re

cDonDavid T. Allen a

aCenter for Energy and Environmental Resources, The UbCollege of Engineering, Center for Environmental Reseac ENVIRON International Corporation, 773 San Marin Dr

t Reyes

d widecitly me and iscomme

Received 26 November 2011Predictions of ozone formation, due to oxidation of alkenes in presence of NOx, generated by the CarbonBond 2005 (CB05) and CB05-TU (CB05 with an updated toluene scheme) condensed chemical mecha-

3through complex chemical reactions. Therefore, a reliable chemicalmechanism is necessary to describe these complex processesrelevant to O3 formation. Carbon Bond (CB; Whitten et al., 1980,

U.S. for various air quality applications to model gas-phase chem-istry related to O3 formation. All chemical mechanisms used in airquality models are simplied and condensed to some degree,especially those for use in 3-dimensional atmospheric modelsneeded to represent the atmospheric reactions of various volatileorganic compounds (VOCs) and nitrogen oxides (NOx) underambient conditions (Dodge, 2000). For example, in CB05 (Yarwoodet al., 2005) and SAPRC-07 (Carter, 2010a), a few model species(e.g., OLE and IOLE in CB05; OLE1 and OLE2 in SAPRC-07) are used

* Corresponding author. Present address: College of Engineering, Center forEnvironmental Research and Technology, University of California, Riverside, CA92521, USA. Tel.: 1 951 781 5708; fax: 1 951 781 5790.

Contents lists available at

Atmospheric E

journal homepage: www.else

Atmospheric Environment 59 (2012) 141e150E-mail address: gookyoung.heo@gmail.com (G. Heo).AlkeneAlkene chemistryOzoneChemical mechanismCB05

experiments and box modeling for four cases (1 for propene and 3 for isobutene) showed that theperformance of CB05 and CB05-TU in simulating ozone formation from propene and isobutene can beimproved by modeling propene and isobutene using the new condensed reactions for propene andisobutene developed in this work. The results of this study indicate that the capability of condensedchemical mechanisms in simulating ozone formation can be improved by (1) examining the relativeimportance of VOCs based on their emissions and reactivity, (2) separately representing relativelyimportant VOCs in the mechanism, (3) modeling less important compounds using reactions of lumpedmodel species shared by multiple compounds, and (4) evaluating mechanisms with experimental datasuch as environmental chamber data.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Ozone (O ) is not directly emitted but formed in the atmosphere

2010; Gery et al., 1989; Yarwood et al., 2005) chemical mechanisms,along with Statewide Air Pollution Research Center (SAPRC; Carter,1990, 2000, 2010a,b,c) chemical mechanisms, have been used in theReceived in revised form21 May 2012Accepted 25 May 2012

Keywords:

nisms were tested by simulating 138 environmental chamber experiments carried out in 7 differentenvironmental chambers and by running box modeling with four cases. CB05 and CB05-TU reasonablysimulated ozone formation from propene under typical urban conditions and for cases with moderatelyelevated propene concentrations. Chamber simulations of 47 propene e NOx and 5 isobutene e NOxd Smog Reyes, PO Box 518, 112 Mesa Road, Poin

h i g h l i g h t s

< The CB05 chemical mechanism is use< New reactions are developed to expli

to represent various alkenes (Table 1), and, instead of detailedreactions of real compounds, highly condensed reactions of thesemodel species are used in air quality models. In this way, a limitednumber of reactions represent the atmospheric reactions of manycompounds (such as alkenes), reducing computational burdens ofair quality modeling (e.g., computational time) and allowingavailable resources to be used for improving other components ofthe air quality modeling (e.g., representing diffusion and transportin a better way).

Alkenes are a major contributor to O3 formation in many areasdue to their relatively high reactivity (Calvert et al., 2000) and largeemissions from various sources (Simon et al., 2010). For example,propene and isobutene react with the hydroxyl radical (OH) fasterthan most alkanes and aromatics commonly observed in urbanareas (Atkinson and Arey, 2003; Calvert et al., 2000). In southeastTexas, 7 alkenes (ethene, propene, 1,3-butadiene, 1-butene, iso-butene, trans-2-butene, cis-2-butene) are classied as HighlyReactive Volatile Organic Compounds (HRVOCs), and their emis-sions from point sources, including event emissions from petro-chemical facilities (Murphy and Allen, 2005), are regulated by TexasAdministrative Code, Title 30, Part 1, Chapter 115 (Texas Commis-

G. Heo et al. / Atmospheric Envir142sion on Environmental Quality (TCEQ), 2011). Since the rateparameters, products and their yields in the CB05 and SAPRC-07condensed mechanisms are based on assumptions about thetypical urban air composition of alkenes (Seila et al., 1989), differ-ences between the assumed average atmospheric compositionused during mechanism development and the real-world atmo-spheric composition affect the performance of the mechanisms. Forexample, in Houston, Texas, propene was found to frequentlyexceed its typical concentrations observed in most urban areas(Jobson et al., 2004; Gilman et al., 2009). As a result, in Houston, theatmospheric composition of alkenes is often markedly differentfrom the typical composition in most urban areas in the U.S.(e.g., see Table 3 of Heo et al. (2010)). Therefore, using the limitednumber of reactions based on the typical composition of alkenescan lead to inaccurate predictions of air pollutants in areas such asHouston. Heo et al. (2010) showed that using the current reactionsfor OLE1 (a model species for most terminal alkenes) in the xed-parameter version of SAPRC-07 (Carter, 2010a,c) leads to under-predictions of O3 concentrations under conditions where thehydrocarbon reactivity is dominated by propene. Using environ-mental chamber simulations and box modeling, Heo et al. (2010)

Table 1Examples of alkene speciation for CB05. Speciation for SAPRC-07 is also shown forcomparison.

