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Aero
sol A
l (ng
m-3
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0
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7000Agadir LaayouneDakhla
A case study of aerosol trace element deposition to Moroccan coastal waters
RachelShelley*,GéraldineSarthou,GeorgesTymen*,RémiLosno,LuisTitodeMorais,AliBenhra,andFa>maZohraBouthir
*[email protected];[email protected]
Ra$onaleAerosoldeposi>onisanimportantsourceoftraceelements(TEs)tothesurfaceocean.ProximitytotheSaharaDesert/SahelmeansthattheNorthAtlan>creceivessomeofthehighestinputsofmineraldustglobally(100-220Tgyr-1;[1];[2]).ThisinputofTEscontributestothegreaterefficiencyoftheCanaryCurrentUpwellingSystemrela>vetoitsPacificcounterpart[3];bothofwhichsupportsocio-economicallyimportantfisheries.However,mineraldustisjustonecomponent,humanac>vi>es(e.g.fossilfuelburning,agriculturalprac>ces)alsocontributetotheatmosphericload.
Inthestudyregionupwellingwateralsofuelshighprimaryproduc>onviathesupplyofmacro-nutrient-rich(N,PandSi)deepwater.ThisdeepwatermayalsobeenrichedinTEs,suchasthosewithwatercolumndistribu>onsthatmirrorthemacronutrients,e.g.Cd&P.AtangibleconcernforthefisheriesoffWestAfricaisthepoten>alfortoxiceffectsatalllevelsofthemarinefoodweb,duetoinputsofCd(andotherpoten>allytoxicTEs).Theaimofthisprojectwastoinves>gatetheTEcomposi>onandrela>velevelsofenrichmentofaerosolsdeliveredtotheAtlan>ccoastofMorocco.
Methods
Conclusions§ NostrongseasonalpaaerninaerosolAlconcentra>on/dustdeposi>on,buttendencytowardshigherfrequencyofdusteventsinwinter§ Mineraldustpropor>onatelycontributesleasttothebulkaerosolinAgadir,mostinDakhla§ DusteventsarecharacterisedbyaerosolsthatoriginatefromsourcesinSahara/Sahel§ Intermsofelementalra>os,Cd/Allookslikeotherloca>onsintheregion,butitwassignificantlyenrichedinsamplesfromallloca>ons&hadliale
tonomineraldustsourceduringthisstudy§ MineraldustdilutesanthropogenicTEaerosolsduringdustevents
AcknowledgementsThisworkwassupportedbytheConseilGénéralduFinistère,&theEPUREprogramme.AirmassbacktrajectoriesweresimulatedusingtheNOAAHYSPLITmodel(hap://ready.arl.noaa.gov)
References[1]Prosperoetal.,1996;[2]Kaufmanetal.,2005;[3]Carretal.,2003;[4]RudnickandGao,2003;[5]Gelado-Cabelleroetal.,2012;[6]Fombaetal.,2013;[7]Torres-Padronetal.,2002;[8]Guieuetal.,2010
Threesamplingsites100-800mfromthecoast,upwindof:
• Agadir• Laayoune• Dakhla Aerosolsampleswerecollectedusingalow-volume
aerosolsamplermanufacturedin-house:• twodownward-facing,openface,polycarbonate
filterholders(NILU)coupledtotwopumps(NOVAIRNTV0625)&twovolumetriccounters(Gallus2000)operatedinseries,interfacedwithalaptop
• windspeed(>0.9ms-1)§or-controlledinordertominimisethesamplingoflocalindustrialemissions&sampleaerosolsdeliveredfromspecificsectors(seetable)
Sectors AGADIR LAAYOUNE DAKHLAMAR 230-350° 230-10° 270-10°TER1 10-110° 20-120° 30-150°TER2 110-170° 120-220°
Theredlinepointsnorth(0°)forreference
(30.46°N,9.67°W)
(27.10°N,13.41°W)
(23.78°N,15.90°W)
Examplesofaerosolsamples:ler&middle=Dakhla,right=AgadirSamplingfromMarch2015-March2016• Agadir=44samples• Laayoune=66samples• Dakhla=56samples
Aerosolconcentra>onsfollowing:• Sequen>aldigests:(1)5:1HNO3:HF,
