An Analysis of the Benzene Scheme for Three Versions of the SAPRC
Mechanism
Draft Report to the California Air Resources Board
Contract No. 07-730
By Wendy Goliff
January 6, 2012
Center for Environmental Research and Technology
College of Engineering
University of California
Riverside, California 92521
1
Abstract
Three versions of the SAPRC mechanism (SAPRC07, SAPRC11 and SAPRC11A) were
used to simulate chamber experiments with benzene under both low NOx and high NOx
conditions in the EUPHORE chamber, which were run as part of the EXACT campaign.
Only the SAPRC07 mechanism provided predicted maximum ozone values within the
stated error of observed values for the low NOx experiment. Model performance was not
as good for the high NOx experiment, with only SAPRC07 predicting ozone values
within measurement error of observations. Regarding the HOx species, each mechanism
predicted peak radical levels close to observed values, although they underpredicted HOx
concentrations during the afternoon hours. For model performance for product species
such as glyoxal and measured ring-retaining products, the mechanisms were not able to
predict peak concentration values close to those of observed values. Recommendations
are as follows: use different ring-opening products for benzene and the other aromatics so
that each degradation pathway may be tuned for each aromatic. For example, methyl
glyoxal is not observed from the oxidation of benzene, and yet it is formed in amounts
comparable to glyoxal by each version of the SAPRC mechanism. Also, it would be
useful to compare SAPRC mechanism predictions to more product information (such as
the sort provided by the EXACT campaign), especially if the modeling of particulate
formation is desired.
Introduction
Benzene, the simplest of the aromatic compounds, has a relatively low reactivity in the
atmosphere, with a tropospheric lifetime of 9.5 days (Atkinson and Arey, 2003) with
respect to reaction with hydroxyl radical (OH). In spite of this, benzene is of interest in
air toxics modeling, and has been classified as class 1 carcinogen by the International
Agency for Research on Cancer (IARC). Although extensive research has been
performed recently to elucidate its mechanism of photooxidation, the degradation of
benzene in the atmosphere is still poorly understood.
Volkamer et al. (2002) studied the hydroxyl-initiated oxidation of benzene in the outdoor
chamber EUPHORE in Spain and an indoor chamber at NIES in Japan. In the
EUPHORE chamber, in which NOx levels and light conditions were representative of the
atmospheric boundary layer, a phenol yield of 53.1 ± 6.6% was reported, and was
independent of NOx and O2 concentrations.
Bloss et al. (2005) used data collected in the EXACT project (Effects of the oXidation of
Aromatic Compounds in the Troposphere) to update the Master Chemical Mechanism’s
(MCM) aromatic scheme. The EXACT project involved the collection of a
comprehensive dataset at the EUPHORE chamber for the elucidation of aromatic
degradation in the atmosphere. Extensive instrumentation were used to collect
measurements of O3, NO, NO2 (using DOAS as well as a NOx monitor), CO, PAN,
phenol, cresols, catechols, glyoxal, methyl glyoxal, HONO, OH, HO2, actinic flux (for
J(NO2)), HCHO and carbonyls. In the case of benzene, the authors found that the
MCMv3.1 predictions for the O3 peak improved compared to the MCMv3 due to the
2
increase in the yield of phenol diverting production away from radical-producing ringopening products. However, this decrease in radical production resulted in a decrease of
predicted OH compared to measured OH (termed “missing OH”) and a decrease in the
oxidation capacity of the system.
Berndt and Böge (2006) studied the hydroxyl-initiated oxidation of benzene in a flowtube reactor. In the absence and presence of NOx, they found a phenol yield of 0.61 ±
0.07. Carbonyls detected were glyoxal, cis-butenedial and trans-butenedial with
formation yields of 0.29 ± 0.10, 0.08 ± 0.03 and 0.023 ± 0.007, respectively, measured in
synthetic air and in the presence of NOx. In the absence of NOx, glyoxal, cis-butenedial
and trans-butenedial were also detected, but with distinctly lower yields compared to the
experiments with NOx.
Studies regarding the ring-opening products of the HO-benzene reaction are sparse, with
main aspects of their formation uncertain due to lack of good experimental techniques for
their quantification, a lack of commercially available standards, and their high reactivity.
In addition to the study by Berndt and Böge (2006), Gomez Alvarez et al. (2007)
investigated the photooxidation of toluene and benzene in the EUPHORE chamber. For
the benzene experiments, they found yields of dicarbonyls using SPME fibers, with
values of (42 ± 3) and (36 ± 2)% for the two successive experiments (September 24 and
25, 2003) of glyoxal, (17 ± 9)% for total butenedial [(8 ± 4)% cis-butenedial and (9 ±
5)% trans-butenedial (September 24, 2003)] and (15 ± 6)% total butenedial (September
25, 2003) [(7 ± 3) and (7 ± 3)% for the cis and trans isomers, respectively]. For this
work, cis- and trans-butenedial were synthesized for calibration purposes. The
implications of the work of Gomez Alvarez et al. 2007 were that the dicarbonyl reaction
route was confirmed, and fast ring-cleavage was observed, due to a peak in observed dicarbonyls shortly after the chamber was opened to sunlight. Also, high yields of
dicarbonyls (e.g., glyoxal) imply a high formation rate of HO into the system.
In contrast to previous studies, Birdsall and Elrod (2011) did find that yields of phenol
were NOx dependent. They also found that the dienedial yields were larger for benzene
than for any other aromatic system, and were observed to increase significantly with NO,
a result previously observed by Berndt and Boge, albeit at a significantly higher NO
concentration than in the present work.
A continuing problem in chemical mechanism development is the over-prediction of
ozone and underprediction of radicals in a variety of cases, especially with aromatic
degradation (Bloss et al., 2005), even for the most widely known mechanisms such as the
Regional Atmospheric Chemistry Mechanism (RACM), the Statewide Air Pollution
Research Center mechanism (SAPRC99, SAPRC07), the Carbon Bond mechanism
(CB05) and the Master Chemical Mechanism (MCM) (Bloss et al., 2005; Chen et al.,
2010). In this work the chemical mechanism of benzene is explored with respect to
ozone formation, radical and NOx concentrations, and product formation using the
SAPRC mechanisms. To this end the chamber data acquired from the EXACT project
was obtained from Dr. Pilling of Leeds University for both benzene experiments: low
3
NOx and high NOx. In the low NOx experiment, the VOC to NOx ratio was 42
ppm/ppm, while in the high NOx experiment the VOC to NOx ratio was 5.6 ppm/ppm.
EXACT Benzene Experiments
The initial conditions for the experiments performed for the EXACT project were chosen
to consider both the technical constraints (such as sufficient concentrations of VOC for
accurate measurements) and to come as close as possible to atmospheric conditions
(Bloss et al., 2005). The initial conditions for the benzene/high NOx and benzene/low
NOx experiments are given in Table 1 (the NOx levels for the “high NOx” EXACT
experiments are not as high as in the UCR database) . The chamber used for the project
was the EUPHORE facility, which consists of two FEP foil hemispherical chambers with
an approximate volume of 200 m3, and for which the transmission of both visible and UV
light through the chamber walls is high (85–90% for wavelengths 500–320 nm, and
around 75% at 290 nm). A complete description of the instrumentation used may be
found in Bloss et al. (2005).
Table 1. Initial conditions used in simulations, concentrations measured before opening
chamber to sunlight (Bloss et al., 2005)
Initial Concentration
Benzene/low NOx (ppm)
Benzene/high NOx (ppm)
(VOC/NOx = 42 ppm/ppm)
(VOC/NOx = 5.6 ppm/ppm)
Benzene
1.986
1.014
NO
0.045
0.100
NO2
0.0022
0.026
HONO
0.0001 (a)
0.0555 (b)
O3
0.006
0
HCHO
0
0
HNO3
0
0.0068
Glyoxal
0
0.0005
CO
0.751
0.615
(a) Estimated by authors
(b) Measured concentration.
Key compounds that were measured during the EXACT campaign fall into three groups
according to their importance for the assessment of model performance. The first group
contains the parent aromatic (in this case, benzene), ozone, NO and NO2. These
compounds provide information on the ozone production and the oxidative capacity in
the system. The second group contains compounds that deliver important information on
the NOy budget, HOx production or the branching ratios for major reaction routes:
HNO3, PAN, HCHO, glyoxal, methylglyoxal, cresol isomers and benzaldehyde (Bloss et
al., 2005). The third group of compounds contains reaction products, such as butenedial.
Typical uncertainties (2) were ±10% for NO2, aromatics, HCHO and ±5% for O3 and
NO, and 26% (1) for the HOx species. (Volkamer et al., 2002; Bloss et al., 2004).
