Atmospheric Environment Vol. 32, No. 14/15, pp. 2443—2451, 1998Published by Elsevier Science Ltd.

Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(98)00045–4

PREPARATION OF AUTOMOBILE ORGANIC EMISSIONSURROGATES FOR PHOTOCHEMICAL MODEL

VALIDATION

FRANK BLACK,*,- SILVESTRE TEJADA- and TADEUSZ KLEINDIENST‡-U.S. Environmental Protection Agency; Research Triangle Park, N C, U.S.A.; and ‡ManTech Environ-

mental Technology, Inc., Research Triangle Park, NC, U.S.A.

(First received 23 April 1997 and in final form 23 December 1997. Published June 1998)

Abstract—Irradiation chamber studies of the photochemistry of motor vehicle emissions generally userelatively simple surrogates (often fewer than 30 compounds) of complex emissions (often greater than 250compounds). The question is often asked: Can the photochemistry of complex organic mixtures beadequately simulated by chamber studies using simple surrogate mixtures? The complexity of the composi-tional profile of motor vehicle organic emissions results in large part from the complexity of the fuel (i.e., thecombustion derived compounds are relatively small in number). An experimental protocol is described bypermitting preparation of representative complex (greater than 250 compounds) emission surrogatesadding a small number (fewer than 30) of selected compounds to the vehicle fuel. Irradiation chamber runsof actual motor vehicle exhaust samples are contrasted with similar runs of associated complex mixtureemission surrogates demonstrating the accuracy with which the exhaust photochemistry can be simulatedusing the surrogate. Time-resolved profiles of NO, NO

x—NO, and ozone concentrations, and selected 12.5 h

product concentrations, are presented as examples of the comparability of the exhaust and surrogatephotochemistry. The observed exhaust and surrogate concentration profiles product distributions are verysimilar. Published by Elsevier Science Ltd.

Key word index: Autoemissions surrogates, ozone photochemistry.

INTRODUCTION

Many urban areas in the United States continue to beplagued with ozone concentrations that exceed theNational Ambient Air Quality Standard (NAAQS)(U.S. EPA, 1996). Unlike most regulated pollutants,ozone is not emitted directly, but is produced in theatmosphere by secondary photochemical processes.The general mechanism for ozone formation in thepolluted troposphere suggests formation throughnonlinear processes involving reactive nonmethaneorganic gases (NMOG) and oxides of nitrogen (NO

x)

(Bowman and Seinfeld, 1994). Motor vehicle emis-sions contribute significantly to national ozone pre-cursor inventories (about 37% of anthropogenicNMOG and 49% of NO

x) (U.S. EPA, 1996); although

these contributions can be greater in cities exceedingthe ozone NAAQS, and can vary seasonally and geo-graphically (OTA, 1989).

The complexity of issues associated with ozoneabatement has long been recognized and the subjectof much research (National Research Council, 1991).Since tropospheric ozone is produced by a complex

*Author to whom correspondence should be addressed.

series of reactions involving NMOG, NOx, and UV

sunlight, rational decisions on regulating either orboth of the precursor emissions rely on fundamentalunderstanding of the details of the ozone formationmechanism. Recently summarized Research Needs forevaluation of the potential impact of alternative trans-portation fuels on ozone and other toxic air pollutantsinclude: (1) characterization of ‘‘life cycle’’ transporta-tion associated emission rates and chemical composi-tions associated with fuel production, distribution,and use (including motor vehicle tailpipe, evaporative,and running-loss emissions with ‘‘real-world’’ vehicles,fuels, and driving scenarios), and (2) characterizationof the contribution of this source category to the totalozone precursor emission inventory (for specific ur-ban areas/regions being evaluated); (3) developmentand evaluation of chemical mechanisms for predictingthe atmospheric transformations by which emitted pol-lutants contribute to ozone and other secondary pollu-tants; (4) refinement and evaluation of urban/regionalairshed models for predicting changes in atmosphericozone resulting from changes in precursor emissions;(5) assessment of associated uncertainties; and (6)ozone air quality monitoring/measurements to evalu-ate or ‘‘truth test’’ model performance (Atkinson,1988; Black et al., 1992; Carter et al., 1994).

