Real-world measurements of exhaust and evaporative emissions in the Cassiar tunnel predicted by chemical mass balance modeling

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Environmental Science & Technology, 30, 10, pp. 3001-3009, 1996-01

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Real-world measurements of exhaust and evaporative emissions in the

Cassiar tunnel predicted by chemical mass balance modeling

Mclaren, Robert; Gertler, Alan W.; Wittorff, Dave N.; Belzer, Wayne; Dann,

Tom; Singleton, Donald L.

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Real-World Measurements of

Exhaust and Evaporative

Emissions in the Cassiar Tunnel

Predicted by Chemical Mass

Balance Modeling

R O B E R T M C L A R E N *

Departm en t of Ch em istry an d Cen tre for Atm osph eric Ch em istry, York Un iversity, 4700 Keele Street, N orth York , On tario M3J 1P3, Can ada

A L A N W . G E R T L E R A N D D A V E N . W I T T O R F F

Desert Research In stitu te, P.O. Box 60220, Ren o, N evada 89506

W A Y N E B E L Z E R

Scien ce Division , En viron m en t Can ada, 700-1200 W est 73rd Aven u e, Van cou ver, British Colu m bia V6P 6H9, Can ada

T O M D A N N

Pollu tion Man agem en t Division , En viron m en tal Tech n ology Cen tre, Ottaw a, On tario K1A OH3, Can ada

D O N A L D L . S I N G L E T O N

In stitu te for Ch em ical Process an d En viron m en tal Tech n ology, N ation al Research Cou n cil, Ottaw a, On tario K1A 0R6, Can ada

The chemical mass balance model has been used to

separate non-methane hydrocarbon emission factors

measured in the Cassiar tunnel study into exhaust and

evaporative emission factors. The local gasoline

composition has been used as a real-w orld surrogate

profile for exhaust emissions and has been

demon-strated to result in vastly improved model

performance compared to the performance obtained

w ith the use of an exhaust profile derived from

dynamometer testing. Because of the approach used,

the combustion and unburned gasoline components

of exhaust emission gases could be estimated

separately. Unburned gasoline w as found to comprise

63.4 ( 7.0% of exhaust gases for light-duty vehicles

operating in steady-state driving conditions in this

study. On-road benzene emissions w ere found to split

71%/27%/2% betw een the combustion, unburned

gasoline, and evaporative sources. Evaporative

non-methane hydrocarbons w ere found to represent 10.3

( 0.8% of the total on-road emission rate on average.

The apportionment of total NM HC emission factors

to exhaust and evaporative emission factors allow ed

a detailed comparison to exhaust and on-road

evaporative emission factors predicted by the

M OBILE4.1C and M OBILE5C models.

Introduction

Air p ollu tion an d it’s im p act on h u m an h ealth con tin u es to b e a con cern in m an y u rb an areas tod ay. Desp ite m an y efforts to red u ce p recu rsor em ission s th at con trib u te to ozon e form ation , th e p rob lem still exists, largely b ecau se of m isd irected ozon e con trol strategies in th e p ast (1). Ph otoch em ical m od elin g is gen erally u sed to estab lish relation sh ip s b etween p recu rsor em ission s an d ozon e form ation (2). Th is m od elin g req u ires th e u se of d etailed em ission in ven tories, th e errors an d u n certain ties of wh ich are freq u en tly b lam ed for p oor m od el p erform an ce an d p reviou s failu res to solve th e ozon e p rob lem . Motor veh icles are kn own to accou n t for sign ifican t fraction s of th e n itrogen oxide (NOx), volatile organ ic com p ou n d (VOC), an d carb on m on oxid e (CO) em ission s in th ese in ven tories. For region al in ven tories, m otor veh icle exh au st em ission s are cu rren tly estim ated by th e p rodu ct of th e em ission factor for a given p ollu tan t (g/ m i) an d th e estim ated veh icle m iles traveled (VMT) for a given veh icle typ e an d set of d rivin g con d ition s. Th e em ission factor is con sid ered to b e m u ch m ore u n certain th an th e VMT estim ate an d is gen erally a fu n ction of m an y variab les. Em ission factors for m ost em ission in ven tories are cu rren tly estim ated b y m otor veh icle em ission factor m od els su ch as th e U.S. EPA’s MOBILE m od els an d th e EMFAC m od el in u se in Californ ia. In Can ad a, th e MOBILE m od el is u sed , b u t with m od ifica-tion s m ad e to th e b ase em ission rates of veh icles to accou n t for d ifferen ces in em ission s con trol tech n ology b etween U.S. an d Can ad ian veh icles in p ast years.

An early com p arison of em ission factors m easu red in a tu n n el with th ose p red icted b y an em ission factor m od el were m ad e followin g th e Van Nu ys Tu n n el stu d y (3, 4). Tu n n els are con ven ien t for veh icle em ission m easu rem en ts b ecau se th ey rep resen t a con trolled en viron m en t in wh ich veh icles, op eratin g u n d er “real-world ” con d ition s, can b e m on itored an d th eir p ollu tan ts con cen trated in a flowin g air m ass. On e of th e early tu n n el exp erim en ts p erform ed d u rin g th e Sou th ern Californ ia Air Qu ality Stu d y (SCAQS) in dicated th at th e EMFAC m odel was p erform in g reason ably well for th e p red iction of NOx em ission factors b u t was u n d erestim atin g VOC an d CO em ission factors to a sign ifican t exten t, b y a factor of 2-4 (3). Sin ce th en , sev-eral u p d ates h ave b een m ad e to th e em ission factor m od els, an d several oth er tu n n el stu d ies h ave b een p erform ed (5,

6). Som e of th ese m ore recen t stu d ies in d icate th at th e

n ewer version s of th e em ission factor m od els are n o lon ger u n d erestim atin g to su ch a great exten t an d m ay in fact b e overestim atin g u n d er certain con d ition s.

On e su ch stu d y, th e Cassiar Tu n n el stu d y (7), was p erform ed on a section of h igh way in Van cou ver, B. C, d u rin g th e 1993 Lower Fraser Valley Oxid an ts Stu d y. A p reviou s p u b lication h as ou tlin ed th e gen eral fin d in gs of th at stu d y for NOx, n on -m eth an e h yd rocarb on s (NMHC), an d CO em ission factors (6). In th is p ap er, we exten d th ose fin d in gs to fu rth er focu s on NMHCs an d to rep ort real-world em ission factors of d econ volu ted exh au st an d evap orative NMHC em ission s an d to com p are th em to em ission factors calcu lated with th e Can ad ian version s of

* Au th or to wh om corresp on d en ce sh ou ld b e ad d ressed ; fax: 416-736-5936; e-m ail ad d ress: tu rin g.sci.yorku .ca.

Environ. Sci. Technol.1996,30,3001-3009

S0013-936X(96)00036-3 CCC: $12.00 1996 Am erican Chem ical Society VOL. 30, NO. 10, 1996 / ENVIRONM ENTAL SCIENCE & TECHNOLOGY93 0 0 1

Downloaded via NATL RESEARCH COUNCIL CANADA on July 13, 2018 at 15:59:21 (UTC).

th e U.S. EPAMOBILE m odels, MOBILE4.1C an d MOBILE5C. Th e total m easu red NMHC em ission factors, given b y th e su m of over 100 sp eciated h yd rocarb on s, are d econ volu ted th rou gh th e u se of th e ch em ical m ass b alan ce (CMB) m od el (8).

