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Impact of the dropping activity with vehicle age on air pollutant emissions

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Road transport is a major source of air pollution especially in cities. Detailed calculations are needed to support road transport emission inventories due to the variance of technologies and operating conditions encountered on the roads. The annual distance driven by cars in relation to their characteristics is an important variable in such calculations. In this work, a large amount of mileage data were collected from second-hand car sellers in Italy and were then analyzed in order to understand the influence of vehicle age on annual mileage driven. The available data enabled the development of dropping functions of annual mileage with vehicle age. It was found that the average mileage of 10 year old cars is only approximately 40% of the mileage driven on year one. This drops to approximately only 10% for 20-year old cars. The findings are of paramount importance in environmental calculations as road transport NOx and PM emissions drop by more than 20% when the corrected functions are used compared to using a constant mileage. Not introducing such a correction may result to an approximately 8% higher nation-wide NOx emissions with negative implications towards meeting the national emission ceilings. In terms of policy implications, the dropping activity with age results to a decrease in the importance of accelerated scrappage schemes and of environmental zones in air quality.
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Impact of the dropping activity with vehicle age on air pollutant
emissions
StefanoCaserini1,CinziaPastorello1,2,PietroGaifami1,3,LeonidasNtziachristos4
1DIIAR
SezioneAmbientale,PolitecnicodiMilano
PiazzaL.daVinci,32
20133Milano,Ital
y
2Presentaddress:EEAEuropeanEnvironmentAgency,KongensNytorv6,1050Copenhagen,Denmark
3Presentaddress:RedecamGroup,PiazzaMontanelli,2020099SestoS.Giovanni,Italy
4LaboratoryofAppliedThermodynamics,AristotleUniversity,POBOX458,GR54124Thessaloniki,Greece
ABSTRACT
Roadtransportisamajorsourceofairpollutionespeciallyincities.Detailedcalculationsareneededtosupport
roadtransportemissioninventoriesduetothevarianceoftechnologiesandoperatingconditionsencounteredon
theroads.Theannualdistancedrivenbycarsinrelationtotheircharacteristicsisanimportantvariableinsuch
calculations.Inthiswork,alargeamountofmileagedatawerecollectedfromsecond–handcarsellersinItalyand
werethenanalyzedinordertounderstandtheinfluenceofvehicleageonannualmileagedriven.Theavailable
dataenabledthedevelopmentofdroppingfunctionsofannualmileagewithvehicleage.Itwasfoundthatthe
averagemileageof10yearoldcarsisonlyapproximately40%ofthemileagedrivenonyearone.Thisdropsto
approximatelyonly10%for20–yearoldcars.Thefindingsareofparamountimportanceinenvironmental
calculationsasroadtransportNOXandPMemissionsdropbymorethan20%whenthecorrectedfunctionsare
usedcomparedtousingaconstantmileage.Notintroducingsuchacorrectionmayresulttoanapproximately8%
highernation–wideNOXemissionswithnegativeimplicationstowardsmeetingthenationalemissionceilings.In
termsofpolicyimplications,thedroppingactivitywithageresultstoadecreaseintheimportanceofaccelerated
scrappageschemesandofenvironmentalzonesinairquality.
Keywords:Airquality,emissions,NOX,roadtransport,vehicleactivity
CorrespondingAuthor:
Cinzia Pastorello
:+4533367298
:+4533367128
:cinzia.pastorello@eea.europa.eu
ArticleHistory:
Received:07February2013
Revised:24April2013
Accepted:07May2013
doi:10.5094/APR.2013.031
1.Introduction
Roadtransportisoneofthemainsourcesofpollutionin
urbanareasinEurope,accountingfor39%,and15%oftotalNOX
andPM2.5emissions,respectively(EEA,2010).Duringthelast
years,benefitsfromtheprogressivereplacementofuncontrolled
gasolinecarswithnewonesequippedwiththreewaycatalystsare
counterbalancedbytheincreasingpenetrationofdieselcarsand
theirhigheremissionlevels,inparticularNOX,comparedtotheir
gasolinecounterparts.Urbansprawlingandthegeneralassociation
ofpersonalmobilitywithqualityoflifeandeconomicdevelopment
(Uhereketal.,2010)haveincreasedthemeanannualdistance
travelledbycars(EC,2012)until2008;thenaslightdecreasehas
beenvisibleduetotheeconomiccrisis.