As in any smog chamber, air is lost throughout the experiment due to small leaks and
withdrawal of air for samples for analysis. In the EUPHORE facility, clean air was
4
added to compensate for this and some dilution of the reactants and products occurred as
a result. To measure the dilution rate SF6 was added to the reaction mixture as an inert
tracer. Dilution rates were 1.18 x 10-3 min-1 and 1.19 x 10-3 min-1 for the low and high
NOx experiments, respectively (http://eurochamp-database.es/Data_Base.htm).
Photolysis rates for this study were calculated using the photolysis program in Dr.
Carter’s box model, using the appropriate latitude, sun declination and time start and end
times. In addition, scaling factors to take into account the transmission through the walls
and backscatter from the aluminum chamber floor were taken from the MCM web site.
Chamber related reactions
When modeling smog chamber experiments, it is always necessary to take into account
wall effects. Current practice is to minimize wall effects through the use of inert
materials and cleaning programs, but walls may still be a significant source and sink of
contaminants, as well as serve as a reaction site for heterogeneous chemistry (Killus and
Whitten, 1990). There are three major ways wall effects can interact with experimental
photochemistry: (1) the introduction of free radicals, (2) trace NOx species can allow the
formation of O3 and PAN and serves as a radical sink, and (3) organic off-gassing that
convert hydroxyl radicals (OH) to HO2. An auxiliary mechanism describing the wall
effects for the EUHPORE chamber was constructed for the EXACT campaign and tested
with two characterization experiments using ethylene in high and low NOx regimes.
Discrepancies between the model and chamber data for key reactants and products such
as ethylene, ozone, NO and NO2 were interpreted as influenced by wall reactions. The
auxiliary mechanism was tuned to fit the discrepancies between model and experimental
data for ethylene and then was used to describe the wall effects in simulations for the
aromatic experiments. The auxiliary mechanism is listed in Table 2. Wall reactions that
produced less than 1% change in the maximum O3 concentration were not used in the
auxiliary mechanism to minimize complexity (Bloss et al., 2005). Because ethylene
degradation is well-understood, the MCM and SAPRC mechanisms perform similarly for
these simulations. Therefore one may use the auxiliary mechanism constructed for the
MCM for the SAPRC mechanisms as well.
Table 2. Tuned auxiliary mechanism used for EXACT campaign.
Process
Tuned reaction rate
NO2 = HONO
0.7 x 10-5 s-1
NO2 = wHNO3
1.6 x 10-5 s-1
O3 = wO3
3 x 10-6 s-1
Mechanism Evaluation
The SAPRC07, SAPRC11 and SAPRC11A mechanisms were evaluated against data
collected during the EXACT campaign for the benzene experiments (high NOx and low
NOx) obtained from Dr. Pilling of the University of Leeds. This data set contains
information regarding the time events for adding reactants and opening the chamber roof,
reactant and product information (e.g., benzene, phenol and formaldehyde), as well as
5
nitrogen-containing species (NO, NO2 and nitric acid) and OH and HO2 concentrations
(measured hydroxyl concentrations agreed well with calculated [OH] from the rate of
decay of the aromatic).
SAPRC07
The low and high NOx benzene experiments conducted for the EXACT campaign were
simulated using the SAPRC07 mechanism (Carter, 2010). The benzene scheme for
SAPRC07 is listed in Table 3, for which the AFG1 and AFG2 species represent the
highly photoreactive mono-unsaturated dialdehydes and aldehyde-ketones, and AFG3
represents the less photoreactive unsaturated diketones and di-unsaturated dicarbonyls.
For the low NOx experiment, the rate of ozone formation during the simulation was
slower than in the experiment, with a modeled O3(max) 7.5% lower than the
experimental data (Figure 1). Modeled NO2 peaked 45 minutes later and 28% lower than
the measured NO2 in the EUPHORE chamber, and modeled NO did not decrease as fast
as in the chamber experiment (Figures 2 and 3). In the case of the HOx species,
SAPRC07 underpredicts HO2 by 37%, and OH by 56% (Figures 4 and 5).
Bloss et al. (2005) reported a peak concentration of 22.4 ppb for glyoxal in their benzene
experiment. SAPRC07 predicted a peak value of 14.0 ppb. However, SAPRC07 has a
yield of 0.29 glyoxal in the BENZENE + OH reaction. Volkamer et al. (2005) report a
yield of glyoxal to be 32% +/- 5% for this reaction, with negligible contribution from
secondary glyoxal formation pathways. Therefore the yield of GLY was adjusted to 0.32
Table 3. Benzene scheme for the SAPRC07 mechanism.
Rate Parameters Reactants
Products
A = 2.33e-12
BENZENE + OH = #.116 OH + #.29 {RO2C + xHO2} + #.024
Ea = 0.38
{RO2XC + zRNO3} + #.57 {HO2 + CRES} +
#.116 AFG3 + #.290 xGLY + #.029 xAFG1 +
#.261 xAFG2 + #.314 yRAOOH + #-.976 XC
k(300) =
AFG1 + OH =
#.217 MACO3 + #.723 RO2C + #.060
7.40e-11
{RO2XC + zRNO3} + #.521 xHO2 + #.201
xMECO3 + #.334 xCO + #.407 xRCHO +
#.129 xMEK + #.107 xGLY + #.267 xMGLY
+ #.783 yR6OOH + #.284 XC
k(300) =
AFG1 + O3 =
#.826 OH + #.522 HO2 + #.652 RO2C + #.522
9.66e-18
CO + #.174 CO2 + #.432 GLY + #.568 MGLY
+ #.652 xRCO3 + #.652 xHCHO + #.652
yR6OOH + #-.872 XC
Phot Set= AFG1
AFG1 + HV =
k(300) =
7.40e-11
AFG2 + OH =
#1.023 HO2 + #.173 MEO2 + #.305 MECO3 +
#.500 MACO3 + #.695 CO + #.195 GLY +
#.305 MGLY + #.217 XC
#.217 MACO3 + #.723 RO2C + #.060
{RO2XC + zRNO3} + #.521 xHO2 + #.201
xMECO3 + #.334 xCO + #.407 xRCHO +
6
k(300) =
9.66e-18
AFG2 + O3 =
Phot Set= AFG1
k(300) =
9.35e-11
AFG2 + HV =
AFG3 + OH =
k(300) =
1.43e-17
AFG3 + O3
#.129 xMEK + #.107 xGLY + #.267 xMGLY
+ #.783 yR6OOH + #.284 XC
#.826 OH + #.522 HO2 + #.652 RO2C + #.522
CO + #.174 CO2 + #.432 GLY + #.568 MGLY
+ #.652 xRCO3 + #.652 xHCHO + #.652
yR6OOH + #-.872 XC
PROD2 + #-1 XC
#.206 MACO3 + #.733 RO2C + #.117
{RO2XC + zRNO3} + #.561 xHO2 + #.117
xMECO3 + #.114 xCO + #.274 xGLY + #.153
xMGLY + #.019 xBACL + #.195 xAFG1 +
#.195 xAFG2 + #.231 xIPRD + #.794
yR6OOH + #.938 XC
#.471 OH + #.554 HO2 + #.013 MECO3 +
#.258 RO2C + #.007 {RO2XC + zRNO3} +
#.580 CO + #.190 CO2 + #.366 GLY + #.184
MGLY + #.350 AFG1 + #.350 AFG2 + #.139
AFG3 + #.003 MACR + #.004 MVK + #.003
IPRD + #.095 xHO2 + #.163 xRCO3 + #.163
xHCHO + #.095 xMGLY + #.264 yR6OOH +
#-.575 XC
Benzene - low NOx experiment
0.25
Ozone (ppm)
0.2
0.15
O3 chamber
O3 SAPRC07
0.1
0.05
0
9:36
-0.05
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 1. Ozone concentrations for the benzene experiment – EXACT Campaign:
measured versus modeled. The blue diamonds are observed values with 5% (2 error
bars, and pink squares are predictions from SAPRC07.
7
NO2 - benzene/low NOx experiment
0.045
0.04
0.035
NO2 (ppm)
0.03
0.025
NO2 chamber
NO2 SAPRC07
0.02
0.015
0.01
0.005
0
9:36
10:48
12:00
13:12
14:24
15:36
Time
Figure 2. NO2 concentrations for the benzene/low NOx experiment. Measurements
(with 10% error bars) in blue diamonds and SAPRC07 predictions in pink squares.
NO - benzene/low NOx experiment
0.05
0.045
0.04
NO (ppm)
0.035
0.03
NO chamber
0.025
NO SAPRC07
0.02
0.015
0.01
0.005
0
9:36
10:48
12:00
13:12
14:24
15:36
Time
Figure 3. NO concentrations for the benzene/low NOx experiment. Measurements in
blue diamonds and SAPRC07 predictions in pink squares.