Compound CB05b SAPRC-07c

Ethene (CH2]CH2)a ETHa Ethenea

Propene (CH3CH]CH2) OLE PAR OLE11-Butene (CH3CH2CH]CH2) OLE 2 PAR OLE11-Pentene (CH3CH2CH2CH]CH2) OLE 3 PAR OLE13-Methyl-1-butene (CH3CH(CH3)CH]CH2) OLE 3 PAR OLE11,3-Butadiene (CH2]CHCH]CH2) 2 OLE OLE22-Butene (CH3CH]CHCH3) IOLE OLE22-Pentene (CH3CH2CH]CHCH3) IOLE PAR OLE2Isobutene ((CH3)2C]CH2) FORM 3 PAR OLE22-Methyl-2-butene (CH3CH]C(CH3)2) ALD2 3 PAR OLE22-Methyl-2-pentene (CH3CH2CH]C(CH3)2) ALDX 4 PAR OLE2a ETHENE in SAPRC-07 and ETH in CB05 are a model species only for ethene.b In CB05, OLE and IOLE represent terminal C]C bonds and C-4 structures

(CeCCeC) having an internal C]C bond, respectively. PAR represents parafniccarbons. FORM is formaldehyde (HCHO), ALD2 is acetaldehyde (CH3CHO), and ALDXrepresents propanal (CH3CH2CHO) and higher aldehydes. For details, see Yarwoodet al. (2005).

c In SAPRC-07, OLE1 represents alkenes with kOH < 7.0 104 ppm1 min1, and

OLE2 representsmore reactive alkenes except isoprene and terpenes. For details, seeCarter (2010a).showed that separately modeling reactions of individual alkenes(especially propene, for southeast Texas) in the SAPRC condensedmechanism has the potential to lead to more accurate simulationsof O3 formation in Houston. Since similar results may be expectedfor CB05, this study assesses the capability of CB05 and CB05-TU(CB05 with an updated toluene scheme; Whitten et al., 2010) insimulating O3 formation from oxidation of propene against envi-ronmental chamber data, and presents newly developedcondensed reactions that can be used to separately model propenein CB05 and CB05-TU. Note that CB05-TU is also tested in this studybecause CB05-TU showed better performance in simulating ozoneformation from aromatics than CB05 against toluene e NOxchamber experiments (Whitten et al., 2010).

Isobutene ((CH3)2C]CH2) is often the most abundant branchedterminal alkene found in the atmosphere of Houston (Jobson et al.,2004; Gilman et al., 2009) and is one of the 7 HRVOCs regulated byTCEQ (TCEQ, 2011). How isobutene is mapped intomodel species inCB05 can also inuence the accuracy of ozone predictions by CB05(Table 1). For a description of speciation of alkenes in CB05, refer tothe Supplementary Material. The current modeling approach forisobutene in CB05, modeling isobutenes reactions with reactions ofFORM and PAR (Table 1; Yarwood et al., 2005), could lead to inac-curate model predictions. Therefore, isobutene was selected fromthese terminal alkenes having a branch at their C]C bond, andcondensed reactions to separately model isobutene were devel-oped and evaluated against environmental chamber data to betterrepresent O3 formation from isobutene.

In this study, four tasks were performed. First, the capability ofCB05 in modeling O3 formation from oxidation of propene wasevaluated against 47 propeneeNOx chamber experiments. Second,new condensed reactions for propene and isobutene were devel-oped for use in CB05 and CB05-TU, and these new reactions weretested against 47 propene e NOx experiments and 5 isobutenee NOx experiments. Third, CB05-TU with the new reactions forpropene and isobutene were tested against 86 surrogate VOCmixture e NOx chamber experiments where at least 8 differentVOCs including propene and toluene were injected to examine theoverall impact of the updated alkene chemistry (this study) andtoluene chemistry (Whitten et al., 2010) on the mechanismperformance in modeling O3 formation. Fourth, box modeling wascarried out using four cases to examine the impact on simulated O3of using the newly developed reactions for propene and isobuteneunder more realistic conditions compared to chamber conditions.

2. Data and methods

2.1. Chemical mechanisms used

Model simulations were conducted using four versions of theCB05 chemical mechanism, two that were developed previously(CB05 and CB05-TU) and two that were developed in this work(CB05-OLE and CB05-TU-OLE). CB05 is the standard CB05 mecha-nism as documented by Yarwood et al. (2005). CB05-TU is a modi-ed version of CB05 developed by Whitten et al. (2010) with anupdated representation of the reactions of toluene and otheraromatics. CB05-TU is the same as CB05 for alkenes and other non-aromatic species, but was found to show improved performance insimulating O3 formation against chamber experiments wheretoluenewas injected (Whitten et al., 2010). CB05-OLE and CB05-TU-OLE were developed in this work to assess the effects of repre-senting the reactions of propene and isobutene separately, ratherthan using lumped model species as shown in Table 1.

Reactions for propene and isobutene for use with CB05 andCB05-TU were developed using only two new model species, PRPE

onment 59 (2012) 141e150for propene and IBTE for isobutene, and are given in Table 2.

Table 2Reactions for propene (PRPE) and isobutene (IBTE) for use with CB05 and CB05-TU. Note: Reactions of OLE (for terminal C]C bonds) in CB05 are also shown for comparison.

Reactants Productsa Rate constantb Note

1 PRPE OH 0.985 (ALD2 FORM XO2 HO2) 0.015 XO2N 0.045 PAR FALLOFF: F 0.50, N 1.13 cko: 8.00E-27$(T/300)3.50

kinf: 3.00E-11$(T/300)1.00

2 PRPE O3 0.33 OH 0.38 HO2 0.54 ALD2 0.73 FORM 0.528 CO 0.0425 MEOH 0.075 CH4 0.15 XO2 0.04 (H2O2 AACD) 0.185 FACD 0.28 PAR

5.50E-15$Exp(1888/T) d

3 PRPE NO3 0.5 (NO2 NTR) 0.96 XO2 0.48 HO2 0.04 XO2N 0.5 FORM 0.625 ALD2 4.60E-13$Exp(1155/T) e4 PRPE O 0.2 ALD2 0.3 ALDX 0.3 HO2 0.1 OH 0.2 (XO2 CO FORM)

0.01 XO2N 1.2 PAR1.02E-11$Exp(280/T) f

5 IBTE OH 0.90 (FORM HO2 XO2) 0.10 XO2N 2.70 PAR 9.47E-12$Exp(504/T) g6 IBTE O3 0.594 OH 0.048 HO2 0.546 (XO2 C2O3) 0.238 MEO2 1.246 FORM