Th e CMB m od el h as recen tly b een ap p lied to oth er tu n n el stu d ies an d to region al stu d ies of VOC sou rces

(9-14). In th e m od el, each sou rce is rep resen ted b y a sou rce

p rofile th at sp ecifies th e fraction al con trib u tion of each VOC sp ecies to th e m ass em ission s from th at sou rce. In all th ese stu d ies, m otor veh icle exh au st is th e m ost sign ifican t VOC sou rce an d is gen erally rep resen ted b y p rofiles d erived from d yn am om eter testin g of veh icles. Th e lim itation in th e u se of d yn am om eter-b ased p rofiles is th at th ey are sp ecific to th e tech n ology, age, an d op eratin g con d ition of th e veh icle as well as th e com p osition of th e fu el u sed in th e veh icle. Recen t literatu re h as p rovid ed in form ation ab ou t th e p rob lem s associated with th e u se of th ese p rofiles alth ou gh few oth er op tion s h ave b een exp lored (13). In p articu lar, on e recen t stu d y fou n d th at th e sou rce allocation b y th e CMB m od el was very sen sitive to th e ch oice of fittin g sp ecies an d p articu larly to th e relative ab u n d an ces of com b u stion p rod u cts in th e exh au st gas p rofile su ch as acetylen e an d eth ylen e (13). Th e relative p rop ortion of acetylen e is h igh ly d ep en d en t on th e state of rep air, th e age, an d th e op eration m od e of th e veh icle. Th e p rop ortion of oth er ligh t olefin sp ecies is also d ep en d en t on th e fu el com p osition . Becau se of th e u n certain ties associated with th e u se of d yn am om eter-d erived exh au st p rofiles, several recen t p u blication s h ave in dicated th e n eed for d evelop m en t an d u se of real-world exh au st p rofiles in sim ilar CMB ap p lication s (9, 13, 15). Th e su ggested op tion s for th is in clu d e th e site-sp ecific u se of a lin ear com b in ation of p u b lish ed exh au st p rofiles, d erived from veh icles with d ifferen t em ission s con trol tech n ology, or th e u se of road sid e m easu red p rofiles. Th e form er ap p roach will still su ffer from th e fact th at th e com p osition of th e fu el u sed in th e em ission s testin g m ay still n ot m atch th e com p osition fou n d in th e geograp h ic area to wh ich th e CMB m od el is b ein g ap p lied . Th e latter op tion will resu lt in a p rofile th at is rep resen tative of th e local gasolin e com p osition b u t also in clu d es on -road evap orative em ission s.

In th is stu d y, we h ave exp lored a n ew ap p roach for exh au st rep resen tation in th e CMB m od el. Sin ce exh au st em ission s are kn own to con tain sign ifican t q u an tities of u n b u rn ed gasolin e as well as com b u stion sp ecies (16-19), we su ggest th at th e local gasolin e com p osition can act as a su rrogate for a real-world exh au st p rofile. Th e com bu stion com p on en t of th e exh au st gases can th en b e estim ated b y th e d ifferen ce b etween th e ob served an d th e CMB-calcu lated em ission factors for id en tified com b u stion sp ecies. Th is m eth od relies on a good d escrip tion of th e local gasolin e com p osition th at com p rises ∼60% of th e exh au st gas m ixtu re as op p osed to th e relian ce on “tracer” com b u stion sp ecies su ch as acetylen e an d eth ylen e in th e con ven tion al ap p roach wh ere a d yn am om eter test p rofile is u sed .

In th is p ap er, th is n ew m eth od ology was tested an d valid ated th rou gh com p arison to th e u se of a con ven tion al exh au st p rofile in th e CMB m odel. Followin g th is validation , th e resu lts for 16 in d ivid u al tu n n el ru n s are p resen ted . As an ad d ition al b en efit to th e n ew ap p roach , we were ab le to estim ate on a ru n by ru n basis th e p ercen tage con tribu -tion of gasolin e to th e exh au st gases. Th ese resu lts are d iscu ssed an d com p ared to recen t d yn am om eter-b ased

stu d ies. Th e p ap er con clu d es with a d etailed com p arison of exh au st an d evap orative em ission factors d eterm in ed b y th e exp erim en tal/ CMB ap p roach an d b y em ission factor m od els.

Tunnel Experiment

Th e Cassiar Tu n n el is an u rb an two-b ore tu n n el, 730 m in len gth , with two lan es of traffic p er b ore. It is situ ated on th e Tran s-Can ad a High way in Van cou ver, BC. Traffic is gen erally h eavy d u rin g th e d ay with an average sp eed of ∼90 km / h an d a high proportion of light-duty vehicles. Th e m eth od ology u sed for th e calcu lation of m ob ile sou rce em ission factors in tu n n els h as b een d escrib ed in d etail b y Pierson (20). Th e d etails for th is p articu lar stu d y h ave b een ou tlin ed p reviou sly (6, 7). Em ission factors were m easu red for a total of 16 1-h ru n s in th e n orth b ou n d b ore d u rin g Au gu st 13-18, 1993. Table 1 h igh ligh ts th e con dition s en cou n tered for each tu n n el ru n in th e stu d y. Th e d etailed gasolin e com p osition s sold d u rin g th e stu d y p eriod in th e areas of Van cou ver an d Wh atcom Cou n ty, WA, were ob tain ed from a gasolin e sam p lin g p rogram con d u cted in th e Lower Fraser Valley (LFV).

Pollu tan t sam p lin g was p erform ed at th e en tran ce an d exit of th e tu n n el to an alyze for NOx, CO, CO2, m eth an e,

an d NMHCs. Acan ister sam p ler was u sed to collect sam p les of NMHC (C2-C10) in electrop olish ed stain less steel can

-isters. Th e can isters were an alyzed b y GC-MSD at En vi-ron m en t Can ad a’s River Road En vivi-ron m en tal Tech n ology Cen tre. Th e an alytical m eth od ology h as b een ou tlin ed p reviou sly (21). In total, 102 h ydrocarbon s were qu an tified.

Emission Profiles

NMHC em ission p rofiles for in p u t to th e CMB m od el were created for each tu n n el ru n u sin g th e em ission factor m eth od ology ou tlin ed ab ove. Th e CMB also req u ires an estim ation of th e u n certain ty for each m easu rem en t valu e. Th is u n certain ty, σi, was estim ated as

wh ere CV is th e coefficien t of variation for th e em ission factor m easu rem en ts, con servatively estim ated to b e ab ou t 20%, an d EDL is th e em ission factor d etection lim it,

estim ated to b e ab ou t 0.1 m g/ m i. TA BLE 1

Details of Individual Runs in the Cassiar Tunnel

Study

run time date vehicles temp (°C)

1 02:00-03:00 8/13/93 125 13.4 2 06:00-07:00 8/13/93 1678 13.3 3 10:00-11:00 8/13/93 1821 15.4 4 15:00-16:00 8/13/93 2502 17.0 5 09:00-10:00 8/14/93 1470 15.3 6 09:00-10:00 8/15/93 948 15.3 7 02:00-03:00 8/16/93 93 14.2 8 06:00-07:00 8/16/93 1622 14.6 9 08:00-09:00 8/16/93 1859 12.8 10 02:00-03:00 8/18/93 100 12.8 11 06:00-07:00 8/18/93 1650 15.3 12 08:00-09:00 8/18/93 2074 13.4 13 10:00-11:00 8/18/93 1769 19.4 14 12:00-13:00 8/18/93 1850 20.0 15 14:00-15:00 8/18/93 1977 21.3 16 16:00-17:00 8/18/93 2975 22.5 σ_i)

x

Th e em ission factors were calcu lated for each sp ecies an d ru n givin g 16 NMHC em ission factor d istrib u tion s to b e u sed in th e CMB m od elin g. In ad d ition to th is, an extra distribu tion was created in an attem p t to sep arate th e effects of ligh t-d u ty (p rim arily gasolin e sp ark ign ited ) an d h eavy-d u ty veh icles (p rim arily eavy-d iesel). Th is last em ission eavy-d istri-b u tion was created istri-b y p erform in g a regression an alysis on th e em ission factors for all of th e 16 tu n n el ru n s with th e fraction of ligh t-d u ty veh icles in each tu n n el ru n as th e in dep en den t variable. Th is m eth odology h as been ou tlin ed in d etail p reviou sly (22). Th e resu ltan t ligh t-d u ty em ission p rofile is exp ected to h ave less con trib u tion from d iesel veh icles sin ce th e h eavy-du ty em ission factors are exclu ded.