2443

Research protocols for development and evaluationof photochemical reaction mechanisms often involveuse of irradiation chambers (Jeffries et al., 1992; Carteret al., 1994). Commonly, chamber studies of the ozonepotential of motor vehicle emissions use relativelysimple surrogates of the emissions (e.g. surrogate mix-tures containing fewer than 60 compounds, oftenfewer than 30 compounds, simulating emission mix-tures generally containing more than 250 compounds)(Jeffries and Sexton, 1995; Carter et al., 1993). Thisresults, in part, from the fact that very few laboratoriesin the world have facilities permitting photochemicalchamber studies of actual motor vehicle emissions(i.e. collocated motor vehicle dynamometers andphotochemical chambers), and that relatively simplereaction mixtures require less sophisticated analyticalsystems to monitor chamber chemistry, and observa-tions are easier to interpret. However, there exists anuncertainty about how well the atmospheric chemistryof a complex emissions mixture is simulated by a simplesurrogate mixture. When mathematical models areused to predict the atmospheric transformation ofchemical precursors to ozone, uncertain extrapolationsof limited known chemical mechanisms are necessaryfor large numbers of compounds with similar chemicalstructures existing in real environmental mixtures (i.e.a large number of compounds for which no empiricalmechanistic and kinetic data exist are grouped witha relatively small number of compounds of similarchemical structure for which empirical data does exist).

The purpose of this study is to develop a procedurefor preparing exhaust surrogates that approximatethe full compositional profile of actual emissions. Theresulting complex emission surrogates can be used inchamber studies to validate model performance withmixtures approximating the complexity and composi-tion of actual vehicle organic emissions. In this paper,irradiation chamber runs of actual motor vehicle ex-haust samples are contrasted with similar runs of theassociated complex surrogates demonstrating the ac-curacy with which the exhaust photochemistry can besimulated using the surrogate.

EXPERIMENTAL

Surrogate preparation

Examination of automobile exhaust suggests that greaterthan 50% of emitted organic compound mass results fromuncombusted fuel (Leppard et al., 1993). Further, most of thecomplexity of the exhaust mixture results from the fuel (i.e.the combustion derived compounds are relatively small innumber). These observations suggest that a fully representa-tive surrogate can potentially be derived from appropriateaddition of a relatively small number of compounds to thefuel mixture. A spreadsheet model (available from theauthors on request) has been developed wherein the com-positions of the fuel and associated exhaust emissions areused to calculate the composition and concentration ofa mixture that when added to the fuel produces a combinedmixture with composition near equivalent to the exhaust(Tejada and Black, 1997).

The spreadsheet model aligns the corresponding speciatedcomponents of replicate fuel and exhaust samples, computesthe average concentration (ppmC) of each component in thefuel and in the exhaust, and then normalizes each componentto 100% of the respective total NMOG concentration.Ozone-forming potentials of the fuel and exhaust compo-nents are calculated using Carter Maximum Incremental Re-activities (MIR, discussed below). The resulting data set issorted twice; first, to place CO at the top of the list andmethane at the bottom; and second, to arrange the remain-ing data set in decreasing order of the relative ozone formingpotential of the exhaust NMOG components. Concentra-tions of the added NMOG components are calculated ini-tially using the criterion that an NMOG component is to beadded to the fuel only if that component is present in theexhaust and not in the fuel, or if the relative concentration ofthe component is greater in the exhaust than in the fuel. The‘‘added’’ NMOG components are combined with the fueland the combined data set is again NMOG-normalized to100 to give the compositional profile of the surrogate. Theexact proportions of the fuel and ‘‘added’’ components in thenormalized surrogate are calculated as well as the specificreactivities of the exhaust and the surrogate. A second circu-lar iteration is performed that automatically applies a signifi-cance criterion (defined by the user) on the acceptable level ofagreement between the relative ozone forming potential ofthe surrogate and the exhaust. Added components that failthis significance test are deleted. The experimental complex-ity of preparing the surrogate is greatly reduced by eliminat-ing ‘‘added’’ components that are insignificant contributorsto the ozone potential of the surrogate.

In this study, a California phase 2 reformulated gasolinefuel was used to create the reference exhaust samples used todefine the surrogate composition. Triplicate exhaust ana-lyses identified 278 NMOG compounds. Associated analyseswere performed using previously described chromatographicprocedures (Braddock and Crews, 1992; Zweidinger et al.,1993; Siegl et al., 1993). These procedures have been routine-ly used to analyze vehicle exhaust organic mixtures by thislab and by the participants of the Auto/Oil Air QualityImprovement Research Program (AQIRP, 1997), and others(Hoekman et al., 1995; Tejada et al., 1997).