Emissionmodelsareusedtocalculateemissionsfromroad
transport.ExamplesofsuchmodelsinEuropeincludeHBEFAused
inGerman–speakingcountries(Hausbergeretal.,2009),VERSIT+
usedintheNetherlands(Smitetal.,2007),LIPASTOinFinland
(lipasto.vtt.fi),andCOPERT4whichisusedin22outofthe27
EuropeanUnionmemberstates(Ntziachristosetal.,2009).Insuch
modelsvehicleactivityismultipliedwithemissionfactorsto
calculatetotalemissions.Vehicleactivityisestimatedasthe
numberoffleetvehiclespertypedistinguishedineachmodel
timestheannualmileagedrivenbyeachvehicletype.Vehiclefleet
dataarereadilyavailableinseveralcountries.Forexample,official
statisticsinItaly(ACI,2011)providethenumberofvehicles
distinguishedperfueltypeused,enginecapacity,andemission
controlregulation(EUROcategories).
Withregardtomileageestimates,Andreetal.(1999)pointed
outtheimportanceandthedifficultiesofestimatingannual
mileageanditstrendasafunctionofvariousfactors(suchas
vehicleage)fordifferentvehiclecategories.Onthebasisof
inspectionandmaintenancemonitoringprograms,Beydounand
Guldmann(2006),Washburnetal.(2001),andBin(2003)demon
stratedthatthetotalmileageofavehicleisstronglyassociatedto
emissionandtestfailurerates.Finally,Sawyeretal.(2000)
underlinedtheimportanceofhavingaccurateactivitydatafor
obtaininganimprovedtransportemissioninventory.
Despiteitssignificancethough,annualmileageisusually
availableasanaveragevaluefortheentiregasolineordieselcar
fleet,withoutdistinguishingintomoreemission–relevantcriteria,
suchastypeofroadorlengthofservicethataffecttheemission
assessment(Ntziachristosetal.,2008).Often,mileagevaluesused
innationalemissioninventoriesarenotbasedonmeasureddata
butarejustcalibratedvalues,estimatedsoastoachieveabalance
betweenthefuelconsumptioncalculatedbythemodelandofficial
statisticsonfuelsold.Althoughthisproducesaninventorywhichis
consistentwithtotalenergystatistics,itdoesnotguaranteea
correctallocationofconsumptiontothedifferentvehicletypes,
neitherthattotalactivityiscorrectlydisseminatedtothevarious
vehicletypesandages.
Caserini et al. – Atmospheric Pollution Research (APR) 283
Anestimateofthemileageofdifferentvehicletypesasa
functionofageisthereforeanimportantinputtoroadtransport
environmentalmodels,inordertoaccuratelyestimatethetotal
emissionsproduced.Itisalsonecessaryinordertoaccurately
predictthereal–worldimpactofpolicymeasurestargetingspecific
vehicletechnologies.Forexample,implementingpoliciestargeting
theremoval(scrappage)orbanoftravelling(environmentalzones)
ofoldvehicletypeswillbelesseffectivethanplannedincasethat
adecreasingfunctionofmileagewithvehicleageisestablished.
Inthisstudyweareproposingamethodologyforprecise
mileageestimationasafunctionofvehicleagethatcanbe
introducedtoemissionmodelsforimprovingthequalityofthe
calculations.Asanexamplecase,weapplythemethodinthecase
ofItalytodemonstratetheextentbywhichmileagemisallocation
betweendifferentvehicleclassesmayaffectemissionsestimates.
Thiscanalsoserveasameasureoftheuncertaintyofaninventory,
whenmileagevaluesarenotbasedonmeasureddata.
2.Material
Real–worlddataonpassengercarmileagewascollectedby
visiting32950individualsecond–handpassengercarsalesonthe
internet(seetheSupportingMaterial,SM,TableS1).Thecollection
andanalysisofthedatatookplaceintheperiodbetweenJuneand
September2010.Thefinaldatasetconsistedof18652gasoline
carsregisteredintheperiodfrom1994to2010and14298diesel
cars,registeredbetween1996and2010.Oldervehicleswere
practicallynotforsaleastheyarenotallowedtocirculatein
severalItalianregionsduringthewinter,duetoair–quality
limitations.Thedatasetconstructedincludedfuelused,yearof
firstregistrationandodometerreading.TableS2(seetheSM)
showssomekeystatisticsofthedataset.
