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Sensors2020,20,1111;doi:10.3390/s20041111www.mdpi.com/journal/sensors
Article
VerticalProfilesofPollutionParticleConcentrations
intheBoundaryLayeraboveParis(France)fromthe
OpticalAerosolCounterLOACOnboarda
TouristicBalloon
Jean‐BaptisteRenard1,*,VincentMichoud2andJérômeGiacomoni3
1LPC2E,CNRS/Universitéd’Orléans,45071OrléansCEDEX2,France
2LISA,CNRS/UniversitéParis‐Est‐Créteil,UniversitédeParis,InstitutPierreSimonLaplace(IPSL),
94010CréteilCEDEX,France;Vincent.Michoud@lisa.u‐pec.fr
3AerophileSAS,75015Paris,France;giacomoni@aerophile.com
*Correspondence:jean‐baptiste.renard@cnrs‐orleans.fr;Tel.:+33‐6‐3291‐7742
Received:21December2019;Accepted:15February2020;Published:18February2020
Abstract:Atmosphericpollutionbyparticulatematterrepresentsasignificanthealthriskandneeds
continuousmonitoringbyairqualitynetworksthatprovidemassconcentrationsforPM10and
PM2.5(particleswithdiametersmallerthan10mand2.5m,respectively).Wepresenthereanew
approachtomonitortheurbanparticlescontent,usingsixyearsofaerosolsnumberconcentration
measurementsforparticlesinthe0.2−50msizerange.Thesemeasurementsareperformedbythe
LightOpticalAerosolsCounter(LOAC)instrumentonboardthetetheredtouristicballoon“Ballon
deParisGenerali”,inParis,France.Suchmeasurementshaveallowedusfirsttodetectatgrounda
seasonalvariabilityintheparticulatemattercontent,duetotheoriginoftheparticles
(anthropogenicpollution,pollens),andsecondly,toretrievethemeanevolutionofparticles
concentrationswithheightabovegroundupto150m.Measurementswerealsoconductedupto
300mabovegroundduringmajorpollutionevents.Theverticalevolutionofconcentrationsvaries
fromoneeventtoanother,dependingontheoriginofthepollutionandonthemeteorological
conditions.Thesemeasurementshaveshowntheinterestofperformingparticlenumber
concentrationsmeasurementsfortheairpollutionmonitoringincomplementwithregulatorymass
concentrationsmeasurement,tobetterevaluatetheintensityofthepollutioneventandtobetter
considertheeffectofsmallestparticles,whicharemoredangerousforhumanhealth.
Keyworks:particulatematter;urbanpollution;numberconcentrations;tetheredballoon
_______________________________________________________________________________
1.Introduction
Atmosphericpollutionbyparticulatematter(PM)isagrowingconcern,particularlyinurban
environmentsthatconcentratealargeportionofthepopulationandtheparticle’semissionsources.
Theseparticlescanbeprimary,directlycomingfromnaturalsources(dusts,salts,pollen)andfrom
anthropogenicsources(transport,heating,industries,agriculture)orsecondary,comingfrom
chemicalreactionsinvolvingsunlightoratmosphericoxidants.
Suchparticlesrepresentasignificanthealthrisk[1–3].ThePM10fraction(particleswith
aerodynamicdiametersmallerthan10m)canpenetratebeyondthenasopharyngealtractinthe
bronchianduptothepulmonaryalveoli.Thesmallerparticles,below1m,candiffuseinthebody
andbefoundinvarioushumanorgans[4–6].Thesmallertheparticlesare,thedeepertheycan
penetrateanddiffuseinthebody.Thus,detectingandcountingthesubmicronicpollutionparticles
isamajorareaofinterestforpublichealth.ExceedingtheWHO(WorldHealthOrganization)
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guidelinevaluesforPM2.5(particleswithaerodynamicdiametersmallerthan10m)of10g∙m−3isthe
causeforabout20,000prematuredeathsormoreeachyearinmajorEuropeancities[7].Inparticular,
ithasbeenestimatedthatifthesestandardswouldhavebeenreacheditwouldhaveresultedina
gainofsixmonthsoflifeexpectancyformorethan11millioninhabitantsoftheParisregion[8].