8
HO2 - benzene/low NOx experiment
2.5E-04
HO2 (ppm)
2.0E-04
1.5E-04
HO2 SAPRC07
HO2 ppm chamber
1.0E-04
5.0E-05
0.0E+00
9:36:00
AM
10:48:00
AM
12:00:00 1:12:00 PM 2:24:00 PM 3:36:00 PM
PM
Time (minutes)
Figure 4. HO2 concentrations for the benzene/low NOx experiment. Measurements in
blue diamonds and SAPRC07 predictions in pink squares.
OH - benzene/low NOx
1.E-06
1.E-06
OH (ppm)
8.E-07
6.E-07
HO ppm chamber
OH ppm SAPRC07
4.E-07
2.E-07
0.E+00
9:36:00 AM
-2.E-07
10:48:00
AM
12:00:00
PM
1:12:00 PM 2:24:00 PM 3:36:00 PM
-4.E-07
Time (minutes)
Figure 5. OH concentrations for the benzene/low NOx experiment. Measurements in
blue diamonds and SAPRC07 predictions in pink squares.
9
for the BENZENE + OH reaction. This gave a predicted peak GLY of 16.9 ppb. In an
attempt to match model predictions to observations, the yield of GLY was then adjusted
to 0.45, the upper limit of the glyoxal yield reported by Volkamer et al. 2001. This
generated a predicted peak concentration for glyoxal of 22.3 ppb, well within the
experimental error of the observed peak in glyoxal concentration for the benzene
experiment. However, because Volkamer et al. (2001) state that observed secondary
glyoxal formation from benzene oxidation was negligible, GLY yields from the oxidation
of AFG1, AFG2 and AFG3 were lowered to 0.001. Combined with a 0.45 yield of
primary glyoxal from the initial benzene oxidation, this resulted in a predicted glyoxal
peak concentration of 20.4 ppb, still within experimental error of the observed peak
value.
There are three routes for glyoxal degradation in the atmosphere: reaction with OH,
reaction with NO3 (only important in the dark) and photolysis (see Table 4). (There are 2
photolysis channels for glyoxal in SAPRC07 rather than 3 as is recommended by
NASA/JPL (Sander et al., 2006). The third channel, GLY + HV = H2 + #2 CO, has a
low quantum yield compared to the other two channels, so its absence is not significant in
SAPRC07.). While the GLY + OH reaction products are not in agreement with the
current IUPAC recommendation, this is a relatively unimportant reaction with little
impact on ozone or PAN formation in the benzene/NOx degradation scheme. One
photolysis channel for glyoxal leads to formation of formaldehyde (HCHO). Figure 6
shows the effects of varying glyoxal yields on HCHO formation for the benzene-low
NOx experiment: higher GLY yields result in HCHO predictions that are closer to
observed values. Figure 7 illustrates the predicted and measured O3 concentrations for
each GLY yield described above. With higher GLY yields, the ozone formation rate is
closer to observed values. Figures 8 and 9 show the OH and HO2 concentrations for each
yield of glyoxal described above, with increasing glyoxal yields correlating with
increasing OH and HO2 which more closely match observations.
Table 4. Degradation scheme for glyoxal in SAPRC07.
Rate Parameters
Reactants
Products
k(300) = 1.10e-11
GLY + OH =
#.63 HO2 + #1.26 CO + #.37
RCO3 + #-.37 XC
A = 2.80e-12 Ea = 4.722
GLY + NO3 =
HNO3 + #.63 HO2 + #1.26 CO
+ #.37 RCO3 + #-.37 XC
PF=GLY-07R
GLY + HV =
#2 {CO + HO2}
PF=GLY-07M
GLY + HV =
HCHO + CO
Product information for phenol, catechol, and nitrophenol are also available for the
benzene/low NOx experiment. In SAPRC07, the model species CRES contains the
grouping of phenols and cresols, so this species was compared to the sum of the catechol
and phenol concentrations observed in the EUPHORE chamber (see Figure 10 for the
low NOx simulation). SAPRC07 underpredicted the sum of catechol and phenol by 37%
at its peak concentration. The reason for this underprediction is unclear, as the phenol
yield from the benzene plus hydroxyl reaction is in agreement with literature values, and
the catechol yield measured in the EXACT campaign is a small fraction (~2%) of the
10
total phenol + catechol yield. In the case of nitrophenol, SAPRC07 (which contains the
model species NPHE which represents all nitrophenols) overpredicts nitrophenol by a
factor of 3.6 (see Figure 11). One possible reason for this overprediction is the yield of
BZO in the CRES + OH reaction is 0.2, while the Master Chemical Mechanism v3.1 (for
which the aromatic scheme is based upon the EXACT campaign) has a BZO yield of 0.06
(Coeur-Tourneur et al. 2006 measured the yield of 6-methyl-2-nitrophenol 4.7±0.8%
from o –cresol). Therefore the CRES + OH reaction in SAPRC07 was adjusted to give a
yield of 0.06. As a result, the NPHE peak concentration lowered by 25%, to 4.9 ppb, a
factor of 2.5 higher than observed.
SAPRC07 was also used to simulate the high NOx experiments with benzene for the
EXACT campaign. The original unaltered SAPRC07 mechanism (here termed “base
case”) overpredicted the peak ozone concentration by 8.6%. When the product formation
of glyoxal was increased to 0.35, the mechanism overpredicted O3 by 12%. When the
glyoxal yield was increased to 0.45, O3 was overpredicted by 17%. When the glyoxal
formation was restricted to primary only (with a yield of 0.45), the resulting modeled
ozone was 15% higher than observed values. Figure 12 shows the ozone formation
observed in the EUPHORE chamber as well as ozone values predicted by each version of
the SAPRC07 mechanism described above. The trend of increasing ozone with
increasing GLY yield is expected because glyoxal degradation creates many radicals
leading to ozone formation. This is illustrated in Figures 13 and 14, in which the OH and
HO2 radical concentrations are compared. The high initial concentration of OH was due
to the injection of HONO into the chamber prior to opening the roof. For each case,
hydroxyl radical is underpredicted by SAPRC07. Although the difference between
model and observation is approximately 50%, the variation between versions of
SAPRC07 varies by only a few percent. SAPRC07 underpredicts HO2 concentrations by
24% (within the stated uncertainty of HO2 measurements). By increasing the GLY yield
for the benzene reaction with hydroxyl, the modeled HO2 concentrations come within
16% of measured values. It is interesting to note that in the case of 0.45 GLY yield with
secondary production, the predicted HO2 concentrations increase at a higher rate at the
beginning of the experiment compared to observations. Figure 15 illustrates the decay of
benzene during the high NOx experiment. In each case predicted values fall within the
stated uncertainty of the measured value (10%). Figure 16 shows the NO concentration
for the benzene/high NOx experiment. In each case the modeled NO concentrations are
lower than the observed values, ranging from 25% for the base case to 28% for the 0.45
GLY yield. It is expected for the NO concentration to be lower when GLY yields
increase due to the increased production of radicals from photolysis of HCHO and GLY,
which will in turn react with NO. The same effect is observed with the NO2 yields (see
Figure 17).
Other products measured for the benzene/high NOx experiment include: catechol, phenol,
nitrophenol and nitric acid. The SAPRC07 species CRES was compared to the sum of
the catechol and phenol concentrations observed in the EUPHORE chamber (see Figure
18 for the high NOx simulation). SAPRC07 performed similarly here to the benzene/low
NOx experiment, underpredicting the sum of catechol and phenol by 30% at its peak
11
HCHO - benzene/low NOx expt
0.016
0.014
0.012
HCHO (ppm)
0.01
0.008
HCHO chamber
HCHO ppm model base
0.006
HCHO.35 GLY yield
0.004
HCHO .45 GLY yield
HCHO .45 GLT yield primary only
0.002
0
9:36:00
-0.002
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time
Figure 6. Comparison of HCHO yields for benzene-low NOx experiment for varying
yields of glyoxal (GLY) to chamber measurements (with 10% error bars).
O3 Concentrations: low NOx - benzene
0.25
0.2
O3 chamber
O3 (ppm)
0.15
O3 SAPRC07
O3 with 0.35 GLY
0.1
O3 .45 GLY
0.05
O3 .45 GLY no secondary GLY
0
9:36
10:48
12:00
13:12
14:24
15:36
-0.05
Time
Figure 7. Comparison of O3 concentrations for benzene-low NOx experiment for
varying yields of glyoxal (GLY) to chamber measurements (with 5% error bars).
12
OH with varying yields of GLY
1.E-06
1.E-06
HO ppm chamber
OH (ppm)
8.E-07
OH ppm SAPRC07
6.E-07
OH with .35 GLY
4.E-07
OH with .45 GLY
2.E-07
0.E+00
9:36:00
-2.E-07
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
OH with .45 GLY primary
formation only
-4.E-07
Time (minutes)
Figure 8. Comparison of OH concentrations for benzene-low NOx experiment for
varying yields of glyoxal (GLY) to chamber measurements (with 10% error bars).