Source Profiles

Th e CMB m od el req u ires sou rce p rofiles in ad d ition to th e m easu red recep tor d istrib u tion s (Ei). In th is ap p lication ,

we wish ed to allocate total NMHC em ission s to exh au st em ission s an d evap orative em ission s. As m en tion ed p reviou sly, we u sed th e gasolin e com p osition p rofile as a su rrogate for a real-world exh au st p rofile. Th e evap orative em ission s were rep resen ted b y a gasolin e vap or p rofile. Several organ ic sou rce p rofiles were develop ed. Twen ty-fou r gasolin e sam p les were collected on b oth sid es of th e b ord er in th e LFVairsh ed d u rin g PACIFIC ‘93. Twelve were

from refin ers in th e Van cou ver region , an d 12 were sam p les collected from retail station s in Wh atcom Cou n ty, WA. Th e gasolin e was sam p led an d an alyzed for Reid vap or p ressu re (RVP) an d d etailed ch em ical com p osition b y GC-MSD at th e lab oratories of Esso Petroleu m Prod u cts in Sarn ia, On tario. Sales d ata b y grad e an d b y b an n er, ob tain ed from a m arketin g su rvey, were u sed to calcu late a sales weigh ted average gasolin e com p osition for th e Van cou ver region an d th e Wh atcom Cou n ty region . Th e two p rofiles are lab eled VANGAS an d WHAGAS in Tab le 2. Sin ce gasolin e h ead sp ace sam p les were n ot taken , th e gasolin e vap or com -p osition corres-p on d in g to th e average gasolin e for each sid e of th e b ord er was calcu lated from th erm od yn am ic p rin cip les u sin g Raou lt’s law b y a m eth od ou tlin ed p revi-ou sly (14). Excellen t agreem en t h as b een ob served recen tly b etween vap or p rofiles calcu lated in th is m an n er an d p rofiles of th e vap or (h ead sp ace an alysis) m easu red ab ove th e sam e gasolin e sam p le (23, 24). Th e two gasolin e vap or p rofiles were calcu lated at 20°C an d are listed as VANVAP an d WHAVAP in Tab le 2.

Un certain ty estim ates for each sp ecies in th e gasolin e an d gasolin e vap or p rofiles were calcu lated in th e followin g m an n er. Th e gasolin e sam p les were sep arated in to th e th ree grad es: regu lar, m id grad e, an d p rem iu m . Th e stan d ard d eviation of th e fraction of each sp ecies was th en TA BLE 2

Source Profiles (% NMHC) Used in CMB Model

HC species F1b _F2b _F3b _Exh801c _Vangasd _Vanvape _{W hagas}f _{W havap}g

2 ethylene * 9.90 0.000 0.000 0.000 0.000 3 acetylene * 2.78 0.000 0.000 0.000 0.000 7 isobutane * * * 0.920 0.428 5.678 0.116 1.793 10 butane * * * 6.440 2.222 19.92 1.861 19.48 15 isopentane * * * 4.930 7.630 25.65 7.615 29.89 18 pentane * * * 2.210 4.959 12.30 4.402 12.75 23 2,2-dim ethylbutane * * * 0.440 0.218 0.334 0.192 0.345 26 cyclopentane * 0.477 0.724 0.522 0.925 27 2,3-dim ethylbutane * * * 0.810 1.089 1.216 1.080 1.410 29 2-m ethylpentane * * * 1.910 3.816 3.829 2.483 2.910 31 3-m ethylpentane * * * 1.370 2.449 2.197 2.161 2.265 33 hexane * * * 0.920 3.200 2.269 2.720 2.252 40 m ethylcyclopentane * * * 0.920 2.125 1.370 1.976 1.487 41 2,4-dim ethylpentane * * * 0.820 1.006 0.460 1.159 0.619 43 benzene * 3.910 2.114 0.928 2.370 1.216 44 cyclohexane * 0.354 0.160 0.554 0.293 47 3-m ethylhexane * * * 0.000 1.652 0.466 1.415 0.466 49 2,2,4-trim ethylpentane * * * 2.910 1.298 0.293 3.800 1.002 51 heptane * 1.032 0.214 0.901 0.218 55 m ethylcyclohexane * 0.595 0.126 0.526 0.130 57 2,4-dim ethylhexane * 0.349 0.048 0.460 0.073 58 2,3,4-trim ethylpentane * 0.752 0.091 1.559 0.221 59 toluene * * * 7.100 9.907 1.265 9.994 1.491 60 2-m ethylheptane * 0.492 0.045 0.416 0.044 61 4-m ethylheptane * 0.252 0.023 0.242 0.026 62 3-m ethylheptane * 0.585 0.050 0.457 0.046 65 2,2,5-trim ethylhexane * 0.381 0.028 0.136 0.012 67 octane * * * 0.440 0.446 0.027 0.365 0.026 73 ethylbenzene * * * 0.790 1.643 0.068 1.861 0.090 74 m/p-xylene * * * 3.320 7.214 0.267 7.548 0.326 76 o-xylene * * * 2.220 2.775 0.079 2.851 0.095 78 nonane * * * 0.310 0.122 0.002 0.149 0.003 81 n-propylbenzene * 0.537 0.008 0.577 0.010 82 3-ethyltoluene * * * 1.630 1.911 0.023 2.010 0.029 83 4-ethyltoluene * * * 0.000 0.823 0.010 0.870 0.013 84 1,3,5-trim ethylbenzene * * * 0.000 1.067 0.011 1.073 0.013 85 2-ethyltoluene * * * 0.280 0.724 0.007 0.746 0.009 92 1,2,3-trim ethylbenzene * 0.746 0.005 0.731 0.005

a_{Asterisk (* ) indicates a fitting species.}b_{Fitting species codes F1, F2, and F3 are defined here and used to describe different fits in Table 3.} c_{Exh801a is derived from FTP tests on 46 in-use passenger vehicles, ref 26.}d_{Vancouver gasoline com position profile.}e_{Vancouver gasoline vapor}

calcu lated for each gasolin e grad e. Th ese stan d ard d evia-tion s were th en p rop agated th rou gh to th e fin al estim ate of u n certain ty for each sp ecies in th e sales weigh ted average gasolin e p rofiles an d th e corresp on d in g vap or p rofiles. In th is m an n er, th e stan d ard d eviation s in clu d e u n certain ty from th e an alytical m easu rem en ts as well as from variability in local gasolin e com p osition .