TheParisregionischaracterizedbyrelativelyfewindustries.Ataregionalscale,theurban
backgroundconditionsaredrivenbylong‐distancetransportofpollutants,accountingfor70%in
averageofPM2.5mass‐concentration[9–13].Localsourcesofprimaryparticlesaredominatedin
massbytrafficemissionandbyresidentialheatinginwinter,asinmanyotherurbanenvironments
[14,15].Thespatialdistributionofparticlesconcentrationsatthesurfacecanstronglydifferatthe
urbanscale,alsodependingonair‐massdispersionconstrainedbytheurbantopography.
Atground,thePMcontentiscontinuouslymonitoredbyairqualitynetworks,whichprovide
themassconcentrationsofPM10usingmicrobalancesorsimilarinstruments.Somestationscanalso
provideanestimateofPM2.5massconcentrations.InParisandthe“IledeFrance”region(which
includesthecityofParis)theseregulatorymeasurementsareconductedbyAirparif[16].However,
thetechniquesdeployedacrosstheairqualitymonitoringnetworksarebylaworientedtowardthe
measurementoftheaerosolmassconcentration,whileitisknownthatthesmallestparticles,which
penetratedeeperintothehumanorganism,mostlydominatethenumberconcentrationand
contributeonlyweaklytothemass.Then,complementarymeasurementsprovidingparticlenumber
concentrationsinadditiontomassconcentrationscanbeproposedtomonitortheurbanparticle
contentandbetterunderstanditsdistributionanddynamics.Verticalprofilesmeasurementsofthe
aerosolscontentabovemajorcities,performedduringspecificcampaigns,canbeusedtobetter
understandtheverticaltransportandthedispersionoftheparticles[17−19].
Theaimofthispaperistopresentsixyearsofaerosolsnumberconcentrationmeasurements
performedbytheLightOpticalAerosolsCounter(LOAC)instrumentonboardatetheredtouristic
ballooninParis.Suchconditionsofmeasurementsallowustobetterdeterminethemeanseasonal
variationsofparticlescontentfordifferentsizeclasses,andalsotoevaluatethemeanvertical
evolutionofparticlesandtofocusonsomespecificpollutionevents.
2.TheLightOpticalAerosolsCounter(LOAC)
TheLOACisaprototypeinstrumentdevelopedforgroundandballoon‐basedmeasurements[20].
Particlesaredrawnuptotheopticalchamberthroughanisostatictubebyasmallpumpandcrossa
laserbeamworkingat650nm.Thescatteredlightisrecordedbytwophotodiodesatscatteringangles
of~15°and~65°,andphotonstraveldirectlytothephotodiodesthoughpipeswithoutalens(Figure1,
updatedfrom[21]).Atotalof19sizeclassesaredefinedintheparticlediameterrangebetween0.2
and~50m.Thesizeclassesarechosenasagoodcompromisebetweentheinstrumentsensitivityand
theexpectedsizedistributionofambientairparticles.Fora10minintegrationtime,theuncertaintyof
totalconcentrationsisabout±20%forconcentrationshigherthan10particle∙cm−3upto±60%for
concentrationslowerthan10−1particle∙cm−3.Theuncertaintiesinthesizedeterminationisof±0.025m
forparticlessmallerthan0.6m,5%forparticlesinthe0.7−2mrange,andof10%forparticlesgreater
than2m.
Themeasurementsat15°arealmostinsensitivetotherefractiveindexoftheirregularshaped
aerosolparticlesandcanbeusedtodeterminetheconcentrations[22].Ontheotherhand,
measurementsatside‐scatteringaround65°areverysensitivetotherefractiveindexoftheirregular
particles[23].Anindicationofthetypologyofsuchparticlescanbeobtainedusinga“speciation
index”,retrievedbycombiningthe15°and65°channelsmeasurements.Thisindexissensitivetothe
imaginarypartoftherefractiveindexoftheparticles,andthustotheiropticalabsorbingproperties.
