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A Smog Chamber and Modeling Study of the Gas Phase NO,-Air Photooxidation
of Toluene and the Cresols
ROGER ATKINSON Shell Research Ltd., Thornton Research Centre, Chester, United Kingdom, and
Statewide Air Pollution Research Center, University of California, Riverside, California 92521
and
WILLIAM P. L. CARTER,* KAREN R. DARNALL,** ARTHUR M. WINER, and JAMES N. PITTS, Jr.
Statewide Air Pollution Research Center, University of California, Riverside, California 92521
Abstract
An experimental and modeling study of irradiated toluene-NO,-air, toluene-benzalde- hyde-NO,-air, and cresol-NO,-air mixtures a t part-per-million concentrations has been carried out. These mixtures were irradiated a t 303 f 1 K in a 5800-liter Teflon-lined, evac- uable environmental chamber, with temperature, humidity, light intensity, spectral distri- bution, and the concentrations of 0 3 , NO, NOz, toluene, PAN, formaldehyde, benzaldehyde, o-cresol, m-nitrotoluene, and methyl nitrate being monitored as a function of time. For the toluene and toluene-benzaldehyde-NO,-air runs a variety of initial reactant concentrations were investigated. Cresol-NO,-air runs were observed to be much less reactive in terms of 0 3 formation and NO to NO2 conversion rates than toluene-NO,-air runs, with the relative reactivity of the cresol isomers being in the order meta >> ortho > para. The addition of benzaldehyde to toluene-NO,-air mixtures decreased the reactivity, in agreement with previous studies. Alternative mechanistic pathways for the NO, photooxidations of aromatic systems in general are discussed, and the effects of varying these mechanistic alternatives on the model predictions for the toluene and o-cresol-NO,-air systems are examined. Fits of the calculations to most of the experimental concentration-time profiles could be obtained to within the experimental uncertainty for two of the mechanistic options considered. In both cases it is assumed that (1) 0 2 adds to the OH-toluene adduct -75% of the time forming, after a further addition of 0 2 , a C7 bicyclic peroxy radical, and (2) this C7 bicyclic peroxy radical reacts with NO -75% of the time to ultimately form a-dicarbonyls and conjugated y-dicar- bonyls (e.g., methylglyoxal + 2-butene-1,4-dial) and -25% of the time to form organic nitrates. The major uncertainties in the mechanisms concern (1) the structure of the bicyclic peroxy intermediate, and (2) the y-dicarbonyl photooxidation mechanism. Good fits to the o-cresol concentration-time profiles in the toluene-NO, runs are obtained if it is assumed that o-cresol
* To whom correspondence should be addressed. * * Present address: Sandia National Laboratories, Albuquerque, New Mexico 87185.
International Journal of Chemical Kinetics, Vol. XII, 779-836 (1980) 8 1980 John Wiley & Sons, Inc. 0538-8066/80/0012-0779$05.80
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780 ATKINSON ET AL.
reacts rapidly with NO3 radicals. However, it is observed that the model underpredicts ni- trotoluene yields by a factor of -10, but this is in any case a minor product. It is concluded that further experimental work will be required to adequately validate the assumptions in- corporated in the aromatic photooxidation mechanisms presented here.
Introduction
Aromatic hydrocarbons are recognized to be an important constituent of anthropogenic emissions in polluted urban atmospheres [l], and, for instance, toluene levels of -250 ppb (parts per billion) have been reported in Japan [2] and levels of -40 ppb are common in the California South Coast Air Basin [3,4].
Hence, in order to develop chemical kinetic computer models of photo- chemical air pollution as an integral part of urban air shed models for the formulation of emission control strategies, the chemistry involved in the photooxidation of these aromatic hydrocarbons under atmospheric con- ditions must be reasonably well understood [5,6]. As a first step, such computer kinetic models must, as far as possible, be validated against ac- curate smog chamber data, even though it is recognized that there are sig- nificant wall effects associated with smog*chambers [7]. At the present time, while reasonably validated detailed chemical kinetic computer models exist for the n-butane (a representative short chain [IC,] alkane) and propene (a representative a1kene)-NO,-air systems [7,8], the chemistry occurring in the NO, photooxidations of the aromatic hydrocarbons is still very incompletely understood [9].
In recent years laboratory studies have shown that under atmospheric conditions the major initial reaction of the aromatics is with the hydroxyl radical [9-201, and data are now available concerning both the overall OH radical rate constants [9-181 and the relative amounts of OH radical ad- dition and H atom abstraction [9,15,16,19,20] for many of these compounds. Additionally, the reactions of ozone with the aromatic hydrocarbons, leading to the production of a-dicarbonyls [21], have been shown to be negligible under atmospheric conditions [21-241.
Several product studies of irradiated NO,-aromatic-air systems (mainly of the NO,-toluene-air) have been carried out 125-341, although quanti- tative measurements of a large variety of products and of the reactants under carefully controlled experimental conditions have not yet been made available. Although products retaining the aromatic ring, including benzaldehyde, benzyl nitrate, cresols, nitrotoluene and nitrocresols in the gas phase [25,26,28-341, and dihydroxynitrotoluenes in the aerosol phase [31], are observed, the formation of large yields of products such as PAN, CO, and HCHO in the toluene-NO, irradiations [28,31,32,34] and the recent observations of significant yields of the a-dicarbonyls glyoxal, methylgly- oxal, and biacetyl in irradiated NO, -0 -xylene-air mixtures [35,36] indicate the importance of ring cleavage.
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 781
Recently, Hendry and co-workers 181 have formulated a detailed chemical computer model for the NO, photooxidation of toluene involving aromatic ring cleavage only after cresol formation, i.e., OH radical addition to toluene yields mainly o-cresol, with the o-cresol reacting further with OH radicals to yield a variety of ring cleavage products. However, Darnall, Atkinson, and Pitts [35] have shown that the rate-determining step for the formation of the ring cleavage product biacetyl from the reaction of OH radicals with o-xylene was that for the initial reaction of OH radicals with o-xylene, and that its production from the reaction of OH radicals (or ozone) with a hy- droxyxylene would have led to an incorrect biacetyl concentration-time profile.
In this work, product studies of toluene-NO,-air, toluene-benzalde- hyde-NO,-air, and cresol-NO,-air mixtures irradiated under controlled conditions in an environmental chamber are presented, and a reaction scheme that employs the available kinetic and mechanistic information and is based in part upon a previously developed chemical computer model for the n-butane-propene-NO,-air systems 171 is formulated. Some of the detailed chemistry is, by necessity, speculative, but this reaction scheme fits the data reasonably well over a significant range of initial reactant levels.
Experimental
The apparatus and general techniques used have been described previ- ously 134,371. Irradiations were carried out in a 5800-liter evacuable, Teflon-lined, thermostatted environmental chamber [38] using a 25-kW solar simulator [38,39]. Prior to each run the chamber was evacuated to pressures of torr, and was then filled with purified matrix air [40] humidified to -50% relative humidity (RH) at 303 f 1 K. Toluene or the cresols, and NO and NO2 were injected into the chamber using precision bore syringes, while benzaldehyde was expanded into the evacuated chamber from known benzaldehyde-N2 mixtures prepared in a 5.5-liter bulb. All reactants were allowed to mix for a t least 30 min before t = 0. During a run the temperature of the irradiated mixture was maintained at the desired value by means of the chamber refrigeration or heating system.
The parameters monitored and the methods employed were: 0 3 by UV absorbance (Dasibi model 1212); NO, Nos, and NO, by chemilumines- cencel (Thermo Electron model 14B); CO by gas chromatography (Beck- man 6800); RH by a thin film, bulk effect humidity sensor (Brady Array); light intensity and spectral distribution by a photodiode and an absolute radiometer (EG & G), and by a double monochromator-photomultiplier
NO2 and NO, values are corrected for response to peroxyacetyl nitrate and alkyl nitrates ~411.
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782 ATKINSON ET AL.
combination, respectively [34]; and HCHO using an improved chromotropic acid method [34,37,42]. The absolute light intensity was determined pe- riodically using the rate of photolysis of NO2 in N2 to determine k l [43].
Benzaldehyde, benzyl nitrate, cresols, nitrotoluenes, and hydroxyni- trotoluenes were monitored by gas chromatography (GC) with flame ion- ization detection (FID) [34] using either (1) a 10-ft X l/*-in. (2-mm inner diameter) borosilicate glass column packed with 60/80 mesh Tenax GC, temperature programmed from 373 to 523 K at 20 K /min, or (2) a 6-ft X Y 4 - h . 0.1% SP 1000/Carbopack C (Supelco) column, temperature pro- grammed from 423 to 523 K at 20 K/min. In both cases 10.8- or 3.6-liter gas samples from the chamber were drawn through a '/4 X 3i-in. borosilicate glass trap packed with @in. Tenax GC 60/80 mesh. The sample was then transferred by the carrier gas a t 528 K from this trap to the column head which was at 373 or 423 K, followed by the temperature programming of the columns as noted above.
Peroxyacetyl nitrate (PAN) and methyl nitrate were monitored by GC using an 18 X l/8-in. Teflon column of 5% Carbowax 400 on Chromosorb G (SO/lOO mesh) operated at 301 K with electron capture detection (ECD) [44,45]. Toluene and the oxygenates were monitored employing a 10-ft X l/s-in. stainless steel (SS) column of 10% Carbowax 600 on C-22 firebrick (100/120 mesh) [34,37,46]. In later runs, toluene and benzaldehyde were monitored employing a 5-ft X l/8-in. SS column of 5% Carbowax 600 on C-22 firebrick (100/115 mesh) for which the sampling trap and injection valve were heated to 368-373 K [34]. Acetylene and background alkanes and alkenes were monitored on Porapak N or 2,4-dimethylsulfolane columns as described previously [34,37,46].
The reliability of the reactant and product analyses employing these techniques is estimated [34,37] to be as follows: 0 3 and NO, f5%; NO2, a t best f5%, but more uncertain because of uncertainties in the measure- ments of interfering compounds (see footnote 1) and the possibility of in- terferences from unmonitored compounds; CO, f 10%; toluene, &lo%, ar- omatic products monitored on the SP-1000 or the Tenax GC system, at best f25%; benzaldehyde on the 5 f t Carbowax 600 system, &lo%; PAN, f20%; formaldehyde, +10 to -30%; and methyl nitrate, 3~50%.
In addition, qualitative analyses of the aromatic products (benzaldehyde, benzyl nitrate, cresols, nitrotoluenes, and hydroxynitrotoluenes) were carried out using a Finnigan 3100 combined gas chromatograph-mass spectrometer (GC-MS) with a 6100 data system [47]. A 26-m X 0.29-mm i.d. glass capillary column coated with Ucon-50-HB-5100 polypropylene glycol, temperature programmed from 313 to 492 K at 8 K/min, was em- ployed for these analyses.