HO2 with varying yields of GLY
HO2 (ppm)
3.0E-04
2.5E-04
HO2 ppm chamber
2.0E-04
HO2 SAPRC07
HO2 .35 GLY
1.5E-04
HO2 .45 GLY
1.0E-04
HO2 with .45 GLY primary
formation only
5.0E-05
0.0E+00
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time (minutes)
Figure 9. Comparison of HO2 concentrations for benzene-low NOx experiment for
varying yields of glyoxal (GLY) to chamber measurements (with10% error bars).
13
Phenol + catechol yields - benzene/low NOx expt
4.00E-02
3.50E-02
CRES (ppm)
3.00E-02
2.50E-02
Catechol + Phenol
2.00E-02
CRES - SAPRC07
1.50E-02
1.00E-02
5.00E-03
0.00E+00
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time
Figure 10. Sum of catechol and phenol concentrations compared to SAPRC07 species
CRES for low NOx experiment.
nitrophenol yield - benzene/low NOx expt
0.007
0.006
NPHE (ppm)
0.005
0.004
Nitrophenol
NPHE - SAPRC07
0.003
0.002
0.001
0
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time
Figure 11. Nitrophenol chamber measurements compared to NPHE species output by
SAPRC07 for low NOx experiment.
14
Ozone concentrations for benzene/high NOx experiment
0.3
0.25
O3 (ppm)
0.2
Ozone (ppm)
O3 SAPRC07 (ppm)
0.15
O3 .35 GLY
O3 .45 GLY
0.1
O3 .45 GLY primary only
0.05
0
9:36
10:48
12:00
13:12
14:24
15:36
16:48
Time
Figure 12. Ozone concentrations for benzene/high NOx experiment. Blue boxes with
error bars (+/- 5%) are chamber measurements. The pink square is the SAPRC07 base
case, triangle is the 0.35 GLY yield, x is the 0.45 GLY yield, and the circle is the 0.45
GLY yield with primary formation only.
OH concentrations for benzene/high NOx experiment
2.0E-06
OH (ppm) chamber
1.5E-06
OH (ppm)
OH SAPRC07 base case
1.0E-06
OH 0.35 GLY
5.0E-07
OH 0.45 GLY
0.0E+00
9:36 10:48 12:00 13:12 14:24 15:36 16:48
OH 0.45 GLY primary
only
-5.0E-07
Time
Figure 13. Hydroxyl radical concentrations for benzene/high NOx experiment. Blue
boxes with error bars (+/- 26%) are chamber measurements. The pink square is the
SAPRC07 base case, triangle is the 0.35 GLY yield, x is the 0.45 GLY yield, and the
circle is the 0.45 GLY yield with primary formation only.
15
HO2 - benzene/high NOx experiment
2.0E-04
1.8E-04
HO2 chamber (ppm)
1.6E-04
HO2 SAPRC07 base case
HO2 (ppm)
1.4E-04
1.2E-04
HO2 SAPRC07 0.35 GLY
1.0E-04
8.0E-05
HO2 SAPRC07 0.45 GLY
6.0E-05
4.0E-05
HO2 SAPRC08 0.45 GLY
primary only
2.0E-05
0.0E+00
9:36
10:48
12:00
13:12
14:24
15:36
16:48
Time
Figure 14. HO2 radical concentrations for benzene/high NOx experiment. Blue boxes
with error bars (+/- 26%) are chamber measurements, the pink square is the SAPRC07
base case, triangle is the 0.35 GLY yield, x is the 0.45 GLY yield, and the circle is the
0.45 GLY yield with primary formation only.
Benzene decay during high NOx experiment
1.2
Benzene (ppm)
1
Benzene (ppm)
0.8
BENZENE - SAPRC07 base case
BENZENE 0.35 GLY
0.6
BENZENE 0.45 GLY
0.4
BENZENE 0.45 GLY primary
only
0.2
0
10:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00
Time
Figure 15. Benzene decay during the high NOx experiment. The blue diamonds are the
chamber measurements, the pink squares are the SAPRC07 base case, triangles are the
0.35 GLY yield, x are the 0.45 GLY yield, and the circles are the0.45 GLY yield with
primary formation only.
16
NO concentrations for benzene/high NOx experiment
1.E-01
NO_ECO (ppm)
NO (ppm)
1.E-01
NO - SAPRC07
8.E-02
NO .35 GLY
6.E-02
4.E-02
NO .45 GLY
2.E-02
NO .45 GLY primary only
0.E+00
10:48
12:00
13:12
14:24
15:36
16:48
Time
Figure 16. NO concentrations for the benzene/high NOx experiment. The blue
diamonds are the chamber measurements (with 5% error bars), the pink squares are the
SAPRC07 base case, triangles are the 0.35 GLY yield, x are the 0.45 GLY yield, and the
circles are the0.45 GLY yield with primary formation only.
NO2 concentrations for benzene/high NOx experiment
NO2 (ppm)
1.4E-01
1.2E-01
NO2 (ppm) chamber
1.0E-01
NO2 - SAPRC07 - base
case
NO2 SAPRC07 0.35 GLY
8.0E-02
6.0E-02
NO2 SAPRRC07 0.45 GLY
4.0E-02
NO2 SAPRC07 0.45 GLY
primary only
2.0E-02
0.0E+00
10:48
12:00
13:12
14:24
15:36
16:48
Time
Figure 17. NO2 concentrations for the benzene/high NOx experiment. The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC07 base case, triangles are the 0.35 GLY yield, x are the 0.45 GLY yield, and the
circles are the0.45 GLY yield with primary formation only.
17
CRES formation for benzene-high NOx experiment
0.025
catechol+phenol (ppm)
CRES SAPRC07
CRES (ppm)
0.02
0.015
0.01
0.005
0
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time
Figure 18. CRES concentrations for the benzene/high NOx experiment. The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC07 predictions.
Nitrophenol formation for benzene-high NOx experiment
0.018
0.016
NPHE (ppm)
0.014
0.012
0.01
nitrophenol (ppm) chamber
NPHE - SAPRC07
0.008
0.006
0.004
0.002
0
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Time
Figure 19. Nitrophenol concentrations for the benzene/high NOx experiment. The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC07 predictions (NPHE).
18
concentration. In the case of nitrophenol, SAPRC07 (which contains the model species
NPHE which represents all nitrophenols) overpredicts nitrophenol by a factor of 4 (see
Figure 19). SAPRC07 also modestly overpredicted HNO3 in the chamber, by 36% at its
peak concentration value. One possible reason for this is the reaction rate for OH + NO2
in SAPRC07 is 14% higher than the rate recommended by Mollner et al. (2010).
SAPRC11
SAPRC11 is based upon SAPRC07 with an updated aromatic scheme. Table 5 lists the
benzene scheme included in SAPRC11 (Carter, 2012). The benzene scheme was also
updated to give explicit reaction products phenol (PHEN) and catechol (CATL).
Surrogate species AFG1, AFG2 and AFG3 are used to represent ring-opening product,
while BZO represents lumped phenoxy radicals and CNDCA represents condensable
products. SAPRC11 was employed to simulate the benzene low NOx and high NOx
experiments performed for the EXACT campaign. Figure 20 shows the SAPRC11
simulation for the low NOx experiment compared to observed values of ozone. The
mechanism overpredicts ozone concentrations during the entire length of the simulation,
with the time of the simulated ozone peak later than the observed peak by one hour. One
possible reason for this is the SAPRC11 mechanism contains methyl glyoxal formation
due to reactions by AFG1, AFG2 and AFG3, which was not observed in the EXACT
experiments. However, when methyl glyoxal was removed from the benzene
mechanism, ozone values did not change significantly.
Regarding the HOx species, SAPRC11 predicted an OH peak concentration 32% above
the observed value but OH concentrations decrease more quickly than observed once the
peak is reached. For HO2, the modeled peak concentration was 8.5% higher than the
observed value, just one hour early (see Figures 21 and 22). In an effort to match
simulated ozone values with observations, the yield of xHO2 was lowered in the reaction
BENZENE + OH from 0.31 to 0.15. This improved the agreement for ozone for the first
few hours of the simulation, but didn’t significantly lower the predicted maximum ozone
concentration (see Figure 23), although it did improve the agreement for HO2 regarding
the timing of the peak (Figure 24).
SAPRC11 gave a predicted peak glyoxal concentration of 23.2 ppb, 3.6% higher than the
observed peak concentration and within the uncertainty in the measured value (± 10%).
Formaldehyde concentrations predicted by SAPRC11 for the low NOx experiment were
45% higher than observed values (Figure 25), with the time of the maximum
formaldehyde concentration one hour earlier than the observed peak.
