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Theimportanceofcharge-transferinteractionsindeterminingchromophoricdissolvedorganicmatter(CDOM)opticalandphotochemicalpropertiesCharlesM.Sharpless*aandNeilV.Blough*bAbsorptionofsunlightbychromophoricdissolvednaturalorganicmatter(CDOM)isenvironmentallysignificantbecauseitcontrolsphoticzonedepthandcausesphotochemistrythataffectselementalcyclingandcontaminantfate.Boththeoptics(absorbanceandfluorescence)andphotochemistryofCDOMdisplayunusualpropertiesthatcannoteasilybeascribedtoasuperpositionofindividualchromophores.Theseinclude(i)broad,unstructuredabsorbancethatdecreasesmonotonicallywellintothevisibleandnearIR,(ii)fluorescenceemissionspectrathatallfallintoasingleenveloperegardlessoftheexcitationwavelength,and(iii)photobleachingandphotochemicalquantumyieldsthatdecreasemonotonicallywithincreasingwavelength.Incontrasttoasimplesuperpositionmodel,thesephenomenaandotherscanbereasonablywellexplainedbyaphysicalmodelinwhichcharge-transferinteractionsbetweenelectrondonatingandacceptingchromophoreswithintheCDOMcontroltheopticalandphotophysicalproperties.ThisreviewsummarizescurrentunderstandingoftheprocessesunderlyingCDOMphotophysicsandphotochemistryaswellastheirphysicalbasis.EnvironmentalimpactTheuseofspectroscopictechniquestomonitorchromophoricdissolvedorganicmatter(CDOM)transportandfatehasrisensteadily,buoyedbyrelationshipsbetweenCDOMsourceandopticalproperties.ResearchalsoshowsthatCDOMphotochemicalreactivitydependsstronglyonitsopticalproperties.WhiletheseempiricalrelationshipsbetweenCDOMsource,optical,andphotochemicalpropertiesrepresentpotentiallyusefultoolsforstudyingandpredictingratesandprocessesofCDOMtransformationandphotochemicalreactions,theirunderlyingphysicalbasisisnotrmlyestablished.Suchknowledgewouldimproveinterpretationsbasedontheempiricalrelationshipsandprovideafoundationonwhichtoreneandexpandthem.Thisreviewsummarizescurrentunder-standingofhowCDOMcompositioncontrolsitsopticalandphotochemicalpropertiesandindicatesareasforfurtherresearch.1.IntroductionChromophoricdissolvedorganicmatter(CDOM)isthelight-absorbingfractionofdissolvedorganicmatter.Itcanoriginatefrombothaquaticmicrobialsourcesandterrestrialsourcessuchasligninthathavedegradedanddissolved;forpurposesofthisreview,werefertothesematerialsasmicrobialandterrestrial,respectively.AbsorptionofsunlightbyCDOMcontrolsphoticzonedepths1–6andinitiatesavarietyofphotochemicalreactionsthatconvertCDOMtoinorganiccarbon6–14andbiologically-available,lowmolecularweightcompounds,5,15–18affecttrace-metalredoxspeciation,7,19–21inuencethefateofaquaticcontaminants,22–29andinsomeinstancescancontributesignif-icantlytoaquaticoxygendemand.30–33TheopticalandphotochemicalpropertiesofCDOMvarywithseason,CDOMsource,andwaterchemistry.2,34–41Anoutstandingchallengeforenvironmentalscientistsistounderstandthesevariationsandpredicttheirimpactonelementalcyclingandthefateofanthropogeniccompounds.ThecolorofCDOMcomespredominantlyfromhumicsubstances(HS),whichcomprisehumicacids(HA,solubleabovepH2)andfulvicacids(FA,solubleatallpHvalues).42AbatteryofanalyticaltechniqueshasbeenemployedtogainstructuralinformationaboutCDOMincludingpyrolysis-massspectrometry,43–45high-andultrahigh-resolutionmassspectroscopy,46–5013Cnuclearmagneticresonancespectros-copy,48,51,52electronspinresonancespectroscopy,53,54acidi-metrictitrations,55–57andelectrochemistry.58,59ThesestudiesindicatethatCDOMcontainsavarietyofstructuresincludingcarboxylicacidsandcarboxyl-richalicyclicmolecules,substitutedphenols,ketones,aldehydes,quinones,carbo-hydrates,saturatedandunsaturatedhydrocarbons,andnitrogenousmaterial.60,61aDepartmentofChemistry,UniversityofMaryWashington,Fredericksburg,VA22401,USA.E-mail:csharple@umw.edu;Fax:+15406541405;Tel:+15406541405bDepartmentofChemistry,UniversityofMaryland,CollegePark,MD20742,USA.E-mail:neilb@umd.edu;Fax:+13013149121;Tel:+13014050051Citethis:Environ.Sci.:ProcessesImpacts,2014,16,654Received31stOctober2013Accepted30thJanuary2014DOI:10.1039/c3em00573arsc.li/process-impacts654|Environ.Sci.:ProcessesImpacts,2014,16,654–671Thisjournalis©TheRoyalSocietyofChemistry2014EnvironmentalScienceProcesses&ImpactsCRITICALREVIEWPublishedon07February2014.DownloadedbyPekingUniversityon7/21/20183:40:08AM.ViewArticleOnlineViewJournal|ViewIssueAlloftheevidencesuggeststhatCDOMabsorptionandphotochemistryoriginatesfromaromaticchromophores.However,thecomplexityofthemixturemakesitexceedinglydifficulttoidentifytheprecisestructuresthatproducethesephenomena.Furthermore,CDOMexhibitsunusualopticalandphotochemicalpropertiesthatareinconsistentwiththebehaviorofindividualchromophoresoreventhebehaviorofacollectionofindependentchromophores.Thisreviewsumma-rizesthesepropertiesandcurrentunderstandingoftheirmolecularbasis.Weadvancetheideathatmuchoftheobservedbehaviorcanbeexplainedbyelectronicinteractionsbetweenchromophores,whichproduceanarrayofphotophysicalandphotochemicalreactionchannels.WefurtherarguethattherelativeimportanceofeachchannelvarieswithCDOMsourceandenvironmentalconditionsduetovaryingabundanceofelectrondonatingandacceptinggroupsandt
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