LaboratoryreferencesforthespeciationindexeshavebeendeterminedwithLOACfor4naturesof
particles:carbonaceous,mineraldust,salts,andliquiddroplet[20].Thespeciationindexesobtained
fromLOACobservationsintheambientairarecomparedtotheselaboratorydatatoderivethe
estimateddominanttypologyofparticlesindifferentsizeclasses.Theidentificationofthenatureof
theparticlesworkswellincaseofahomogeneousmediumbutismorequestionableincaseofa
heterogeneousmediumthatcausethespeciationindextobemorescattered.
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Figure1.LightOpticalAerosolsCounter(LOAC)principleofmeasurementsattwoscatteringangles
(updatedfromRenardetal.,2018).
Finally,theconcentrationscanbeconvertedtomassconcentrations(ing∙m−3)assumingspherical
particlesandmassdensitybetween1.2and2.2g∙cm−3dependingonthedetectedtypologyofthe
particles.Forcarbonaceousparticlessmallerthan1m,themassdensityisassumedtobe2.0g∙cm−3,
whilethedensityisassumedtobeof1.2g∙cm−3forparticlesgreaterthan1m.Formineralparticles
thedensityisassumedtobeof2.2g∙cm−3;incaseofnoidentification,thedensityisassumedtobeof
2.0g∙cm−3(thesevaluesareupdatedfrom[20]).Theerrorbarsarecalculatedconsideringthe
uncertaintiesinsizedetermination.TheLOACaveragemass‐concentrationsaccuracyisofabout
±5g∙m−3whencomparedtomicrobalancemeasurementsinlaboratory[20].
TheperformancesofLOAChavebeenestablishedduringnumeroussessionsofinter‐comparison
withotherinstrumentsdedicatedtothecounting,sizedistribution,extinctions,andmass
concentrationsofsolidaerosolsintheatmosphere[20].ThesesessionshaveshownthatLOACcan
beusedforstudiesonurbanaerosols.
3.MeasurementsConditionsattheTouristic“BallondeParisGenerali”inParis(France)
TheLOAChasbeenmountedonthegondolaofthetouristictetheredballoon“BallondeParis
Generali”(Figure2)inthepark“AndréCitroën”inthesouth‐westofParis,France(48.8414°N,
2.2740°E),sincemid‐2013andworkscontinuously[24].Dependingonthemeteorologicalconditions,
150to200daysperyeararefavorableforflying.TheheightismeasuredbyaGPS,withavertical
accuracyofabout±15m.Theballoonnominalmaximumheightabovegroundis150mandupto50
flightscanbeperformedperday.Someflightscanalsobeconducteduptoaheightof300mwhen
thewindspeedisverylow(<fewm−1).Theurbanpollutioneventsoccurmainlyduringanticyclonic
conditionswhenthewindspeedislow;thus,theballooncanoftenflyduringsuchevents.
DuetotheballoonlocationinoneofthelargestparksinPariswithnomainhighwaysinthe
directvicinity,theground‐basedmeasurementscanbereferredasbackgroundurbanconditions
(Figure3).Thehighestbuildingsclosetotheparkhaveaheightbelow50m.Therefore,wecanexpect
tomeasuremeanpersistenturbanpollutionwhentheballoonheightisabove50m.Nevertheless,
thesemeasurementsareconductedjustatonelocationinParis,thustheresultspresentedbelow
couldbenotrepresentativeofthegeneralpicture.
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Figure2.(a)LOACmountedonthegondolaofthe“BallondeParisGenerali”;(b)Theballooninthe
parkAndréCitroën(Paris,France).