1.005 PAR 0.153 CO 0.111 FACD 0.035 H2O22.70E-15$Exp(1632/T) h

PAR

00.3

LD2(XO

G. Heo et al. / Atmospheric Environment 59 (2012) 141e150 1437 IBTE NO3 0.96 (FORM XO2 NO2) 0.04 XO2N 2.888 IBTE O 0.5 ALD2 0.5 ALDX 2.0 PARe OLE OH 0.8 FORM 0.33 ALD2 0.62 ALDX 0.80 XO2e OLE O3 0.1 OH 0.44 HO2 0.74 FORM 0.18 ALD2

0.22 XO2 PARe OLE NO3 NO2 0.91 XO2 0.09 XO2N FORM 0.35 Ae OLE O 0.2 ALD2 0.3 ALDX 0.3 HO2 0.1 OH 0.2

0.01 XO2N 0.2 PARFootnotes to this table provide detailed information on how thesecondensed reactions were derived. For more details of CB05 modelspecies used in Table 2, refer to Yarwood et al. (2005). Most rateparameters for PRPE in Table 2 are from IUPAC (2010). The reactionsof IBTEwith OH, O3 and NO3 in Table 2 are based on rate parametersfrom Atkinson and Arey (2003).

a FORM, ALD2 and ALDX are HCHO, CH3CHO and higher aldehydes; PAR and NTR repreand acetic and higher carboxylic acids, respectively. The general framework used to approXO2 and XO2N are an operator for representing NO to NO2 conversion from alkylperoxy (RRO2, respectively.

b Rate constants are in units of molecule$cm3$second1.c For reaction PRPE OH, rate parameters are from IUPAC (2010) and organic nitrate y

are major products (Calvert et al., 2000), for which a yield of 0.985 (1.0e0.015) was gid Rate parameters are from IUPAC (2010). The product side was constructed as follows:

CH2OO* and CH3CHOO* were represented as 0.37 HCOOH 0.16 (OH HO2) 0.12 H(CH3OH CO) 0.15 CH4, respectively, based on IUPAC (2010). CH3CHOO, a stabilized Crits dominant reaction with H2O (Sauer et al., 1999; IUPAC, 2010). CH2CHOwas represente(2005) and Fig. S1 of Heo et al. (2010), and CHOeCHO (glyoxal) was represented as FORMCH3CHO, CH3OH, HCOOH and CH3OOH were renamed as FORM, ALD2, MEOH, FACD and

e Rate parameters are from IUPAC (2010), and product yields were derived asHCHO NO2 XO2)} 0.04$XO2N. There are various uncertainties in product distributirepresented as 0.5 (NTRNO2) 0.48 (HO2 2 XO2) 0.5 FORM 0.625 ALD2 0.04XO2

f From OLE O reaction of Yarwood et al. (2005); rate constant was slightly modiedg Rate parameters from Atkinson and Arey (2003); organic nitrate yield (10%) from Tua

(Calvert et al., 2000), for which a yield of 0.90 (1.0e0.10) is given. Acetone is relative uh Rate parameters from Atkinson and Arey (2003); product yields from Neeb and M

IBTE O3 0.7 {(CH3)2COO* HCHO} 0.3 {CH2OO* ACET}. (CH3)2COO* was represenCH3 CO2} 0.05 (ACET H2O2) 0.78 {XO2 FORM C2O3 OH} 0.34 MEO2. (dominant products after reaction with H2O (Sauer et al., 1999; IUPAC, 2010), and A(OH HO2) 0.12 H2 0.51 CO based on IUPAC (2010).

i Rate parameters are from Atkinson and Arey (2003), and products were formulated asbased on Berndt and Bge (1995).Acetone was represented as 3 PAR.

j Rate parameters are from Calvert et al. (2000). Due to lack of experimental data, thebalancing carbon.

k Directly from Yarwood et al. (2005).

Table 3An overview of indoor environmental chambers at UCR and TVA used for mechanism ev

Chamber Chamber ID Reactor type Reactor

Evacuable Chamber at UCR EC Single w5.8Indoor Teon Chamber at UCR ITC Single w6.4Ernies Teon Chamber at UCR ETC Single w3.0Dividable Teon Chamber at UCR DTC Dual w5.0 (XCE-CERT Teon Chamber at UCR CTC (11e82a) Single w5.0CE-CERT Teon Chamber at UCR (rebuilt) CTC (>82a) Dual w2.5 (XUCR EPA chamber EPA Dual w90 (XTVA indoor chamber TVA Single w28

References: Dodge (2000), Carter (2000, 2010a), Carter et al. (2005).a Run number of the chamber experiment.3.44E-13 i

1.14E-11$Exp(130/T) j

.95 HO2 0.70 PAR 3.20E-11 k2 ALDX 0.33 CO 6.50E-15$Exp(1900/T) k

0.56 ALDX PAR 7.00E-13$Exp(2160/T) k2 CO FORM) 1.00E-11$Exp(280/T) k2.2. Environmental chamber data

A database of chamber experiments continually expanded andevaluated for w35 years by William Carter at the University ofCalifornia at Riverside (UCR) was used for the chamber simulationsin this study (Carter, 2000, 2010a). Most UCR chamber data (w2000

sent parafnic carbons and organic nitrates; MEOH, FACD, AACD are CH3OH, HCOOHximate the product side is the same as in Gery et al. (1989) and Yarwood et al. (2005).O2) radicals, and an operator for representing NO to organic nitrate conversion from