A d yn am om eter-b ased exh au st p rofile was also u sed in th e CMB an alysis for com p arison of th e CMB p erform an ce u sin g th e con ven tion al ap p roach an d th e n ew su rrogate ap p roach ou tlin ed in th is work. Profile Exh 801a in Tab le 2 was d erived from FTP tests of 46 in -u se p assen ger veh icles for 1975-1982 m odel years (25). Profile Exh 801a was recalcu lated b y th e Californ ia Air Resou rces Board from th e origin al m easu rem en ts to p rovid e a m ore com p lete ch em ical b reak d own . Prop an e/ p rop en e, b en zen e/ cyclo-h exan e, an d tolu en e/ 2,3-d im etcyclo-h ylcyclo-h exan e were n ot sep a-rately rep orted in th e origin al work, so ratios of 3:22, 19:1, an d 1:0 were d erived from tu n n el m easu rem en ts (13) an d u sed for th ese p airs of sp ecies, resp ectively. Th is m od ified exh au st p rofile h as b een u sed in several recen t CMB m od elin g stu d ies (9, 13, 14), an d p erform an ce valu es h ave typ ically b een b etter th rou gh its u se com p ared to oth er exh au st p rofiles. All p rofiles are in d icated in Tab le 2. For b revity, on ly th ose sp ecies u sed as fittin g sp ecies in th e m od el h ave b een in d icated in Tab le 2.

Application of the Chemical Mass Balance Model to

Tunnel Runs

Th e ch em ical m ass balan ce m odel (CMB) con sists of a least-sq u ares solu tion to a set of lin ear eq u ation s th at exp ress th e m easu red recep tor con cen tration of ch em ical sp ecies as a lin ear su m of p rod u cts of th e sou rce p rofile fraction s for th e sp ecies an d sou rce con tribu tion s. Th e sou rce p rofile fraction s an d th e recep tor con cen tration s, each with realistic u n certain ty estim ates, serve as in p u t data to th e CMB m odel. Th e ou tp u t con sists of th e con trib u tion s of each sou rce to total m easu red NMHC as well as to in d ivid u al NMHC con cen tration s. Stan d ard u n certain ties are calcu lated as well. In p u t data u n certain ties are u sed to weigh t th e relative im p ortan ce of th e in p u t d ata to th e m od el solu tion an d to estim ate u n certain ties in th e sou rce con trib u tion s. Cal-cu lation s were m ad e u sin g th e U.S. EPA/ DRI Version 7.0 of th e CMB m od el (8).

Th e CMB m od el was u sed to estim ate evap orative an d exh au st sou rce con trib u tion s to ob served NMHC em ission factor p rofiles in th e tu n n el. Th e Van cou ver gasolin e p rofile was u sed to rep resen t th e exh au st sou rce, wh ile th e corresp on d in g vap or p rofile was u sed to rep resen t on -road evap orative em ission s. A caveat to th is m eth od is th at on -road evap orative em ission s, as th ey are d efin ed in th e MOBILE m od els, m ay in clu d e som e em ission s th at are n ot th e sam e in com p osition as a gasolin e vap or in eq u ilib riu m with th e liq u id . For on -road evap orative em ission s, we exp ect th ese con tribu tion s to be sm all. Th e u se of a gasolin e p rofile to rep resen t exh au st em ission s is n ew in th is CMB ap p lication . Previou s stu d ies u sin g d ifferen t m eth od s h ave sh own th at u n b u rn ed fu el rep resen ts a sign ifican t com -p on en t of exh au st NMHC for variou s fu els (16) an d is estim ated to b e 50-65% for gasolin e u sed in cu rren t tech n ology veh icles (18). Th e oth er con trib u tion to exh au st NMHC is from com b u stion sp ecies su ch as acetylen e, eth ylen e, olefin s, an d ben zen e. Wh ile som e of th ese sp ecies su ch as acetylen e an d eth ylen e are key tracers for au

to-m ob ile exh au st, ou r ap p roach h ere h as b een to elito-m in ate all com b u stion sp ecies from th e an alysis b y n ot u sin g th em as fittin g sp ecies in th e CMB m od el. Th e trad eoff in th is ap p roach is to ob tain m u ch b etter m od el p erform an ce b y relyin g on a very good d escrip tion of th e ca. 60% gasolin e p resen t in exh au st as op p osed to relyin g on th e gen eric com b u stion tracers.

Th e d isad van tage to th is ap p roach is th at th e CMB calcu lated em ission factor d oes n ot accou n t for th e m ass of em itted com b u stion sp ecies in exh au st. It on ly accou n ts for on -road evap orative em ission s an d th e u n b u rn ed fu el com p on en t of exh au st. Th is was overcom e b y allocatin g th e excess m easu red m ass for iden tified com bu stion sp ecies (ab ove th at calcu lated b y th e CMB m od el) to th e com b u s-tion sou rce. Th is is valid sin ce th e em ission factor m eth od ology u sed in th e tu n n el exp erim en t en su res th at th e total m ass of NMHC origin ates from veh icles op eratin g in th e tu n n el. Th e com b u stion com p on en t for selected sp ecies was th erefore calcu lated extern al to th e m od el as th e d ifferen ce b etween th e exp erim en tal em ission factor an d th e CMB calcu lated em ission factor. Th e su m of th ese differen ces over all sp ecies defin ed to be com bu stion sp ecies h as b een lab eled th e com b u stion com p on en t in th is stu d y an d is largely d erived from th e low m olecu lar weigh t olefin s an d b en zen e. Th e tru e exh au st sou rce can th en b e calcu lated as th e su m of th e gasolin e com p on en t an d th e com b u stion com p on en t.

To test an d valid ate th is ap p roach , n u m erou s fit were p erform ed on several of th e Cassiar em ission d istrib u tion s u sin g d ifferen t com b in ation s of sou rce p rofiles an d fittin g sp ecies. Th e resu lts for m od el sim u lation s p erform ed with on e em ission factor d istrib u tion , th e average ligh t-d u ty veh icle p rofile ob tain ed b y least sq u ares regression , are sh own in Tab le 3. For each m od el sim u lation , Tab le 3 in d icates th e n u m b er an d typ e of fittin g sp ecies u sed , p erform an ce m easu res, an d th e d ifferen t sou rce con trib u -tion s (in u n its of g/ m i). Fou r p erform an ce m easu res are in d icated . R2 _{rep resen ts th e good n ess of fit b etween}

calcu lated an d m easu red em ission factors, in d icatin g th e fraction of th e varian ce in th e m easu red factors th at is exp lain ed b y th e varian ce in th e calcu lated factors. Th e red u ced χ2_{valu e is eq u al to th e weigh ted su m of sq u ares}

of th e d ifferen ces b etween m easu red an d calcu lated con cen tration s for th e fittin g sp ecies. Lower valu es in dicate a b etter fit. Valu es of χ less th an 1.0 in d icate a very good fit to th e m easu red d ata b y th e sou rce p rofiles (27). Th e p ercen t m ass recovery (MASS) is th e p ercen t ratio of total NMHC calcu lated b y th e m od el vs th e m easu red valu e. Valu es close to 100% in d icate th at th e sou rces u sed for m od elin g ad eq u ately accou n t for all th e m easu red m ass. Th e last m easu re in clu d ed , collin earity (COL), in d icates th e p resen ce or ab sen ce (Y/ N) of sim ilarity/ u n certain ty clu sters in th e fit as in d icated b y th e m od el ou tp u t (28). Th ese clu sters resu lt from collin earity b etween sou rce p rofiles or excessive u n certain ty in th e sou rce an d recep tor p rofiles an d in d icate th at resolu tion of th e sou rce con tri-b u tion s m ay n ot tri-b e ad eq u ate.