Product assignments were made by comparison of the retention times (FID and ECD) and/or mass spectra (GC-MS) [47] with authentic samples, wherever possible.
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NO+-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 783
Computational Methods
Most calculations were carried out at Riverside as discussed previously [7]. Input parameters consisted of the initial concentrations of the reac- tants, a reaction mechanism consisting of a list of reactions and their rate constants, or photolysis rates, which is given in Appendix A together with the relevant notes and references, and the dilution rate. The set of dif- ferential equations resulting from this reaction mechanism were integrated using the Gear algorithm [48,49]. Initial exploratory calculations were carried out on a Univac 1108 computer at Shell Research Ltd., Thornton Research Centre, Chester, U.K., using an implicit one-step method with single precision to solve stiff ordinary differential equations [50].
Reaction Mechanism
The reaction scheme used in our model calculations, along with associ- ated references and notes, is given in Appendix A. The inorganic reactions [reactions (1)-(34)] are essentially identical to those used recently by Carter et al. [7] for the NO,-n-butane-propene-air system, apart from a minor updating of rate constants. Hence, the inorganic reaction scheme will not be discussed in any detail here; [7] and Appendix A should be consulted for further details.
In previous modeling studies [7,8] of alkane- and/or alkene-NO,-air irradiations done in the same chamber as the runs reported here, it was found that model simulations could not fit the data unless the model con- tained a provision for an unknown chamber radical source. This radical source, represented by reaction (34) in our model (see Appendix A), is at- tributed to photolysis of highly photoreactive contaminants (such as ni- trous acid) offgassing from the chamber wall [7], and its rate has been generally adjusted for each run in order to fit the observed hydrocarbon consumption rates. However, for the runs considered here, the uncer- tainties in the aromatic photooxidation mechanism (see below) are such that these chamber radical input rates should not be independently ad- justed. Consequently, the chamber radical input rates were not treated as adjustable parameters in this study but were estimated based on our previous model simblations [7] of our alkane- and alkene-NO,-air irra- diations (see note 8 of Appendix A). The problem of unknown chamber radical sources is discussed in detail elsewhere [7] and is not considered further here.
The organic reaction scheme is given in Appendix A as reactions (35)- (166); reactions (46)-(53) and (70)-(77) represent overall processes whose detailed reactions are given for aromatic systems in general in Appendix B. The general reactions listed in Appendix B were applied to both the toluene-NO,-air and the o-cresol-NO,-air mechanisms, and are intended to be applicable to the NO,-air photooxidations of all substituted benzenes
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784 ATKINSON E T AL.
(see notes 14-19,26, and 27 of Appendix A and the discussion below). A number of possible alternative mechanisms, discussed below, were exam- ined, and the appendices list all reactions considered, including those that were subsequently found to give unacceptable fits to the data. Throughout the discussion, unprimed reaction numbers [e.g., (35), (36), etc.] will refer to those in Appendix A, while primed reaction numbers [e.g., (l’), (2’), etc.] will refer to specific examples of the general reactions in Appendix EL
Results
Experimental Results
Initial conditions and selected reactivity parameters for the experiments reported here are given in Table I. In order to check reproducibility, several duplicate irradiations were carried out. In those cases, very similar reac- tivity parameters were observed (especially for runs carried out with the same solar simulator lamp [34,37], which was changed between runs EC-290 and EC-327), and product yields agreed to within their stated uncertainty limits (compare runs EC-264 to EC-266 and EC-327 and EC-340).
The products observed in the toluene-NO,-air runs for which quanti- tative results were obtained (in approximate order of yield) were CO, formaldehyde, PAN, benzaldehyde, o-cresol, m-nitrotoluene, acetylene, and methyl nitrate. Also detected 1471 were small amounts of m - and p - cresol, o- and p-nitrotoluene, benzyl nitrate, and 2,3-, 2,5-, 3,2-, 3,4-, and 4,3-hydroxynitrotoluenes. In the cresol-NO,-air runs the products de- tected were CO, PAN, and 2,3- and 2,5-hydroxynitrotoluenes from o-cresol; CO, PAN, and 3,2- and 3,4-hydroxynitrotoluenes from m-cresol, and CO, PAN, and 4,3-hydroxynitrotoluene from p-cresol. The organic product yields in the cresol runs were very low, and the PAN yields were much lower than those observed in the toluene runs. The maximum yields of the major organic products observed in the toluene-NO,-air and toluene-benzal- dehyde-NO,-air irradiations are given in Table 11.
Figure 1 compares the concentration-time profiles observed for 0 3 , NO, NOz, PAN, and the reactant cresol in the o-, m-, and p-cresol-NO,-air runs. (The nonzero initial ozone values observed are believed to be due to a cresol interference on the UV absorbance 0 3 monitor.) In addition, concentration-time profiles for selected species observed in the o -cresol and in selected toluene and toluene-benzaldehyde runs are plotted in Figures 2-7, along with results of model calculations (discussed below). In general, it can be seen that the cresol runs and the added benzaldehyde runs have a lower overall reactivity than comparable toluene runs, and that rn-cresol is by far the most reactive of the cresol isomers. These results can be rationalized in terms of the mechanism which is discussed below.
Detailed data tabulations, giving concentrations of all organic and in-
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TA
BL
E I.
Initi
al c
ondi
tions
and
sel
ecte
d re
activ
ity p
aram
eter
s fo
r th
e ir
radi
ated
tolu
ene-
NO
,-air,
to
luen
e-be
nzal
dehy
de-N
O,-a
ir,
and
cres
ol-
NO
,-air
mix
ture
s.
Run
Rea
ctan
ts
(ppm
)
Tol
uene
o-C
reso
l
m-C
reso
l
p-C
reso
l
Ben
zald
ehyd
e
NO
NO2
NO
X
Pn
ysi
cal
Par
amet
ers
kl
(min
-1)
Ave
. T
erp.
(K
) A
ve.
RH (X
)
Rea
ctiv
ity
Par
amet
ers
Max
. O3
(p
pd
Tim
e 0
max
. (m
in.)
a 6-
hour
val
ue g
iven
.
7
:C-2
64 -
1.15
6
0.42
0
0.05
6
0.47
0
0.35
304.
5
42
.0
0.41
9
210 - -
EC
-265
7
1.0
70
0.43
5
0.0
48
0.4
82
0.3
5
302.
6
54.0
0.3
93
210
- -
EC
-266
-
1.19
6
0.43
2
0.05
9
0.49
4
0.3
5
302.
8
47
.5
0.4
03
215
7
I_L
EC
-269
-
0.5
66
0.3
98
0.07
4
0.47
2
0.3
5
302.
9
46.5
a ,0
.298
,360
- E
C-2
71 -
1.14
6
0.18
6
0.0
29
0.21
5
0.37
303.
0
49.5
0.29
6
90
- E
C-2
73 -
0.58
7
0.09
6
0.01
4
0.11
0
0.37
30
3.6
49.5
0.21
5
80
- E
C-2
81 -
0.39
4
0.45
8
0.02
0
0.48
0
0.35
302.
9
48.0
a PO
. 064
>36
0
c_
EC
-289
-
0.29
7
0.40
4
0.0
45
0.44
9
0.39
303.
0
56.5
0.13
7
210
_I
-
EC
-290
-
0.36
4
0.4
13
0.06
7
0.4
81
0.39
30
3.1
52.4
a 20
.068
2360
II
-
EC
-327
0.5
73
0.35
7
0.09
6
0.44
8
0.4
0
303.
4
-60
0.37
6
-360
- -
EC
-337
-
0.95
9
0.17
2
0.32
2
0.12
4
0.44
8
0.39
303.
0
65.6
0.32
5
255
I_
EC
-339
-
0.53
6
0.18
7
0.3
41
0.1
02
0.44
5
0.39
303.
4
64
.0 a
>0.
225
1360
-
3C-3
40 -
0.53
7
0.3
33
0.09
6
0.4
30
0.39
30
3.1
63.2
0.34
6
360
-
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TA
BL
E 11.
Obs
erve
d an
d ca
lcul
ated
a max
imum
pro
duct
yie
lds
in th
e to
luen
e an
d to
luen
e-be
nzal
dehy
de-
NO
,-air
runs
.
RUn
Pro
du
cts
(ppb
)
EC-2
66b
EC-2
69
EC
-271
Obs.
Cal
c.
Obs
. C
alc.
O
bs.
Cal
c
Aro
mat
ic
Ben
zald
ehyd
e
o-C
reso
l
m-C
reso
l
p-C
reso
l
o-N
itro
tolu
ene
m-N
itro
tolu
ene
p-N
itro
tolu
ene
C H
yd
rox
yn
itro
tolu
enes
Ben
zyl
nit
rate
'
PBzN
23
18
NCf
NC NC
0.2
NC
NC
14
9 60
22c
9c
2c
2c
OC
3c
4c _-
-- --
gd
7 1' 0.7'
1' lC
0.9'
6.7'
0.8'
--
--
Oth
er O
rgan
ic N
itra
tesg
yh
I --
12
10
NC
NC
NC 0.1
NC
NC
lo
9 La
17
6
13
--
--
0
.1
-- --
-- -- --
--
(e)
11
12
-_
NC NC
-- N
C
2 0.
2
-_
NC
__ N
C 10
39
-_
__ __
18
14
NC
NC
NC
<O.l
NC
NC
8 3 35
(e )
5 8
-- N
C
--
NC
-- NC
0.7
0.1
--
NC
--
NC
--
7
--
35
EC
-273
Obs
. C
alc
16d
10
38
0.2
NC
50.6
NC
0.6
NC
0.3
0
.3
0.3
N
C
NC
4 2 21
-_
__ --
-- -_
4
s
1 -- 33
I --
44
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Fra
ymen
tati
on
PAN
Oth
er p
erox
y n
itra
tesi
Gly
oxal
Met
hyl
gly
ox
al
2-B
uten
e-l.
4-di
al
For
mal
dehy
de
Met
hyl
nit
rate
'
Ace
tyle
ne'
coj
75
91
-- 67
18
21
--
75
75
31
1
0.6
500
202
--
NC
-- -_
so
52
56
--
19
lo
39
40
26
1
0.6
-300
21
1 --
NC
__ __ __
41
61
--
46
15
8
__ -- -- 57
65
26
0.6
0.
5
(ef
2.4
NC
24
32
32
--
14
--
4 __ -_
33
48
19
-- 0.