Figure3.Mapofthevicinityofthe“ParcAndréCitroën”(thegreendotrepresentstheballoon
location,mapfrommappy.com).
LOACprovidesmeasurementsevery10s.Duringtheballoonflight,datamustbeintegrated
overatleast30storeducethemeasurementnoise.Sincetheballoonascentspeedisof1m∙s−1,the
verticalresolutionforanindividualLOACconcentrationsprofileisofabout30m.Individualprofiles
canbealsodailyaveragedtoreducethenaturaldispersionoftheconcentration’smeasurements.For
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measurementsatground,thedatacanbeaveragedover10mintoreducethedispersionofthe
measurements.
4.MeasurementsatGround
Figure4presentsthe2013−2019evolutionoftheparticlenumberconcentrationswithtimefor
the19sizeclassesofLOACwhentheballoonisatgroundandoutsidemajorLOACand/orballoon
maintenanceperiod.Dataduringfogandheavyraineventshavebeenremovedbecauseofthe
presenceofdropletsthatcanskewtheretrievalofthesolidparticles.Forclarityreason,thedatahave
beensmoothedbyaslidingsmoothingprocedurewithawidthof2days.Thehighestconcentrations
correspondingtostrongpollutionseventswererecordedatthebeginningofwinter2013,andatthe
beginningof2014and2015.Itcanbenoticedthatthemeanlevelofconcentrationsforparticles
smallerthan10mmeasuredbyLOACishigherinthe2013periodthanafter(Figure4),duetostrong
buildingactivitiesclosetotheParcAndréCitroën(unfortunatelynoAirparifstationisclosetothis
locationtovalidatetheseobservations).
Figure4.Temporalevolutionofnumberconcentrationsforthe19LOACsizeclasseswhentheballoon
isatground(thedatahavebeensmoothedbyaslidingsmoothingprocedurewithawidthof2days).
ThePM10massconcentrationsobtainedbytheAirparifairqualitynetworkforastationinthe
suburbofParis(Vitry,backgroundurbanconditions,48.7778°N,2.3779°E)andforastationinarural
areainthesouthofParisregion(RuralSouth,48.3667°N,2.2333°E)alsoshowtheconcentrations
enhancementsduringpollutionevents(Figure5).ThemassconcentrationsderivedfromtheLOAC
measurementsarealsoplottedinFigure5andaregloballyingoodagreementwiththeAirparif
measurements(allthedatahavebeenalsosmoothedwiththesameprocedureasforLOACnumber
concentrations).Excludingthe2013periodwiththebuildingactivitiesclosetothepark,themean
LOACmassconcentrationsmeasurementsare6.5m∙m−3and0.5m∙m−3lowerthantheVitryandRural
Southstations,withastandarddeviationof10m∙m−3and9.5m∙m−3respectively.
Whenconsideringthesixyearsofmeasurements,aseasonalcycleseemstobepresentforthe
particlessmallerthanabout1m,withmaximumconcentrationsinwinterandminimum
concentrationsinsummer(Figure6).Thiscouldberelatedtotheheatingandtrafficduringwinter
butalsototheseasonalvariationoftheboundarylayerheight[25]andtothemeteorological
conditions.Theconcentrationsforparticleslargerthanabout15mexhibitalsoaseasonalcycle,
anticorrelatedwiththeseasonalcycleofthesmallerparticles;thehighestconcentrationsareobtained
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duringsummer(Figure6).Apossibleexplanationisthedetectionofpollens,sincemostofthepollens
areindeedinthe15−50msizerangeandtheirseasonalityissimilartotheonedetectedbyLOAC.
Figure5.TemporalevolutionofthePM10massconcentrationsfromLOACatthe“BallondeParis
Generali”andfromtheAirparifairqualitynetworkforstationsinthesuburbofParis(Vitry)andin
theruralareainthesouthofParisregion(thedatahavebeensmoothedbyaslidingsmoothing
procedurewithawidthof2days).