The previously described model identifies 16 NMOGcompounds that when added to the fuel produce an exhaustsurrogate providing nearly 97% of the ozone potential of theactual exhaust mixture. The criteria used to prioritize com-pounds for addition is relative contribution to exhaustCarter MIR Ozone Potential. While a number of reactivityschemes have been developed over the years, a recent schemeby Carter is the most widely used (Carter et al., 1987; andLour and Carter, 1990; Carter, 1994). The reactivity scaledeveloped by Carter recognizes that the ability of a singleorganic compound to produce ozone depends upon the rateat which the compound initially reacts (usually with hy-droxyl radicals), the mechanism of its decomposition, andthe characteristics of the environment into which the com-pound is emitted. This environment is typically referred to asthe ‘‘base case’’. The base case represents a composite mix-ture of emissions from all sources in the reaction scenario.Reactivity factors have been developed for a number ofscenarios (i.e. 2 airshed environments). The two scales mostfrequently used are the maximum incremental reactivity(MIR) scenario and the maximum ozone reactivity (MOR)scenario. The MIR scale is used where the NMOG input (i.e.the organic compound emission rate and composition) hasthe maximum incremental effect on ozone formation, and isused in this experimental protocol to prioritize compoundsfor addition to the fuel. The reactivity weighted ozone form-ing potential of the emissions is estimated using the relation-ship

R"

n+

i/1

(MIR)iw

i

2444 F. BLACK et al.

Fig. 1. Liquid organic mixture injection-vaporization and dilution system.

Table 1. Volumetric distribution of liquid-phase hy-drocarbons added to fuel during surrogate prepara-

tion

Compound Volumetric %

Toluene 25.8Ethylbenzene 11.1m,p-Xylene 27.6o-Xylene 9.01-Methyl-3-ethylbenzene 8.31-Methyl-4-ethylbenzene 3.81,3,5-Trimethylbenzene 3.41-Methyl-2-ethylbenzene 3.51,2,4-Trimethylbenzene 7.6

where R is the normalized ozone-forming potential forNMOG&CO emissions (g O

3formed per g of emitted

NMOG and CO), (MIR)ithe Carter maximum incremental

reactivity of species i (g O3

formed per g of species i emitted)and w

ithe weight fraction of species i in the emissions.

The model identifies a 16 component NMOG mixture,totaling 34 ppmC, that when added (with CO) to 100 ppmCof the fuel collectively produces a surrogate with similarMIR ozone forming potential to that of the reference ex-haust. Having defined the ‘‘target’’ ppmC concentration dis-tribution of the components of the fuel and added NMOGmixture that are combined to obtain the exhaust surrogate,relative liquid volumes (for those compounds that are reas-onably handled as liquids at room temperature) must becalculated to provide for physical preparation of a fuel plusNMOG liquid mixture (for subsequent controlled injection-vaporization and dilution). Compounds more reasonablyhandled in the gas phase are processed to the irradiationchamber from an appropriately prepared compressed gascylinder with N

2as the diluent. The previously mentioned

spreadsheet model can be used to determine the liquid vol-umetric distribution of fuel and added NMOG componentsfrom the previously described ppmC distribution. For theexample scenario, a mixture of 9.5 ml of a liquid hydrocar-bon blend distributed as described in Table 1 is combinedwith 100 ml of fuel. This liquid mixture excludes ‘‘added’’ethylene, propylene, isobutylene, trans-2-butene, 1,3-bu-tadiene, formaldehyde and acetaldehyde, that are more read-ily processed in the gas phase. Formaldehyde andacetaldehyde are highly soluble in water, and can also beprocessed to the chamber using the controlled liquid injec-tion-vaporization and dilution techiques to be described forthe gasoline and ‘‘added’’ liquid NMOG components of thesurrogate organic mixture (Stump et al., 1996).

Figure 1 illustrates apparatus suitable for controlled injec-tion-vaporization and dilution of the liquid-phase mixtures.This is an adaptation of previously described equipment andprocedures for controlled injection of known concentrationsof organics into emissions sampling systems providing forquality assurance tests of sample integrity during complexmixture transport (Stump et al., 1996). Solution A is a mix-ture of liquid gasoline and ‘‘added’’ liquid phase NMOG,and solution B could be a mixture of the water-solubleconstituents of the emissions. HPLC pumping systems per-mit ready adjustment of the relative flow rates of solutionsA and B. Alternatively, motorized syringe pumps can beused. Target initial chamber gas phase organic compounds,CO and NO

xconcentrations are achieved by direct injection

into the irradiation chamber from appropriate compressedgas cylinders.

