DOM的光化学转化

ChemicalandopticalphototransformationofdissolvedorganicmatterStevenLoisellea,b,*,DavideVionec,d,ClaudioMineroc,ValterMaurinoc,AntonioTognazzia,b,ArduinoM.Dattiloa,b,ClaudioRossia,b,LucaBracchinia,baDip.FarmacoChimicoTecnologico,Universita`degliStudidiSiena,ViaAldoMoro2,53100Siena,ItalybCSGI(Inter-UniversityCenterforColloidandSurfaceScience),ViadellaLastruccia3,50019SestoFiorentino(FI),ItalycDipartimentodiChimicaAnalitica,Universita`degliStudidiTorino,ViaPietroGiuria5,10125Torino,Italy1dCentroInterdipartimentaleNatRisk,Universita`degliStudidiTorino,ViaLeonardodaVinci44,10095Grugliasco(TO),Italy2articleinfoArticlehistory:Received19December2011Receivedinrevisedform24February2012Accepted29February2012Availableonline23March2012Keywords:PhotochemistryPhotogeneratedtransientsDirectandsensitizedphotolysisAbsorptionspectralslopePhotobleachingabstractDissolvedorganicmatterrepresentsthemainreservoiroforganiccarboninmostaquaticecosystems.Inthepresentstudy,wedeterminedtheopticalchangesandthequantumyieldsoftransientspeciesformationforchromophoricdissolvedorganicmatter(CDOM)samplesundergoingphotodegradation.Theresultsshowthatthetripletstates3CDOM*arepotentiallykeyplayersinCDOMphotodegradationandthatsuchtransformationsarestronglyinfluencedbysmalldifferencesinCDOMsourcesandsinks.Incontrast,
OHradicalsareveryunlikelytoplayakeyroleinphototransformation.Theseresultsrepresentanimportantfirststepincombiningopticalandtransientspeciesanalysestounderstandphotodegradationprocessesofdissolvedorganicmatter.ª2012ElsevierLtd.Allrightsreserved.1.IntroductionDissolvedorganicmatter(DOM)isacomplexmixtureoforganiccompoundswithspatialandtemporaldistributionsthatresultfrommultiplesourcesandsinks(Tranviketal.,2009;Bracchinietal.,2011).DOMsourcesinmostecosys-temscanbedistinguishedasautochthonous(producedinsitubybiologicalactivityfromaquaticmacrophytes,phyto-plankton,zooplankton,bacterioplankton,.)orallochtho-nous(ofterrestrialorigin,usuallyoriginatingfromthedegradationofterrestrialorganicmatter).ThesetwoDOMpoolsdifferintheiropticalandchemicalcharacteristics(SulzbergerandDurisch-Kaiser,2009;Zhangetal.,2009).DominantsinksofDOMincludedirectphotodegradation,reactionwithphotogeneratedtransientspecies,biologicaldegradationandflocculation(Moranetal.,2000;Chenetal.,2007).TheimportanceofeachsinkwilldependonthelocalconditionsaswellasthechemicalandopticalcharacteristicsoftheDOMpresent.Solarultravioletradiationcandirectlymodifypropertiesofthechromophoricfractionofdissolvedorganicmatter(CDOM)throughphotolysis.However,indirect*Correspondingauthor.Dip.FarmacoChimicoTecnologico,Universita`degliStudidiSiena,ViaAldoMoro2,53100Siena,Italy.Tel.:þ390577234360;fax:þ390577234177.E-mailaddress:[e-mail](S.Loiselle).1:(2012)3197e32070043-1354/$eseefrontmatterª2012ElsevierLtd.Allrightsreserved.doi:10.1016/j.watres.2012.02.047phototransformationmayalsoplayanimportantroleinalteringCDOMcomposition(Vioneetal.,2009a,2010a).Thetermindirectphotodegradationhasbeenusedwithtwodifferentmeaningsinthescientificliterature.Oneconcernsthelossinabsorptionatwavelengthsthatarenotpresentintheincidentirradiance(DelVecchioandBlough,2004).Theotherislinkedtotheformationandsubsequentreactivityoftransientspeciessuchashydroxylradicals(
OH),singletoxygen(1O2)andtripletexcitedstatesofdissolvedorganicmatter(3CDOM*)thataregeneratedduringirradiationofCDOM(Boreenetal.,2008).ThefactthatsomeormostofthereactivespeciesthatcouldbeinvolvedinthetransformationofDOM/CDOMarephotochemicallygeneratedbyCDOMitself,suggeststhatCDOMisatthesametimeaproducerandascavengerofreactivephototransients(Hoigne´,1990;CanonicaandFreiburghaus,2001;AlHousarietal.,2010;Minellaetal.,2011).FewattemptshavebeenmadesofartoassociatetheproductionoftransientspecieswithopticalchangesinCDOM.Inthepresentstudy,wecomparedchangesinCDOMopticalpropertiestotheproductionoftransientspeciesincontrolledphotodegradationexperiments.RatesoftransientformationandratesofphotodegradationwerecomparedtoidentifythetransientspeciesthatdominateCDOMdynamicsinasmallcoastallake.2.Experimentalsection2.1.SamplingSamplingandmeasurementsweremadeinFoglianoLakeinMay2009(Fig.1).Thelakeisasmallcoastallake(4km2)intheNationalParkofCirceoincentralItaly.Twosamplingsites(F1andF2)werechosentoexaminetheCDOMvariabilityduetosmalldifferencesinsourcesandsinks.TheF1sitewaslocatedinthecenterofthelake,200mfromthetidalseagatethatconnectsthelaketotheTyrrhenianSea.TheF2sitewaslocatedinthenortheastpartofthelakewhichisborderedbyagricultureareasbuthasnodirectsurfacefreshwaterinputs.Theaveragedepthofthelakeisaround1mandthelakebottomischaracterizedbyextensiveareasofsubmergedmacrophytes(RuppiamaritimaandZoosteranoltii),particularlyevidentinthenortheastpartofthelakeduringthesummer(Signorinietal.,2008).ThespatialdistributionofverticallightattenuationshowsthatthewateroftheF2areahasahigherUVattenuation(Bracchinietal.,2005).Hydrodynamicmo