Comparisons of predictions of catechol and phenol concentrations by SAPRC11 were
also performed, see Figures 26 and 27. The SAPRC11 prediction for the peak catechol
concentration was 2.4 ppb 110% higher than the observed value of 1.1 ppb. The peak
phenol concentration predicted by SAPRC11 was 31.9 ppb, just 0.6% higher than the
observed value of 31.7. In the case of nitrophenol, agreement between SAPRC11 and
observed concentrations was poor, with SAPRC11 overpredicting the peak nitrophenol
concentration by 173% (Figure 28). One possible reason for the overprediction of
19
Table 5. Degradation scheme for benzene and its degradation products in SAPRC11.
Rate
Reactants
Products
Parameters
A = 2.33e-12,
BENZENE +
OH =
Ea = 0.384
A = 4.70e-13,
PHEN + OH =
Ea = -2.424
k(298) =
3.80e-12
PHEN + NO3
=
k(298) =
2.00e-10
CATL + OH =
k(298) =
1.70e-10
CATL + NO3
=
#.027 RO2XC + #.31 RO2C + #.57 HO2 + #.31 xHO2 + #.027 zRNO3
+ #.337 yR6OOH + #.57 PHEN + #.31 xGLY + #.13 xAFG1 + #.18
xAFG2 + #.093 OH + #.093 AFG3 + #.065 AFG3C + #1.929 XC
#.700 HO2 + #.100 BZO + #.110 xHO2 + #.090 OH + #.110 RO2C +
#.700 CATL + #.090 AFG3 + #.055 xAFG1 + #.055 xAFG2 + #.110
xGLY + #.110 yR6OOH + #-.200 XC
#.100 HNO3 + #.900 XN + #.700 HO2 + #.100 BZO + #.110 xHO2 +
#.090 OH + #.110 RO2C + #.700 CATL + #.090 AFG3 + #.055 xAFG1
+ #.055 xAFG2 + #.110 xGLY + #.110 yR6OOH + #-.200 XC
#.400 HO2 + #.200 BZO + #.200 xHO2 + #.200 OH + #.200 RO2C +
#.400 CNDCA + #.200 AFG3 + #.100 xAFG1 + #.100 xAFG2 + #.100
xGLY + #.100 xMGLY + #.200 yR6OOH + #-.500 XC
#.200 HNO3 + #.800 XN + #.400 HO2 + #.200 BZO + #.200 xHO2 +
#.200 OH + #.200 RO2C + #.400 CATL + #.200 AFG3 + #.100 xAFG1
+ #.100 xAFG2 + #.100 xGLY + #.100 xMGLY + #.200 yR6OOH + #.500 XC
Ozone - benzene/low NOx run
0.25
Ozone (ppm)
0.2
0.15
O3 chamber
O3 SAPRC11
0.1
0.05
0
9:36
-0.05
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 20. Ozone concentrations for the benzene/low NOx experiment. The blue
diamonds are the chamber measurements (with 5% error bars), the pink squares are the
SAPRC11 predictions.
20
OH - benzene/low NOx
1.2E-06
1.0E-06
OH (ppm)
8.0E-07
6.0E-07
OH ppm chamber
4.0E-07
OH SAPRC11
2.0E-07
0.0E+00
9:36:00 10:48:00 12:00:00 13:12:00 14:24:00 15:36:00
-2.0E-07
-4.0E-07
Time (minutes)
Figure 21. OH concentrations for the benzene/low NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares).
HO2 - benzene/low NOx
0.0003
HO2 (ppm)
0.00025
0.0002
HO2 ppm chamber
0.00015
HO2 SAPRC11
0.0001
0.00005
0
9:36:00 10:48:00 12:00:00 13:12:00 14:24:00 15:36:00
Time (minutes)
Figure 22. HO2 concentrations for the benzene/low NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares).
21
Ozone - benzene/low NOx run
0.25
Ozone (ppm)
0.2
0.15
O3 chamber
0.1
O3 SAPRC11
0.05
O3 SAPRC11 with 0.15 xHO2
0
9:36
-0.05
10:48 12:00 13:12 14:24 15:36
Local Time
Figure 23. Ozone concentrations for the benzene/low NOx experiment. The blue
diamonds are the chamber measurements (with 5% error bars), the pink squares are the
SAPRC11 predictions, and the triangles are the modified SAPRC11 predictions.
HO2 - benzene/low NOx
3.0E-04
HO2 ppm chamber
2.5E-04
HO2 (ppm)
HO2 SAPRC11
2.0E-04
HO2 SAPRC11 with 0.15
xHO2
1.5E-04
1.0E-04
5.0E-05
0.0E+00
9:36:00 10:48:0 12:00:0 13:12:0 14:24:0 15:36:0
0
0
0
0
0
Time (minutes)
Figure 24. HO2 concentrations for the benzene/low NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares), and the modified
SAPRC11 with lower xHO2 yields.
22
HCHO - benzene/low NOx experiment
0.02
HCHO (ppm)
0.015
0.01
HCHO (measured)
HCHO SAPRC11
0.005
0
9:36
-0.005
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 25. Formaldehyde concentrations for the benzene/low NOx experiment. . The
blue diamonds are the chamber measurements (with 10% error bars), the pink squares are
the SAPRC11 predictions.
Catechol - benzene/low NOx experiment
0.0030
Catechol (ppm)
0.0025
0.0020
Catechol
CATL SAPRC11
0.0015
0.0010
0.0005
0.0000
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 26. Catechol concentrations for the benzene/low NOx experiment. . The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC11 predictions.
23
Phenol (ppm)
Phenol - benzene/low NOx experiment
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
9:36
Phenol
PHEN SAPRC11
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 27. Phenol concentrations for the benzene/low NOx experiment. . The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC11 predictions.
Nitrophenol (ppm)
Nitrophenol - benzene/low NOx experiment
0.0045
0.0040
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
9:36
Nitrophenol
NPHE SAPRC11
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 28. Nitrophenol concentrations for the benzene/low NOx experiment. . The blue
diamonds are the chamber measurements (with 10% error bars), the pink squares are the
SAPRC11 predictions.
24
catechol may be losses to the chamber walls, although this does not seem likely
considering the size of the chamber (~200 m3). One explanation for the overprediction of
nitrophenol is there may not be as many NOx sink products formed from the phenoxy +
NO2 reactions as are predicted by SAPARC11.
For the high NOx experiment (in which HONO was injected into the chamber at the
beginning of the experiment), SAPRC11 overpredicted ozone by 63% (Figure 29).
SAPRC11 also overpredicted HCHO formation by 149% (see Figure 30). Concentrations
predicted by SAPRC11 for the products phenol (PHEN), and nitrophenol (NPHE) were
overpredicted by 53% and 206%, while catechol (CATL) was underpredicted by 71% for
the high NOx experiment, respectively (Figures 31, 32 and 33). In the case of OH and
HO2 for the high NOx experiment, SAPRC11 predictions were 7.8% below and 67%
above peak observed values, respectively (Figures 34 and 35), with the peak HO2
occurring 1 ½ hours before the observed peak.
When the benzene/high NOx experiment was performed employing the changes
mentioned above for the low NOx experiment, the maximum predicted ozone was
lowered by 11 ppb, still well above observed values (Figure 36).
O3 (ppm)
Ozone - benzene/high NOx expt
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
9:36
Ozone (ppm)
O3 SAPRC11
12:00
14:24
16:48
Local Time
Figure 29. Ozone concentrations for the benzene/high NOx experiment: chamber
measurements (with 5% error bars - blue diamonds), SAPRC11 (pink squares).
25
Formaldehyde - benzene/high NOx expt
0.035
Formaldehyde
0.03
0.025
0.02
formaldehyde (ppm)
HCHO SAPRC11
0.015
0.01
0.005
0
9:36:00 10:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00
Time
Figure 30. Formaldehyde concentrations for the benzene/high NOx experiment:
chamber measurements (with 10% error bars - blue diamonds), SAPRC11 (pink squares).
Phenol - benzene/high NOx expt
0.025
Phenol (ppm)
0.020
phenol (ppm)
PHEN SAPRC11
0.015
0.010
0.005
0.000
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Local Time
Figure 31. Phenol concentrations for the benzene/high NOx experiment: chamber
measurements (with 10% error bars - blue diamonds), SAPRC11 (pink squares).
26
Catechol - benzene/high NOx expt
0.006
Catechol (ppm)
0.005
0.004
catechol (ppm)
CATL SAPRC11
0.003
0.002
0.001
0
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 32. Catechol concentrations for the benzene/high NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares).
Nitrophenol (ppm)
nitrophenol - benzene/high NOx expt
0.01
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
9:36
nitrophenol (ppm)
NPHE SAPRC11
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 35. Nitrophenol concentrations for the benzene/high NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares).
