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Envlron.Sci.Technol.1985,Grosjean,D.;Fung,K.Anal.Chem.1982,54,1221.(20)Kuwata,K.;Urebori,M.;Yamasaki,Y.J.Chromatogr.Sei.1979,17,264.Tuazon,E.C.;Graham,R.A.;Winer,A.M.;Easton,R.R.;(21)Pitts,J.N.,Jr.;Hanst,P.L.Atoms.Environ.1979,12,865.Matthews,T.G.;Howell,T.C.Anal.Chem.1982,54,1495.(22)Beasley,R.K.;Hoffmann,C.E.;Rueppel,M.L.;Worby,J.W.Anal.Chem.1980,52,1110.Kim,W.S.;Geraci,C.L.;Kupel,R.E.Am.Ind.Hyg.Assoc.(23)J.1980,41,334.(24)Anderson,G.;Anderson,K.;Nilsson,C.A.;Levin,J.0.;19.74-81Rodriguez,S.T.;Olson,P.B.;Lund,V.R.“ColorimetricAnalysisofFormaldehydeCollectedonaDiffusionMonitor”;3MCo.:St.Paul,MN,1981.Kuwata,K.;Urebori,M.;Yamasaki,H.;Kuge,Y.;Kiso,Y.Anal.Chem.1983,55,2013.ThermoElectronCorp.“ProductLiteratureforThermo-sorb/NAirSamplingCartridges”;ThermoElectronCorp.:Waltham,MA,1983.Lipari,F.Anal.Chem.1984,56,1820.Hodgson,A.T.;Geisling,K.L.;Girman,J.R.;Remijn,B.“ValidationofaPassiveSamplerforDeterminingForm-aldehydeinResidentialAir”.paperpresentedattheThirdAnnualEPANationalSymposiumonRecentAdvancesinPollutantMonitoringofAmbientAirandStationarySources,Raleigh,NC,May3-6,1983.Chemosphere1979,10,823.Kennedy,E.R.;Hill,R.H.,Jr.Anal.Chem.1982,54,1739.DuPont“Pro-TekColorimetricAirMonitoringBadgeSystemLaboratoryValidationReport,FormaldehydeBadge,Series11,TypeC-60”,E.I.duPontdeNemours&Co.:Wilmington,DE,1981.AirQualityResearchInc.“ProductLiteratureforPF-1PassiveFormaldehydeMonitor”;AirQualityResearch,Inc.:Berkeley,CA,1982.ReceivedforreviewMarch12,1984.RevisedmanuscriptreceivedAugust2,1984.AcceptedAugust21,1984.Thispaperwaspresentedatthe187thNationalMeetingoftheAmericanChemicalSociety,St.Louis,MO,April8-13,1984.PhotosensitizedTransformationsInvolvingElectronicEnergyTransferinNaturalWaters:RoleofHumicSubstancesRlchard0.Zepp,”PatriciaF.Schiotzhauer,andR.MerrittSlnkEnvironmentalResearchLaboratory,US.EnvironmentalProtectionAgency,Athens,Georgia30613Studiesarereportedofthekineticsofphotosensitizedreactionsinvolvingthetransferofabsorbedlightenergyfromhumicsubstancestovarioustraceorganicchemicalsinwater.Thephotoisomerizationofl,&pentadieneandthephotooxygenationof2,5-dimethylfuranwereusedtoprobethenatureandconcentrationsoftheexcitedspeciesthatmediatehumus-sensitizedphotoreactionsofaquaticpollutants.Evidenceispresentedthatthephotosensitizedreactionsofpentadieneanddimethylfurandonotinvolvebindingofthechemicalsbythehumicsubstances.Kineticresultsindicatethatthekeystepsinbothphotoreactionsinvolvethetransferofelectronicenergyfromtripletstatesofthehumicsubstances.Uptohalfofthesetripletsareestimatedtohaveenergiesofatleast250kJ/mol,suffi-cientlyhightotransferenergytopolycyclicaromatichy-drocarbons,nitroaromaticcompounds,conjugateddienes,andothertypesofchemicals.Steady-stateconcentrationsofthehumus-derivedtripletsareestimatedtotypicallyfallinthe10-15-10-13Mrangeintheupperlayersofsun-light-irradiatednaturalwaters.TheconcentrationsareproportionaltoUVabsorptioncoefficientsofthewater.Actionspectraforthephotosensitizedreactionsindicatethatsolarultravioletradiationismostimportantinin-ducingthereactions.Resultsofthestudyareusedtoestimatemaximumratesofreactionsphotosensitizedbyhumicsubstances.IntroductionAssunlightpenetratesdownintofreshwaterandcoastalmarinewaters,thegreatbulkoftheradiationisabsorbedbynaturaldissolvedorparticulatesubstances.Initialstudiesoftheinfluenceofthesenaturalsubstancesonratesofphotochemicalandphotobiologicalprocessesinaquaticenvironmentshavefocusedonlightattenuationeffects(1,2).Resultsofthesestudieshavebeenusedtopredictratesofpollutantphotoreactionsthatareinitiatedthroughdirectabsorptionofsunlightbythepollutant(1).Anumberofrecentinvestigationshaveshownthattheinfluenceofnaturalsubstancesonphotoreactionsinfreshwatersandseawaterisnotlimitedtolightattenuation(3-13).Sunlight-inducedreactionsinvolvingfreeradicalsmaybeinitiatedthroughphotolysisofnaturalinorganicconstituentssuchasnitrite(3)andhydrogenperoxide(4,5).Moreover,asignificantportionofthesolarradiationabsorbedbyfreshwaterhumicsubstancesresultsinfor-mationofelectronicallyexcitedmoleculesthatarecapableofparticipatinginavarietyofreactionswithaquaticpollutants(8-15).Thesephotosensitizedreactionscangreatlyacceleratethelight-inducedtransformationoftracechemicalsinnaturalwaters,insomecasesresultinginrapidphotoreactionofcompoundsthatarestabletosun-lightindistilledwater(6-13).Humicsubstanceshavebeenshowntosensitizeoxy-genationsandotherphotoreactionsoforganicchemicalsinvolvingelectronicenergytransfer(9),butthereisadearthofinformationconcerningfactorsthatinfluencetheratesoftheseprocessesinaquaticenvironments.Inthispaper,humus-sensitizedphotoreactionsoftwoorganicchemicals,2,5-dimethylfuranandl,&pentadiene,areusedtoexplorethenatureandconcentrationsoftheexcitedspeciesthatareinvolvedintherate-determiningstepsoftheenergy-transferprocesses.Resultsofthisstudyareusedtodefinetypesofpollutantsthatmaybesusceptibletophotosensitizedreactionsinvolvingenergytransferandtoestimatemaximumratesofthesephotoreactionsinnaturalwaters.Asusedinthispaper,theterms“humicsubstances”or“humus”refertothecoloredorgan