The apparatus illustrated in Fig. 1 includes equipment forprocessing the injection solution to a 170°C vaporizationoven. Varian HPLC pumps with Scientific Instrumentsmodel LP-21 flow pulse dampeners are used to manageliquid flow rates. Diluent nitrogen flow is managed bya 10 lmin~1 mass flow controller/meter. The nitrogen isheated in the oven by passing through a 50 ft by 0.125 incopper tubing heat exchanger. The hot nitrogen is used to

Photochemical model validation 2445

Table 2. Compound distribution of exhaust and surrogate Carter MIR ozone potential (30 most significant NMOGcompounds and CO)

Compound CarterMIR

Surrogategasoline

contribution,% MIR ozone

potential

Added NMOG,CO

contribution,% MIR ozone

potential

Surrogate,% MIRozone

potential

Exhaust,% MIRozone

potential

Carbon monoxide 0.054 8.985 8.985 8.846m&p-Xylene 7.38 8.425 3.407 11.831 11.649Ethylene 7.29 10.138 10.138 9.982Propylene 9.40 8.844 8.844 8.707Isobutylene 5.31 0.003 6.241 6.244 6.148Toluene 2.73 3.216 1.352 4.568 4.4981,2,4-Trimethylbenzene 8.83 3.330 1.014 4.343 4.276o-Xylene 6.46 2.830 0.987 3.817 3.758Formaldehyde 7.15 3.208 3.208 3.1591-Methyl-3-ethylbenzene 7.20 2.283 0.884 3.168 3.1191,3,5-Trimethylbenzene 10.12 1.818 0.511 2.329 2.293Ethylbenzene 2.70 1.253 0.503 1.756 1.7291,3-Butadiene 10.89 1.468 1.468 1.4451-Methyl-4-ethylbenzene 7.20 1.041 0.398 1.439 1.417Isopentane 1.38 2.020 2.026 1.411Isooctane 0.93 1.210 1.210 1.091Acetaldehyde 5.52 1.084 1.084 1.0671-Methyl-2-ethylbenzene 7.20 0.677 0.376 1.053 1.0372,3-Dimethylpentane 1.51 1.206 1.206 1.017trans-2-butene 9.94 0.022 0.832 0.854 0.8411,2,3-Trimethylbenzene 8.85 0.603 0.603 0.7231,2-Diethylbenzene 6.45 0.575 0.575 0.697cis-2-Butene 9.94 0.017 0.017 0.6412-Methylpentane 1.53 0.785 0.785 0.6062,3,4-Trimethylpentane 1.60 0.667 0.667 0.5902,4-Dimethylpentane 1.78 0.569 0.569 0.5541,3-Dimethyl-2-ethylbenzene 9.07 0.436 0.436 0.530Propadiene 7.29 0.000 0.522¹rans-2-Pentene 8.80 0.606 0.606 0.454Acetylene 0.50 0.000 0.4033-Methylpentane 1.52 0.521 0.521 0.3993-Methylhexane 1.40 0.442 0.442 0.385Sum 34.561 50.232 84.794 83.993Total alkanes 11.860 10.230Total alkenes 34.150 33.500Total aromatics 39.430 40.610Total alkynes 0.000 1.060Total oxygenates 5.580 5.730

vaporize the injection liquid after it has been aerosolized bythe illustrated nebulizer. The mixture is then passed througha 9 in]0.25 in SS tube to a 0.375 in]2 in SS tube filled withSS fragments (1/8 in SS ferrules) and a 4 f]0.25 in SS heatexchanger before passing to the 115°C heated line used totransport the gaseous mixture to the dilution system. The gasstream is initially directed to a rhodium catalyst loop (wherethe organic compounds are oxidized to CO

2and H

2O),

while system flows and temperatures equilibrate; and afterequilibration, directed to the dilution system. An appropri-ate aliquot of the vaporized and diluted organic mixture istransferred from the dilution system to the chamber.