27
OH -- benzene/High NOx expt
1.6E-06
1.4E-06
OH (ppm)
1.2E-06
1.0E-06
8.0E-07
OH (ppm) chamber
OH SAPRC11
6.0E-07
4.0E-07
2.0E-07
0.0E+00
-2.0E-079:36
10:48 12:00 13:12 14:24 15:36 16:48
Local Time
Figure 34. Hydroxyl (OH) concentrations for the benzene/high NOx experiment:
chamber measurements (blue diamonds), SAPRC11 predictions (pink squares).
HO2 -- benzene/High NOx expt
3.0E-04
HO2 (ppm)
2.5E-04
2.0E-04
HO2 chamber (ppm)
HO2 SAPRC11
1.5E-04
1.0E-04
5.0E-05
0.0E+00
9:36
12:00
14:24
16:48
Local Time
Figure 35. HO2 concentrations for the benzene/high NOx experiment: chamber
measurements (blue diamonds), SAPRC11 predictions (pink squares).
28
Ozone - benzene/high NOx expt
0.40
0.35
Ozone (ppm)
O3 (ppm)
0.30
0.25
O3 SAPRC11
0.20
0.15
O3 SAPRC11 -- lower
xHO2
0.10
0.05
0.00
9:36
10:48
12:00
13:12
14:24
15:36
16:48
Local Time
Figure 36. Ozone concentrations for the benzene/high NOx experiment: chamber
measurements (with 5% error bars - blue diamonds), SAPRC11 (pink squares), modified
SAPRC11 (triangles) with lower xHO2.
SAPRC11A
SAPRC11A is based upon the improvements made with SAPRC11, using the same base
mechanism but with an updated benzene scheme containing the benzene-OH adduct (here
called BENZOH) which may react with both O2 and NO2 under high NOx conditions
found in some chamber experiments (>200 ppb NO2), with ring-opening products used in
SAPRC07 (e.g., AFG1) plus new ring-opening products (e.g., AFG3C) which react
further to form condensable products (CNDAP). Table 6 shows the benzene scheme for
SAPRC11A (Carter, 2012). For the low NOx run the SAPRC11A mechanism predicts
the peak ozone concentration to be 226 ppb, 20% higher than the observed peak value of
189 ppb (see Figure 37).
SAPRC11A predictions for hydroxyl (OH) and HO2 were also compared to observed
values for the low NOx run (Figures 38 and 39, respectively). Predicted OH and HO2
concentrations peaked at a time comparable to observations, and the peak height of both
are in good agreement with measurements.
SAPRC11A output was compared to chamber observations of nitrophenol, phenol,
catechol and formaldehyde as well (Figures 40 through 43). Figure 40 shows the
SAPRC11A predictions for nitrophenol, for which the maximum concentration predicted
by SAPRC11A was 29% higher than what was measured. One possible reason for this
difference is the similar as for SAPRC07 – the yield of BZO for the PHEN + OH reaction
is 0.1, rather than 0.06. When the yield of BZO was lowered to 0.06, the modeled peak
NPHE concentration was lowered to 0.0015 ppm, 15% lower than the original
SAPRC11A mechanism, and 32% higher than the observed peak concentration.
29
Table 6. Benzene scheme of SAPRC11A.
Rate
Reactants
Products
Parameters
A = 2.33e-12
BENZ + OH =
BENZOH
Ea = 0.384
k(300) =
BENZOH + NO2 =
NO + HO2 + AFG3
3.60e-11
k(300) =
BENZOH + O2 +
#.027 RO2XC + #.31 RO2C + #.57 HO2 +#.31
4.30e-18
#BZNF =
xHO2 + #.027 zRNO3 + #.337 yR6OOH +
#.57 PHEN + #.31 xGLY + #.17 xAFG1 +
(BZNF = 0.02)
#.14 xAFG2 + #.093 OH +#.093 AFG3 +
#.065 AFG3C + #1.929 XC
k(300) =
AFG3C + OH =
CNDAP
9.35e-11
k(300) =
AFG3C + O3 =
1.43e-17
Figures 41 and 42 compare the measured catechol and phenol concentrations with the
SAPRC11A predictions for the benzene/low NOx run. Unlike SAPRC07, SAPRC11A
overpredicts the catechol concentrations with the maximum concentration occurring at
the end of the run. This implies there is insufficient removal of catechol in the
mechanism. Meanwhile, SAPRC11A underpredicts phenol concentrations by 32%,
mainly due to the fact that phenol is not formed in the competing aromatic-OH adduct +
NO2 reaction.
Figure 43 shows the measured concentrations of formaldehyde and the SAPRC11A
predictions throughout the duration of the benzene/low NOx experiment. The model
overpredicted the peak HCHO concentration by 91% (the stated 2 uncertainty for
HCHO was 10%).
For the benzene/high NOx experiment, SAPRC11A generated a maximum ozone
concentration of 393 ppb, while the observed ozone peak ozone value was 225 ppb, a
difference of 74% (Figure 44). Figures 45 and 46 show the OH and HO2 concentrations
for the benzene/high NOx experiment with both chamber measurements and SAPRC11A
output. In both cases, the simulated values peaked earlier than the observed values.
Figure 47 illustrates the formaldehyde concentrations, comparing the measured values for
the high NOx run and the predictions generated by SAPRC11A. The model
overpredicted the peak formaldehyde concentration by 239%. Figure 48 shows the
comparison for nitrophenol for the benzene/high NOx case, in which SAPRC11A
underpredicted the peak concentration by 7%. In the case of catechol and phenol,
SAPRC11A underpredicted the peak concentration by 92% and 42%, respectively (see
Figures 49 and 50).
30
Ozone - benzene/low NOx run
0.25
Ozone (ppm)
0.20
0.15
O3 chamber
O3 SAPRC11A
0.10
0.05
0.00
9:36
-0.05
10:48
12:00
13:12
14:24
15:36
Time
Figure 37. Ozone concentrations for the benzene/low NOx experiment. The blue
diamonds are the chamber measurements (with 5% error bars), the pink squares are the
SAPRC11A predictions.
OH - benzene/low NOx
1.2E-06
1.0E-06
OH (ppm)
8.0E-07
6.0E-07
OH ppm chamber
OH SAPRC11A
4.0E-07
2.0E-07
0.0E+00
9:36
-2.0E-07
10:48
12:00
13:12
14:24
15:36
-4.0E-07
Time (minutes)
Figure 38. Comparison of hydroxyl radical concentrations for the benzene/low-NOx experiment.
The chamber observations are in blue diamonds and model predictions in pink squares.
31
HO2 - benzene/low NOx expt
0.00025
HO2 ppm chamber
HO2 SAPRC11A
HO2 (ppm)
0.0002
0.00015
0.0001
0.00005
0
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 39. Comparison of HO2 concentrations for the benzene/low-NOx experiment.
The chamber observations are in blue diamonds and model predictions in pink squares.
Nitrophenol (ppm)
Nitrophenol - benzene/low NOx experiment
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
9:36
Nitrophenol
NPHE SAPRC11A
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 40. Nitrophenol concentrations for benzene/low NOx run. Blue diamonds are
chamber measurements and pink squares are SAPRC11A model output.
32
Catechol (ppm)
Catechol - benzene/low NOx experiment
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
9:36
Catechol
CATL SAPRC11A
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 41. Measured catechol concentrations (blue diamonds) compared to the
SAPRC11A species CATL (pink squares).
Phenol (ppm)
Phenol - benzene/low NOx experiment
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
9:36
Phenol
PHEN SAPRC11A
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 42. . Measured phenol concentrations (blue diamonds) compared to the
SAPRC11A species PHEN (pink squares).
33
HCHO - benzene/low NOx experiment
0.03
HCHO (ppm)
0.025
0.02
HCHO (measured)
HCHO SAPRC11A
0.015
0.01
0.005
0
-0.0059:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 43. Measured formaldehyde concentrations for the benzene/low NOx experiment
(blue diamonds) compared to the SAPRC11A species HCHO (pink squares).
O3 (ppm)
Ozone - benzene/high NOx expt
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
9:36
Ozone (ppm)
O3 SAPRC11A
10:48
12:00
13:12
14:24
15:36
16:48
Local Time
Figure 44. Ozone concentrations (with 5% error bars for observed values) for
benzene/high NOx experiment compared to SAPRC11A: chamber measurements (blue
diamonds), SAPRC11A (pink squares).
34
OH -- benzene/High NOx expt
1.6E-06
1.4E-06
OH (ppm)
1.2E-06
1.0E-06
8.0E-07
OH (ppm) chamber
OH SAPRC11A
6.0E-07
4.0E-07
2.0E-07
0.0E+00
-2.0E-079:36 10:48 12:00 13:12 14:24 15:36 16:48
Local Time
Figure 45. Hydroxyl (OH) concentrations for the benzene/high NOx experiment.
Chamber measurements are blue diamonds, the SAPRC11A simulation are pink squares.