4
(e)
1.0
NC
42
47
51
-- 1
6
__
8 36
__ __ 48
23
(el
0.5
0.5
1.7
NC
a C
alcu
late
d us
ing
mod
el A
. Y
ield
s giv
en a
re m
axim
um c
alcu
late
d at
the
tim
e of
com
plet
e NO
2 co
nsum
ptio
n,
Dup
licat
e ru
ns d
one
gave
eith
er le
ss w
ell c
hara
cter
ized
pro
duct
dat
a, o
r dat
a th
at w
ere
cons
iste
nt w
ith re
sults
Onl
y se
miq
uant
itativ
e G
C-M
S da
ta [
47] a
re a
vaila
ble.
or a
t 6 h
r, w
hich
ever
cam
e fi
rst.
give
n he
re.
d C
hrom
atog
raph
ic c
olum
n em
ploy
ed c
ould
not
sep
arat
e be
nzal
dehy
de a
nd b
enzy
l ni
trat
e, s
o re
port
ed d
ata
are
benz
alde
hyde
+ be
nzyl
nit
rate
. Pr
esen
t as
a re
acta
nt in
this
run.
N
C =
not
cal
cula
ted;
assu
med
to b
e ne
glig
ible
in
the
mod
el.
g R
eact
ions
con
sum
ing
this
pro
duct
wer
e no
t inc
lude
d in
the
mod
el, s
o th
e ac
tual
cal
cula
ted
yiel
ds s
houl
d be
sm
alle
r. Pr
imar
ily C
70N
02 fo
rmed
in r
eact
ions
(46)
-(53
). Pr
imar
ily H
CO
CH
=CH
C03
N02
fo
rmed
in r
eact
ion
(112
). J B
ackg
roun
d le
vels
sub
trac
ted.
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788 ATKINSON ET AL.
% OZONE % CRESOL *
A It
* A
Q 0 m
m ' 0 I
120 zuo 360
NITRIC OXIDE =! P A N I
b Q 0
- rn Q Q x I Q i L r n 5 N
u A z 0
Q 0
* 0
A
u o 0
0 360
NITROGEN DIOXIDE
" 0 120 240 360
TIME (MINI Figure 1. Concentration-time profiles observed in cresol-NO,-air runs. (*) o cresol, run EC-281; (0) m-cresol, run EC-289; ( A ) p-cresol, run EC-290.
organic species monitored, and values of measured physical parameters as a function of time, including the spectral distribution of the photolyzing light, are available from the authors upon request [51].
Results of Model Calculations
Calculations were carried out with varying mechanistic pathways and rate constant ratios. The various mechanistic options, which are described in more detail in the discussion, concern (1) the reactions of the species formed by the addition of hydroxyl radicals and then 0 2 to the aromatic ring, (2) the reactions of the predicted conjugated y-dicarbonyl products, (3) the relative importance of the chain-terminating formation of organic
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 789
e e
e n e
. .
nitrates from the ROz + NO reaction [e.g., reactions (38) and (4')], (4) the photolysis of benzaldehyde, and (5) the reaction of o-cresol with NO3 radicals. The varying assumptions made concerning these options and the designations given to the calculations using the different sets of as- sumptions are given in Table 111.
Figures 2 and 3 show the effect of using different alternate mechanisms concerning the reactions of the OH-aromatic-02 adduct and the reactions of the y-dicarbonyl products, compared with the experimental data for a representative toluene run and the o-cresol run; and Figure 4 shows the effect of varying the relative importance of the formation of nitrate from
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TA
BL
E 111.
Sets
of m
echa
nist
ic o
ptio
ns e
xam
ined
in m
odel
cal
cula
tions
and
thei
r des
igna
tion
sym
bols
.
Mec
han
isti
c O
pt io
ns
Tri
cy
cli
c
Rad
ical
Cg
HgC
HO
k
(N0
3+
cres
ol)
D
esig
nat
ion
A
rom
OH
-02
Aro
nrO
H-O
2 RC
OCH
=CH
C03
Dec
om
po
siti
on
k(
C70
2+N
0+ c
70N
02)
Ph
oto
lysi
s Sy
mbo
l A
dduc
t Fo
rmed
C
ycl
izat
ion
R
ecy
cliz
atio
n
Cy
cliz
atio
n
Mec
hani
sm
k(C
,02+
NO
) M
echa
nism
(c
c-m
olec
ule-
1 se
c-1)
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Non
-Rad
ical
Rad
ical
No
Ph
oto
lysi
s
Non
-Rad
ical
1.5
x
1.5
x
10-l
'
1.5
x
lo-''
1.5
x
1.5
x
LO-''
1.5
x
1.5
x 10-l'
1.5
x
1.5
x
1.5
x
1.5
x
lo-''
1.5
x 0
a 3-
Add
uct:
k4
5 >>
k4
4 (t
olue
ne sy
stem
); k6
9 >>
k6
8 (c
reso
l sys
tem
); 1
-add
uct:
kp
4 >>
k4
5, k
s8 >>
k69
. R
efer
s to
stru
ctur
e of
bic
yclic
ring
syst
em. [
2.2.
2]:
k; >>
k;;
[3.2
.1]:
k; >>
k;.
Y
es:
ki6 >
> k$[
NO
] or k
;7 >>
k,[N
O] o
r k
;8 >>
k;,[
NO
]; N
o:
k;6
, k
;7, k
;8 n
eglig
ible
. Y
es:
kloe
>> (
kllo
[NO
) + kl
~z[N
Oz]
) and
klos
>> (
klll
[NO
] + k1
13[N
0~])
; No:
kl
oa, k
log
negl
igib
le.
NA
= N
ot a
pplic
able
. In
term
edia
tes
invo
lved
not
form
ed in
this
set o
f opt
ions
. N
E =
No
effe
ct.
The
sam
e in
term
edia
tes o
r pro
duct
s ar
e fo
rmed
rega
rdle
ss o
f the
ass
umpt
ions
mad
e
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NO,-AIR PHOTOOXIDATION OF
s OZONE , TOLUENE AND THE CRESOLS 791
S mTno CRESOL
2 WllRDGEN OIDXIOE
T I M E I t t l N I
Figure 3. Experimental and calculated concentration-time profiles for reac- tants and products in o-cresol-NO, run EC-281. Effect of alternate mechanisms for OH-aromatic adduct fragmentations. (*) Experimental data; (-) calculat- ed using mechanisms A-H.
the RO2 + NO reaction. From these figures it can be seen that best fits are obtained if it is assumed that the aromatic-OH-02 adducts undergo cy- clization but not recyclization (see Discussion), and that the C7 bicyclic peroxy radicals formed in the toluene and cresol photooxidations react with NO -25% of the time to give nitrates. Both calculations A (assuming ex- clusively 3-addition of 0 2 to the aromatic-OH adduct, [2.2.2] cyclization of this aromatic-OH-02 adduct, and no cyclization of the RCH=CHCOB radicals formed from the y-dicarbonyl products) and calculations F ([3.2.1] cyclization of either the 1 - 0 2 - or 3-02-aromatic-OH adduct, and rapid cyclization of RCH=CHC03) fit the data reasonably well, and the pre- dictions of these two mechanisms are compared with the data from other toluene and toluene-benzaldehyde runs in Figures 4-7. In general, cal- culations A predict an initial reactivity which is slightly high, while calcu-
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792 ATKINSON E T AL.
O Z O N E O R l M C C R E S U L
T I M E 1MlNl
Figure 4. Experimental and calculated concentration-time profiles for selected reactants and products in toluene-NO, run EC-266. Effect of varying the effi- ciency of alkyi nitrate formation from ROz + NO from 10% (A10) to 50% ( A ~ o ) . (*) Experimental data; (-, - - -) calculated.
lations F predict a slightly low initial reactivity, although in both cases the calculated initial reactivity can be made to fit experimental results by re- spectively reducing or increasing the adjustable radical input rate from chamber sources [7] (see note 8 of Appendix A).
Figure 5 shows the effect on model calculations of varying assumptions concerning benzaldehyde photolysis for a toluene-benzaldehyde run. It can be seen that in order to fi t the benzaldehyde concentration-time pro- files and the overall reactivity, it is necessary to assume that benzaldehyde undergoes photolysis at a significant rate, but that it forms nonradical products.
Figure 8 shows the effect that a reaction of NO3 with o-cresol has on the 0 3 , Nos, and o-cresol concentration-time profiles in the o-cresol run and
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 793
TOLUENE FORnRLDEHlOE
. BENZRLOCHYOE I IETHYL N I l R R l E
_ - - , ,
, 6 0 Bo , i o , b O rba ria 3 i a d o 60 I20 110 2 " l YO0 160
T I M E l l l lN1
Figure 5. Experimental and calculated concentration-time profiles for selected reactants and products in toluene-henzaldehyde-NO, run EC-337. Effect of as- suming 100% efficiency of radical production in henzaldehyde photolysis (AR), and effect of neglecting henzaldehyde photolysis (ANP). (*) Experimental data; (- , - - - ) calculated.
in a toluene-benzaldehyde run. (Predicted concentration-time profiles of other monitored reactants and products are less affected by this reaction). It can be seen that the data are much better fit by assuming that the reac- tion occurs with a rate constant on the order of (1-2) X cm3/molecule sec than by assuming that it is negligible.
Table I1 lists the experimental and calculated (using model A) yields of the major products monitored, and gives the maximum calculated yields of major products predicted by the model which were not experimentally monitored. Among the products monitored, the major discrepancy appears to be that the observed nitrotoluene yields are higher by a factor of -10 than those calculated, despite the fact that the calculations generally fit the cresol
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794 ATKINSON ET AL.
RUN 269 R U N 271
OZUNE OZONE
T I H E I M I N I
Figure 6. Experimental and calculated concentration-time profiles for selected reactants and products in toluene-NO, runs EC-269 and EC-271. (*) Experi- mental data; (-, - - -) calculated.
data and used the experimentally determined ratio (k42/k43) for cresol versus nitrotoluene formation obtained by Kenley et al. [19]. In addition, the predicted formaldehyde yields are slightly low, relative to the experi- mental data. This may be due to the neglect of glyoxal photolysis in our mechanism (see note 29 of Appendix A). For the other major products, the fits of calculated to experimental yields can be considered to be within the range of the experimental uncertainties. Although the model predicts the formation of products which were not experimentally observed [mainly C7 organic nitrates, a-dicarbonyls, 2-butene-1,4-dial, peroxybenzoyl nitrate (PBzN), and nitrophenols], we have no evidence that these species are not formed in the yields predicted. Indeed, the low carbon balance observed for these systems suggests a significant formation of unmonitored products,
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND T H E CRESOLS 795
RUN 2 7 3 RUN 339 OZONE O Z O N E
c 0 1 /' . r _ - -
P R N PRN
T I M E I H I N I
Figure 7. Experimental and calculated concentration-time profiles for selected reactants and products in toluene-NO, run EC-273 and in toluene-benzalde- hyde-NO, run 339. (*) Experimental data; (-, - - -) calculated.
and thus, except for the nitrotoluene yields (a very minor product), our model simulations can be considered to be generally consistent with the experimental data reported here.
Discussion
Initial Toluene Reactions
The only significant chemical loss process of toluene under simulated polluted atmospheric conditions is now known to be by reaction with the OH radical [8,9,11-15,17,19-211. This reaction proceeds via two pathways: H atom abstraction from the substituent methyl group [reaction (35)], and OH radical addition to the ring [reaction (41)] [8,9,12,15,17,19,20,28-331,
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796 ATKINSON ET AL.