Datacollectedrepresentabout1.1%ofthe2.8–3.0millionof
usedcarssoldinItalyeveryyear.Thiswasasmuchascouldbe
collectedfromthesecondhandmarketwithcompleteinformation
onageandmileage.Noparticularcriteriaweresettoselectthe
vehiclesinthesample.Hence,weexpectthemtomakea
representativesampleoftheactualvehiclestock.Thisisalso
verifiedbythefactthatthedistributionofvehiclesinthedifferent
ageclassesisquitesimilartothedistributionofthecompletestock
ofvehiclesregisteredinItaly.ThisisdemonstratedinTable1
whichcomparestheagedistributionofthesampleusedinthe
currentstudywiththeItalianstockdistribution,basedondata
takenfromtheItaliancarassociationdatabase(ACI,2011).The
frequencydistributionsarequitesimilarbetweenthetwodatasets.
Onemightexpectthattheratioofcarsbeingsoldvs.carsin
operationwouldincreaseastheageincreases.Thiswaspartly
visibleinthecaseofdieselbutnotforgasolinecars.Several
reasonscouldcontributetothesedifferenttrends,suchas
generallylongerlifetimefordieselcars,scrappageandexportingof
vehiclesvs.secondhandsalewithinthecountry,etc.Anyway,the
differencesintheratiobetweensoldandregisteredcarinevery
yeararelow(from0.08%to0.2%fordieselandfrom0.09%to
0.13%forgasoline)andthesamplesizeperageclassis
satisfactory,withtheminimumnumberperclassstillabove400
vehicles(diesel,14yearsold).
Questionsmayariseonthereliabilityoftheodometerreading
asamethodtoinferthemileagedrivenbyeachvehicle.Thisis
onlyreportedbytheownerandsuspicionsmayariseregardingthe
effectofclockingofthecars,i.e.thefraudulentwindingbackof
theodometerreadingtomakethecarappearyoungerthanit
reallyisandnegotiateabetterpricewiththepotentialbuyer.A
recentstudyontheimpactofmileagefraudwithusedcars(EREG,
2010)identifiedmileagefraudasaseriousproblem,butnodata
weremadeavailabletodefinehowmuchthisissuecouldaffectthe
variationofaveragemileagewithtime.Inthiswork,wehave
assumedthatthedistortioneffectisproportionaltothelengthof
service.Thatis,wehaveassumedthatcarclockingtakesplaceat
thesamefrequencyfornewandoldcarsandthatthemileage
correctionisproportionaltotheodometerreading.Inotherwords,
theassumptionisthattheintentiontoimprovethesellingpriceof
thevehicleisthesame,regardlessoftheactualageofthecar,
whichtendstobeareasonableapproach.Hence,thiswillhavean
impactonthetotalmileagereportedbutwillnotaffecttherelative
relationbetweenmileageandage,whichisofimportancetothis
study.Alldatacollectedwerepooledtogetherandastatistical
analysiswasconducted.Theresultsofthisstatisticalanalysisand
themethodsusedarepresentedinthefollowingsection.
Table1.FrequencydistributionofvehiclesintheavailabledatasetandofvehiclesregisteredinItaly,accordingtotheirage
GasolineDiesel
VehicleageDataavailableVehiclesregisteredinItaly
VehicleageDataavailableVehiclesregisteredinItaly
Number(%)Number(%)Number(%)Number(%)
111426.1%10692126.1%111708.2%9378197.3%
212606.8%12681727.2%211848.3%9526487.4%
311236.0%10803426.2%311438.0%11182398.7%
411486.2%11158226.4%411848.3%142517611.1%
511836.3%9825595.6%511037.7%137571910.7%
611035.9%9280705.3%611077.7%130943310.2%
711756.3%9358305.3%710897.6%130626010.1%
811886.4%11284976.4%810697.5%10584848.2%
911706.3%12469257.1%911077.7%8996567.0%
1011916.4%14038958.0%1010847.6%7566015.9%
1111776.3%13799627.9%1110627.4%6606565.1%
1211836.3%12111306.9%129916.9%4943713.8%
1313077.0%12220497.0%135954.2%3461322.7%
1412916.9%11988616.8%144122.9%2428331.9%
1511506.2%7059164.0%
168614.6%6662463.8%
Total18652100%17543488100%Total14300100%12884027100%
16<age<305298935 14<age<301016079

Caserini et al. – Atmospheric Pollution Research (APR) 284
3.CalculationandResults
3.1.Cumulativeandannualmileages
Theaveragecumulativemileage(ACMk)referstothetotal
distancecoveredonaveragebyeachcarofthesameage,orina
moretechnicalway,withthesamelengthofservice(k–inyears).