Figure6.Annualevolutionoftheconcentrationsforthesmallestparticles(a)andthebiggestparticles
(b)detectedbyLOACforthesix‐yearperiod;theverticalbarscorrespondtothemeanabsolute
deviationoftheconcentrations.
5.VerticalProfiles
5.1.DailyProfiles
Duringthe2013−2019period,976dayswithflightsperformedduringdaytimeareavailable.
Amongthem148dayswithflightsuptotheheightof300mwereperformed.Sincemostoftheflights
wereconducteduptotheheightof150m,wewillconsidertheseflightstoestimatethemean
evolutionoftheconcentrationswithheight.Ontheotherhand,thefewflightspermonthuptothe
heightof300mcanbeusedtostudysomespecificevents.
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Tostudythemeanverticaltrend,theconcentrationsareintegratedover7heightranges:0−20m,
20−40m,40−60m,60−80m,80−100m,100−120m,and120−150m.Theverticalprofilesaredaily
averagedtoreducethevariabilityoftheindividualmeasurements.Thisvariabilityoriginatesfrom
themeasurementaccuracy(Poissondistribution)butalsofromvariationsinthewindconditions.
Globally,thedailystandarddeviationoftheconcentrationprofilesisinthe20%−60%range.Thus,
thedailyprofilespresentedinthefollowingsubsectionscontainserrorbarsthatrepresentthe
standarddeviationforeachsizeclassandheightrange.
5.2.BackgroundConditions
Thedailyprofilesexhibitatemporalvariabilityofabouttwoordersofmagnitudedependingon
theweatherandonthepollutionconditions.Asexamples,Figure7presentstwonumber
concentrationsprofilesobtainedinsummertime,8July2016and20September2017,duringlow
pollutionconditions(theLOACPM10massconcentrationswereofabout10g∙m−3atgroundforboth
casesandinthe5−15g∙m−3rangeinflight).Thetwoprofilesexhibitdifferentverticalevolutionsand
sizedistributions;inparticular,theprofileofthe8July2016presentsanexcessoflargeparticlesin
the10−30msizerange,probablyduetoapollenepisode.
Figure7.Evolutionwithheightofnumberconcentrationsforthe19sizeclassesofLOACwhenthe
urbanpollutionislow.(a)8July2016;(b)20September2017.
Ateachheightrange,thehistogramofthenumberconcentrationsobtainedduringthesixyearsof
measurementsdonotfollowagaussiandistribution.Ontheotherhand,agaussiandistributioncanbe
obtainedwhenconsideringthelogarithmoftheconcentrations.Fourbroad‐rangeclassesaredefined:
0.2−1.0m,1−3µm,3−10m,and15−50m.Then,foreachheightrangeandforthe4broad‐rangeclasses,
thehistogramofthelogarithmoftheconcentrationsisfittedbyagaussianfunction,toestimatethe
modeofnumberconcentrations(correspondingtothemostfrequentconcentrationsoverthefulltime
periodofmeasurements,examplesaregiveninFigure8).
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Figure9presentstheevolutionwithheightofthemorefrequentconcentrationsforthe4
broad‐rangeclasses.Itmustbenoticedthattheseverticalprofilesmightnotbeindividually
consistentandarenotthemodalverticalprofilesbutareasequenceofindividualmodal
concentrationforeachheightrange.Forthesubmicronicparticles,noevolutionisdetected,whilethe
concentrationsslightlydecreaseofabout20%inthefirsttensofmetersforthelargestparticles.This
couldresultoftheparticlessensitivitytoupwarddiffusion,dependingontheirsizes,shapes,and
densities,butthisanalysisrequiresfurtherinvestigation.
Figure8.Examplesofthehistogramofthelogarithmoftheconcentrationsattwoheightrangesfor
thesixyearsofmeasurements,forthe1−3msizerange.(a)measurementsatground;(b)
measurementsattheheightof120−150m.