OZONE ;1
RUN 281 RUN 337
=! OZONE
N I T R O G E N D I O X I D E x NITROGEN D I O X I D E
% ORTHO CRESOL 0
0 N
O
X 0 0
0 120 zvo 360 0-1
N
0
I c
2 ORTHO CRESOL
;̂1 / ' A N N - - -
/
?Ks, 0 0
0 120 zvo 360 0
T I M E ( M I N I Figure 8. Experimental and calculated concentration-time profiles for 0 3 , NOz, and o-cresol in the o-cresol-NO, run EC-281 and in the toluene-benzaldehyde- NO, run EC-337. Effect of neglecting the NO3 + o-cresol reaction (A"). (*) Experimental data; (-, - - -) calculated.
(35 ) OH + 0 -- @ + H,O
with k35/(k35 + h41) N 0.15 [9,15,19,20] and (1235 + 1241) N 6 X cm3/ molecule sec [9,12,13,15] at room temperature. Under atmospheric con- ditions the benzyl radical will react to form benzaldehyde and benzyl nitrate
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 797
[reactions (36)-(40)], as has been discussed previously [8,9,19,20,28-321. The reasonably good fits of the model calculations to the experimental benzaldehyde yields (see Table 11) indicate that our data are consistent with the reported ratios [9,15,19,20] for OH abstraction versus addition to the aromatic ring.
OH Radical Addition Pathway: Formation of Cresols and Nitrotoluene
The major pathway (-85%) for reaction of OH radicals with toluene is via the initial formation of an OH-toluene adduct [9,15,19,20]. By analogy with the position of O(3P) atom addition to toluene [52-541 and based on the available experimental product data [19,20,28,30-331, the OH radical is presumed to add primarily at the ortho position. The thermalized ad- duct (I) can react under atmospheric conditions with 0 2 via two pathways [9]: H atom abstraction to form the cresol [reaction (42)], or reversible addition to form the peroxy radical (11) [reactions (44) and (45)]:
CH,
(42)
(44,45)
r & + HO,
(11)
or it can react with NO2 to form the nitrotoluene:
Hendry and co-workers [19,20] concluded from their discharge flow study of the reaction of OH radicals with toluene that the cresol isomers and rn-nitrotoluene [i.e., reactions (42) and (43)] are the sole initial products of the OH radical addition pathway, and that k 4 3 / k 4 2 = (4.4 f 0.5) X lo3. If the discharge flow results [19,20] are applicable to the present experi- mental study, the only significant initial products of toluene photooxidation would be benzaldehyde and the cresols, with the latter being formed -85% of the time. Because the cresols are consumed more rapidly in the NO,-air irradiations than toluene, owing to their higher rate constant for reaction with the OH radical [9,16,18], this would mean that the chemistry of the toluene-NOx system would be dominated by that of the cresol-NO, sys- tem. However, the present smog chamber results indicate that in terms of ozone and PAN formation and NO to NO2 conversion rates, the cresol-
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798 ATKINSON ET AL.
NO,-air systems are far less reactive than is the toluene system. The ex- cess reactivity of the toluene system cannot be attributed to benzaldehyde reactions, since, as is discussed below, benzaldehyde acts as .an inhibitor [55-571. Thus, the initial formation of products other than these is nec- essary to account for the relatively high reactivity of toluene runs, and for the high yields of products such as PAN, as well as for the observation of biacetyl [35,36] and glyoxal and methylglyoxal [36] as initial products in o-xylene-NO,-air irradiations. We find that the o-cresol data are best fit by assuming that it is formed to the extent of only -20% of the total OH + toluene reaction, with products other than cresol, benzaldehyde, benzyl nitrate, and nitrotoluenes being formed -65% of the time.
The apparent inconsistency in terms of predicted cresol and nitrotoluene yields between the results of the discharge flow studies [ 191 and our smog chamber studies may be due to the fact that the former was carried out at a total pressure of -6-15 torr of (Ar + 0 2 ) [19] against a total pressure of 1 atm of air in the smog chamber studies. 0 2 addition to (I) to form (11) and the reaction of (I) with NO2 to form nitrotoluenes both may not have been in the limiting high-pressure second-order kinetic regime under the conditions of the discharge flow study [19], and thus could be significantly faster a t 1-atm total pressure. This is indeed assumed to be the case for the addition reaction of (I) with 0 2 , but the model still uses the low-pres- sure-derived value for the (I) + NO2 addition reaction [reaction (43)]. However, the fact that the model significantly underpredicts nitrotoluene yields suggests that h 4 3 is pressure dependent as well.
Structure and Reactions of the OH-Aromatic-02 Adducts
be at the 1,3, or 5 position: The structure of the radical (11) is not known; addition of 0 2 to (I) may
(44)
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND T H E CRESOLS 799
0 2 addition at the 5 position should be the least thermochemically fa- vored, since the double bonds are nonconjugated. Addition a t the 1 posi- tion [reaction (44)] is estimated [58] to be slightly more favored thermo- chemically over addition to the 3 position [reaction (45)], although the re- sulting radical (IIa) will also have the most steric hindrance. In the cal- culations the consequences of assuming exclusively l addition or exclusively 3 addition are both considered.
Based on thermochemical estimates [15,58], 0 2 addition to radical (I) is expected to be only -10 kcal/mol exothermic. This means that signifi- cant back decomposition of radicals (11) to radical (I) may be occurring, which could account for the relatively low effective rate constant for 0 2
addition to radical (I) required to fit the cresol data (see note 13, Appen- dix A).
A t the present time, little is known about the other reaction pathways of the OH- aromatic - 0 2 adducts. There are a number of possible alter- native pathways, and these are listed for a general aromatic adduct in Ap- pendix B and are discussed below for the case of adduct (IIb) (substituent 6 in Appendix B = CH3, substituents 1-5 = H). Reactions of the other adducts are assumed to be analogous.
If the OH- aromatic - 0 2 adducts react in a manner analogous to other peroxy radicals, the following scheme would appear to be the most rea- sonable:
( IIh) followed by either reaction with 0,
or ring opening &) P H - C H t -% HCOC(CH,)=CH-CH=CH-CHO + HO,
0.
However, this scheme is inconsistent with the observation of a-dicarbonyls as primary products in the NO,-o-oxylene-air system [35,36], and no reasonable mechanism accounting for those observations [35,36] could be derived without assuming formation of bicyclic radicals. It is thus assumed
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800 ATKINSON E T AL
that the OH- aromatic - 0 2 adduct primarily undergoes cyclization reac- tions such as those shown below:
(7')
(IIIC)
In addition to accounting for the ring cleavage products observed, these reactions are expected to dominate on the basis of thermochemical and kinetic estimates. If we assume that the rate constant for the reaction of the peroxy radical (IIb) with NO is -8 X cm3/molecule sec (see note 9 of Appendix A), then under realistic atmospheric conditions [[NO] < 1 ppm (2 X 1013 molecule/cm3)] cyclization should dominate if its rate con- stant is >160 sec-'. If we further assume that (1) the A factors for the cyclization reactions are at a minimum sec-l[58], (2) the activation energy for cyclization is [S9 kcal/mol (based on the activation energies of bimolecular radical + double bond addition reactions [59]) + the reaction endothermicity (if any)], and (3) AH (cyclization) (ring strain -14) kcal/mol, based on thermochemical calculations using group additivity principles [58], then cyclization should dominate if the ring strain of the bicyclic radicals is less than -16 kcal/mol. Although such ring strains are not known, this would appear to be likely [58].
Of the radicals (1IIa)-(IIIc), (IIIa) will probably have the least ring strain, while (IIIb) will have some resonance stabilization energy by virtue of its allylic character. Since we cannot determine, a priori, which of these structures is favored, it is again necessary to consider both options in the model calculations, i.e. (IIIa) and (IIIb).
These cyclized aromatic-OH-02 adducts are then expected to add 0 2
to form radicals of the type:
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 801
(IV) (Va) ( V h )
which could react with NO to either form an alkoxy radical or yield a stable nitrate (analogous to the >-C4 alkylperoxy cases [7,60], for example:
( VII )
The reactions of the bicyclic peroxy organic nitrates such as (VII) are unknown; they may undergo molecular decomposition to form HN03 or other organic nitrates. In our model we assume that their reactions, if any, have a negligible effect on the overall mechanism and are thus ignored. The bicyclic peroxy alkoxy radicals such as (VI) are expected to undergo a series of highly favorable 0-scission fragmentations, resulting in the formation of a-dicarbonyl and conjugated y-dicarbonyl products. For example, (VI) is assumed to decompose as shown, with the other isomers, resulting from the reaction of NO with (Va) and (Vb), reacting analogously:
HO 0. I I
CH-C--CHJ - CH,COCHCHCH=CHCHO -+
(VI) I ( 5’) H CH ,COCHOH + HCOCH=CHCHO
( 6’) CH,COCHO + HO,
The relative importance assumed for nitrate formation versus alkoxy radical formation has an important effect on model simulations, since the former process is radical terminating and is an effective nitrogen sink, while the latter is radical propagating. This effect is shown in Figure 4, where
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802 ATKINSON ET AL
k4fl(k3t + k 4 r ) (and also the analogous rate constant ratios in the cresol system and for the benzyl peroxy radical) is varied from 0.1 to 0.5. It can be seen that toluene-NO,-air data are best fit by assuming that nitrate formation occurs -25% of the time; this is true for the toluene-benzalde- hyde-NO,-air and the cresol-NO,-air systems as well. Assuming that nitrate formation is negligible results in a significant overprediction of ozone maxima regardless of the other mechanistic options used.
A possibility, which must also be considered, is that the bicyclic peroxy radicals may further cyclize, such as shown below 0 2 adduct to radical (IIIb)]:
[for radical (IV) and the
If formed, these doubly cyclized radicals should react with 0 2 and then NO, forming alkoxy radicals or alkyl nitrates by reactions analogous to those shown above for (IV). The tricyclic bridged-peroxy alkoxy radicals are also expected to rapidly fragment via consecutive @-scissions:
0 O O H O H 11 HO 0.
I I I
- (CHO), + HCOCHCCHO
CH, OH I
H C ~ H C O C H ~ + HCO “PP’ / HCO~HCCHO
I bHJ ‘m
HCOCHOH + CH,COCHO (VIII) I
k ( C H O ) , + HO,
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 803
The relative importance of the two possible modes of decomposition [e.g., reactions (31’) and (33’)] of species such as (VIII) is not known, so both possibilities are considered.