HO2 -- benzene/High NOx expt
2.5E-04
HO2 (ppm)
2.0E-04
1.5E-04
HO2 chamber (ppm)
HO2 SAPRC11A
1.0E-04
5.0E-05
0.0E+00
9:36
12:00
14:24
16:48
Local Time
Figure 46. HO2 concentrations for the benzene/high NOx experiment. Chamber
measurements are blue diamonds, the SAPRC11A simulation are pink squares.
35
O3
Formaldehyde - benzene/high NOx expt
0.045
0.04
0.035
0.03
0.025
0.02
0.015
0.01
0.005
0
9:36:00
formaldehyde (ppm)
HCHO SAPRC11A
10:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00
Time
Figure 47. Formaldeyde concentrations: chamber measurements (blue diamonds) and
SAPRC11A predictions (pink squares).
nitrophenol - benzene/high NOx expt
0.0035
Nitrophenol (ppm)
0.003
0.0025
nitrophenol (ppm)
NPHE SAPRC11A
0.002
0.0015
0.001
0.0005
0
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 48. Nitrophenol concentrations: chamber measurements (blue diamonds) and
SAPRC11A predictions (pink squares).
36
Catechol - benzene/high NOx expt
0.006
Catechol (ppm)
0.005
0.004
catechol (ppm)
CATL SAPRC11A
0.003
0.002
0.001
0
9:36:00
10:48:00
12:00:00
13:12:00
14:24:00
15:36:00
Local Time
Figure 49. Catechol concentrations: chamber measurements (blue diamonds) and
SAPRC11A predictions (pink squares).
Phenol - benzene/high NOx expt
0.016
Phenol (ppm)
0.014
0.012
0.01
phenol (ppm)
PHEN SAPRC11A
0.008
0.006
0.004
0.002
0
9:36:00
10:48:00 12:00:00 13:12:00 14:24:00 15:36:00
Local Time
Figure 50. Phenol concentrations: chamber measurements (blue diamonds) and
SAPRC11A predictions (pink squares).
37
The SAPRC11A mechanism was designed to accommodate both low and high NOx
chamber experiments, but does not provide adequate agreement with observations
obtained during the EXACT campaign. The mechanism allows for the aromatic-OH
adduct to react with either O2 or NO2 by allowing the total rate constants for these
competing reactions to be treated as an adjustable parameter to minimize the NOxdependence on the model error in simulating the aromatic - NOx chamber experiments.
This adjustable parameter, BZNF, was varied for this study, with values ranging from
0.001 to 1.00, to explore its influence on ozone predictions. The results are illustrated in
Figure 51, showing that BZNF values between 0.02 to 1.00 gave similar values for ozone
during the simulation, while BZNF values of 0.001 and 0.005 give results that provide
better agreement with the timing of the ozone peak, although the actual value of the
maximum of the simulated ozone peak is not much changed.
simulated O3 (ppm)
Ozone - benzene/high NOx expt
5.0E-01
4.5E-01
4.0E-01
3.5E-01
3.0E-01
2.5E-01
2.0E-01
1.5E-01
1.0E-01
5.0E-02
0.0E+00
9:36
O3
O3
O3
O3
O3
O3
12:00
14:24
BZNF = 1.00
BZNF = 0.80
BZNF = 0.20
BZNF = 0.02
BZNF = 0.005
BZNF = 0.001
16:48
Local Time
Figure 51. Simulated ozone concentrations for benzene/high NOx experiment compared
with varying BZNF values for SAPRC11A.
Comparison of the three versions of the SAPRC mechanism: SAPRC07, SAPRC11
and SAPRC11A for ozone and NOx.
Figure 52 shows the comparison of the three versions of SAPRC output with the chamber
data for ozone for the benzene/low NOx experiment. SAPRC07 provided the peak ozone
concentration closest to the measured value and closest to the time of the peak, while
SAPRC11 and SAPRC11A overpredicted ozone values throughout the duration of the
simulation. The mechanisms performances were mirrored for NO (Figure 53) with
SAPRC07 predicting NO decay at the slowest rate, and SAPRC11 and SAPRC11A
underpredicting the observed NO concentrations. Figure 54 shows the NO2
concentrations for the low NOx experiment, comparing observations to model
38
Ozone - benzene/low NOx run
0.25
0.2
Ozone (ppm)
O3 chamber
0.15
O3 SAPRC07
O3 SAPRC11
0.1
O3 SAPRC11A
0.05
0
9:36
-0.05
10:48 12:00 13:12 14:24 15:36
Local Time
Figure 52. Ozone concentrations for the benzene/low NOx experiment: chamber
measurements with 5% error bars (2) (blue diamonds), SAPRC07 (pink squares),
SAPRC11 (triangles), and SAPRC11A (blue circles).
NO - benzene/low NOx
5.0E-02
4.5E-02
4.0E-02
NO (ppm)
3.5E-02
NO chamber
NO SAPRC07
NO SAPRC11
NO SAPRC11A
3.0E-02
2.5E-02
2.0E-02
1.5E-02
1.0E-02
5.0E-03
0.0E+00
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 53. NO concentrations for the benzene/low NOx experiment: chamber
measurements (blue diamonds), SAPRC07 (pink squares), SAPRC11 (green diamonds),
and SAPRC11A (orange circles).
39
NO2 - benzene/low NOx
4.0E-02
3.5E-02
NO2 (ppm)
3.0E-02
NO2 chamber
NO2 SAPRC07
NO2 SAPRC11
NO2 SAPRC11A
2.5E-02
2.0E-02
1.5E-02
1.0E-02
5.0E-03
0.0E+00
9:36
10:48
12:00
13:12
14:24
15:36
Local Time
Figure 54. NO2 concentrations for the benzene/low NOx experiment: chamber
measurements (blue diamonds), SAPRC07 (pink squares), SAPRC11 (triangles), and
SAPRC11A (green circles).
predictions. SAPRC11 and SAPRC11A predicted the peak NO2 to be earlier than
observed, while SAPRC07 predicted the peak NO2 value to be 30 minutes later than
observed. All three mechanisms underpredicted the peak NO2 value by approximately
25%.
Figures 55 through 57 show the comparisons between chamber measurements and
predictions of three SAPRC mechanisms for the benzene/high NOx experiment. For the
case of ozone, SAPRC07 most closely matches the chamber observations with a
maximum ozone concentration of 244 ppb while the maximum observed was 225 ppb, a
difference of 8.6%. SAPRC11 and SAPRC11A overpredicted ozone by 74% and 87%,
respectively (Figure 55). These trends are mirrored in the NO predictions, with
SAPRC07 most closely matching observed values while SAPRC11 and SAPRC11A
predictions are lower than observed values (Figure 56). Regarding NO2, SAPRC07 most
closely matched observations, with SAPRC11 and SAPRC11A underpredicting NO2
concentrations for most of the simulation (Figure 57).
40
Ozone - benzene/high NOx expt
0.45
0.40
O3 (ppm)
0.35
0.30
Ozone (ppm)
O3 SAPRC07
O3 SAPRC11
O3 SAPRC11A
0.25
0.20
0.15
0.10
0.05
0.00
9:36
10:48
12:00
13:12
14:24
15:36
16:48
Local Time
Figure 55. Ozone for high NOx experiment with 5% error bars (2) on chamber measurements:
chamber (blue), SAPRC07 (pink squares), SAPARC11 (triangles), and SAPRC11A (green
circles)
NO - benzene/high NOx expt
0.12
NO (ppm)
0.10
0.08
NO (measured)
NO SAPRC07
NO SAPRC11
NO SAPRC11A
0.06
0.04
0.02
0.00
9:36
12:00
14:24
16:48
Local Time
Figure 56. NO measurements and predictions for high NOx experiment: chamber (blue
diamonds), SAPRC07 (pink squares), SAPARC11 (triangles), and SAPRC11A (green circles).
41
NO2 - benzene high NOx
0.12
0.10
NO2
0.08
NO2 (measured)
NO2 SAPRC07
NO2 SAPRC11
NO2 SAPRC11A
0.06
0.04
0.02
0.00
9:36
12:00
14:24
16:48
Time
Figure 57. NO2 measurements and predictions for high NOx experiment: chamber (blue
diamonds), SAPRC07 (pink squares), SAPARC11 (triangles), and SAPRC11A (green
circles).
Conclusions and Implications
Three versions of the SAPRC mechanism (SAPRC07, SAPRC11 and SAPRC11A) were
used to simulate chamber experiments with benzene under both low NOx and high NOx
conditions in the EUPHORE chamber, which were run as part of the EXACT campaign.
For the low NOx experiment, only SAPRC07 predictions were within 10% of the
observed peak ozone concentrations. All three mechanisms were less successful in
predicting product yields, both ring-opening (e.g., glyoxal) and ring-retaining (e.g.,
phenol).