ield (1.5%) is from OBrien et al. (1998). Under high NOx conditions, ALD2 and FORMven. 0.45 PAR was added for balancing carbon (i.e., 3 carbons on each side).PRPE O3 0.5 (HCHO CH3CHOO*) 0.5 (CH3CHO CH2OO*). Criegee biradicals2 0.51 CO and 0.16 CH3CHOO 0.4 (OH CH2CHO) 0.06 CH2]C]O 0.085iegee biradical, was represented as 0.5 ALD2 0.5 CH3COOH 0.5 H2O2 by assumingd as 0.25 (OH FORM CO) 0.75 {XO2 HO2 CHOeCHO} based on Kuwata et al.COHO2 (Whitten et al., 2010); CH2]C]Owas represented as FORM CO. HCHO,AACD, respectively, and 0.28 PAR was added for balancing carbon.follows: PRPE NO3 (1e0.04)$0.5 {(NTR XO2 HO2) (CH3CHO ons for reactions of alkenes with NO3 (Calvert et al., 2000), so the product side wasN to be consistentwith the updated OLENO3 reaction for CB6 (Yarwood et al., 2010).based on Calvert et al. (2000); 1 PAR was added for balancing carbon.zon et al. (1998). Under high-NOx conditions, acetone and FORM are major productsnreactive based on its k(OH) (IUPAC, 2010) and was represented as 3 PAR.oortgat (1999) and IUPAC (2010). The product side was constructed as follows:ted as follows: (CH3)2COO* 0.05 (CH3)2COO 0.78 {CH3C(O)CH2 OH} 0.17 {2CH3)2COO, a stabilized Criegee biradical, was represented as acetone and H2O2, itsCET was represented as 3 PAR. CH2OO* was condensed as 0.37 HCOOH 0.16

follows: IBTE NO3 0.04$XO2N (1 0.04)$(CH3C(O)CH3 HCHO NO2 XO2)

product side was tentatively given based on Carter (2010a). 2.0 PAR was added for

aluation (Heo, 2009).

volume (m3) Light source Relative humidity Operation period

Xenon arc w50% 1975e84Blacklight w50% 1982e86Blacklight dry (

against (1) 47 propene e NOx experiments to examine theircapability in simulating O3 formation from propene, (2) 5 isobutenee NOx experiments to examine their capability in simulating O3formation from isobutene, and (3) 86 surrogate VOCmixtureeNOxexperiments wheremajor components of urban atmospheres otherthan propene were also injected.

Running chamber simulations with a chemical mechanismrequires wall mechanisms that characterize chamber-dependenteffects such as chamber-dependent radical sources and NOx off-gasing from the chamber walls (Jeffries et al., 1992; Carter et al.,2005). For chamber simulations to evaluate the CB05 variantsused in this study, wall mechanisms used for evaluating CB05 byYarwood et al. (2005) and CB05-TU by Whitten et al. (2010) wereused. Chemical systems such as CO e NOx lack chemical processesgenerating new radicals (e.g., hydroperoxy radical (HO2) formationfrom the photolysis of formaldehyde) and are sensitive to chamber-dependent radical sources. However, chemical systems (such aspropene e NOx, isobutene e NOx, and surrogate VOC mixturee NOx) that have a strong internal radical source are relativelyinsensitive to chamber-dependent radical sources.

2.4. Comparing mechanisms using box modeling

nvironment 59 (2012) 141e150experiments, not including very recent experiments) and chamberdata of the Tennessee Valley Authority (TVA,w60 experiments) areavailable in this chamber database that is publicly available athttp://www.cert.ucr.edu/wcarter/SAPRC/SAPRCles.htm (Carter,2010a). The chamber data used in this study were extracted fromthis large UCR database (April 23, 2010 version), and includechamber experimental data from 138 experiments carried out in 6different chambers at UCR and in one chamber at TVA. Table 3provides an overview of these 7 environmental chambers.

Experiments of single test compound e NOx are useful intesting each component of a chemical mechanism (e.g., propenechemistry), at least for reactive compounds such as propene andisobutene. On the other hand, for testing a chemical mechanism asawhole, surrogate VOCmixture e NOx experiments are useful. Ina surrogate mixture experiment, a mixture simulating a targetatmospheric composition (e.g., an urbanmixture) is injected. In thisstudy, two types of chamber experiments were used: single testVOC e NOx and surrogate VOC mixture e NOx experiments.

Chamber effects (e.g., wall reactions such as NOx offgasing,which are difcult to accurately describe (Jeffries et al., 1992;Dodge, 2000; Carter et al., 2005; Heo, 2009), must be considered inselecting chamber experiments to be used in evaluating mecha-nisms. In order to minimize the impact of chamber effects onmechanism evaluation, experiments that are expected to have beensignicantly inuenced by chamber effects in general should not beused. In selecting UCR and TVA experiments, criteria for the initialNOx level ([NOx]0), the ratio of O3 formed toNO oxidized (Max(O3)/[NO]0), and the chamber light source were used to makea compromise between increasing the total number of selectedexperiments and minimizing uncertainty due to chamber effectsand other artifacts (e.g., high oxygen atom concentration due topresence of high NOx; Paulson et al., 1992) (Whitten et al., 2010).Criteria applied for selecting single test VOCeNOx experiments forpropene and isobutene are (1) 0.010 [NOx]0 0.500 parts permillion (ppm), and (2) Max(O3)/[NO]0 1.0. Criteria applied forselecting surrogate VOC mixture e NOx experiments are (1)0.010 [NOx]o 0.200 ppm, and (2) Max(O3)/[NO]0 1.0. Black-light experiments were not used when sufcient data fromcomparable experiments with arc lights are available because thearc light source is considered to be the better representation ofnatural sunlight (Carter et al., 2005). However, we are not aware ofstrong direct evidence showing that there are systematic differ-ences in mechanism performance between evaluation usingblacklight experiments and evaluation using experiments with anargon arc light or natural sunlight. After applying these criteria, 47experiments of propene e NOx and 86 experiments of surrogateVOC mixture e NOx were selected. For the surrogate VOC mixturee NOx experiments, in most cases, 8 different VOCs (n-butane,n-octane, ethene, propene, trans-2-butene, toluene, m-xylene, andformaldehyde) were injected. For the ve TVA experiments, 54different VOCs simulating ambient VOCmixtures were injected. Fortesting isobutene chemistry, chamber data for only 2 isobuteneeNOx blacklight experiments and 3 isobutenee other VOCseNOxblacklight experiments were available. Thus, all 5 experimentswere used. Tables S1, S2 and S3 in the Supplementary Materialprovide additional information on the experiments for propene,isobutene and surrogate mixtures, respectively.