As can be seen from Figures 2 and 3, the data do not clearly indicate which aromatic-OH-02 adduct is formed, or whether it undergoes [2.2.2] or [3.2.1] cyclization, but they do indicate that recyclization probably is negligible. If recyclization occurs, the assumption made concerning the fragmentation of radical (VIII) is important [Assuming that reaction (33’) dominates results in overprediction of reactivity in the toluene and the o -cresol systems, while assuming that reaction (31’) dominates results in the reactivity in the toluene system being underpredicted, but gives rea- sonably good fits to the o-cresol system.] It is clear, however, that no ad- justment of the k311lk33’ ratio would allow both the toluene-NO, data and the o-cresol-NO, data to be fit by models assuming recyclization. Model A assumes [2.2.2] cyclization and predominant addition of 0 2 to the 1 po- sition of the OH-aromatic adduct, although 3 addition should not be completely negligible. If [3.2.1] cyclization occurs, as assumed in the other model which gives reasonable fits to the data (F), formation of the 1 adduct or the 3 adduct is immaterial, since each adduct cyclizes to form the same intermediate [i.e., radical (IIIb)].
The aromatic ring-opening mechanisms that best fit the toluene and o-cresol data, when applied to the o-xylene system, yield biacetyl after OH radical addition a t the 1 or 2 position; for example,
&H & [ CHBCOCOCHj H_ OH + 0 2 -.+ OH -% $- CHOCH=CHCHO
This is, as noted previously, in accord with the recent observation of sig- nificant yields (18 f 4%) of biacetyl from irradiated NO,-o-cresol-air systems [35], and the magnitude of the biacetyl yield is then consistent with the mode of O(3P) atom addition to o-xylene [53].
Benzaldehyde Reactions
The reaction of benzaldehyde with OH radicals must, because of the magnitude of the OH radical rate constant [56], proceed predominately via H atom abstraction from the substituent CHO group [8,9,20] to form the benzoyl radical:
CHO CO I I
(54) OH + @) -+ H,O + 0
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804 ATKINSON ET AL.
This CGH5CO radical is then expected to add 0 2 [reaction (56)] to form the peroxybenzoyl radical which can react with NO or NO2. Reaction with NO2 [reaction (57)] will lead to the formation of peroxybenzoylnitrate (PBzN), which was not observed, but which may have been formed and not detected. PBzN thermally decomposes [61] with an analogous mechanism to PAN [62-641, to lead to the ultimate formation of, among other products, o- and p-nitrophenols [reactions (58)-(65)] [57]. Note that in each case the ultimate result of the OH-benzaldehyde reaction is radical termination, which is consistent with these and previously reported observations [55- 571.
While little is known about benzaldehyde photolysis under atmospheric conditions, its photolysis at rates approximately five times greater than that used for the aliphatic aldehydes [7] is included in the mechanism to account for the observed benzaldehyde loss rates in the toluene-benzal- dehyde runs (see note 20 of Appendix A). As noted previously (see Fig. 5), the data are best fi t by assuming that benzaldehyde photolysis results in the formation of nonradical products. The most likely pathway would thus appear to be formation of benzene + CO, but although small yields of benzene (-0.5-1 ppb) are indeed observed in the toluene-benzaldehyde runs, the yields are far less than the amounts predicted (50-60 ppb) if it were the major product of benzaldehyde photolysis.
Cresol Reactions
The cresols, represented in this model by o -cresol alone (substantiated to a large extent by the smog chamber data reproduced here), are known to react rapidly with OH radicals [9,16,18], with this reaction being a major cresol loss process under simulated atmospheric conditions [HI. The slow reaction with 0 3 [18,20] has been neglected in the present model since it will contribute 15-10% of the total cresol loss rate under the experimental conditions employed in the NO,-toluene-air and NO,-o -cresol-air irra- diations. The irradiated NO,-o- cresol-air data yield little evidence as to the reaction mechanisms-very little PAN is formed, and the major organic products observed are hydroxynitrotoluenes in low (<5%) yields. The reaction of OH radicals with o-cresol can proceed via OH radical addition to the ring or by H atom abstraction [9,16].
The abstraction pathway, which accounts for -8% of the total OH radical reaction [9,16], probably proceeds mainly via H atom abstraction from the substituent-OH group, since the abstraction rate constant for o-cresol is a factor of -4 higher a t 400 K than that for toluene [15,16].
p 3 $a
OH + & -+ H,O + &"
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 805
Reactions of NO2 with radical (IX) can, by analogy to the phenoxy radical reaction (64) [57], lead to formation of the observed hydroxynitrotoluene isomers:
This minor H atom abstraction pathway has been neglected in the model calculations since its inclusion would have an insignificant effect on the calculated time-concentration profiles of the major species.
The major OH radical reaction pathway is via addition to the aromatic ring [9,16], and thermochemical calculations [58] show that radical (X) is the most thermochemically favorable adduct. Its formation is thus as- sumed to be the sole pathway:
(65) OH + 0 (XI
Species such as (X) can also react with NO2 to give rise to the observed hydroxynitrotoluene isomers:
-- -H,O 0 0 2
(IX)
However, the dominant reaction of (X) is expected to be with 02:
The subsequent reactions of radicals (XI) are assumed to be analogous to those of radical (11) formed in the toluene system. The various possible
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806 ATKINSON ET AL.
detailed reactions for these species are listed in Appendix B (substituent 1 = OH, 2 = CH3 for the l-adduct; substituent 1 = OH, 6 = CH3 for the 3-adduct), and the overall processes are given in Appendix A as reactions (70)-(77). The best-fit models predict that the major overall processes are as shown below:
HO, + CH,COCO,H + HCOCH=CHCHO or
HO, + 1/2(HCOCO,H + CH,COCH=CHCHO) + 1/2(CH,COCO,H + HCOCH=CHCHO)
I,( There is evidence that there is an additional significant loss process for
cresols in NO,-air systems. It has been independently observed by O'Brien [65] and in our laboratories [66] that although the cresols do not react at significant rates with either 0 3 [18,20,65] or NO2 [65,66], they do appear to rapidly disappear in chambers dosed with NO2 and 0 3 together. Since O3 and NO2 react rapidly to form NO3 and N205 [67],
(5) 0 3 + NO2 + NO3 + 0 2
these observations can be attributed to rapid reaction of cresols with either NO3 or N205. Since NO3 is known to react with alkenes [68], it is probable that it is also NO3 which is reacting with the cresols. We have recently obtained, using a relative rate technique, a rate constant for the reaction of No3 radicals with o-cresol of -1.5 X 1O-I' cm3/molecule sec [66]. As can be seen in Figure 8, this rate constant is sufficiently high that reaction with NOs becomes a significant sink for o-cresol after 0 3 formation begins, and accounts for the rapid decrease in the o-cresol concentrations observed after their initial rise in the toluene-NO,-air runs. The observation that
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NOr-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 807
NO3 radicals react rapidly with phenol and the cresols, but only very slowly with toluene and methoxybenzene [66], indicates that the reaction with phenol and the cresols proceeds via H atom abstraction from the substit- uent -OH group to form radical (IX) [66]:
Hence, this reaction is another explanation for the formation of the hy- droxynitrotoluene isomers observed.
As noted previously, the order of the overall reactivity of the cresol iso- mers in NO,-air systems is meta >> para > ortho (see Fig. 1). This effect is larger than can be attributed to the relatively small difference in the OH + cresol rate constants [9,18] (meta/ortho = 1.4; para/ortho = l.l), but it can readily be rationalized in terms of the mechanism. On the basis of the mechanisms which best fit the toluene-NO, and the o-cresol-NO, data, m- and p-cresol are expected to react as shown:
1
&OH 4 OH
&; -
OH or
00 &; H -
CH I R
O?, NO -4
HO, + HOCWHO +
HCOC=CHCHO or
CH3COCH= CHCHO
HOCOCHO 9 +oH-HoQoH 4 @to.= + OH HO OH
CHOC02H is expected to be more reactive toward OH radicals than the CH3COCOzH formed in the o-cresol system, since it has the labile -CHO
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808 ATKINSON ET AL.
group. In addition, both 2-methyl-2-butene-l,4-dial and 2-pentene- 1,4-dial are expected to give rise to methylglyoxal, a powerful photoinitiator, in their photooxidation mechanism [see Appendix A, reactions (100)-( 141)], which is not formed from the 2-butene-1,4-dial predicted in the o-cresol mechanism [the latter instead forming glyoxal, which is believed to have far lower quantum yields for photodecomposition (see note 29 in Appendix A)]. Furthermore, in the m-cresol system, the OH radical may add to a significant extent at the 2 position, since the radical formed is stabilized by both the OH and the -CH3 groups, unlike analogous adducts for the other cresol isomers. Applying our mechanism, this adduct is expected to give rise to methylglyoxal directly, which may explain the far greater reactivity of m-cresol relative to the 0- and p-isomers.
Subsequent Reactions of Ring-Opening Products
The subsequent reactions of the ring cleavage products formed in the toluene and the cresol photooxidations are given in Appendix A. In many cases the mechanisms are uncertain and rate constants have had to be es- timated; these are discussed in the associated notes. The following points concerning the reactions of these products should be noted:
In order for the model to fit the smog chamber data, it must include the formation of radical initiator(s) in the toluene photooxidation mech- anism which are not formed in the o-cresol system. The major initiator appears to be methylglyoxal, whose photolysis is also primarily responsible for the observed production of PAN, HCHO, and CH30N02. No other product predicted by our mechanism is believed to photodecompose to give radicals as rapidly as methylglyoxal (see notes 20,29, and 33 in Appendix A for brief discussions of benzaldehyde, glyoxal, and conjugated y-dicar- bony1 photolyses, respectively).
There is a significant uncertainty in the photooxidation mechanism of the conjugated y-dicarbonyls which are predicted to be formed as major products. These products are expected to react predominately with OH radicals, giving rise to unsaturated a-carbonyl peroxy radicals:
(1)
(2)
OH + RCOCH=CHCHO - H20 + RCHOCH=CHCO RCOCH=CHCO + o2 % RCOCH=CHCO~)
(XI11 These radicals could react with NO and NO2 [e.g., reactions (110)-(113)], in a manner analogous to the reactions of acyl peroxy radicals, forming unsaturated PAN analogs such as HCOCH=CHC03NO2. However, these unsaturated peroxy radicals may also undergo cyclization:
RCO 6H
RCOCH=CHCO,* - >C’ ‘CO LO-0’ (XQ
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 809
resulting in the ultimate formation of glyoxal (R = H) or PAN + methyl- glyoxal (R = CHB) (see Appendix A). It is not known at the present time whether this cyclization is rapid enough to compete with reaction of (XII) with NO or NO2. The recent observation of methylvinylketone and methacrolein formation during the early stages (before significant ozone formation) of the NO, photooxidation of isoprene [69] indicates that in the case of the unsaturated alkyl peroxy radicals, (XIII), cyclization is negligible compared to reaction with NO:
C H 2 4 R - E* '--CH,OH (R= H or CH; R' =CH3 or H)
(xm)
However, it is not clear that the unsaturated acylperoxy radicals such as (XII) formed in this system will react analogously, and hence it is necessary to consider both mechanistic options.