Th e first five fits in Tab le 3 were p erform ed with a p relim in ary set of files con tain in g 33 sp ecies in ten d ed to b e u sed for con ven tion al CMB m od elin g with an exh au st sou rce p rofile. Th e last two fits were don e with an exp an ded set of files con tain in g 84 sp ecies. To b e con sisten t in ou r com p arison s, n on e of th e fittin g sp ecies in th e m od el sim u lation s in clu ded olefin s greater th an two carbon atom s, an y sp ecies with greater th an n in e carbon atom s, an d several

sp ecies th at were su sp ected to h ave ch rom atograp h ic in terferen ce. Th e first fit sh own u ses two sou rce p rofiles, th e con ven tion al exh au st p rofile an d a vap or p rofile. Twen ty-six fittin g sp ecies were u sed (Tab le 2), in clu d in g th e com b u stion sp ecies: acetylen e, eth ylen e, an d b en zen e. Th e p erform an ce m easu res are reason ab ly p oor for th is fit (R2_{) 0.62, χ ) 3.68, MASS ) 61.3%, N}

f) 26). Th e secon d

fit, still u sin g com b u stion fittin g sp ecies, in d icates th at a b etter fit can b e ob tain ed b y in clu sion of th e local gasolin e p rofile. Th e gasolin e sou rce is d om in an t over th e exh au st sou rce, an d th e com b u stion sp ecies, still retain ed as fittin g sp ecies, are greatly u n d erp red icted . Th e reten tion of th ese com b u stion sp ecies are likely resp on sib le for th e still reason ab ly p oor R2_{valu e (0.84). Th e n ext two fits (3 an d}

4) were p erform ed with th e sam e sou rce p rofiles as th e first two b u t with th e com b u stion sp ecies (acetylen e, eth ylen e an d b en zen e) n ot in clu d ed as fittin g sp ecies. Th is d oes n ot sign ifican tly im p rove th e fit p erform ed with th e exh au st an d vap or p rofile (3 com p ared to 1) b u t d oes sign ifican tly im p rove th e fit th at in clu des th e extra gasolin e sou rce p rofile (4 com p ared to 2). Th e im p rovem en t in th is fit is u n d er-stan d ab le sin ce th e com b u stion sp ecies are n ot p resen t in th e gasolin e p rofile. Th e fifth fit, p erform ed with ju st th e gasolin e an d vap or p rofiles, resu lted in an excellen t fit.

Th is good CMB p erform an ce m ad e it clear th at th e gasolin e com p osition cou ld b e u sed su ccessfu lly as a su rrogate for gasolin e exh au st. With th is fact estab lish ed , exp an d ed sou rce an d recep tor files were th en created con tain in g 84 com m on sp ecies. From th is list of 84 sp ecies, fittin g sp ecies selection was p erform ed in th e followin g way. As a b ase, all sp ecies were selected . Th en all p oten tial com b u stion sp ecies were rem oved in clu d in g eth an e, acetylen e, eth ylen e, p rop an e, oth er olefin s, an d b en zen e. Several sp ecies with ch rom atograp h ic in terferen ces were th en rem oved (2-m eth ylh exan e/ 2,3-d im eth ylp en tan e; 2,5-d im eth ylh exan e/ 2,2,3-trim eth ylp en tan e) alon g with all sp ecies th at h ad greater th an n in e carb on atom s b ecau se of p oten tial in terferen ce from d iesel em ission s. Th is p rocess left 35 fittin g sp ecies. It is n oted th at tolu en e, wh ich h as b een id en tified as a p oten tial com b u stion sp ecies (17), was retain ed as a fittin g sp ecies in th is stu d y sin ce in itial sim u lation s in d icated th at m ost of th e tolu en e m ass (∼90-93%) cou ld be accou n ted for by th e exh au st an d evap orative com p on en ts. Th is was well with in g th e error ob served for oth er n on com b u stion sp ecies. Su b seq u en t sen sitivity sim u lation s sh owed th at th e exclu sion of tolu en e as a fittin g sp ecies h as a very m in or effect on th e resu lts. Th e fit u sin g th e VANGAS an d VANVAP sou rce p rofiles with 35 fittin g

sp ecies is sh own on lin e 6 of Tab le 3. Th e p erform an ce m easu res are excellen t (R2_{) 0.97, χ ) 0.24, MASS ) 107.9%,}

Nf) 35). A revised defin ition of m ass recovery h as been

u sed for fits 6 an d 7 to in clu d e th e m ass of com b u stion sp ecies calcu lated extern al to th e m od el. Th e calcu lated an d m easu red fittin g sp ecies em ission factors for fit 6 are sh own in Figu re 1.

Th e im p roved CMB p erform an ce u sin g gasolin e com -p osition as a su rrogate for exh au st arises from th e fact th at exh au st con tain s sign ifican t qu an tities of u n b u rn ed gaso-lin e. Ad d ition ally, th e gasogaso-lin e com p osition p rofile u sed in th e m od el was sp ecific to th e region an d tim e in wh ich th e Cassiar stu d y was p erform ed . In trin sically, th e Exh 801a p rofile also con tain s gasolin e com p osition in form ation , b u t th e gasolin e com p osition u sed in th ose d yn am om eter tests is likely d ifferen t from th at availab le in th e LFV, Tab le 2. We m ay th erefore exp ect th at gasolin e p rofiles th at are n ot sp ecific to th e region will give p oorer m od el p erform an ce. To test th is th eory, an extra fit was d on e (fit 7 in Tab le 3) in wh ich th e Wh atcom Cou n ty gasolin e an d vap or sou rce p rofiles were u sed . Th is is a u n iq u e test for th e m od elin g ap p roach ou tlin ed h ere sin ce th e two gasolin e p ools are p resen t in th e sam e LFV airsh ed , an d yet in term ixin g is lim ited d u e to th e p resen ce of an in tern ation al b ord er. Th e Can ad ian gasolin e is exp ected to d om in ate in th e Cassiar Tu n n el. Wh ile th e sou rce con trib u tion s are sim ilar to th ose ob tain ed with th e b ase fit (fit 6), th e p erform an ce m easu res are d eteriorated (R2_{) 0.93, χ) 0.45, MASS ) 111%, N}

f)

35). It is exp ected th at d ifferen ces in fu el com p osition b etween th e two areas an d th e d om in an ce of Can ad ian fu el in th e Van cou ver m arket (an d h en ce th e Cassiar tu n n el) are resp on sib le for th e d ifferen ces in m od el p erform an ce. Som e of th e d ifferen ces in fu el com p osition b etween th e two areas can b e id en tified in Tab le 2.

It is worth n otin g th at th e CMB p erform an ce ob tain ed with th e u se of th e Exh 801a d yn am om eter-b ased p rofile can b e im p roved b y d eselectin g fittin g sp ecies th at sh ow large deviation s between m easu red an d calcu lated em ission factors. Th is ap p roach su ffers from th e fact th at it is som ewh at arb itrary, alth ou gh it is p rob ab ly eq u ivalen t to an ap p roach in wh ich on e d eselects sp ecies for wh ich th e com p osition s d iffer sign ifican tly, b etween th e LFVgasolin e an d th e gasolin e u sed in th e d yn am om eter tests.