TherelationshipbetweenACMkandvehicleageisshownin
Figure1asanaverageforallgasolineanddieselpassengercarsin
oursample.ThefigureshowsthatACMincreaseswithvehicleage
almostinalinearfashionupto4–5yearsofage.Beyondthispoint,
therateofincreaseinACMdrops,denotingadecreaseofthe
annualmileageconducted.After14yearsandapproximately
126thousandkilometersforgasolinevehiclesand13yearsand
approximately180thousandkilometersfordieselcars,thereis
onlylimited(ifany)increaseinthecumulativemileage.
Whenlookingatsinglevehiclesonly,totalmileagecanonly
monotonicallyincreasewithvehicleage.Hence,thestabilizationof
mileageafteracertainagecannotbeexplainedonasinglevehicle
basis.ThereasonformileagestabilizationinFigure1isthat
vehicleswithexcessivemileageareremovedearlierfromthe
stock,eveniftheiragemeasuredinyearsdoesnotjustifythis.
Hence,asthefrequencyofvehiclesbeingscrappedincreaseswith
age,therateofincreaseofmileagewithagegraduallydropsata
fleetwidelevelandasaturationpointisreached.Afterthispoint,
increasingthemeanvehicleagedoesnotcauseanysignificant
increaseinthemileage.Actually,adropinthemileagewouldalso
betheoreticallypossible.
Themileagestabilizationisofimportancetoroadtransport
emissionmodels.Forexample,asaninputinfunctionswhich
correctemissionfactorsaccordingtothemileagecovered.Figure1
showsthat,despitetogeneralbelief,theactualaveragemileageof
afleetofpassengercarsdoesnotincreasebeyondacertainpoint
astheygrowolder,henceemissionfactorsshouldnotdegrade
aboveacertainlevel.
BydividingtheACMvaluewiththelengthofservice,one
obtainstheaverageannualmileage(AAMk)whichistheaverage
annualdistancecoveredbyeachcarofthesamelengthofservice.
Thatis,

(1)
AAMonlydependsonvehicleageanditisthesameforeach
yearduringthelifetimeofthevehicle.AAMkvaluesasafunctionof
vehicleageareshowninFigure2.Thisshowsthattheaverage
mileagedrivenperyeargenerallydropswhilethevehiclesbecome
older.Thefunctiondoesnotdropmonotonicallyfordieselcarsbut
carsofoneyearofageappeartobedrivenlessthancarsoftwo
andthreeyearsofage.Wehavenoevidencethatthisisasample
artifact,howeverasatisfactorynumberofmorethan1000diesel
vehicleswasavailableforeachageclassinourdataset.
Ifvehicleageisnottakenintoaccount,thentheaverage
mileageofourgasolinecarsampleis10636kmand18685kmfor
dieselcars.However,asshowninFigure2,thetrueaveragewill
dependontheaverageageofthevehiclesconsidered.Thisisnot
alwaystakenintoaccountinrelevantstudies.
3.2.Mileagetobeusedforemissionmodeling
Justbecausecarsareuseddifferentlythroughouttheir
lifetime,themaximumlengthofservice,j,i.e.theageinyearsat
whichthevehicleisremovedfromthestock,mayvaryfrom
vehicletovehicle.Themaximumlengthofservicewillhavean
impactontheincreaserateofACMwithage.Thatis,vehicles
whicharescrappedfromthestockearly(shortlengthofservice)
shouldbeexpectedtoaccumulatemileagefasterthanvehicles
withalongermaximumlengthofservice.Therefore,themaximum
lengthofserviceisaparameterthathastobetakenintoaccount
whenexpressingthefunctionofmileagewithvehicleage.
Figure1.
A
veragecumulativemileage(ACM)fordieselandgasolinepassengercars,asafunctionoftheirage.Fittedparabolas
passingthoughtheaxesoriginhavebeendrawnforbothdatasets.
Caserini et al. – Atmospheric Pollution Research (APR) 285
Figure2.
A
verageannualmileage(AAM)drivenbyvehiclesofdifferentage.