As can be seen in Figures 2 and 3, where results of calculations that as- sume cyclization of (XII) is rapid are compared with those that assume that it is negligible (compare model A with B, C with D, or E with F), the un- certainty concerning the rate of this cyclization has a significant impact on model predictions concerning both the toluene and the cresol systems. It is this uncertainty which does not allow us to distinguish unambiguously whether these data are best fit by assuming l2.2.21 cyclization and pre- dominant 3 addition of 0 2 to the OH-aromatic adduct (calculations A) or by assuming that [3.2.1] cyclization dominates (calculations F), since the former assumes that cyclization of (XII) is negligible, while the latter as- sumes that it dominates. If the rate of this cyclization were known, one of these mechanisms could probably be eliminated.
Conclusions
The chemical kinetic computer models for irradiated toluene-benzal- dehyde-o -cresol-NO,-air mixtures, which are discussed and are consistent with available basic kinetic and mechanistic data and with reasonable thermochemical and kinetic estimates, have been shown to be capable of giving predictions of reactant and major and minor product concentra- tion-time profiles which are in reasonably good agreement with results of well-characterized smog chamber irradiations performed with a variety of initial reactant concentrations. However, with the information available to date, it is not possible to choose between various optional sets of equally reasonable reaction sequences. For example, the experimental data appear to be reasonably well fit by the two sets of mechanistic options discussed. In addition, in our initial studies it was found that most of the concentra- tion-time profiles, except for those of 0-cresol, could be equally well fit by
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810 ATKINSON ET AL.
a model giving the same products as A, but assuming that organic nitrate formation from ROe + NO occurs -40% of the time instead of -25%, pro- viding that fragmentation is increased by decreasing the formation of o- cresol from -20 to -8% of the overall reaction, and that the resulting in- creased production of PAN is compensated by changing the ratio K(CH3C03 + NO)/h(CH3C03 + NOz) from 1.5 to 2.0, which is within its range of uncertainty [7].
These chemical kinetic models predict the formation of products and reactive intermediates in aromatic-NO, -air photooxidations which have not yet been detected in such systems, and whose atmospheric chemistry is a t best poorly characterized. Such products predicted in the toluene system include the a-dicarbonyls glyoxal and methylglyoxal, and the y-unsaturated dicarbonyls RCOCH=CHCHO (R=H, CH3) and possibly the corresponding peroxyacyl nitrates RCOCH=CHCO3N02, together with the alicyclic nitrate which is proposed to be formed from the OH- toluene-02 adduct and NO; for example,
Furthermore, quantitative rate and mechanistic information is required for the photolysis of benzaldehyde and the a-carbonyl carboxylic acids, the fraction of the alkyl nitrate from the peroxy radical intermediates in the aromatic photooxidation mechanism, and the general atmospheric chemistry of a-dicarbonyls and y -unsaturated dicarbonyls.
Acknowledgment
Much of the initial modeling work on the toluene-NO, and o-cresol-NO, systems was carried out while one of the authors (R.A.) was at Shell Re- search Ltd., Thornton Research Centre, Chester, England. The authors wish to thank Dr. A. Glangetas for performing the GC-MS analyses, G. C. Vogelaar and F. R. Burleson for carrying out the gas chromatographic analyses, and W. D. Long for valuable assistance in conducting the chamber experiments. This work was supported by the National Science Founda- tion Research Applied to National Needs under Grant No. ENV73- 02904-A04, the U.S. Environmental Protection Agency under Grant No. 800649-19, and the California Air Resources Board under Contract No. A7 -17 5 -30.
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APP
END
IX A
. R
eact
ions
and
rate
con
stan
ts u
sed
in th
e N
O,-t
olue
ne-a
ir m
odel
.
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e co
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APP
END
IX A
(C
onti
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from
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APP
END
IX A
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NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 815
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z A
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HO
+ 112
HC
OC
H=C
HC
HO
+ 1
12 C
H&
OC
H=C
HC
HO
) + 0
.25
C70
NO
z
oo.
+ 112
HO
CO
CO
CH
3
H
NL),4
_ 0.
75(H
CO
+ N
O2
+ (C
HO
h + C
H3C
OC
(OH
hCH
O)
(74)
c + 0
.25
~~
0~
0~
(Slo
w)
Mec
h. A
-E,G
,H
Fast
M
ech.
E,F
17
,27
(Slo
w)
Mec
h. A
-F,H
Fa
st
Mec
h. G
18
,26
(Slo
w)
Mec
h. A
-F,H
Fa
st
Mec
h. G
18
,27
![Page 39: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/39.jpg)
oo@
z -
Ny
), 0,
0 75
(H02
f N
O2
+ 3/2
(CH
0)2
f 1
/2 C
HR
CO
CH
O
(CH
0)z + h
u - Pr
oduc
ts
HC
OC
O - H
CO
+ C
O
OH
+ (C
H0)
z + H
zO +
HC
OC
O
HC
O +
02
4 HO
z + C
O
D.
Met
hyl g
lyox
al (a
nd it
s pro
duct
s)
(84)
(86)
(8
7)
CH
3CO
CH
0 + h
u - CH
3C0
+ HC
O
CH
3CO
C0 - C
H3C
0 + C
O
CH
3C0
+ O2 - C
H3C
03
(85)
O
H +
CH
3CO
CH
O +
H20
+ CH
3CO
CO
(88)
(8
9)
CH
3C03
+ N
O - C
HsC
Oi +
NO2
CH3C
Os +
NOz - CH
3C03
N02
(Slo
w)
Fast
(Slo
w)
Fast
(Slo
w)
2 x
10-1
1 Fa
st
5.6
x 10
-l2
1.0
x 10
-3
1.6
X lo
-" Fa
st
Fast
8
x 1O
-lZ
5 x
10-1
2
Mec
h. A
-G
Mec
h. H
Mec
h. A
-G
Mec
h. H
1.5
X lo
-"
1.6
X lo
-"
19,2
6
19,2
7
28
6 29
30
1,6
30
6 6 21
21
![Page 40: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/40.jpg)
APP
END
IX A (C
ontin
ued
from
prev
ious
pag
e.)
2 03
Rat
e co
nsta
nt
cm m
olec
ule
sec
Rea
ctio
n un
itsav
b
(90)
(9
1)
(92)
(9
3)
(94)
(9
5)
(96)
(9
7)
(98)
(9
9)
CH
3C03
N02
- CH
3C03
+ N
O2
CH
3CO
i - CH
; + C
O2
CH; + 0
2 - CH
302
CH@
2 + N
O - C
H30
+ NO
2 C
H30
+ NO
2 +
C
H30
N02
C
H30
+ NO
2 - HC
HO
+ H
ON
O
CH
30 +
02
- HC
HO
+ H
Oz
HC
HO
+ hv
-H
+ HC
O
HC
HO
+ hv +
H2
+ CO
O
H +
HC
HO
+ H
20 +
He
0
E. C
onju
gate
d y-
dica
rbon
yls
(100
) (1
01)
(102
),( 10
3)d
(!04)
,(105
)d
(106
),(10
7)d
RC
OC
H=C
HC
O
+ 02
+RC
OC
H=C
HC
&
(108
),(10
9)d
OH
+ HC
HO
=CH
CH
O - Hz
O +
HC
OC
H=C
HC
O
OH
+ CH
3CO
CH
=CH
CH
O +
HzO
+ 0
3 + R
CO
CH
=CH
CH
O - Pr
oduc
ts
RC
OC
H=C
HC
HO
+ h
v - Pr
oduc
ts
CH
3CO
CH
=CH
CO
M
RCOC
H=CH
CO, +
R C
H ;h0
(110
),(11
1)d
(112
),(11
3)d
RC
OC
H=C
HC
&
+ NO
2 +
RC
OC
H=C
HC
&
+ NO
- NO
2 +
RC
OC
H=
CH
C~
~
M
RC
OC
H=C
HC
03N
02
8.0
x 10
-4
Fast
Fa
st
8 X
10-
l2
1.3
x lo-
" 2.
1 x
10-1
2 1.
2 x
10-1
5
4.1
x 10
-5
9.4
x 10
-12
3 x
10-1
1 1.
6 X
lo-
"
(Slo
w)
(Slo
w)
Fast
(Slo
w)
Fast
8 X
5 x
10-1
2
Not
e R
ef.
163,
641
6
Mec
h. A
,C,E
M
ech.
B,D
,F
30,3
1 30
.31
32
33
34
21
21
m 4
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n
n
(1 14
),( 1 1
5)d
( 1 16
),( 1 17
)d
(120
),(12
1)d
0
n
0
0
0
0
II R
C mod
:< iko -. RCO
CH(O
.)CH
O +
CO,
n H
CO
CH
(O.)C
HO
- HC
O +
(CH
0)Z
CH
&O
CH
(O-)
CH
O - 1
/2(C
H&
O +
(CH
0)z)
t 1
/2 (C
H3C
OC
HO
+ H
CO
) R
CO
CH
=C
HC
~~
- RCO
CH
=CH
+ c
o2
M
RC
OC
H=C
H
+ 02
-+ R
CO
CH
=CH
Or
RC
OC
H=C
HO
z + N
O +
NO
2 +
RC
OC
H=C
HO
. - RC
OC
HC
HO
M
R
CO
CH
=CH
Oy
+ NO
+ R
CO
CH
=CH
ON
02
M
RC
OC
HC
HO
+ 02
-+ R
CO
CH
(Oz-
)CH
O
RC
OC
H(0
2*)C
HO
+ NO
- RC
OC
H(O
*)C
HO
+ N
O2
RC
OC
H(0
y)C
HO
t N
O -+
RC
OC
H(O
N02
)CH
O
M
Fast
s x 1
0-1
3
9,35
Fast
Fast
Fa
st
Fast
Fast
7 x
10-1
2
s x 1
0-13
Fast
s x 1
0-13
6 6 6
9,35
9,35
P
2
rj co
W
)-L
![Page 42: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/42.jpg)
APP
END
IX A
(C
onti
nued
from
pre
viou
s pa
ge.)
Rat
e co
nsta
nt
cm m
olec
ule
sec
00
N
0
Rea
ctio
n N
ote
Ref
.
(138
) (1
39)
(140
),( 14
1)d
HC
OC
H(0
-)C
HO
- (CH
0)z +
HCO
C
H&
OC
H(O
-)C
HO
- 1/2
(CH
3CO
+ (C
H0)
z)
RC
OC
H=C
HC
03N
Oz
4 R
CO
CH
=CH
C03
+ N
O:!