2.3. Mechanism evaluation using environmental chambersimulations

Chamber simulations were performed using the SAPRC softwarethat has been used for evaluating various versions of the SAPRC andCB mechanisms (Carter, 2000 and 2010a; Heo et al., 2010; Yarwood

G. Heo et al. / Atmospheric E144et al., 2005; Whitten et al., 2010). Each mechanism was evaluatedBoxmodeling was also carried out to characterize the impact onmodeled ozone of using the new reactions for propene and iso-butene (Table 2) under conditions based on atmosphericmeasurements. Four cases were selected (Table 4). LP1.Base isa modeling case for the morning of August 25, 2000 at the La Portemunicipal airport, 30 km southeast of downtown Houston, Texas.On this date,103 parts per billion (ppb) ethene and 90 ppb propenewere measured at around 7:30 CST and winds were relativelystagnant during the morning hours (Heo et al., 2010; Heo, 2009).LP1.Base was used to examine the impact on simulated ozone ofusing the new PRPE reactions (Table 2) while LP1.IBTE, LP1.IB-TEPRPE and LP2.IBTE were used to examine the impact of usingthe new IBTE reactions (Table 2) under various conditions. LP1.IBTEand LP1.IBTEPRPE are variants of LP1.Base where the initialethene and propene concentrations ([ethene]0 and [propene]0)were reduced and the initial isobutene concentration

Table 4Four box modeling cases used to examine the impact onmodeled ozone of using thenew propene and isobutene reactions.

Case ID Description

LP1.Base The La Porte August 25, 2000 case used byHeo et al. (2010). Ethene (103 ppb) and propene(90 ppb) dominated the reactivity. Initial VOCconcentrations were set based on measurementsat the La Porte airport (Jobson et al., 2004) and noadditional emissions were added. For details,see Heo et al. (2010).

LP1.IBTE LP1.Base but with adjusted initial concentrationsfor ethene, propene and isobutene: ethene(decreased from 103 ppb to 5 ppb), propene(decreased from 90 ppb to 5 ppb), and isobutene(increased from 0.7 ppb to 100 ppb).

LP1.IBTEPRPE LP1.Base but with adjusted initial concentrationsfor ethene, propene and isobutene: ethene(decreased from 103 ppb to 70 ppb), propene(decreased from 90 ppb to 70 ppb), and isobutene(increased from 0.7 ppb to 50 ppb).

LP2.IBTE A modeling case based on Faraji et al. (2008). Initialconcentrations and composition of VOC emissionswere based on aircraft measurements, and additionalVOC and NOx emissions were added over themodeling period (Faraji et al., 2008). However,

isobutene was increased from 0.5 ppb to 22.5 ppb.

Fig. 1. Comparison of mechanism performance between CB05 and CB05-OLE against47 propene e NOx experiments: (a) Max(O3), (b) Max(D(O3eNO)), and (c) NOx

nvironment 59 (2012) 141e150 145([isobutene]0) was increased (Table 4). LP2.IBTE is a variant ofthe box modeling case used by Faraji et al. (2008) (Table 4). TheVOC composition used for LP2.IBTE is based on aircraft datameasured on multiple early mornings in August and September of2000 near the La Porte airport (Faraji et al., 2008). For this study,the fraction of VOC for isobutene was increased from0.0011 ppm ppmC1 VOC to 0.050 ppm ppmC1 VOC; as a result,the initial isobutene concentration was increased from 0.5 ppb to22. 5 ppb. However, the initial VOC/NOx ratio was kept at 10 (450ppmC VOC/45 ppm NOx). Box model simulations were performedusing the same simulation software package as used by Heo et al.(2010) (Carter, 2000 and 2010a). Additional information regardingthe box model simulations is available in the SupplementaryMaterial.

3. Results and discussion

This section presents (1) chamber simulation results for testingthe four versions of CB05 against measured data of the 138chamber experiments listed in Tables S1 to S3 in the Supple-mentary Material and (2) box modeling results for the four cases(Table 4). The three performance metrics that were used forquantifying the performance of a chemical mechanism in thechamber simulations are the maximum ozone concentration(Max(O3)), maximum D(O3eNO) (Max(D(O3eNO)), and NOxcrossover time. In this work, Max(O3) is dened as the highest O3concentration by the end of the experiment but no later than 6 hsince the start of chamber irradiation (i.e., t 0) because inmost cases chamber data after hour of 6 were not gatheredand are not quality-assured. The metric D(O3eNO), dened as([O3] [NO])tt ([O3] [NO])t0, quanties the amount of O3formed and NO oxidized since t 0 and is useful even when thereis no signicant O3 production (Carter and Atkinson, 1987).Max(O3) and Max(D(O3eNO)) are useful because a primary goal ofcondensed chemical mechanisms for urban/regional photo-chemical models is accurate prediction of maximum O3 concen-trations; however, these metrics do not provide information onthe rate of O3 formation. The NOx crossover time, dened as thetime elapsed since t 0 before the NO2 concentration becomesequal to the NO concentration, contains information on the rate ofNO oxidation into NO2, which accompanies O3 formation. There-fore, the NOx crossover time is a useful performance metricand was also used in this work. Model errors of Max(O3) andMax(D(O3eNO)) were calculated as {(model e experimental)/experimental} in units of percentages (%); model errors of NOxcrossover times were calculated as (model e experimental) inunits of minutes (min). Means and standard deviations of thesemetrics were used to summarize performance over multipleexperiments.