Inordertointroducethemaximumlengthofservice(j)inthe
calculation,onemaystartbyobservingthatthemileagevaluesin
Figure1seemtofollowaparabolicfunctionwithage.Hence,ACM
canbeapproachedbyabinomialfunctionpassingthroughtheaxes
origin,i.e.afunctionofthetypeshowninEquation(2):

(2)
Thebinomialcurvesthatbestfitthedataresulttotheaandb
parametersareshowninFigure1forbothdieselandgasoline
passengercars.Thisequationcanbeconsideredrepresentativefor
astockofvehicleswithanaverageend–of–lifeageatthepoint
wherethebinomialequationbecomeslevel.Thisismathematically
expressedbythefunction:
0
kj
dACM
dk
(3)
Forsuchacurve,parametersaandbcanbeeasilycalculated
byEquations(2)and(3)as:

(4)
  2(5)
UsingEquations(4)and(5)andthebestfitparametersfor
dieselandgasolinecars,thismethodresultstojvaluesof16(years
ofservice)forgasolinecarsand14yearsofservicefordieselcars,
withcorrespondingACMvaluesof126300kmand172600km,
respectively.Thesevaluesareveryclosetheonesestimatedinthe
previoussectionwhichmeansthatthebinomialfunctionverywell
describestheevolutionoftheACMasafunctionofvehicleage.
Havingestablishedaparabolicdevelopmentofmileagewith
age,withparametersdefinedinEquation(4)and(5),thiscanbe
usedtoestimatetheevolutionofthemileageofvehiclesthathave
amaximumusefullifemorethanwhatisshowninFigure1(see
alsotheSM,FiguresS1andS2,forgasolineanddieselcars,
respectively).Themaximumaveragemileageremainsconstant,as
thiswastheevidencefromtheexperimentaldatainFigure1;
hence,theevolutionofmileagealongthelifeofthevehiclecanbe
predictedhavingtheend–of–lifeageofthevehicleastheonly
independentparameter.Allgasolinecarswhichareyoungerthan
16yearsoldanddieselcarswhichareyoungerthan14yearsold
areexpectedonaveragetofollowtheoriginalcurve.
Theparabolicfunctionsdefinedcaninturnbeusedtomodel
howtheannualmileageofcarsdropswiththeirage.Thiswasnot
possiblebysubtractingtheACMvaluesoftwoconsecutiveyearsas
thisstabilizedafteracertainageandwouldresulttozero,oreven
negativevalues.Withthemodeldeveloped,ifonespecifiesthe
end–oflifeageofvehicles(j),thentheactualaverageannual
mileage(AAAM)thatthesevehiclesconductedwhenatagekwill
be:
,


  
1

1
(6)
Theparametersaandbinthisfunctiondifferforgasolineand
dieselpassengercars.ThemodeldevelopedinEquation(6)canbe
appliedtopredicttheevolutionofmileageoftheItalianpassenger
carstock.Thedistributionofvehiclesaccordingtoyearof
registrationisavailableatnationallevelintheItaliancar
associationdatabase(ACI,2011).Thisdatabaseshowsthatthere
areveryfewvehiclesregisteredabove30yearsofage,whichis
consideredasthemaximumendoflifeageinouranalysis.The
probabilityofvehiclestoreachacertainendoflifeageisrequired
inordertoapplyEquation(6)ontheItalianstockdata.However,
thisisnotknowna–priori.Wecanassumethatthisprobabilityis
equaltothepercentageofvehiclesintheparticularagebin
registeredinItalyin2010.Thisapproximationisactuallyalso
theoreticallyaccurateifthenumberofvehiclesregisteredinItalyis
constantthroughouttheyears.Inreality,intheperiod2000–2011,
theItaliannewpassengercarregistrationshavebeenfallingwith
anaveragerateof2%.Thisisaverymildchangewhichmeansthat
ourapproximationisveryclosetothetheoreticalaccurateandcan
besafelyusedinourcalculations.
Withthisassumption,andthefactthatallvehiclesbelowa
certainage(J*=16forgasolinecarsandJ*=14fordieselcars)
followthesameACMcurve,theAAAMvalueofallvehicles
registeredinItalyasafunctionoftheiragecanbecalculatedby
meansof:
Caserini et al. – Atmospheric Pollution Research (APR) 286
30
*Jj j
30
*Jj jj,k
kf
fAAAM
AAAM (7)
wherefjistheprobabilityofcarstoreachanendoflifeagej.
ThegraphicalrepresentationofEquation(7)isshowninFigure
3.TheAAAMkvaluesdropwithvehicleageandpracticallyreach
zeroatanageof30years.Regressioncurvessplitindifferent
regionshavebeendrawnthatallowusingthesetrendsindifferent
applications.Aquadraticfithasbeenassumeduntilthe22ndyear
ofageandthenalinearfittotheminimumofmileageinthe30th
year.Thishasbeenselectedinordertomaximizethefittingofthe
curveswiththedata.Severalattemptshavebeendoneandthe
split<22and>22yearofageresultedasthebestone.