With th e p reviou s resu lts an d d iscu ssion taken as a validation of th e m eth odology for u sin g gasolin e com p osi-tion as su rrogate for exh au st em ission s, each tu n n el ru n was th en fit u sin g th e VANGAS an d VANVAP p rofiles an d TA BLE 3

Summary of CMB Model Performance for Fitting the LDV Emissions Profile w ith Various Source Profile

Combinations

Nfa R2 χ M ASSb(%) COLc

Exh801

(g/mi) Vangas(g/mi) Vanvap(g/mi) W hagas(g/mi) W havap(g/mi) Combustion d (g/mi) 1 26 (F1) 0.62 3.68 61.3 ( 9.2 N 0.251 ( 0.029 0.102 ( 0.026 2 26 (F1) 0.84 1.32 70.0 ( 9.0 Y 0.061 ( 0.045 0.285 ( 0.043 0.057 ( 0.026 3 23 (F2) 0.63 3.55 58.3 ( 9.0 N 0.224 ( 0.030 0.111 ( 0.026 4 23 (F2) 0.97 0.21 65.9 ( 8.9 Y -0.016 ( 0.050 0.329 ( 0.045 0.066 ( 0.027 5 23 (F2) 0.97 0.21 66.6 ( 8.6 N 0.319 ( 0.033 0.065 ( 0.026 6 35 (F3) 0.97 0.24 107.9 ( 13.5 N 0.318 ( 0.026 0.066 ( 0.025 0.238 ( 0.029 7 35 (F3) 0.93 0.45 111.0 ( 14.0 N 0.329 ( 0.029 0.065 ( 0.027 0.246 ( 0.029 a_N

fis the num ber of fitting species. The fitting species used are indicated by the code in parentheses; cross reference to Table 2.bM ass recovery

(M ASS) for sim ulations 1-5 only account for mass allocated by the model. For simulations 6 and 7, mass recovery is redefined to include mass allocated by the m odel as w ell as the “ Com bustion” com ponent; (gasoline + vapor + combustion)/(total measured NMHC) where gasoline is either Vangas or Whagas and vapor is either Vanvap or Whavap.c_{Presence or absence of sim ilarity/uncertainty clusters in m odel fit.} d_{Com bustion}

35 fittin g sp ecies (F3 in Table 2). Th e CMB resu lts are sh own in Tab le 4 for th e 16 tu n n el ru n s an d th e ligh t-d u ty veh icle em ission factor d istrib u tion . In ad d ition , a veh icle-weigh ted average of 13 valid tu n n el ru n s (AVG) h as b een calcu lated . Th is lin e (AVG), like th e LDV resu lts, is ch aracteristic of th e overall stu d y. Th ere is reason ab le agreem en t b etween th e resu lts listed for AVG, ob tain ed from averagin g th e CMB resu lts from each tu n n el ru n , an d LDV, ob tain ed from th e CMB resu lts for an average d istrib u tion of all tu n n el ru n s. Th e agreem en t b etween th ese two sim u lation s gives fu rth er con fiden ce th at th e CMB m od elin g for th e in d ivid u al ru n s is n ot affected in a sign ifican t way b y d iesel em ission s, sin ce th e LDVem ission factor p rofile, th rou gh correlation , is d om in ated b y gasolin e veh icles.

In gen eral, th e p erform an ce of th e m od el is very good for m ost of th e m od el sim u lation s in Tab le 4. Th ree sim u lation s sh ow p oor p erform an ce (1, 7, an d 10) with low valu es of R2_{, h igh valu es of χ}2_{, an d d ep ressed valu es of}

m ass p ercen t. Not coin cid en tally, th ese ru n s h ave th e lowest veh icle cou n ts (∼100/ h ) an d h igh est d iesel cou n ts resu ltin g in less statistical con fid en ce. It is also n oted th at ru n 1 h ad su sp ected sam p lin g p rob lem s. Th e th ree ru n s were con sid ered to b e in valid for fu rth er an alysis an d were n ot in clu d ed in th e average resu lts listed as AVG. Mass recovery valu es are som ewh at h igh er th an 100%. Th e veh icle-weigh ted average for all th e ru n s is 108.3% wh ile th at for th e LDV p rofile is 107.9%. Th is sligh tly h igh m ass recovery stem s from th e fact th at th e gasolin e p rofiles were d erived from 319 calib rated an d u n calib rated NMHC GC p eaks wh ile th e em ission m easu rem en ts were d erived from on ly 102 calib rated NMHCs. Th e 102 calib rated NMHCs are exp ected to rep resen t a sign ifican t p ortion of th e C2

-C12m ass wh ereas th e 319 p eaks are likely to rep resen t closer

to 100% of th e m ass. It was fou n d th at a su b set of th e total 319 GC p eaks, n am ely, th e 102 h yd rocarb on s m easu red in th e tu n n el, accou n t for 81% of th e gasolin e m ass. Th u s th e total m ass allocated to th e LFVGAS sou rce b y th e CMB

FIGURE 1. Comparison of measured and CM B-calculated emission factors for the average light-duty emission distribution. The CM B fit w as performed w ith tw o profiles; the local gasoline composition and it’s calculated vapor composition. See LDV in Table 3 for further details.

TA BLE 4

Emission Rates Apportioned by CMB Model for 16 Tunnel Runs and Averages

run R2 _{χ M ASS (%)}a _{LFVGAS (g/mi) LFVVAP (g/mi) combustion}b_{(g/mi) vapor}c_{(%) gasoline in exhaust}d_(%)

1 0.59 5.60 73.2 ( 8.5 0.846 ( 0.054 -0.073 ( 0.019 0.561 ( 0.052 -5.4 ( -1.5 60.1 ( 5.0 2 0.95 0.62 106.1 ( 12.0 0.413 ( 0.025 0.046 ( 0.016 0.280 ( 0.025 6.2 ( 2.1 59.6 ( 4.7 3 0.94 0.73 110.2 ( 12.6 0.297 ( 0.019 0.135 ( 0.023 0.211 ( 0.020 21.0 ( 3.8 59.6 ( 4.7 4 0.96 0.47 110.3 ( 12.4 0.277 ( 0.017 0.039 ( 0.012 0.176 ( 0.015 8.0 ( 2.4 61.2 ( 4.8 5 0.96 0.52 107.8 ( 12.2 0.218 ( 0.013 0.021 ( 0.008 0.149 ( 0.013 5.4 ( 2.0 59.5 ( 4.8 6 0.95 0.54 110.2 ( 12.4 0.244 ( 0.015 0.020 ( 0.008 0.147 ( 0.013 4.9 ( 2.0 62.5 ( 4.9 7 0.91 1.11 99.0 ( 11.4 0.157 ( 0.011 0.062 ( 0.011 0.268 ( 0.023 12.7 ( 2.4 36.9 ( 3.3 8 0.94 0.77 105.5 ( 11.9 0.338 ( 0.021 0.048 ( 0.014 0.260 ( 0.023 7.4 ( 2.2 56.5 ( 4.6 9 0.95 0.65 107.9 ( 12.1 0.530 ( 0.032 0.059 ( 0.019 0.329 ( 0.029 6.4 ( 2.1 61.7 ( 4.8 10 0.78 3.02 94.5 ( 10.8 0.185 ( 0.014 0.077 ( 0.016 0.252 ( 0.019 15.0 ( 3.2 42.4 ( 3.9 11 0.95 0.58 110.7 ( 12.6 0.457 ( 0.029 0.135 ( 0.027 0.252 ( 0.023 16.0 ( 3.3 64.4 ( 5.2 12 0.95 0.59 109.5 ( 12.4 0.468 ( 0.029 0.080 ( 0.021 0.292 ( 0.026 9.5 ( 2.6 61.6 ( 4.9 13 0.96 0.49 109.0 ( 12.3 0.271 ( 0.017 0.055 ( 0.014 0.196 ( 0.017 10.5 ( 2.7 58.0 ( 4.7 14 0.96 0.48 108.7 ( 12.3 0.214 ( 0.014 0.046 ( 0.011 0.160 ( 0.014 10.9 ( 2.7 57.2 ( 4.7 15 0.97 0.37 107.8 ( 12.3 0.177 ( 0.012 0.038 ( 0.009 0.140 ( 0.012 10.7 ( 2.7 55.9 ( 4.7 16 0.97 0.40 108.3 ( 12.3 0.445 ( 0.028 0.117 ( 0.026 0.337 ( 0.029 13.0 ( 2.9 56.9 ( 4.6 AVGe _{0.953 0.581 108.3 ( 3.5 0.344 ( 0.006 0.068 ( 0.005 0.235 ( 0.006 10.3 ( 0.8 59.1 ( 1.4} LDVf _{0.97 0.24 107.9 ( 13.5 0.318 ( 0.026 0.066 ( 0.025 0.238 ( 0.029 10.5 ( 4.1 57.2 ( 6.2}