3.1. Evaluating CB05 and CB05-OLE against propene chamberexperiments

CB05 reasonably simulated O3 formation from oxidation ofpropene for experiments with relatively low and moderately highpropene concentrations (EPA and TVA experiments listed inTable S1) as shown in Fig. 1. Note that results for CB05-TU andCB05-TU-OLE are not shown in Figs. 1 and 2 because their repre-sentation of alkene chemistry is the same as in CB05 and CB05-OLE, respectively. CB05 simulated Max(O3) with model errorswithin 25% for most of the 47 experiments while simulating betterfor experiments with their measured Max(O3) lower than0.250 ppm than for experiments with their measured Max(O3)larger than 0.250 ppm (Fig. 1a). The average model errors

G. Heo et al. / Atmospheric Ecommitted by CB05 for the 47 experiments were 13% (16%) forcrossover time.

ne b7, et, ethCB05refe

G. Heo et al. / Atmospheric Envir146Max(O3), 10% (10%) for Max(D(O3eNO)), and 18 min (17 min)for the NOx crossover time (Table 5). Assumptions used in devel-oping the OLE reactions of CB05 (Table 2) can explain these resultsdemonstrating that ozone formation from propene is reasonably

Fig. 2. Comparison of performance in simulating O3 formation from oxidation of isobute3 experiments of isobutene e other VOCseNOx. Note: For ETC253, ETC255 and ETC25ethene (0.678), m-xylene (0.096), n-hexane (0.407); for ETC255, isobutene (0.195 ppm)ethene (0.745), m-xylene (0.092), and n-hexane (0.389). CB05(s1:OLE 2 PAR) isCB05(s2:IOLE) is CB05 but with speciation of isobutene into IOLE. For more detail,well simulated by the OLE reactions (which are reactions not justfor propene but also for many other terminal alkenes (moreexactly, terminal C]C bonds)). The reaction parameters such asrate constants for the OLE reactions in CB05 are heavily based onthe oxidation mechanisms of propene although two model speciesfor aldehydes (ALD2 and ALDX) are used as products in the OLEreactions (Table 2, Yarwood et al., 2005; Gery et al., 1989). Note thatALD2 (CH3CHO) is produced from the reaction of propene and OHbut ALDX (model species in CB05 for higher aldehydes such aspropanal (CH3CH2CHO)) is not and that propene is assumed toconstitutew35% (0.33/(0.33 0.62)) of OLE in CB05 as implied bythe ALD2 and ALDX yields in reaction OLE OH in Table 2. Previousstudies on evaluating the OLE chemistry of CB05 with chamberdata also heavily relied on propene e NOx experiments, andevaluation of the OLE chemistry against chamber experiments forterminal alkenes other than propene was limited (Yarwood et al.,2005; Gery et al., 1989), in part due to lack of such chamber dataavailable for mechanism evaluation. In principle, the reactions ofOLE could be re-derived to exclude propene. In practice, thechemistry of terminal alkenes is dominated by the double bond

Table 5Comparison of model errors between CB05 and CB05-OLE for 47 propene e NOxexperiments. CB05TU is the same as CB05 for propene.

Max(O3) [%] Max(D(O3eNO) [%] NOx crossovertime [min]

CB05 &CB05TU

CB05-OLE CB05 & CB05TU CB05-OLE CB05 &CB05TU

CB05-OLE

Average 13 2 10 2 18 5Std. dev. 16 13 10 9 17 13allowing one model species (OLE) to represent the family ofcompounds. Therefore, it is both reasonable and practical to leavethe OLE reactions unchanged regardless of whether propene istreated using the explicit species PRPE.

etween CB05 and its variants: (top) for 2 experiments of isobutene e NOx; (bottom) forhene, m-xylene and n-hexane were also injected. For ETC253, isobutene (0.211 ppm),ene (0.737), m-xylene (0.0943), n-hexane (0.388); for ETC257, isobutene (0.108 ppm),but with speciation of isobutene into OLE 2 PAR instead of FORM 3 PAR.

r to section 3.2.

onment 59 (2012) 141e150In comparison, overall, CB05-OLE performed better than CB05(Fig. 1, Table 5). For example, the average model error of Max(O3)was reduced from 13% (16%) to 2% (14%), which resultedfrom the reduced model errors for experiments with theirmeasured Max(O3) larger than 0.250 ppm (Fig. 1a). CB05-OLEshowed performance comparable to SAPRC-07 (adjustable) insimulating O3 formation from propene for these 47 chamberexperiments (Heo et al., 2010). For example, the average modelerrors in simulating Max(O3) were 2% (14%) for CB05-OLE and4% (16%) for SAPRC-07 (adjustable), which is the adjustable-parameter version of SAPRC-07 described in Carter (2010a,b).The results conrm that CB05-OLE has potential to better simulateO3 formation from propene than CB05 under conditions that theemissions of terminal alkenes dominate the hydrocarbon reac-tivity and the relative contribution of propene to those terminalalkenes is more than assumed during development of a simpliedand lumped chemical mechanism (e.g., 80% (Table 3 of Heo et al.(2010)) instead of 35% (Yarwood et al., 2005)). Similar resultswere seen in chamber simulations and box modeling usingdifferent representations for propene in SAPRC-07 (Heo et al.,2010).

3.2. Evaluating CB05 and CB05-OLE against isobutene chamberexperiments

Fig. 2 shows the time proles of O3 concentrations simulated byCB05 and CB05-OLE against measurements of 5 chamber experi-ments for isobutene: 2 experiments of isobutene e NOx and 3experiments of isobutene e other VOCs e NOx. Although onlyblacklight experiments were used, the results show that usinga separate model species for isobutene (IBTE) and using the IBTE

Fig. 3. Comparison of mechanism performance between CB05, CB05-TU andCB05-TU-OLE against 86 surrogate VOC mixtureeNOx experiments: (a) Max(O3),

nvironment 59 (2012) 141e150 147reactions listed in Table 2 could improve the performance of CB05and CB05-TU in simulating O3 formation from oxidation of iso-butene. Based on chamber simulations of the two isobutene e NOxexperiments (DTC052B and ITC694), modeling isobutene oxidationwith FORM 3 PAR in CB05 leads to prediction of faster O3formation in the early stage but slower O3 formation later,compared to observations (Fig. 2). This early O3 over-prediction ismore apparent in the O3 time series for the three VOC mixturee NOx experiments (ETC253, ETC255 and ETC257) for which VOCsother than isobutene were also initially injected (Fig. 2). The over-predicted O3 formation in the early stage can be explained byarticial radical production from FORM (formaldehyde) due tousing FORM 3 PAR to represent isobutene in CB05. When thefour reactions listed in Table 2 for IBTE were used to model theoxidation of isobutene, this problem of early O3 over-prediction andmid- and late-stage O3 under-prediction for DTC052B and ITC694was mitigated as shown by time proles of O3 for CB05-OLE inFig. 2.