Depending upon the volume and concentration of surro-gate required by the application of interest, it may be morepractical to prepare the entire surrogate mixture in a com-pressed gas cylinder. The gasoline plus added liquid-phasecompound mixture can be vaporized and injected, alongwith the previously mentioned gas-phase compounds, intoan evacuated cylinder for subsequent dilution and compres-sion with N

2(or zero grade air). Quality assurance of similar

sample mixtures for periods up to one year by the Co-ordinating Research Council has demonstrated excellent

stability (Hoekman et al., 1995). After one year, ppmC con-centration losses ranged 2—5% with no clear evidence ofindividual species differences.

Table 2 provides a list of the 30 most significant ozoneforming exhaust NMOG compounds (resulting from com-bustion of Calif. Phase 2 reformulated gasoline and weightedby relative contribution to Carter MIR ozone potential).Aligned with the exhaust compounds are the fuel and addedNMOG components that are combined to produce a sur-rogate mixture with a similar relative distribution ofozone-forming compounds. After CO, the exhaust organiccompounds are ordered by decreasing contribution to MIRozone potential. In this example, 16 NMOG compounds(and CO), when added to the fuel mixture, provide 97% ofthe exhaust MIR ozone potential. As indicated, some of theadded compounds are combustion products, not present inthe fuel; and several are present in the fuel, but requireconcentration manipulation to agree with the exhaust rela-tive abundances. Note that the % MIR ozone potentials ofthe exhaust and surrogate compounds with the largest con-tributions to mixture ozone potential are very similar, as arethe total alkane, alkene, aromatic, alkyne, and oxygenate

2446 F. BLACK et al.

Fig. 2. Photochemical irradiation chamber and associated analytical equipment.

distributions. The 30 most significant NMOG compoundsand CO collectively account about 85% of the total MIRozone potential of exhaust and surrogate mixtures. The 16added NMOG compounds and CO account for 50.2%of the surrogate MIR ozone potential, with the remainder ofthe ozone potential associated with the fuel components ofthe mixture.

Criteria other than MIR ozone potential can be developedfor prioritizing compounds for addition to the fuel when theapplication requires motor vehicle emissions surrogates forpurposes other than ozone photochemistry studies.

Photochemical chamber irradiations

Facilities have been developed to examine the atmo-spheric photochemistry of motor vehicle exhaust. A brief over-view of the facilities and test protocol will follow, as detaileddescriptions have been previously provided (Kleindienstet al., 1992, 1994 and 1997). The approach involves operatinga vehicle on a chassis dynamometer while collecting analiquot of the diluted exhaust, adjusting the NMOG to NO

xratio to about 5.5 and processing this mixture to the irradia-tion chamber achieving initial target conditions of about2.2 ppmC NMOG and 0.400 ppmV NO

x. Controlled ir-

radiations of these mixtures have been remarkably reproduc-ible. This high degree of experimental reproducibility haspermitted detailed direct comparisons of the photochemistryof the previously described surrogate mixtures with the auto-motive exhaust mixtures on which they were based.

A schematic diagram for the experimental apparatus re-quired to conduct these studies is shown in Fig. 2. Theprimary components of this system are the dynamometer(not shown) for generating the required exhaust emissions,the photochemical chamber with associated apparatus forreactant introduction and product sampling, and the ana-lytical instrumentation for making chemical and physicalmeasurements.

Automobile exhaust is generated using a chassis dyna-mometer (for road load simulation while operating the ve-hicle over an urban transient driving simulation) in an areaadjoining the irradiation chamber facility. The dynamometer

is a mechanically-coupled twin-roll, DC electric chassisdynamometer manufactured by Burke-Porter (GrandRapids, MI). The entire vehicle exhaust is transferredthrough a heated (100°C), stainless-steel line to a 20 m3min~1 constant volume sampler (CVS) system providingexhaust dilution and cooling with conditioned ambient air.Exhaust samples are collected from the dilution system usingfive heated Metal Bellows pumps (Sharon, MA; ModelsMB-302 and MB-151), operating in parallel, into 250 l Tef-lon bags for transfer to the photochemical chamber.