a_{M ass recovery in this application is defined as the percentage ratio of (LFVGAS + LFVVAP + combustion)/total measured NMHC.}b_{Com bustion}

represents the sum of (m easured - calculated) emission rates for all combustion species not used as fitting species.c_{Vapor percentage is the}

percentage ratio of the CM B predicted evaporative em ission rate to the total em ission rate, LFVGAS + LFVVAP + combustion.d_{Percentage gasoline}

in exhaust is given by the percentage ratio of LFVGAS/(LFVGAS + combustion).e_{AVG is the vehicle-w eighted average values for all 16 tunnel runs.} f_{LDV is the light-duty vehicle NM HC distribution obtained by vehicle-w eighted regression of 16 runs.}

m od el reflects 23% m ore m ass th an was m easu red . Ou r th eoretical m ass % sh ou ld th erefore b e ab ou t 112% for an average tu n n el ru n wh ere 57% of th e m ass is allocated to gasolin e. Th is is close to th e average valu e for m ass recovery of 108% in Tab le 4.

Contribution of Gasoline to Exhaust Emissions

Total exh au st em ission s rates are given b y th e su m of th e gasolin e com p on en t an d th e com b u stion com p on en t. Th e con trib u tion of gasolin e to th e total exh au st is d efin ed as th e p ercen t ratio: LFVGAS/ (LFVGAS + COMBUSTION) an d is in d icated in Tab le 4 for all ru n s. For th e 13 valid ru n s, th is valu e ran ges from 56 to 64% with a veh icle-weigh ted average of 59.1 ( 1.4% an d a ligh t-du ty veh icle valu e of 57.2 ( 6.2%. Th ese valu es assu m e th at all com bu stion sp ecies evolve from gasolin e com b u stion . It is kn own th at th is valu e is im p acted b y th e p resen ce of p rop an e veh icles op eratin g in th e tu n n el sin ce p rop an e is on e of th e com b u stion sp ecies. NMHC em ission s from p ro-p an e veh icles are com ro-p osed of ab ou t 60% ro-p roro-p an e (28). A fu ll an alysis of th e effect of p rop an e veh icles in th e tu n n el stu d y is cu rren tly b ein g com p leted in ord er to q u an tify p rop an e em ission s from veh icles in th e LFV. For th e p u rp oses h ere, we h ave d on e a sen sitivity test u sin g th e LDV em ission factor d istrib u tion an d in clu d ed an extra sou rce p rofile d erived from recen t m easu rem en ts (28) to rep resen t p rop an e veh icle em ission s. Th is sen sitivity test in d icated th at th e in clu sion of a p rop an e p rofile h ad little im p act on th e LFVGAS an d LFVVAP sou rce con -trib u tion s b u t red u ced th e COMBUSTION con -trib u tion sligh tly b ecau se of th e allocation of sign ifican t p rop an e an d som e ligh t olefin s to th e p rop an e veh icles sou rce. Th e test also in d icated th at th e p ercen tage con trib u tion of gasolin e to ligh t-d u ty gasolin e veh icle exh au st NMHC was in creased to 63.4 ( 7.0% as com p ared to th e origin al estim ate of 57.2 ( 6.2%. Th is was largely th e resu lt of a large fraction of th e p rop an e em ission s b ein g allocated to p rop an e veh icles.

Th e average level of u n b u rn ed gasolin e in exh au st d eterm in ed in th is stu d y, 63.4%, is in gen eral agreem en t with th at d eterm in ed in d yn am om eter-b ased m easu re-m en ts (18). An oth er stu d y of th ree n ew veh icles fou n d th e ran ge of u n b u rn ed gasolin e in com p osite en gin e-ou t an d com p osite tailp ip e em ission s to b e 50-55% an d 35-55%, resp ectively, th e d ifferen ce b ein g attrib u tab le to th e effect of th e catalyst (17). In th at stu d y, th e level of gasolin e was determ in ed for each bag of th e Federal Test Procedu re (FTP) Urb an Dyn am om eter Drivin g Sch ed u le (UDDS), an d it was fou n d th at th e level of gasolin e in en gin e-ou t em ission s was m u ch less sen sitive to th e em ission s m od e (b ag 1, b ag 2, or b ag 3) com p ared to th e tailp ip e em ission s. Th is d ifferen ce was also attrib u tab le to th e effect of a fu lly fu n ction in g catalyst. For tailp ip e em ission s, th e level of u n b u rn ed gasolin e was lowest for th e h ot stab ilized p ortion (bag 2) of th e UDDS, 5-25%, in these relatively n ew vehicles. It is exp ected th at th e typ e of d rivin g in th is p ortion of th e UDDS wou ld b e m ost sim ilar to th e d rivin g en cou n tered in th e Cassiar Tu n n el, an d yet, we h ave ob served m u ch h igh er levels of u n b u rn ed gasolin e. Th e m ore recen t p u b lication (18) also ob served su stain ed levels of u n b u rn ed gasolin e in th e b ag 2 p ortion of th e UDDS (∼60%) in agreem en t with ou r m easu rem en ts h ere. Th e d ifferen ces ob served with th e stu d y th at sh owed low levels of gasolin e in b ag 2 an d th is stu d y m ay b e attrib u tab le to d ifferen ces in catalyst typ e, en gin e rep air, or em ission s con trol system .

It also m ay in d icate th at catalysts are n ot fu n ction in g well in real-world veh icles.

Of gen eral in terest in CMB m od elin g is th e ab ility to sp ecify th e con trib u tion of each sou rce to ob served levels of in d ivid u al h yd rocarb on s. A com p lete an alysis is n ot p resen ted h ere, alth ou gh it is u sefu l to exam in e th e resu lts for b en zen e, a m ajor com p on en t of gasolin e, an d a kn own com b u stion sp ecies. Th e p resen ce of b en zen e in exh au st is largely attrib u ted to th e d ealkylation of oth er su b stitu ted arom atics d u rin g th e com b u stion p rocess (16). Th e sep aration of exh au st in to com b u stion an d gasolin e com p on en ts allows u s to determ in e th ese two con tribu tion s sep erately. For th e ligh t-d u ty veh icle p rofile CMB sim u la-tion (LDV in Tab le 4), it was fou n d th at th e total on -road ben zen e em ission factor cou ld be ap p ortion ed to com bu s-tion (exh au st), gasolin e (exh au st), an d evap orative em is-sion s to th e levels of 70.8 ( 38.3%, 26.7 ( 9.9%, an d 2.4 ( 0.9%, resp ectively. Th e fact th at ∼71% of on -road b en zen e com es from th e com b u stion p rocess is in terestin g sin ce it in d icates th at efforts to lower h u m an exp osu re to am b ien t levels of b en zen e can n ot b e con fin ed to th e elim in ation of b en zen e in gasolin e alon e. Th e effort h as to b e com b in ed with a lowerin g of arom atics in th e fu el at th e sam e tim e.

Evaluation of Exhaust and Evaporative Emission

Factors

On e of th e ob jectives of th e Cassiar Tu n n el stu d y was to com p are exp erim en tal em ission factors with th ose calcu -lated b y cu rren t on -road em ission factor m od els. In Can ad a, th e m ost cu rren t m od el u sed in th e calcu lation of em ission factors for in ven tory d evelop m en t is MOBILE5C (30), an ad ap tation of th e U.S. EPAMod el MOBILE5a, wh ile th e p reviou s version u sed was MOBILE4.1C (31). Th e Can ad ian version s of th e m od el h ave b een m od ified to take in to accou n t differen ces in Can adian an d U.S. regu lated em ission con trol tech n ology th at existed for veh icles of m od el year 1981-1987. Becau se of less strin gen t em ission stan d ard s in Can ad a d u rin g th at p eriod , fleets of veh icles from th ose m od el years h ave h igh er em ission factors com p ared to th ose in th e Un ited States.

