylene-oxide
pathway from OH-initiated oxidation of p-xy-
lene.
Fig. 7. Proposed reaction mechanisms leading to the
formation of oligomers.

462 J. Chin. Chem. Soc., Vol. 55, No. 2, 2008
Huang et al.
Fig. 8. Diameter distribution of p-xylene SOA particles after 2 hours and 8 hours photooxidation detected by ATOFMS
(Aerosol diameter: (a): 2 hours photooxidation, (b): 8 hours photooxidation).
into the aerosol, particle phase reactions may take place.28
Recently studies have demonstrated that acid-catalyzed
heterogeneous reactions can occur. Possible reaction mech-
anisms for acid-catalyzed aldehyde reactions include hy-
dration, polymerization, hemiacetal/acetal formation, and
aldol condensation.29 And the polymerization can lead to
the formation of oligomers in the aerosol phase.
organic aerosol particles were also discussed at the same
time. The number distribution of SOA particles, produced
from 2 hours and 8 hours photooxidation of p-xylene, as a
function of particle size is shown in Fig. 8. Under the ex-
perimental conditions mentioned above, the longer the re-
action time was, the larger the particle diameter was. This is
because the fine particles of matter became larger through a
self-nucleation or condense on the pre-existing aerosol
process; also the consequence of acid-catalyzed heteroge-
neous reactions and oligomer formation is that species that
partition between the gas and aerosol phases are converted
to larger compounds of extremely low volatility, thereby
locking gas carbonyl products into the aerosol phase and
increasing the diameter of the particles. And aerosol cre-
ated by p-xylene is predominant in the form of fine parti-
cles, which have diameters less than 2.5 mm (i.e. PM2.5),
researchers have shown that this fine particulate matter is
more easily deposited in the lungs of human beings, and
does great harm to the health.31
Carbonyls and acids have been measured in the gas
and particle phase products of the p-xylene photooxidation
experiments. Methylglyoxal, a C3-dicarbonyl, was found
to be one of the most abundant gas-phase photooxidation
products of p-xylene.11,12,14-17 And the acid products, such
as p-toluic acid and oxalic acid catalyze the heterogeneous
reaction of methylglyoxal on secondary organic aerosols.
The net result is leading to the formation of oligomers in
particles. As shown in Fig. 7, initial dissolution and hydra-
tion of methylglyoxal (1) result in the diol formation (2),
and further hydration produces a tetrol (3). And self-reac-
tions and cross-reaction between diols and/or tetrols pro-
duce oligomers as shown in Fig. 7. Evidence has been pro-
vided for the formation of the dimers (4, 5, and 6) in aque-
ous methylglyoxal solution.30 The produced dimers (4, 5,
and 6) further engage in reaction with diols or tetrols to
yield higher oligomers (7, 8, and 9).
ACKNOWLEDGMENT
This work is supported by the National Natural Sci-
ence Foundation of China (20477043) and the Knowledge
Innovation Foundation of Chinese Academy of Sciences
(KJCX2-SW-H08). The authors express our gratitude to
the referees for their value comments.
Size distribution
When the chemical composition of the aerosol parti-
cles were analyzed, the number distribution of secondary
Received September 6, 2007.

Study on the Components of SOA from p-Xylene Using ATOFMS
J. Chin. Chem. Soc., Vol. 55, No. 2, 2008 463
16. Volkamer, R.; Platt, U.; Wirtz, K. J. Phys. Chem. A 2001,
105, 7865.
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