The reaction chamber is a 9000 l rectangular structurefabricated from 2-mil Teflon film supported by a metalframe. Samples are taken from fittings mounted on a Teflonplate at the bottom of the chamber. A fan is installed toachieve active mixing of the reactants and compounds injec-ted into the chamber. Ultrazero air is used as the base airmatrix and for sample dilution during irradiations. This air isproduced with an Aadco pure air generator (Aadco Inc.,Model 737) with NO

xlevels typically less than 1 ppbv and

hydrocarbon levels less than 50 ppbC. The irradiation cham-ber is typically operated with 2—3 lmin~1 dilution air tocompensate for volume loss due to sampling. The irradiationsource is based on a system of 122 cm fluorescent bulbs.Radiation in the range 300—350 nm is generated with UVA-340 (Q-Panel, Cleveland, OH) fluorescent blacklight bulbs;radiation at wavelengths 350—400 nm is generated with stan-dard UV bulbs (F40-BL). The mixture of bulbs is designed tomatch, to the extent possible, the distribution of solar radi-ation between 300 and 400 nm. The spectral distribution ofthe light setting is measured with a portable spectro-radiometer (LI-COR, Inc., Model LI-1800). During experi-ments, the light intensity is continuously monitored with anintegrating radiometer (Eppley Laboratory, Inc. Newport,RI).

Exhaust was generated using a Dodge Caravan with a V-6, 3.3 l engine. The vehicle was fueled with California Phase2 reformulated fuel. A detailed hydrocarbon distribution ofthis fuel is available from the authors on request. The vehicleexhaust samples for this study were collected during the firsttwo cycles (i.e. the initial 124 s) of Phase 1 of the FederalTest Procedure (CFR, 1994). This mixture is relatively

Photochemical model validation 2447

Table 3. Chamber initial conditions and example 12.5 h product observed concentrations

Initial Units IR109%9) IR110%9) IR111%9) IR117463 IR118463 IR119463

conditions

NOx

ppmV 0.402 0.396 0.397 0.378 0.409 0.405NMOG ppmC 2.166 2.109 2.218 2.225 2.274 2.257NMOG/NO

xratio 5.39 5.33 5.59 5.89 5.56 5.57

Water ppmV 16290 15220 15270 15420 17340 15900CO ppmV 8 8 10 8 9.1 8.3C

2—C

4alkenes ppmC 0.349 0.334 0.327 0.316 0.321 0.319

Relative NMOG, CO MIRozone potential 1.00 0.98 0.97 0.92 0.96 0.94

12.5 h Units IR109%9) IR110%9) IR111%9) IR117463 IR118463 IR119463

products

O3

ppbV 700 696 719 670 701 702PAN ppbV 21.9 20.7 18.1 17.0 17.6 16.7HCHO ppbV 65.8 64.4 55.7 60.3 55.7 60.0CH

3CHO ppbV 20.2 15.6 19.9 19.3 19.0 19.1

Acetone ppbV 36.9 49.7 45.4 43.3 43.4 47.7

hydrocarbon rich, permitting the target chamber initialNMOG/NO

xratio of 5.5 to be achieved without significant

further NMOG manipulation, and thus providing a uniqueopportunity to contrast exhaust reactivity with surrogatereactivity. NO from a 100 ppmV in N

2cylinder mixture

(National Specialty Gases, Raleigh, NC) is injected, whennecessary, to produce equivalent chamber initial conditions.

RESULTS AND DISCUSSION

Triplicate 12.5 h chamber irradiation tests werecompleted with both exhaust and surrogate samples.Table 3 provides an overview of chamber initial con-ditions and some of the observed 12.5 h products. Forall tests, the initial chamber CO, NMOG and NO

xconcentrations were targeted to be 9 ppmV, 2.2 ppmCand 400 ppbV, respectively (NMOG/NO

xratio 5.5).

For the exhaust irradiations, initial chamber NOx

averaged 88.6% NO and 11.4% NO2; and for the

surrogate irradiations, 93.4% NO and 6.6% NO2.

The higher initial NO2

concentrations existing duringthe exhaust runs result from NO oxidation occurringduring the time periods necessary for collection of anintegrated sample of vehicle exhaust, and for the smalladditions (less than 10% of the initial concentrationswith this vehicle and fuel) of NMOG (surrogate) orNO necessary to adjust the mixture initial NMOG/NO

xratio to the target 5.5. During this period NO

2is

being formed by reaction of NO with molecular O2,

a reaction that is second order in NO concentration(the integrated diluted exhaust NO concentrationsranged from 25 to 30 ppmV). The surrogate runsinvolve direct injection of NO into the chamber to aninitial concentration of 0.4 ppmV. With the surrogateruns less NO