+ 1/2
(HC
O +
CH
3CO
CH
O)
F. p
-Hyd
roxy
-P-d
icar
bony
ls
(142
)
(143
)
(145
), (14
6)"
CH
&O
CH
(OH
)CH
O +
OH
- CH3C
OC
(OH
)CH
O +
CH
3CO
C(O
Il)C
HO
+ 0
2 - H
Oz
+ CH
3CO
CO
CH
O
CH
&O
CX
(OH
)CH
O +
OH - C
H&
OC
X(O
H)C
O
CH
3CO
CX
(OH
)C03
+ N
O - C
H~C
OC
X(O
H)C
OZ
HzO
(144
) C
H3C
OC
OC
HO
- CH3C
OC
HO
+ CO
M
(147
),(14
8)"
CH
~CO
CX
(OH
)CO
+ o2 +
CH
~C
OC
X(O
H)C
~~
(1
49),(
150)
" (1
51),(
152)
" C
H3C
OC
X(O
H)C
O3 +
NO2 - CH
3CO
CX
(OH
)-
C03
N02
(1
53),(
154)
' (1
55),(
156)
" (1
57),(
158)
" C
H&
OC
X(O
H)C
03N
Oz +
CH
3CO
CX
(OH
)C&
+
CH
3CO
CX
(OH
)CO
z - CH
3CO
CX
OH
+ CO
z C
H3C
OC
XO
H +
02
- HOz
+ CH
3CO
CO
X
N0z
G
. a-C
arbo
nyl a
cids
(1
59)
(161
)
HO
CO
CH
O +
OH
- HO
CO
CO
+ H
zO
HO
CO
+ 0
2 - HO
z + C
On
(160
)
(162
),(16
3Id
HO
CO
CO
- HOC
O +
CO
HO
CO
CO
R +
hu - Pr
oduc
ts
+ OH -
Plhd
ucts
+ H
Oz
@Lo
*
Fast
Fa
st
8.0
x 10
-4
6 X
10-
l2
Fast
Fa
st
1.6
X lo
-" Fa
st
8 X
10-
l2
5 x
10-1
2
Fast
Fa
st
8.0
x 10
-4
1.6
X 10-l:
Fast
Fa
st
(Slo
w)
-6 X
6 6 36
37
38
30
21
21
6 36
38
30
39
40
![Page 43: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/43.jpg)
IV.
Hyd
rope
roxi
des
(165
) R
Oz‘
+ H
Oz
-+
RO
zH +
02
3 x
10-1
2 6
(166
R
OzH
f + hu
- OH
+ H
Oz
+ Pro
duct
s 5
x 10
-6
6
a R
ate
cons
tant
s ar
e gi
ven
for t
he c
ondi
tions
of t
he sm
og c
ham
ber
runs
list
ed in
Tab
le I
(303
K, 7
35 to
rr, M
= a
ir).
Ph
otol
ysis
rate
con
stan
ts a
re
give
n fo
r the
ligh
t int
ensi
ty a
nd sp
ectr
al d
istr
ibut
ions
ass
ocia
ted
with
runs
EC
-327
-EC
-340
[34
]; pho
toly
sis
rate
s for
runs
EC
-266
-EC
-281
are
som
ewha
t lo
wer
. Firs
t-or
der
reac
tion,
sec
-I;
seco
nd-o
rder
reac
tion,
cm
3/m
olec
ule
sec;
thir
d-or
der
reac
tion,
cm
6/m
olec
ule2
sec;
zer
o-or
der r
eact
ion
[rea
ctio
n (3
4)
only
], m
olec
ule/
cm3
sec.
“F
ast”
mea
ns t
hat t
he re
actio
n is
the
sole
pat
hway
for t
hat p
artic
ular
spe
cies
; the
pre
dict
ions
are
inse
nsiti
ve f
or th
e ac
tual
ra
te c
onst
ant.
“(Sl
ow),”
or r
ate
cons
tant
s giv
en in
par
enth
eses
mea
ns th
at th
e re
actio
n is
ass
umed
to
be n
eglig
ible
or t
hat t
he re
actio
n ha
s a
negl
igib
le
effe
ct o
n th
e pr
edic
tions
of t
he m
odel
for t
he c
once
ntra
tion
regi
mes
stu
died
her
e.
The
reac
tion
as w
ritte
n re
pres
ents
an
over
all p
roce
ss fo
r whi
ch t
he d
etai
led
reac
tions
invo
lved
are
giv
en in
App
endi
x B
. R
eact
ion
num
bers
refe
r to
R =
-H
and
-CH
3,
resp
ectiv
ely.
R
eact
ion
num
bers
refe
r to
X =
-H
and
-OH
, re
spec
tivel
y.
R0z
refe
rs t
o al
l rad
ical
s of
the
form
RO
z or
RC
03 in
the
mec
hani
sm.
RO
zH is
trea
ted
as a
sing
le s
peci
es.
P
Z d 4
T m
![Page 44: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/44.jpg)
APP
EN
DIX
B.
Rea
ctio
ns a
nd r
ate
cons
tant
s use
d in
the
tolu
ene-
NO
,-air
m
odel
: ge
nera
l aro
mat
ic-O
H-0
2 ad
duct
reac
tion
mec
hani
sm.
00 z
Rea
ctio
n R
ate
cons
tant
cm
mol
ecul
e se
c un
itsa.
b N
otes
C
[2.2
.2] C
ycliz
atio
n
[2.2
.2]
Cyc
lizat
ion,
no
recy
cliz
atio
n
(5')
0
OH
I1
I HK), +
6-C-C
-1
sc-c
-1
+ 0, -
Fast
M
ech.
A-D
, (G
,H)d
(S
low
) M
ech.
E,F
Fast
Rat
io
adju
sted
6 x
10-l
2
2 x
10-1
2
Fast
Fas
t
9 38
![Page 45: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/45.jpg)
[3.2
.1] C
ycliz
atio
n
4
(Slo
w)
Mec
h. A
-D
Fast
M
ech.
E,F
,(G,H
)d
on
OH
OH
(8')
&$
u
0,
-+
112
'&&s
+ 11
2 5g
Fast
00'
.OQ
(3
.2.1
1 C
ycliz
atio
n, n
o re
cycl
izat
ion
Rat
io
9 ad
just
ed
(9')
'6
3 +
NO+'&
s +
NO,
6 X
lo-''
00.
0'
(10'
) 2
x 10
-12
00
' O
NQ
,
(12'
) 5 &
OH+
NO
-
00
Rat
io
9 ad
just
ed
6 x
lo-'*
2 x
lo-'?
W
N
w
![Page 46: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/46.jpg)
APP
END
IX B
(Con
tinue
d fr
om pr
evio
us p
age.
) 00
to
rp
Rea
ctio
n R
ate
cons
tant
cm
mol
ecul
e se
c un
itsa,
b N
otes
C
OH 0
I
II - 1-
c--c
-2
+
HO
0
~~
.
I II
(14')
1-C-C
-2 +
0, --f
HQ
, +
Fast
Fast
38
0
OH
Rec
ycliz
atio
n (16'
)
Fast
(Slo
w)
Fast
[Rea
ctio
n (6
'11
Mec
h. A
-F
Mec
h. G
,H
![Page 47: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/47.jpg)
NO,-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 825
t t
* m a
N 3
l s X W
6 z +
4 6 + m
z
3 O W o .O
4 0" +
t 0 z +
N 3
I
X s N
4 + m
![Page 48: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/48.jpg)
APP
END
IX B
(Con
tinue
d fr
om p
revi
ous p
age.
)
w R
eact
ion
Rat
e co
nsta
nt c
m m
olec
ule
sec
units
arb
Not
esC
a
(24'
)
6 X
1OI2
2 x
10'2
,f
Fast
0
OH
0
0
It I
I II
I1
- 6-
C-C-
C-C-
3 f
12
Rat
io
9 ad
just
ed
![Page 49: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/49.jpg)
NO,-AIR PHOTOOXIDATION OF TOLUENE AND T H E CRESOLS 827
m Ill
g*G 3
0
t
t
LO
t 0
N b m O
@'* 3
111 ""9
b -x II
-cz In N
L PI ""7 +
I "=Y "?
t
![Page 50: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/50.jpg)
3 A
PPEN
DIX
B (
Con
tinue
d fr
om p
revi
ous p
age.
) 00
Rea
ctio
n R
ate
cons
tant
cm
mol
ecul
e se
c un
itsa,
b N
otes
C
0
OH
0
0
II
II
U
II
(3
2')
6-C-C
-C-C
-3 --+
12
0
o o
ono
II
- 5
-c-C-
c-C--2
(2
9')
61
0-
0
0
o on
0 0
II I
II !I
6-C-
C--C
-2
+ 3-C
, I 1
Fast
Fast
Fast
3
a-D
icar
bony
l dec
ompo
sitio
n
Fast
(S
low
) M
ech.
G
Mec
h. H
Fast
(S
low
) M
ech.
G
Mec
h. H
![Page 51: A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols](https://reader035.fdocuments.us/reader035/viewer/2022073019/5750057d1a28ab1148a4ad22/html5/thumbnails/51.jpg)
P-H
ydro
xy d
ecom
posi
tion
Qi
o on
6 o
0
OH
II
I I
II I1
I 6-
--c--C
-C-C
-3
-.+
6-C-C
-1 I
1
(34‘
)
ii
no
0
I II
Lt 1-
c-c-
2
3-
2-c-
c-3
If
no
o I1
I I-C
-C-2
+ O
2
,f
I I
(Slo
w)
Fast
M
ech.
G
Mec
h. H
(Slo
w)
Mec
h. G
Fa
st
Mec
h. H
Fast
[R
eact
ion
(14’
)]
Fast
[R
eact
ion
(6‘)
]
a R
ate
cons
tant
s are
giv
en fo
r the
cond
ition
s of
the
smog
cha
mbe
r ru
ns li
sted
in A
ppen
dix
A (T =
303
K, 7
35 to
rr, M
= a
ir).
Ph
otol
ysis
rate
con
stan
ts
are
give
n fo
r the
ligh
t int
ensi
ty a
nd s
pect
ral d
istr
ibut
ions
ass
ocia
ted
with
run
s E
C-3
27-3
40 [
34];
phot
olys
is r
ate
for r
uns
EC
-266
-281
are
som
ewha
t lo
wer
. Firs
t-or
der r
eact
ion,
sec-
I; se
cond
-ord
er re
actio
n, c
m3/
mol
ecul
e se
c.
“Fas
t” m
eans
that
the
reac
tion
is th
e so
le p
athw
ay fo
r tha
t par
ticul
ar sp
ecie
s;
the
pred
ictio
ns a
re in
sens
itive
for
the
actu
al r
ate
cons
tant
. “(
Slow
)” m
eans
that
the
reac
tion
is a
ssum
ed to
he
negl
igib
le, o
r tha
t the
reac
tion
has a
ne
glig
ible
eff
ect o
n th
e pr
edic
tions
of t
he m
odel
for
the
conc
entr
atio
n re
gim
es s
tudi
ed h
ere.
Fo
otno
tes
are
thos
e of
App
endi
x A.
M
echa
nism
s G
and
H pr
edic
t for
mat
ion
of t
he sa
me
prod
ucts
, reg
ardl
ess o
f whe
ther
[2.