For the high NOx experiment, SAPRC07 predictions for peak ozone levels come closest
to agreeing with the observed peak ozone concentrations, while SAPRC11 and
SARPC11A had significantly higher predicted peak ozone values. Regarding both ringretaining and ring-opening products product concentrations, none of the three
mechanisms was able to match observed values within the measurement error. The
greatest discrepancy was with nitrophenol, with the mechanisms overpredicting peak
concentrations by a factor of 3 or more.
There is increased interest recently in predicting secondary organic aerosol (SOA) more
accurately. If this is to be achieved, mechanisms must not only be able to predict ozone
formation from its precursors, but SOA precursors as well. Degradation products of
aromatic oxidation are currently believed to be SOA precursors, such as glyoxal,
42
methylglyoxal, and nitrophenols (Hallquist et al., 2009 and references therein). More
product information from aromatic oxidation is needed in order to predict SOA precursor
concentrations correctly. New product information for the degradation of aromatic
compounds is now becoming available. For example, Arey et al., 2009 report yields of
dicarbonyl species from reactions of hydroxyl radicals with toluene, xylenes and
trimethyl benzenes using a Teflon reaction chamber. However, this data should be used
with caution as these dicarbonyl species are not commercially available, and the analysis
was done by mass spectrometry. Because mass spectrometers do not respond equally to
every species it ionizes, such data are not quantitative.
Recommendations
Recommendations are as follows: the use of different ring-opening products for benzene
and the other aromatics so that each degradation pathway may be tuned for each aromatic
is suggested. For example, methylglyoxal is not observed from the oxidation of benzene,
and yet it is formed during benzene oxidation in amounts comparable to glyoxal by each
version of the SAPRC mechanism. Also, it would be useful to compare SAPRC
mechanism predictions to more product information (such as the sort provided by the
EXACT campaign), especially if the modeling of particulate formation is desired. The
current lack of reliable product information in aromatic schemes in general is a
significant obstacle in forming more speciated and explicit mechanisms which are
necessary for accurate SOA model predictions. Current models which predict SOA
formation are largely parameterized and are based on few studies in which models are
“calibrated” to match observations.
More specific recommendations include lowering the BZO yield from the reaction of
cresol with hydroxyl radicals, removing the product MGLY from the benzene scheme.
While removing MGLY from the benzene scheme would involve creating new ringopening model species, this would allow tuning the degradation scheme to each aromatic
without making compromises which introduce error into the predicted products. While
there is a current lack of information on ring-opening products, model species may be
designed to contribute the necessary radicals (or radical consuming species) to match
current observations of radical formation and termination in laboratory experiments.
An additional recommendation is to compare model output to field observations in
ambient atmospheres, rather than only chamber experiments, which contain
parameterized wall reactions based on the mechanism employed.
Acknowledgements
This work was carried out at the University of California at Riverside (UCR) and was
supported by the California Air Resources Board (CARB) primarily through Contract 07730, with additional support provided through Contract 08-326. Dr. William P. L. Carter
was the Principal Investigator for these projects. The author would to thank Ajith
Kaduwela of the CARB for his support for this project, Dr. Gookyoung Heo of UCR for
helpful discussions, and Dr. William Carter of UCR for assistance with the modeling
43
inputs. However, the contents of this report reflect only the conclusions of the author and
not the CARB or UCR.
References
Arey, J.; Obermeyer, G.; Aschmann, S. M.; Chattopadhyay, S.; Cusick, R. D.; Atkinson,
R., “Dicarbonyl Products of the OH Radical-Initiated Reaction of a Series of Aromatic
Hydrocarbons,” Environ. Sci. Technol., 43, 683, 2009.
Atkinson, R. and J. Arey, “Atmospheric Degradation of Volatile Organic Compounds,”
Chem. Rev., 103, 4605-4638, 2003.
Birdsall A.W. and M. J. Elrod, “Comprehensive NO-Dependent Study of the Products of
the Oxidation of Atmospherically Relevant Aromatic Compounds,” J. Phys. Chem. A,
115, 5397–5407, 2011.
Bloss, W. J., J. D. Lee, C. Bloss, K. Wirtz, M. Martin-Reviejo, M. Siese, D. E. Heard and
M. J. Pilling, “Validation of the calibration of a laser-induced fluorescence instrument for
the measurement of OH radicals in the atmosphere,” Atmos. Chem. Phys., 4, 571, 2004.
Bloss, C., V. Wagner, A. Bonzanini, M. E. Jenkin, K.Wirtz, M. Martin-Reviejo, and M. J.
Pilling, “Evaluation of detailed aromatic mechanisms (MCMv3 and MCMv3.1) against
environmental chamber data”, Atmos. Chem. Phys., 5, 623–639, 2005.
Berndt, Torsten and Olaf Böge, “Formation of phenol and carbonyls from the
atmospheric reaction of OH radicals with benzene,” Phys. Chem. Chem. Phys., 8, 1205–
1214, 2006.
Carter, W.P.L., “Development Of Revised Saprc Aromatics Mechanisms,” Draft Report
to the California Air Resources Board, Contract No. 07-730, 2012.
Chen et al., S. Chen, X. Ren, J. Mao, Z. Chen, W.H. Brune, B. Lefer, B. Rappenglück, J.
Flynn, J. Olson and J.H. Crawford, “A comparison of chemical mechanisms based on
TRAMP-2006 field data,” Atmos. Environ. 44 (33), pp. 4116–4125, 2010.
Coeur-Tourneur, C., Henry, F., Janquin, M.-A., and Brutier, L.: Gas-phase reaction of
hydroxyl radicals with m-, o- and p-cresol, Int. J. Chem. Kinet., 38, 553–562 (2006).
Gomez Alvarez, E. G., Viidanoja, J., Munoz, A., Wirtz, K., and Hjorth, J.: Experimental
confirmation of the dicarbonyl route in the photo-oxidation of toluene and benzene,
Environ. Sci. Technol., 41, 8362–8369, 2007.
Carter, W. P.L., “Development of the SAPRC-07 chemical mechanism”, Atmos. Environ,
In Press, 2010.
Hallquist et al., “The formation, properties and impact of secondary organic aerosol:
44
current and emerging issues,” Atmos. Chem. Phys., 9, 5155, 2009.
IARC, Monographs on the evaluation of carcinogenic risk of chemicals to humans, 29(7),
120.
Killus, J. P. andWhitten, G. Z.: Background reactivity in smog chambers, Int. J. Chem.
Kinet., 22, 547–575, 1990.
Koch, R., Knispel, R., Elend, M., Siese, M., and C. Zetzsch, “Consecutive reactions of
aromatic-OH adducts with NO, NO2 and O2: benzene, naphthalene, toluene, m- and pxylene, hexamethylbenzene, phenol, m-cresol and aniline,” Atmos. Chem. Phys., 7, 2057–
2071, 2007.
Mollner, A.K., Valluvadasan, S., Feng,L., Sprague, M.K., Okumura, M, Milligan,D.B.,.
Bloss, W.J., Sander, S.P.,. Martien, P.T., Harley, R.A. McCoy, A.B., Carter, W.P.L.,
“Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen Dioxide,” Science,
330, 646-649, 2010.
Sander, S. P.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Kurylo, M. J.; Molina,
M. J.; Moortgat, G. K.; Finlayson-Pitts, B. J.; Wine, P. H.; Huie, R. E.; Orkin, V. L.
“Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation
Number 15.” Jet Propulsion Laboratory: Pasadena, CA, 2006.
Volkamer R., Platt, U., and K. Wirtz, “Primary and Secondary Glyoxal formation from
Aromatics: Experimental Evidence for the Bicycloalkyl-Radical Pathway from Benzene,
Toluene, and p-Xylene,” J. Phys. Chem. A, 105, 7865-7874, 2001.
Volkamer, Rainer, Björn Klotz, Ian Barnes, Takashi Imamura, Klaus Wirtz, Nobuaki
Washida, Karl Heinz Becker and Ulrich Platt, “OH-initiated oxidation of benzene. Part I.
Phenol formation under atmospheric conditions”, Phys. Chem. Chem. Phys., 4, 1598–
1610, 2002.
Volkamer, R., Spietz, P., Burrows, J., Platt, U., “High-resolution absorption cross-section
of Glyoxal in the UV/vis and IR spectral ranges,” J. Photochem. Photobiol. A, 172, 35–
46, 2005.
Xiang, B. and L. Zhu, “Absorption cross sections of E,E-2,4-hexadienedial at 248 nm,
and in the 290–430 nm region, and photolysis study at 248, 308, and 351 nm,” Chemical
Physics Letters, 450, 31–38, 2007.
Zádor, J., Wagner, V., Wirtz, K., and Pilling, M. J.: Quantitative assessment of
uncertainties for a model of tropospheric ethene oxidation using the European
Photochemical Reactor, Atmos. Envir., 39, Issue 15, Pages 2805-2817, 2005.
45