2is formed prior to initial chamber

‘‘lights-on’’. As indicated in Table 3, the initial NOx

concentrations were well correlated; and with greatercare, the initial NO and NO

2concentrations could

have been better correlated. As indicated in Table 3

the relative MIR ozone forming potentials of theinitial chamber NMOG and CO mixtures (nor-malized to run IR109 ) suggest run-to-run ozoneforming differences less than 10%. Observed differ-ences in 12.5 h ozone concentrations with similarlysmall, but not well correlated calculated MIR ozonepotential differences. With the possible exception ofrun IR117 (with the lowest calculated MIR ozonepotential and the lowest chamber 12.5 h ozone con-centration), observed 12.5 hr product variability ap-pears reasonable and related to sources ofexperimental variability other than initial mixtureMIR ozone potential differences.

Figures 3a, b, and c illustrate 12.5 h time profilesof observed chamber NO, NO

x—NO, and ozone

concentrations for each exhaust and surrogate run,respectively. The average observed ozone concentra-tions after 12.5 h were 0.705 ppmV with the exhaustsamples and 0.691 ppmV with the surrogate samples.Examination of Fig. 3c ozone data suggests that ex-haust run IR111 is higher and surrogate run IR 117lower than the prevailing near overlay results of theremaining 4 runs. Similar individual plots of NO andNO

x—NO concentration data suggest that during sur-

rogate run IR117 NO oxidation to NO was slowerthan the prevailing data. Examination of the initialchamber organic composition suggests that the vari-ant observations with run IR117 resulted from anexperimental error while injecting HCHO into thechamber; the initial HCHO concentration was 50%below target concentration. There was no similar ex-planation for the higher ozone observed in run IR111.The 12.5 h product concentrations listed in Table 3suggest that the surrogate runs produced 98% of theozone produced in the exhaust runs, 95% of theformaldehyde , 103% of the acetaldehyde, 102% ofthe acetone, and 85% of the PAN.

In this study, ozone formation potential was usedto prioritize compounds for addition to gasoline.

2448 F. BLACK et al.

Fig. 3. (Continued overleaf ) (a). Irradiation chamber NO time—concentration profiles for individual ex-haust and surrogate samples. (b). Irradiation chamber NO

x—NO time—concentration profiles for indi-

vidual exhaust and surrogate samples. (c). Irradiation chamber ozone time—concentration profiles forindividual exhaust and surrogate samples.

Although this mixture performed reasonably well fora number of other products of interest, PAN wasabout 15% low with the surrogate mixture. Withadditional care given to important PAN precursors,PAN performance can also be improved. The ‘‘perfor-mance criteria’’ product of the reported experimentswas ozone, and the results were better than originallyanticipated. The results suggest that motor vehicleexhaust photochemistry can be reasonably simulatedby irradiating a mixture of fuel and a relatively smallnumber of additional compounds.

The described procedures provide for preparationof exhaust organic surrogates with compositionalcomplexity very similar to that of actual environ-

mental mixtures, surrogates that should be usefulfor photochemical chamber validation of ozone pre-dictive model performance. The surrogate mixtureshave complex compositional profiles; however, theexperimental complexity is no greater than that asso-ciated with preparation of simple 30 to 60 compoundsurrogates. The mixtures are easily prepared, ana-lyzed, stored, and processed to chambers for photo-chemistry studies. It is acknowledged that theanalytical complexity is greater than that with simplesurrogates, but the associated analyses are routinelycompleted by qualified chemists using availablechromatographic analyzers and computer-based datamanagement systems.

Photochemical model validation 2449

Fig. 3. Continued (Caption on previous page).

Acknowledgements—The authors wish to acknowledge thesupport of Michael Gurevich, U.S. Department of Energy,for collaboration in the described research under In-teragency Agreement No. DE-AI01-94CE50397; and thetechnical contributions of William Crews (project super-vision), Kimberly Wood, Tholeathcus Grantham, and Col-leen Loomis (analytical chemistry) of Clean Air VehicleTechnology Center; and Eric Corse (irradia-tion chamberoperations), Richard Snow, Ned Perry and Jerry Faircloth(test cell and vehicle operations), of ManTech, Inc.

The research described in this paper has been funded inpart by the U.S. Environmental Protection Agency. Thisreport has been subjected to Agency review and approvedfor publication. Mention of trade names or commercialproducts does not constitute endorsement or recommenda-tion for use.

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