In comparison, representing isobutene by either OLE 2 PARor IOLE instead of FORM 3 PAR (default speciation in CB05)did not result in improved model performance comparable tothat shown by CB05-OLE in simulating O3 for these isobuteneexperiments (Fig. 2). In terms of the carbon bond concept (Whittenet al., 1980), the branched C]C bond of isobutene is not wellmodeled by either OLE (for terminal C]C bond without a branch)or IOLE (for internal C]C bond without a branch). Representingisobutene with OLE 2 PAR was not comparable to modelingwith CB05-OLE in simulating O3 but showed overall somewhatbetter performance in simulating O3 formation from isobutenethan that with FORM 3 PAR (CB05) and that with IOLE(Fig. 2). The new reactions for isobutene (Table 2) could allowcomparison of measured andmodeled concentrations of isobuteneandmore accurate estimation of the impact of isobutene emissionson O3 formation in areas inuenced by industrial isobuteneemissions.

3.3. Evaluating CB05 variants against surrogate mixture chamberexperiments

CB05 and CB05-TU were evaluated to test their capability insimulating O3 formation from surrogate VOCs e NOx mixturesusing data from 86 chamber experiments where at least 8 differentVOCs including propene and toluene were injected. Becausearomatics were injected for these experiments, CB05 and CB05-TUare expected to give different results, unlike the simulations of thealkene experiments discussed above. Then, CB05-TU-OLE wasevaluated using those chamber data to examine the overall impacton model performance in simulating O3 formation of the updatedalkene chemistry described above combined with the updatedaromatic chemistry in CB05-TU (Whitten et al., 2010). For the 86chamber experiments (Table S3), isobutene was not injected exceptfor the 5 TVA chamber experiments where a small amount of iso-butene was injected.

In general, CB05-TU performed better than CB05 in simulatingO3 formation for these experiments (Fig. 3, Table 6), which isconsistent with the results previously reported by Whitten et al.(2010) and attributable to the improved toluene chemistry. Theaverage model errors for CB05-TU vs. CB05 are as follows: 23%(11%) vs.31% (15%) for Max(O3),20% (10%) vs.27% (13%)for Max(D(O3eNO)), and 3 min (8 min) vs. 4 min (11 min) forthe NOx crossover time. Despite the more explicit approach tomodeling propene in CB05-TU-OLE, CB05-TU and CB05-TU-OLEshowed very similar results (Fig. 3 and Table 6) because propenedid not dominate the VOC reactivity in these experiments due to

G. Heo et al. / Atmospheric Epresence of other reactive compounds injected. For the 86(b) Max(D(O3eNO)), and (c) NOx crossover time.

experiments, propene was injected in a smaller amount on a molarbasis than toluene, on average, by about 30% (10%), and otherreactive VOCs such as ethene, trans-2-butene and m-xylene werealso injected. However, based on the results in section 3.1, underatmospheric conditions that are dominated by propene emissions,for example, industrial propene emissions from petrochemicalfacilities, the effect on modeled O3 of the updated propene chem-istry could be larger, which is demonstrated by box modelingresults in section 3.4.

Table 6Comparison of model errors between CB05, CB05-TU and CB05-TU-OLE for 86 surrogate

Max(O3) [%] Max(D(O3eNO)

CB05 CB05-TU CB05-TU-OLE CB05 CB

Average 31 23 22 27 2Std. dev. 15 11 11 13 1

G. Heo et al. / Atmospheric Envir148Fig. 4. Comparison of modeled O3, OH and HO2 between CB05-TU and CB05-TU-OLEfor a modeling case for propene, La Porte 8/25/2000 Base case (LP1.Base): (a) O3, and(b) OH and HO2.3.4. Comparing CB05-TU and CB05-TU-OLE using box modelingcases

CB05-TU and CB05-TU-OLE were compared by carrying out boxmodeling with the four cases based on ambient measurements(Table 4). Results for CB05 and CB05-OLE were not shown in thissection because (1) differences in modeled ozone between the twomechanisms were nearly the same as differences between CB05-TUand CB05-TU-OLE and (2) CB05-TU showed better O3 performancein the chamber simulations (see section 3.3).

Comparison of CB05-TU and CB05-TU-OLE for case LP1.Base (forwhich [propene]0 is 90 ppb) showed that using new reactions forpropene (PRPE) listed in Table 2 increased the modeled 1-h O3concentrations up to 7.9 ppb and the 1-h Max(O3) by 4.6 ppb (3.8%when calculated by ([O3]CB05-TU-OLE e [O3]CB05-TU)/[O3]CB05-TU)(Fig. 4a). These results can be explained by higher OH and HO2concentrations with CB05-TU-OLE than with CB05-TU, on average,by about 10% over the period of 7:30 to 14:00 CST (Fig. 4b), and themagnitude of impact on modeled O3 of separately modeling pro-pene with CB05-TU-OLE is w50% of that shown for SAPRC-07 byHeo et al. (2010). Mao et al. (2010) reported decits in modeled OHand HO2 sources relative to HO and HO2 sinks during the Texas AirQuality Study 2000. Thus, increases in OH and HO2 shown by CB05-TU-OLE compared to CB05-TU are consistent with OH and HO2measurements.