Figure9.Evolutionwithheightofthemodeofnumberconcentrationsforthesixyearsof
measurements.(a)broad‐rangeclass0.2−1.0µm;(b)broad‐rangeclass1−3µm;(c)broad‐rangeclass
3−10µm;(d)broad‐rangeclass15−50µm.
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5.3.MainPollutionEvents
Duringmostofthepollutionevents,thewindswerelow,thusflightsuptoaheightof300mwere
available.WepresentbelowthemainpollutioneventsobservedbyLOACinthe2013−2019period.
5.3.1.WinterEvent
PollutionisoftenencounteredinParisduringanticyclonicconditionsinwinter,typicallyin
December,withparticulatesoriginatingfromtrafficandheating[26].Duringsuchevents,the
concentrationoftheparticlescanbefrom10to100timeshigherthanduringthesummerbackground
conditions.Thestrongerwintereventrecordedinthe2013−2019periodbyLOACoccursinDecember
2013.Figure10presentstheprofilesforthestrongpollutioneventin11December2013.Theprofile
exhibitsastrongconcentrationincreaseataheightof200mforthesmallestparticles<0.4mwhilethe
concentrationsareabouttentimeslowerabove,duetoatemperatureinversionlayerresultingfromthe
anticyclonicconditions.TheLOACPM10massconcentrationsatgroundareofabout50g∙m−3at
ground,ofabout60g∙m−3ataheightof130mduetoanincreaseofconcentrationsofparticlesinthe
5−10µmsizerange,onlyof45g∙m−3ataheightof200malthoughtheincreaseofsubmicronicparticles
concentrations,andofabout35g∙m−3above.Thus,theheightvariabilityinmassconcentrationsdo
notfullyreflectthemorecomplexheightvariabilityofthenumberconcentrationsforthevarious
sizeclasses.
Figure10.EvolutionwithheightofLOACconcentrationsforthe11December2013pollutionevent.
(a)numberconcentrations;(b)PM10massconcentration.
5.3.2.SpringEvent
Anothertypeofpollutionisoftenencounteredinbeginningofspring,typicallyinMarch,mainly
originatingfromthecombinationoflocalpollutionandthetransportofpollutantsfromagricultural
activitiesaroundParis[27,28].TheverticalprofilesinFigure11forthe14March2014and17March
2015differfromthosemeasuredinwinter(Figure10).Theconcentrationsoftheparticlesaremore
constantwithheight,becauseboundarylayeristhickerduetomoreverticallymixedairmasses,
whileconcentrationsofthebiggestparticlesslightlydecreasewithheight.TheLOACPM10mass
concentrationsdecreasefrom110g∙m−3atgroundtoabout65g∙m−3at300mforthe17March2015
anddecreasefromabout40g∙m−3atgroundtoabout20g∙m−3at300m(Figure12).
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Figure11.EvolutionwithheightofLOACnumberconcentrationsduringthespringpollutionevents.
(a)14March2014;(b)17March2015.
Figure12.EvolutionwithheightofLOACPM10massconcentrationsduringthespringpollution
events.(a)14March2014;(b)17March2015.
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5.3.3.SummerEvent
Pollutioneventscanbealsopresentduringsummer,dependingontheweatherconditions.
Althoughnomajorpollutioneventwasdetectedatgroundduringthe2014−2019period,anincreasein
aerosolconcentrationswasdetectedfrom24to30Juneaboveaheightofabout100m.Forthe25June
2019profile,thenumberconcentrationswereupto10timeshigherforparticlessmallerthan5minthe
100−200mheightrangethanatground,themassconcentrationwerefivetimeshigher(Figure13).This
eventwasnotdetectedfromground‐basedstation,showingtheinterestofballoon‐borne
measurements.