3.3.Comparisontootherstudies
Despiteitssignificanceforemissioncalculations,information
onvehiclemileageasafunctionofageisratherscarceinthe
literature.Fewdataareavailable,mostlyestimatedonthe
assumptionthatvehiclesaredrivenforthesameannualdistance
duringtheirwholelives.Asalreadysaid,thisassumptiondoesnot
correctlyrepresenttheknowndependenceofmileageonvehicle
agewhichisdescribedbyAAAM.Ontheotherhand,nottaking
intoaccountthedropofannualmileagewithagelikelyresultsto
anoverestimationoftotalemissionsasthecontributionofolder
vehicletechnologiesisoverstated.
AdatasetofmileagedatahasbeendevelopedatEuropean
levelintheframeworkoftheTREMOVE(EC,2005)andMEET
(Andre,1999;Andreetal.,1999)activities.TREMOVEisapolicy
assessmenttoolthatprovidesthebackgroundcalculationsfor
impactassessmentstudiesintheareaoftransportpolicy
interventions.IntheMEETproject,thedependenceofmileageon
vehicleagewasalsocollectedfromafewnationaldata,without
offeringdistinctiontovehiclecategories(Andreetal.,1999).For
example,forSwedenthedependenceofmileageonvehicleage
(Figure4)wascalculatedusingdatasetsfromtwoconsecutive
years(1987–1988)andmatchingcarwiththeregistrationnumber.
Althoughnoinformationisavailableontheamountofdata
processed,inthiscasetheresultistheactualannualmileage
definedpreviously.
Vehiclesurvivabilityandmileageforpassengercarswerealso
developedonthebasisof1977to2002registrationsand2001
mileagesurveydata(NHTSA,2006)inUS.Thisanalysisshowsthat
atypicalpassengercartravelsforatotaldistanceofapproximately
150000miles,reachedafter25years,whileinthepresentstudy
thelifetimemileageforgasolineanddieselvehicleswasrespect
tively126300km(after14years)and172600km(after16years).
AsitisshowninFigure4,thedecreaseofAAAMmileageinthe
firstyearsofserviceishigherthanfortheothercurves.
Thecomparisonofthedatageneratedinthisworkwith
resultsfromotherstudiesgenerallyshowsthattheAAMvalues
generatedareclosetothemaximumoftherangeofthedata
collected.Actually,therelativeincreaseofdieselcarsupto3years
ofageisuniqueinourdataset.Ontheotherhand,theAAAM
valueswhichshouldcloserreflecttheactualdropinmileagewith
ageareatthelowendoftherangecollected,andcomparableto
datafromUS.Ithasnotbeenpossibletoexactlyidentifywhich
methodhasbeenusedbyotherstudiesforestimatingthemileage
functionwithage.However,thevaluesobtainedinthisworkwith
twodifferentmethodsaregenerallywithintherangeofvalues
reportedelsewhere.
Thiscomparisonshowsthatthedefinitionofthemileage
functionwithageisimportantandthatdifferentdroppingmileage
ratescanbeobtained,dependingonthedefinition.Thisleadsto
twoimportantconclusions.First,themethodusedtoestimatethe
functionofmileagewithagehastobereportedand,second,the
environmentallyrelevantcalculationswilldependonwhich
methodhasbeenusedfortheassessment.
Figure3.
A
ctualaverageannualmileage(AAAM)bydieselandgasolinecarsintheItalianvehiclestock.Bes
t
fitcurves
havebeendrawn,splitintworegions.

Caserini et al. – Atmospheric Pollution Research (APR) 287
4.Discussion
Therelationshipbetweenmileageandvehicleageallowsthe
estimationoftheaveragemileageofvehiclesthatbelongto
specificlegislative(i.e.Euro)classes.Thisisthestandardvehicle
classificationinallEuropeanroadtransportemissionmodels(e.g.
COPERT,HBEFA,VERSIT+).Multiplicationofthismeanmileagewith
thenumberofvehiclesintheclassandwithanappropriate
emissionfactorcalculatedbythemodelleadstotheestimationof
thetotalemissionsproducedbythevehiclesintheparticularclass.