(2)HNO3incapped,Teflon®vialsat130°C
• Solu>onsre-dissolvedin0.48MHNO3andanalysedbyICP-MS(Thermo,Element2)
• Posi>veMatrixFactorisa>onusingEPAPMFv.5forsourceappor>onment
(1)AerosolAlconcentraRons,March2015-2016.90%ofdatafallsbelowthedashedline(2000ngAlm-3).ConcentraRonsof~100-2000ngAlm-3couldbeconsideredasaneffecRvebackgroundconcentraRon
v NostrongseasonalpaaerninaerosolAlconcentra>on,buttendencytowardshigherfrequencyofdusteventsfromDec-Feb
v 8%Alabundance[4]usedtoes>matetotaldustmassfortheyearofstudyv 3400-67000ngm-3(Agadir)v 350-26000ngm-3(Laayoune)v 205-74000ngm-3(Dahkla)
v MaximumAlconcentra>onsinthisstudyareanorderofmagnitudelowerthanobservedonislandsdownwindoftheSaharanplume,e.g.GranCanaria=26200ngm-3[5]&Sal,CapeVerde=26487ngm-3[6]
v Despitelargeinterannualvariabilityindustsupply[7],thisresultwasunan>cipated.Possibili>esthatcouldaccountforthedifference:
v A‘lowdust’yearv DustloadingscouldbehigheroffshoreasthedustistransportedintheSaharanAirLayer(SAL)atal>tudesof
1500-6000minborealsummer.However,theSALisatloweral>tudesinborealwinter,sothishypothesisseemsunlikely
v Weusedlowvolumesamplers(~1m3airfilteredh-1),[5]&[6]usedhighvolumesamplers(~1m3min-1).Lowvolumesamplersrequirelongersamplingdura>onsinordertocollectsufficientmaterialonthefilters.Thiswouldmanifestasareduc>onintheconcentra>onofAlperairvolumefilteredper>meunit,duetothelongerrun>mes
v Weweresamplingfromspecificsectors
Results&Discussion
(2)PMFoutput:factorfingerprintofaerosolsources
Ø Usedforaerosolsourceappor>onment
Ø Mineraldust
Ø =red,otherfactorsdonotnecessarilyrepresentthesamesource,butanaaempthasbeenmadetocolour-codethembasedonsimilargroupingsofelements
Ø Smallestcontribu>onfrommineraldustinAgadir,greatestinDakhla
Ø SomeTEshavelialetonomineraldustsource:Ø Agadir:V,Ni,Zn,Mo,PbØ Laayoune:P,Mo,Cd,PbØ Dakhla:P,Ni,Mo,Cd,Pb
Ø Pisaninteres>ngcase.WetendtothinkofPasbeingderivedfrommineraldust.However,contribu>onofmineraldusttoaerosolPwaslow.PcouldbeaspecialcaseinthisregionduetothepresenceofopencastPmines(Moroccois
secondlargestexporterofPintheworld)
(3)RelaRonshipbetweendustevents&elementalraRos
o Alisaproxyformineraldust
o Dustevents(peaksinAlconcentra>on)havelialeeffectonthemassra>ooflithogenicelementstoAl(e.g.Ti/Al),sugges>ngmineraldustismainsourceoftheseelements
o Dusteventsreducethera>oofanthropogenicelementstoAl(e.g.Cd),sugges>ngtheseelementshavelowerrela>veabundancesinmineraldustthanindustrialemissions
o Onlowdustdays(lowAl)therewerepeaksinra>oofanthropogenicelementstoAl
(4)EnrichmentFactors
q NormalisedtoAl
q Onlyenrichment>10consideredsignificant(markedbyhorizontalline)
q Lithogenicelementsnotsignificantlyenriched(toprow)
q GenerallyhighestenrichmentinAgadirsamples
q PmostenrichedinLaayounesamples
q Cdenrichedatallloca>onsbut…q Cd/Al(mass/mass)atall
sta>ons(Agadir=2.1x10-5-2.7x10-4;Laayoune=8.4x10-6-2.9x10-4;Dakhla=3.0x10-6-2.0x10-4)fallclosetotherangeobservedatnearby
loca>ons(e.g.,CapSpartel=4.6x10-5-2.3x10-4[8]&GranCanaria=4.6x10-5–1.84x10-4[5]q SametrendseenforotheranthropogenicTEs,e.g.Pb/Al
Pumps
Weathervane
Aerosolsampler Filterholder
Filterholder
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