Noexplainableextra‐source(smoke,industrialactivities)wasidentifyduringthisperiodatthe
vicinityofthemeasurementlocation.TheLOACtypologyindicationdiffersstronglyfromthoseof
theotherpollutioneventsanddonotcorrespondtotheusualLOACtypologyforconventionalurban
pollutionparticles(carbonaceous,mineral,sulfate,ammoniumnitrate).Atpresentnoexplanation
canbeproposedfortheoriginofthiseventandforthenatureoftheparticles,andfurther
investigationsareneeded.
Figure13.EvolutionwithheightofLOACconcentrationsforthe25June2019pollutionevent.(a)
numberconcentrations;(b)PM10massconcentration.
6.Discussion
Theconcentrationsoftheparticlesandtheirverticalevolutionvariesfromonepollutionepisode
toanother.Tobetterevaluatethevariability,wecanconsidertheprevious4broad‐rangeclasses,now
appliedtothe6verticalprofilespresentedinFigures7,10,11and13.Afactorofmorethan50occurs
betweenlowpollutionandhighpollutioneventsforthe3firstbroad‐rangeclasses(particleswith
diametersmallerthan10m),asshowninFigure14.Asexpected,thehighestconcentrationsofthe
submicronicparticlesoccurfortheDecember2013eventandfortheMarch2014and2015events.For
theparticlesinthe1−3msizerange,thehighestconcentrationsweredetectedduringtheMarch2014
event,duringtheMarch2015event,andduringtheJune2019eventinflight.Inparticular,theMarch
2015eventwasdominatedbysecondaryaerosols(moreparticularlyofammoniumnitrate,as
reportedin[28]).Theconcentrationofthe1−3mparticlesduringtheDecember2013eventis
significantlylowerthanfortheMarch2014event.Thetendencyforthe3−10mparticlesisrelatively
similartotheoneforthe1−3mparticles,exceptfortheMarch2015concentrationsthatarecloserto
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thebackgroundconditions.Finally,noobviouscorrelationbetweenpollutionandconcentrationsof
thelargestparticles(15−50m)canbepointedout,since,aspreviouslymentioned,theseparticlesare
likelymainlyoriginatingfrompollenevents.
WhenconsideringthemassconcentrationsvaluesfromLOAC,thehighestpollutioneventswere
onMarch2014andMarch2015(withmaximumvaluesupto110g∙m−3andupto90g∙m−3,
respectively).Whenconsideringthenumberofallparticleslargerthan0.2m,thehighest
concentrationsweredetectedonDecember2013,althoughthemassconcentrationswereonlyofup
to80g∙m−3.Thesedifferencescouldbeduetotheoriginofthepollution,typicallyprimary
(carbonaceous)particlesversussecondaryaerosols.
MassconcentrationvariabilityfromlowpollutiontohighpollutionlevelsinParisisaboutone
orderofmagnitude,whilethevariabilityisoftwoordersofmagnitudefornumberconcentration.
Mostofthestudiesonthepollutiontrendincitiesarebasedonmassconcentrations[29].Itisoften
establishedthatthemass‐concentrationofPM10isdecreasinginEuropeancities,mainlydueto
changesindieselenginesandparticlesfilters[30].Nevertheless,possiblechangeinthesizedistribution,
aspossibleincreaseinnumberconcentrationofsubmicronicparticles,cannotbeestimatedwithsuch
measurements.Also,theverticalevolutionofparticlesnumberconcentrationscanchangedepending
ontheoriginofthepollutionevents.Numberconcentrationmeasurementsasthosepresentedabove,
mainlyforsubmicronicthatcanbethemostdangerousforhealth[4–6],maybemoreefficientthan
massconcentrationstoevaluatethehealthimpactofpollutionseventsandtoestablishthetemporal
trendofnumberconcentrationof(anthropogenic)pollutionparticles.Obviously,suchtrendscannotbe
retrievedfrommeasurementsonlyatonelocation;thus,anetworkof(optical)counterinstruments
mustbeimplantedinparallelwithregulatorymassconcentrationinstruments.