Themeanmileagepercategoryisderivedastheweighted
averageofvehiclesofdifferentagewhichcomplywiththesame
emissionlimit,astheyareregisteredintheofficialstatistics(ACI,
2011).Thepercentagedistributionofvehiclesduringdifferent
yearsofage(Table1)wasusedinordertoestimateaveragevalues
ofAAAMseparatelyforthedifferentlegislativeclassesofdiesel
andgasolinevehicles,fromEuro0(non–catalyticvehicles)toEuro
5,aswellasanaveragemileageforthewholefleet(Figure5).It
shouldbenotedthattheestimateisrelatedtotheyearinwhich
theestimationofmileagewascarriedout,thatistheyear2010in
ourcase,andsooneyearoldvehiclesaretheonesregistered
during2009.Aswouldhavebeenexpected,themeanmileagefor
vehiclesregisteredbefore1992(Euro0)isonlyasmallfractionof
newEuro5vehicles.
Figure4.Comparisonbetweentheresultsofthepresentstudyanddatafromtheliteraturereviews:USAdatacollectedbetween1997
and2002(NHTSA,2006),TREMOVEdata(EC,2005)andSwedishdatacollectedbetween1987and1998(Andreetal.,1999).
Figure5.
AAM(1
3km/y)fordifferenteurocategoriesandaverageforthewholefleetintheyear2010.

Caserini et al. – Atmospheric Pollution Research (APR) 288
ThemileagevaluesestimatedinFigure5canthenbeusedas
inputtoemissionmodelstoestimateroadtransportemissions.We
haveusedCOPERT4asanexample,todemonstratetheimpactof
thedroppingmileagewithageontotalemissionestimates.The
maininputdatausedforsuchacalculationareshowninTable2.
Theemissionfactorsfornitrogendioxide(NOX)andparticulate
matter(PM)werederivedasaggregatesfromthedetailedCOPERT
4methodology,usingdetailedactivityandenvironmental
informationcorrespondingtotheItalianconditions.Thenumberof
vehiclessplitbyemissionlegislationwasderivedfromnational
statistics,andtherelativemileagepertechnologystepwas
estimatedusingtheapproachesdescribedabove(constant
mileage,AAMandAAAM).Basically,themileagevaluesestimated
pereachclassbasedeitherontheAAMortheAAAMmethods
wereproportionallyadjustedtoleadtothesamefuelconsumption
astheconstantmileagecase.Thisisthetypicalprocedurefollowed
inemissioninventories,i.e.arelativemileagepertechnologyclass
isfirstestimatedandthenitisproportionallyadjustedtomeetthe
fuelconsumptionreportedbytheofficialstatistics.Withthis
method,allemissionresultsshowninTable3correspondtothe
sametotalfuelconsumption.
Despiteallcalculationscorrespondtothesamefinalenergy
utilization,therearesignificantdifferencesinthetotalemissions
calculationsforbothpollutants.BothNOXandexhaustPM
emissionsdropbymorethan20%whenAAAMisusedinsteadof
fixedmileage.ThedifferenceinNOXmostlycomesfromgasoline
carsasthetrueNOXemissionfactors(Table2)ofdieselcarsdonot
consistentlydropwithanimprovingemissionstandard.Hence,the
allocationofmileagetogasolinevehiclesismoreimportantthan
dieselonesinthecaseofNOX.TheoppositeoccursincaseofPM
emissionswheredifferencesingasolinevehiclePMemission
factorsareminimalandthemainreductionsoriginatefromdiesel
vehiclesonly.RelevantdifferencesalsooccurwhentheAAM
estimateofmileageisused.
Suchemissiondifferencesarenottrivial.Forexample,outof
theelevenmemberstatesthatemittedmorethantheirNOX
targetsaccordingtotheemissionceilingsdirective(EEA,2011),
eightofthemonlyexceededtheirlimitsbylessthan20%.Taking
intoaccountthatroadtransportaloneissome40%oftotal
nationalNOXemissions(EEA,2010)thedifferenceofmorethan
20%thatwecalculatedduetomileageestimationonlyinthis
studywouldbeequivalentofmorethan8%oftotalnation–wide
NOXemissions.Suchadifferencewouldbringanumberof
countriescloserorwithintheirallowedlimits.