In a p reviou s p u b lication , a com p arison was m ad e b etween em ission factors of NOx, total NMHC, an d CO m easu red in th e Cassiar Tu n n el an d th ose calcu lated with th e two Can ad ian MOBILE m od els (6). In th is work, ap p ortion m en t of total NMHCs to exh au st an d evap orative em ission s allows a m ore d etailed com p arison of NMHC em ission factors. Th e m eth od ology u sed for calcu latin g em ission factors in th is stu dy u sin g th e two MOBILE m odels h as b een ou tlin ed p reviou sly (6, 7). Briefly, th e ap p roach u sed was to collect su fficien t d ata so th at each tu n n el ru n cou ld be m odeled sep arately with h ou rly sp ecific in form a-tion . Th e m od el in p u t for each ru n in clu d ed th e local gasolin e RVP, local b y m od el year d iesel fraction s, an d h ou rly average sp eed s, tem p eratu res, an d b y m od el year veh icle d istrib u tion s. Th e MOBILE m od el calcu lates av-erage or m od el year-sp ecific em ission s factors for eigh t veh icle classes. Th e vid eo tap es for each ru n were u sed to sep arate th e ob served veh icles in to th e eigh t veh icle classification s. Th ese cou n ts alon g with th e em ission factors for each veh icle typ e an d m od el year were th en u sed to calcu late a veh icle-weigh ted average em ission factor for each tu n n el ru n .

Th e MOBILE m od els give a b reakd own of NMHC em ission factors b y m od e in clu d in g exh au st, ru n n in g loss,

restin g loss, d iu rn al, an d h ot soak em ission s. By d efin ition , d iu rn al an d h ot soak em ission s are n ot on -road em ission s an d were n ot evalu ated in th is stu d y. Ru n n in g loss em ission s are d efin ed as evap orative em ission s occu rrin g wh ile th e veh icle is b ein g d riven , an d th ey arise from em ission s from variou s en gin e com p on en ts, th e veh icle fu el tan k, an d in adequ ate p u rgin g of th e evap orative con trol can ister. Restin g loss em ission s are an oth er form of evap orative em ission , d efin ed as vap ors p erm eatin g p arts of th e em ission con trol system , m igratin g ou t of th e carb on can ister, an d evap oratin g liq u id fu el leaks. Con trary to ru n n in g loss em ission s, restin g loss em ission s m ay occu r 24 h a d ay. From th is d efin ition , it is clear th at restin g loss em ission s are n ot n ecessarily d istin ct from oth er typ es of evap orative em ission s eith er in d efin ition or in m easu re-m en t. Wh ile th e n are-m e re-m ay ire-m p ly th at restin g loss ere-m ission s occu r off-road wh en th e veh icle is at rest, recen t gu id an ce h as su ggested th at th e in ten tion in th e MOBILE m od els is th at th ese em ission s occu r on -road as well as off-road (32). From th e ab ove d iscu ssion , it is clear th at th ere m ay b e som e am b igu ity over th e ch em ical com p osition of th e on -road evap orative em ission s. It is exp ected th at em ission s from th e fu el tan k an d evap orative can ister will b e sim ilar in ch em ical com p osition to gasolin e vap or as wou ld em ission s from th e escap e or p erm eation of gasolin e vap ors from p arts of th e en gin e. Som e “evap orative” em ission s m ay b e sim ilar in com p osition to u n b u rn ed gasolin e, n am ely, an y fu el leakage or cran kcase em ission s. On e m igh t exp ect th at on -road fu el leakage is sm aller th an off-road leakage sim p ly d u e to tim e of con tact con sid eration s. It is also n oted th at cu rren t estim ates of cran kcase em ission s from ligh t-d u ty veh icles in th e MOBILE m od els are also very sm all. With th ese con sid eration s, we h ave taken th e ap p roach in th e com p arison to b e p resen ted h ere, th at on road evap orative em ission s are m ost sim ilar in com -p osition to gasolin e va-p or. For com -p arison -p u r-p oses, th e MOBILE m odel p redicted equ ivalen t of on -road evap orative em ission s h as b een taken to b e th e su m of ru n n in g loss an d restin g loss em ission s. We ackn owled ge th e p ossib ility for m isid en tification of som e sm all com p on en t of gasolin e exh au st to b e th ose on -road evap orative em ission s th at are sim ilar in com p osition to u n b u rn ed gasolin e.

Th irteen ru n s were in clu d ed for com p arison of em ission factors, n am ely, th ose th at were con sid ered valid th rou gh good p erform an ce in d icators with th e CMB m od el (all ru n s exclu d in g 1, 7, an d 10). Veh icle-weigh ted em ission factors (13 ru n average) were calcu lated for each of th e two MOBILE m od els an d th e exp erim en tal/ CMB com b in ation (Tab le 5). Tab le 5 sh ows b oth ab solu te em ission factors an d th e relative p rop ortion of evap orative em ission s. Also sh own are calcu lated ratios of th e m agn itu de of th e MOBILE m odel over/ u n d erp red iction b ased u p on th e m easu rem en ts in th is stu d y. Differen ces b etween th e exp erim en tal an d

MOBILE valu es are in d icated b y ratios d ifferen t from 1.0. Un certain ties are n ot in clu d ed in th e MOBILE m od els. Th e u n certain ties in th e exp erim en tal/ CMB com b in ation are th ose calcu lated by th e CMB m odel an d p rop agated th rou gh th e calcu lation of a veh icle-weigh ted average resu lt. For ab solu te em ission factors, th e u n certain ties in clu d e th e p rop agation of th e 50% u n certain ty in em ission factors for each tu n n el ru n d u e to exp erim en tal u n certain ties m en -tion ed p reviou sly. Th ose valu es th at are statistically d ifferen t from 1.0 at th e 95% con fid en ce level are italicized in th e tab le.

Th e tab le in d icates th at MOBILE5C overestim ates exh au st, on -road evap orative, an d total NMHC b y +24, +10, an d +22%, resp ectively, with th e associated u n cer-tain ties wh ile th e MOBILE4.1C p ercen tages are -29, +25, an d -24%, resp ectively. Th e u n derestim ation of exh au st an d total NMHCs by th e MOBILE4.1C m odel are sign ifican t. Th e exh au st/ evap orative p ercen tages p redicted by th e CMB m od elin g are less affected b y exp erim en tal u n certain ty. Th ese in d icate th at th e MOBILE5C m od el u n d erp red icts th e evap orative p ercen tage b y 2% wh ile th e MOBILE4.1C m od el overp red icts th is ratio b y 74% in a statistically sign ifican t m an n er. It is clear th at th is overp red iction of th e evap orative ratio b y MOBILE4.1C is largely d u e to an u n d erestim ation of exh au st em ission s.

Acknow ledgments

Th e au th ors wou ld like to th an k F. Prystarz (GVRD) an d D. Wan g (En viron m en t Can ad a) for m easu rem en t exp ertise; B. Th om son (PAES) for sam p lin g exp ertise; G. Ertel (Sh ell), G. Felsky (Esso), an d CPPI for coop eration in an alysis of gasolin e sam p les; an d B. C. Min istry of Tran sp ortation an d High ways an d th e Cap ilan o High ways Services for d ata su p p ly.

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Received for review Jan u ary 17, 1996. Revised m an u scrip t received May 3, 1996. Accepted May 3, 1996.X

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