Figure14.Evolutionwithheightofaerosolsconcentrations.(a)broad‐rangeclass0.2−1.0µm;(b)
broad‐rangeclass1−3µm;(c)broad‐rangeclass3−10µm;(d)broad‐rangeclass15−50µm.
7.Conclusions
ThesixyearsofnumberconcentrationmeasurementsperformedinParis,France,bytheLOAC
instrumentforparticlesinthe0.2−50msizerangeallowedustodetectaseasonalvariabilityinthe
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PMcontent,expectedlyduetotheoriginoftheparticles(anthropogenicpollution,pollens).Sincethe
instrumentismountedonboardthetouristictetheredballoon“BallondeParisGenerali”,themean
evolutionofparticlesconcentrationswithheightwasobtainedforbackgroundconditionsuptoa
heightof150m.Theconcentrationsofsubmicronicparticlesremainconstantwithheight,whilethe
concentrationsslightlydecreaseinthefirsttensofmetersforthelargestparticles.Measurements
werealsoconductedduringmajorpollutionevents,withparticlesconcentrationsseveraltensof
timeshigherthanduringbackgroundconditions.Theverticalevolutionofconcentrationsvariesfrom
oneeventtoanotherone,dependingontheoriginofthepollutionandonthemeteorological
conditions.Inparticular,anaccumulationlayerwassometimesdetectedataheightofabout200m
inwinterduetotemperatureinversionlayer.
Thesemeasurementshaveshowntheinterestofperformingparticlenumberconcentrations
measurementsfortheairpollutionmonitoringincomplementwithregulatorymassconcentrations
measurements,tobetterevaluatetheintensityofthepollutioneventandtobetterconsidertheeffect
ofsmallestparticles.
WehavepresentedherethefirstresultsobtainedwithLOACattheParistouristicballoon.
FuturestudiescouldbeconductedusingthisLOACdatabase,asmodelingworksonthevertical
transportofparticles,studiesonthecorrelationofthevariabilityoftheparticle’sconcentrationsand
theirsizedistributionwiththemeteorologicalconditions,andstudiesontheevolutionoftheparticle
concentrationsafterrain.Finally,aLOACisalsomountedinanothertouristicballoon,the“Ballon
TerraBotanica”inasmallercity,Angers(WestofFrance);itsmeasurementswillbecomparedsoon
tothoseofParistobetterestablishthesimilaritiesandthedifferencesfortheverticalevolutionofthe
pollutionparticlesdependingonthemeasurementlocations.
NewmeasurementscouldbeproposedbymountingtheLOACinstrumentonboardthevarious
touristictetheredballoonsavailableintheworld,tobetterevaluatetheverticaltransportoftheurban
pollutionparticles.
Authorcontribution:TheinstrumentconceptionandthedataanalysiswereconductedbyJ.‐B.Renard.The
measurementsinterpretationwasdonebyJ.‐B.RenardandV.Michoud.Thefundingacquisitionswereconducted
byV.Michoud,J.‐B.RenardandG.Giacomoni.The“BallondeParis”facilitieswereprovidedbyJ.Giacomoni.
Funding:ThisprojectwaspartlyfundedbyADEME(projectMESUrPOP)CORTEA/contractnumber
1762C0004andbytheLabex“ÉtudedesgéofluidesetdesVOLatils–Terre,AtmosphèreetInterfaces–Ressources
etEnvironnement”(VOLTAIRE)(ANR‐10‐LABX‐100‐01)managedbytheUniversityofOrleans.
Acknowledgments:TheregulatorymassconcentrationsareprovidedbytheairqualitynetworkAirparif.The
authorswanttothankthepilotsofthetouristicballooninParis,andG.Berthet,P.Formenti,J.‐F.Doussin,and
IsabellaAnnesi‐Maesanoforfruitfuldiscussionsthathelpimprovingourpaper.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.Thefundershadnoroleinthedesignofthe
study;inthecollection,analyses,orinterpretationofdata;inthewritingofthemanuscript,orinthedecisionto
publishtheresults.
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