Table2.AggregatedNOxandPM10emissionfactorsandmileagevaluesusedforenvironmentalmodeling
FuelLegislative
category
NOX
(mg/km)
PM10
(mg/km)
Vehicle
share(%)
Constant
mileage
(km/y)
AAM
(km/y)
AAAM
(km/y)
Gasoline
euro019582.418%697042191829
euro14262.38.4%697051893038
euro22232.226%697063034909
euro3951.117%697076537854
euro4571.128%6970919312088
euro5461.11.9%6970996314548
Diesel
euro06952164.6%1363861772267
euro1691892.4%1363884704023
euro27345413%13638109846961
euro38044331%136381405711966
euro46003645%136381690017922
euro54331.84.0%136381876722525
Table3.NOxandPM10emissionscalculatedwithdifferentaveragemileageestimates
  NOX  PM
Constant
mileageAAMAAAM Constant
mileage AAMAAAM
Gasoline
euro073.2%63.7%46.6% 24.8%16.4%8.1%
euro17.6%8.1%8.0%11.4%9.3%6.1%
euro212.3%16.0%20.9% 34.4%34.1%30.0%
euro33.4%5.4%9.3%10.6%12.8%14.8%
euro43.4%6.4%14.2%17.6%25.5%37.8%
euro50.2%0.4%1.0%1.2%1.9%3.1%
Total(t/y)678254716027949 243221196
Diesel
euro04.6%2.0%0.8%20.2%9.8%4.2%
euro12.4%1.4%0.7%4.4%2.9%1.6%
euro213.9%10.7%7.3%14.4%12.4%9.2%
euro336.9%36.1%33.3% 27.9%30.9%30.6%
euro439.6%46.5%53.5% 32.9%43.7%54.1%
euro52.5%3.3%4.3%0.1%0.2%0.3%
Total(t/y)129219136159125524 923086067382
Caserini et al. – Atmospheric Pollution Research (APR) 289
Moreover,itwouldbeinterestingtoexplorewhatwouldbe
theimpactofintroducingameasurethateliminatesnon–catalyst
vehiclesfromtheroad.Suchmeasurescouldbeanincentive–
basedscrappageschemeoranenvironmentalzoneenforcedina
partofacity.Ifonemakestheusualassumptionthatallvehicles
aredrivenforthesamedistanceeitherannuallyoronastreet
network,theneliminationofnon–catalystvehicles(Euro0)would
haveledtoassumethat28%ofNOXand20%oftotalPMemissions
fromgasolineanddieselpassengercarsshouldbereducedwith
suchameasure.However,takingintoconsiderationthatolder
vehiclesaredrivenless,theactualimprovementwouldonlybe9%
and4%respectively.Thisentirelychangesthecost–benefitratios
ofsuchmeasures.
Despitesuchsignificantimpacts,widespreadandrobust
estimatesofmileageasafunctionofspeedarestilllackingandour
strongrecommendationisthatsuchinformationhastobemore
reliablyandthoroughlyassessedandcollected.
5.Conclusions
Inordertoimprovetrafficemissionestimatesand,
consequently,fordefiningthestrategiesaimedtocontrolair
pollutionevents,thepresentworkhighlightstheimportanceof
increasingourknowledgeonvehiclemileagebehavior.Basingon
anextensivedataset(morethan33000data),arelationship
betweenvehiclemileageandagehasbeendefined,bothfordiesel
andgasolinepassengercars,withtheexampleoftheItalianstock.
Amethodologyhasalsobeenpresentedwhichcanbeappliedto
thenationalconditionsinothercountries.Theresultsofthis
methodologyshowthatannualmileagedropssignificantlywith
mileageage.Bothdieselandgasolinecarsdrivehalftheannual
distancewhentheyhavereachedanaverageageofapproximately
8years.Vehiclesof20yearsofageonlydriveapproximately10%
oftheannualdistancetheyusedtodrivewhentheyarenew.
Theimpactofthedroppingmileagewithageissignificantin
assessingtheenvironmentalimpactsoftransportandthepotential
impactofenvironmentalpolicies.NOXandPMemissionsof
passengercarsdropbymorethan20%whenadecreasingfunction
ofmileagewithageisused,insteadofafixedmileageforeach
environmentalclass.Also,theemissioncontributionfromold
vehiclesdecreaseswhichworsensthecost–effectivenessofair
qualityrelatedpolicymeasurestargetingsucholdvehicles.These
findingsdemonstratetheimportanceofperformingprecise
estimatesofmileagepervehicleclassifrobustroadtransport
emissioninventoriesneedtobeproduced.
SupportingMaterialAvailable
Listofwebsitesusedtocollectdata(TableS1);Meanmileage
(km)andstandarddeviation(km)ofthevehiclesampleperagebin
(TableS2);Averagecumulativemileageofgasolinecarsasa
functionofend–of–lifeage(FigureS1);Averagecumulative
mileageofdieselcarsasafunctionofend–of–lifeage(FigureS2).
ThisinformationisavailablefreeofchargeviaInternetat
http://www.atmospolres.com.
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