2.2]
or [
3.2.
1] cy
cliz
atio
n oc
curs
. Su
bseq
uent
reac
tions
of t
his p
rodu
ct a
re ig
nore
d.
Subs
eque
nt re
actio
ns of
the
se sp
ecie
s or
inte
rmed
iate
s ar
e sh
own
in A
ppen
dix
A.
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830 ATKINSON ET AL.
Notes
1. k l was determined from the photolysis of NO2 in Nz [34,37,43]. All other photolysis rate constants were derived as described previously [7] from the absorption coefficients of the photolyzing species, the quantum yields, and the relative spectral distribution of the photolysis light [34].
2. The rate constant is pressure dependent and is given for 1 atm of air.
3. k g was derived from the equilibrium constant of Graham and Johnston [67] combined with the Nz05 decomposition rate constant; k l o was derived by interpolating the data of Connell and Johnston [71], which were reported a t various NZ number densities, to 760 torr of N2.
4. Value given is consistent with that required for the NO,-n-bu- tane-propene-air system [7]. It is also consistent with the gas phase upper limit of k11 5 1.3 X cm3/molecule sec obtained by Morris and Niki ~ 1 .
5. Value given is specific for the SAPRC evacuable chamber. 6. Based on the assignment or estimate of Carter et al. [7] for the same
or for analogous reactions. 7. Pseudo-third-order reaction added to account for the observed ef-
fects of humidity on this reaction [80,86]. sistent with the data of Hamilton and Lii [80] and the HOyHz0 equilibrium constant calculated by Hamilton and Naleway [87]
8. Reaction (34) is required in the model to account for the excess radical initiation associated with all of our smog chamber experiments [7,8]. In modeling our NO,-propene and/or n-butane-air chamber runs [7], we found that the values of k34 required for calculations to fit the data generally depended on initial NO, NO2, and humidity values. Based on these cor- relations [7,88], the following values of k34 have been assigned: k 3 4 2 0.2 ppb/min for run EC-273; 0.3 ppb/min for runs EC-271 and EC-281; and 0.4 ppb/min for runs EC-266, EC-269, EC-293, EC-339, and EC-340.
9. The total RO2 + NO rate constant used was assumed to be equal to that for the CH3Oz + NO rate constant kg3. Recently, Plumb et al. [89] obtained a value of k93 = (8 f 2) x cm3/molecule sec a t room tem- perature; Adachi and Basco [go] obtained hg3 = (3.0 f 0.2) X cm3/ molecule sec; and Cox and Tyndall [91] obtained (6.5 f 2.0) X cm3/molecule sec. Since Adachi and Basco [go] may have had problems with the product CH30NO in their uv absorption system, and since HOz formed from HCO + 0 2 may have caused problems in the study of Plumb et al. [89], the data to date are ambiguous. We have used a value of k93 of 8 X cm3/molecule sec, identical to that for the reaction of HOz radicals with NO, and consistent with the above-mentioned rate constant studies. The relative rates of alkoxy formation versus alkyl nitrate formation are unknown for these systems, but based on the results of Darnall et al. [60] alkyl nitrate formation probably occurs a t least 10% of the time in Cg+
was calculated to be con
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NO*-AIR PHOTOOXIDATION OF TOLUENE AND T H E CRESOLS 831
systems. For these calculations the efficiency of alkyl nitrate formation was assumed to be the same for all Cs and C7 peroxy radicals, and the best fits to the data are obtained when -25% alkyl nitrate formation is assumed. These reactions have also been discussed by Hoshino et al. [30].
10. Ortho addition is assumed to predominate, based on experimental product data [19,30,31], and results of the analogous addition of O(3P) atoms to toluene [52-541.
11. Reaction (42) is assumed to have the same rate constant as RCH20 + 0 2 - RCHO + HO2 [e.g., reaction (96)], since they have similar ex- othermicities [58] and probably have similar transition states.
12. Derived from 1242 and the h42/k43 ratio of Kenley et al. [19]. However, the nitrotoluene yields would be better fit if h43/(h42 + h43 + 1244 + 1245) were a factor of -10 higher.
13. The total rate constant of 0 2 addition to the toluene-OH adduct was the same for all mechanisms considered, and was adjusted to fit the ob- served maximum cresol yields in the runs modeled. Since the OH-aro- matic-02 adduct can back decompose to the 0 2 + the OH-aromatic ad- duct [9] or cyclize, this rate constant is a composite of h ( 0 2 + OH-aro- matic-adduct) X h (cyc1ization)lh (back-decomposition). Since h ( 0 2 + OH-aromatic-adduct) is expected, by analogy with other alkyl radicals, to be cm3/molecule sec [70], then this implies that the ratio of dis- sociation/cyclization for the OH-aromatic-02 adduct is -300. This means that the OH-aromatic adduct and the OH-aromatic-02 adduct are probably essentially in equilibrium under atmospheric conditions.
14. Detailed reactions shown in Appendix B for substituent 2 = CH3, 1,3-6 = H.
15. The overall reaction occurs as shown if 12.2.21 cyclization predomi- nates and recyclization is negligible [reactions (1’)-(6’) in Appendix B]. hy l (h3~ + k4’) is assumed to be 0.75.
16. Detailed reactions shown in Appendix B for substituent 6 = CH3, 1-4 = H.
17. The reaction occurs as shown if [3.2.1] cyclization predominates and recyclization is negligible [reactions (7’)-( 159, (6’) in Appendix B). h g ~ / ( k y + 1210.) = h l l f / ( h l l ~ + h 1 ~ ) is assumed to be 0.75.
18. The reaction occurs as shown if recyclization predominates and if 0-scission of a-carbonyl, /3-hydroxy alkoxy radicals results primarily in formation of carbonyl radicals. The same products are formed regardless of whether [3.2.1] or [2.2.2] cyclization predominates [reactions (l’), (29, (79, (89 , (16’)-(32’)]. k214(&21r + h 2 ~ ) = k234(k231 + 1224’) is assumed to be 0.75.
19. The reaction occurs as shown if recyclization predominates and 0-scission of a-carbonyl, ,8-hydroxy alkoxy radicals results primarily in formation of a-hydroxy radicals. The same products are formed regardless of whether [3.2.1] or [2.2.2] cyclization predominates [reactions (l’), (2’), (7’), (8’1, (16’)-(30’), (33’), (34’1, (6’), (14’)]. hzi~/(k21( + h22O = k23f/(h23’ + h24’) is assumed to be 0.75.
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832 ATKINSON ET AL.
20. Estimated from the observed disappearance rates of benzaldehyde in runs EC-337 and EC-339, after correcting for reaction of benzaldehyde with OH [9,56] and for benzaldehyde formation from OH + toluene [9,15,19]. The hydroxyl radical concentrations were estimated using the toluene disappearance rates, and the OH + toluene rate constant [9,15]. It is necessary to assume formation of nonradical products, or the overall reactivities of runs EC-337 and EC-339 are overpredicted.
21. RC03 is assumed to react with NO with the same rate constant as CH3Oz (see note 9). The k(RC03 + NO)/k(RC03 + NO2) ratio assigned by Carter et al. [7] was used.
22. Assumed to occur with the same rate constant as that assigned for ROZ + HO2 and NO3 + HO2 [7].
23. OH addition as shown is estimated [58] to be at least 3-7 kcal/mol more thermochemically favorable than addition at any other position, and is therefore assumed to predominate.
24. Assumed to have same rate constant as assigned for reaction (43) (see note 12).
25. Assumed to be analogous to 0 2 addition to the OH-toluene adduct, i.e., k68 = k44 and k69 = k45 (see note 13).
26. Detailed reaction shown in Appendix B for substituent 1 = OH, 2 = CH3,3,4, and 6 = H.
27. Detailed reactions shown in Appendix B for substituent 1 = OH, 6
28. Included in the mechanism to account for the observed rapid dis- appearance of cresols in the presence of 0 3 + NO2 [65,66], and the loss of cresol formed in NO,-toluene-air runs following NO consumption. The rate constant was derived from subsidiary experiments [66]. The 2-methyl phenoxy radical assumed to be formed [66] is expected to react primarily with NOz, giving rise to nonradical products (i.e., hydroxynitrotoluenes). Mechanisms which assume that NO3 + cresol gives radicals result in sig- nificant overprediction of reactivity in NO,-cresol-air runs.
29. Although glyoxal has large absorption coefficients in the 340-460 wavelength region [92] where the intensity of the photolyzing light is high, it appears to have low photodecomposition quantum yields at atmospheric pressure [92].
30. Assumed to react with a similar rate constant as other aldehydes
31. This mechanism is assumed to be primarily H abstraction as shown since OH radical addition to the double bond would be anticipated, by analogy with the reaction of O(3P) atoms with acrolein and crotonaldehyde [93] (when compared to O(3P) and OH + ethene [9,70]), to have a rate constant of <5 x 10-12 cm3/molecule sec.
32. 0 3 addition to the double bond is anticipated to be slow, by analogy with the reaction of O(3P) atoms with acrolein and crotonaldehyde [93], when compared to ethene [9,70].
= CH3,2-5 = H.
[91.
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NOx-AIR PHOTOOXIDATION OF TOLUENE AND THE CRESOLS 833
33. The photochemistry of these species is assumed to be analogous to that of acrolein and crotonaldehyde [92] which are believed to be photo- chemically stable under atmospheric conditions.
34. These initial additions to the double bond are estimated to be exo- thermic by -3 kcal/mol[58]. If the estimates of Demerjian et al. [94] (as- suming an A factor of sec-’ and an activation energy of 11 kcal/mol) are correct, these reactions should dominate over competing processes. However, if their activation energy estimate is low by 2 5 kcal/mol, which is within the uncertainty range of their estimation, the reaction would be expected to be unimportant.
35. Nitrate formation is arbitrarily assumed to occur -10% of the time, based on results for Cq and C5 alkyl radicals [60].
36. Assumed to decompose with the same rate constant as peroxya- cetylnitrate (PAN) [63,64].
37. Assumed t‘o occur with the same rate constant as OH + isopropanol
38. Reaction with 0 2 forming HO2 and carbonyls has been shown to be the primary fate of a-hydroxy radicals in air [95].
39. The rates of photolysis for these acids are unknown, although there is evidence that photodecomposition to C02 + CHsCHO is efficient in the gas phase at 353 K [92]. If they photolyze to form radicals rapidly (anal- ogously to methylglyoxal), model calculations greatly overpredict the re- activity of the o-cresol-NO, system, regardless of which mechanistic option is used.
40. Assumed to react with the same rate constant as OH + toluene. This reaction is only significant in affecting primary nitrotoluene yields.
~91.
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NO%-AIR PHOTOOXIDATION OF TOLUENE AND T H E CRESOLS 835
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836 ATKINSON ET AL
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Received December 4,1979 Accepted April 18,1980