Simulations with 3 modeling cases for isobutene, LP1.IBTE([propene]0 5 ppb, [isobutene]0 100 ppb), LP1.IBTEPRPE([isobutene]0 100 ppb, [propene]0 70 ppb), and LP2.IBTE([isobutene]0 22.5 ppb, [propene]0 1.7 ppb) (Table 4), showedthat using the new reactions for isobutene (IBTE) listed in Table 2increased the 1-h Max(O3) by 20.3 ppb (21.6%), 9.3 ppb (7.7%) and3.4 ppb (3.1%), respectively (Fig. 5). The magnitude of impact washighest for LP1.IBTE for which isobutene dominated the reactivityand was lowest for LP2.IBTE for which [isobutene]0 was relativelymoderate compared to 100 ppb for LP1.IBTE and 60 ppb forLP1.IBTEPRPE. For case LP1.IBTE, representing IBTE as OLE 2 PARresulted in Max(O3) closer to that predicted by CB05-TU-OLE thanusing the default speciation of FORM 3 PAR (Fig. 5a). However,speciation into OLE 2 PAR resulted in 1-h Max(O3) higher by only1.4 ppb (1.1%) than that with CB05-TU for case LP1.IBTEPRPE(Fig. 5b) and resulted in 1-h Max(O3) over-predicted by 10.3 ppb(9.1%) compared to that with CB05-TU-OLE for case LP2.IBTE(Fig. 5c). Based on the results presented in Figs. 2 and 5, speciatingisobutene into OLE 2 PAR is not a completely satisfactory solutionto the under-prediction of O3 formation from isobutene by CB05and CB05-TU, compared to using the isobutene (IBTE) reactions in

VOC mixture e NOx experiments.

[%] NOx crossover time [min]

05-TU CB05-TU-OLE CB05 CB05-TU CB05-TU-OLE

0 20 4 3 30 10 11 8 8

onment 59 (2012) 141e150Table 2.In combination with chamber simulation results described in

sections 3.1 and 3.2, the box modeling results for propene (1 case)and isobutene (3 cases) indicate that modeling propene and iso-butene using the reactions listed in Table 2 can improve theperformance of CB05 and CB05-TU in simulating O3 under ambientconditions inuenced by industrial propene and isobutene emis-sions. However, further studies under ambient conditions (e.g., 3-dimensional air quality modeling) are needed to clarify conditionsunder which propene or isobutene needs to be separately modeled.

Fig. 5. Comparison of modeled O3 between CB05-TU, CB05-TU-OLE andCB05-TU(OLE 2 PAR) for 3 box modeling cases for isobutene: (a) LP1.IBTE,(b) LP1.IBTEPRPE, and (c) LP2.IBTE.

G. Heo et al. / Atmospheric Envir4. Conclusions

In CB05 and CB05-TU, ozone formation from alkenes isdescribed by highly condensed chemical reactions of a few modelspecies. These mechanisms were tested against environmentalchamber data for their capability in simulating O3 formation fromoxidation of propene and isobutene. The chamber simulation andbox modeling results show that (1) CB05 and CB05-TU reasonablysimulate O3 formation from propene under typical urban condi-tions and for cases inuenced by moderately elevated propeneconcentrations, (2) separately modeling O3 formation from pro-pene and isobutene using condensed reactions for propene andisobutene (e.g., reactions for PRPE (propene) and IBTE (isobutene)listed in Table 2) has potential to improve the performance of CB05and CB05-TU in modeling O3 under conditions where the hydro-carbon reactivity is dominated by propene and/or isobutene, (3)CB05-TU is generally better than CB05 in simulating O3 formationfor the 86 surrogate VOCs e NOx experiments, which is consistentwith results reported by Whitten et al. (2010) for toluene e NOxexperiments, and (4) the characteristics of chamber experiments(e.g., VOC composition) inuence the magnitude of impact onmodeled ozone of using more explicit alkene reactions.

Historically, developers of chemical mechanisms for use in3-dimensional air quality models have represented atmosphericreactions of many different VOCs with highly condensed reactionsof a limited number of model species by using assumptions on theaverage air compositions of major VOC classes (e.g., alkenes andaromatics) or major carbon bond types (e.g., terminal carbondouble bonds). However, the air composition changes over timeand is spatially different. Certain compounds may need to get moreattention due to changes in emissions over time, and certaincompounds may warrant more attention for some regions than forother regions (e.g., HRVOCs in southeast Texas). Developingcondensed chemical reactions (e.g., OLE reactions in CB05) opti-mized only for one region may result in inaccurate model predic-tions because inmany cases the air quality modeling domain coversmultiple states or much of the entire continental U.S. Therefore,a practical but more scientically robust alternative approach is(a) ranking the relative importance of compounds based on theiremissions and reactivity, (b) separately modeling more importantcompounds (e.g., ethene and propene), (c) lumping other lessimportant compounds into model species shared by multiplecompounds and developing condensed reactions for those modelspecies. For example, propene is separately modeled in the toxicsversion of the SAPRC-07 mechanism (SAPRC-07T, Hutzell et al.,2012). As demonstrated in this work, chamber data are crucial todeveloping and evaluating chemical mechanisms because chamberdata produced under well-characterized and well-controlledconditions allow testing of chemical mechanisms withoutinvolving uncertainties in emissions and meteorology. Lack ofuseful chamber data for mechanism evaluation is a critical obstacleto developing reliable mechanisms. For example, among the7 alkenes (ethene, propene, 1,3-butadiene, 1-butene, isobutene,trans-2-butene, cis-2-butene) regulated as HRVOCs in southeastTexas, robust chamber data for mechanism evaluation are availableonly for ethene and propene. The reliability of chemical mecha-nisms used in air quality models can be strengthened by conduct-ing targeted mechanism improvements using experimental data asillustrated by improving mechanisms for propene and isobutene inthis work.

Acknowledgments

This study was in part funded by the Texas Air Research Center

onment 59 (2012) 141e150 149(TARC) through TARC project 079UTA0102A, Implementation of

Modied Carbon Bond Mechanisms in CAMx. The authors thankall investigators who contributed to producing the chamber dataused in this study.

Appendix A. Supplementary material

Supplementary material associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.atmosenv.2012.05.042.

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Modeling ozone formation from alkene reactions using the Carbon Bond chemical mechanism1. Introduction2. Data and methods2.1. Chemical mechanisms used2.2. Environmental chamber data2.3. Mechanism evaluation using environmental chamber simulations2.4. Comparing mechanisms using box modeling

3. Results and discussion3.1. Evaluating CB05 and CB05-OLE against propene chamber experiments3.2. Evaluating CB05 and CB05-OLE against isobutene chamber experiments3.3. Evaluating CB05 variants against surrogate mixture chamber experiments3.4. Comparing CB05-TU and CB05-TU-OLE using box modeling cases

4. ConclusionsAcknowledgmentsAppendix A. Supplementary materialReferences