Comparative Measurements Of CO2 flux Over A Forest Using Clo

专业文献

COMPARATIVEMEASUREMENTSOFCO2FLUXOVERAFOREST

USINGCLOSED-PATHANDOPEN-PATHCO2ANALYSERS

YUKIOYASUDA andTSUTOMUWATANABE

ForestEnvironmentDivision,ForestryandForestProductsResearchInstitute,P.O.Box16,

Tsukuba-Norin,Ibaraki305-8687,Japan

(Receivedin nalform18December2000)

Abstract.Aneddycovariancesystemusingaclosed-pathCO2analyserwasconstructedforlong-termCO2 uxmeasurementsaboveaforest,anditstotalfrequencyresponsewasevaluatedexperimentally.Theamplitudeandphaseresponsesofthesystemwereexaminedthroughapre-liminarytest,inwhichaprescribedpatternofCO2 uctuationwasinputtothesystem.Theresultshowedthattheamplitudeoftheoutputfromthesystemwasattenuatedasfrequencyincreased,withahalf-powerfrequencyof0.3Hz.Thephasewasdelayedbytheairsamplingthroughalongtube,butthedelayinphasedecreasedasfrequencyincreased.Wethenpresentedanewtechniqueforthecorrectionof uxlossduetotheinadequatesystemresponsefortheeddycovariancemeasurementsofCO2 http://www.77cn.com.cningthepresentsystemandthecorrectiontechnique,diurnalvariationsinCO2 uxweremeasuredoveratemperatedeciduousforestonthreedaysin1997.TheresultswerecomparedwiththeCO2 uxesmeasuredwithafastresponseopen-pathgasanalyser.TheCO2 uxesfromtheclosed-pathsystemagreedwiththosefromtheopen-pathsystemaftertheWebb,PearmanandLeuningcorrectionwasmadeforthelatter.Inthepresenttestoveraforest,thecontributionofthefrequency-responsecorrectiontotheCO2 uxeswassmallanditsaveragedpercentagewasonly3%inthedaytime.However,thepercentagewouldlikelyincrease,ifthesystemwereappliedtoashortervegetationsitewherehighfrequencycomponentsaremoreimportant.Thecomparisoncon rmedthatwecanobtaincorrectmeasurementsofCO2 uxusingthepresentclosed-pathsystemandthecorrectiontechnique.

Keywords:Closed-pathanalyser,CO2 ux,Eddycovariancemethod,Forest,Frequencyresponse,Open-pathanalyser.

1.Introduction

EddycovariancemeasurementsofCO2 uxhavebeenmadeeversinceafastresponseCO2analyserwasdeveloped(Ohtaki,1982).ThefastresponseCO2ana-lyserhasanopenopticalpathformeasuringCO2concentration.Theopen-pathCO2analysercandetectsmallandfast uctuationsinCO2concentration,whichisadesirablecharacteristicfortheeddycovariancemethod.However,someoftheopen-pathanalysersavailableonthemarketstillhavedisadvantages.Theycannotbeexposedtorainbecauseoftheirimperfectwaterproo ng,andneedextens-ivemaintenanceforconstantoperation.Forlong-termmeasurements,open-pathanalysersareoftenunreliable.

E-mail:

yassan@ffpri.affrc.go.jp

Boundary-LayerMeteorology100:191–208,2001.

©2001KluwerAcademicPublishers.PrintedintheNetherlands.

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192YUKIOYASUDAANDTSUTOMUWATANABE

Aclosed-pathCO2analyser,ontheotherhand,isanordinaryCO2analyserthathasclosedopticalpathsformeasuringCO2concentration.Manyresearchershaveconductededdycovariancemeasurementsusingclosed-pathanalysers(Fanetal.,1990;Shurpalietal.,1995;Gouldenetal.,1996;Valentinietal.,1996;Malhietal.,1998andothers).Advantagesofusingaclosed-pathanalyserincludethestabilityandaccuracyofmeasurement,thesimplicityofoperation,andtheconvenienceofperiodiccalibrationusingstandardgases.Withtheseadvantages,aclosed-pathCO2analyserissuitableforlong-termmeasurements.However,aclosed-pathCO2analyseralsohasdisadvantages.Tomakeacontinuousmeas-urementoftheatmosphericCO2withaclosed-pathanalyser,wehavetodrawthesampleairusinganairpumpintoasamplecellintheanalyserthroughasamplingtube.Thissamplingprocesscausesattenuationof uctuationsinCO2concentration,whichthenleadstotheunderestimationofCO2 ux.Thedegreeoftheattenuationdependsonthreemainfactors:thetubelength,thetubediameterandthestateofair ow(laminarorturbulent)withinthesamplingtube(Leeetal.,1996).Theattenuationof uctuationsinCO2isalsocausedbytheslowresponseoftheanalyserandthebuffereffectofthesamplecellintheanalyser.LeuningandMoncrieff(1990)andSuykerandVerma(1993)representedtheseattenuationprocessesastubetransferfunctionsandshowedtechniquesforthecorrectionof uxlosscausedbytheattenuationprocesses.Inadditiontotheprocessesofthetubetransfer,LeuningandJudd(1996)discussedtheeffectsofelectronicband-widthlimitationsand lteringdueto owthroughthesamplecell.Theircorrectiontechniqueshavebeensuccessfullyusedinsome uxmeasurements(Shurpalietal.,1995;Valentinietal.1996;Malhietal.,1998).Moncrieffetal.(1997)introducedasystemforeddycovariancemeasurementswithaclosed-pathanalyserandsum-marizedthecorrectiontechniques.Withthecorrections,theclosed-pathanalysercanbeused,inprinciple,forlong-termmeasurementsofCO2 ux.

Inthispaper,wedescribeasystemforeddycovariancemeasurementsusingaclosed-pathanalyserdesignedforlong-termstudies,andreporttheperformanceofthesystemtestedagainstanopen-pathsystem.Althoughseveralresearchershaveconductedsimilartestsovershortvegetation(LeuningandMoncrieff,1990;LeuningandKing,1992;SuykerandVerma,1993),weconductedthetestsoveraforestandobtainedfurtherinformationaboutthetechniquesofmeasuringandcomputingCO2 ux.

2.Theory

Inthissection,wesummarisethetheoryfordeterminingCO2 uxbytheeddycovariancemethod.Ingeneral,wehavetoincludesomecorrectionsforeliminatingthedensityeffectsfromsensibleheatandwatervapourtransferinthecalculations(Webbetal.,1980).ThetechniquesofthecorrectionsaredifferentdependingonthetypeofinstrumentsusedformeasuringCO2density.

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COMPARATIVEMEASUREMENTSOFCLOSED-ANDOPEN-PATHCO2FLUXES193

2.1.1.Open-PathCO2Analyser

Foreddycovariancemeasurements,thetotal uxofCO2(Fc)canberepresentedas

¯aw ξc ,Fc=ρ(1)

whereρaisthedryairdensity,wistheverticalwindvelocity,andξcisthemixingratioofCO2relativetothedryair(Webbetal.,1980).Theoverbarandtheprimedenoteatime-averagedvalueand uctuationfromtheaverage,respectively.How-ever,theopen-pathCO2analysermeasuresthedensityofCO2(ρc)ratherthanthemixingratio.TheCO2 uxhastobecalculatedinadifferentmanneraccordingly.ThecorrectedCO2 ux,Fcc,iscalculatedas(Webbetal.,1980)

+µ(ρ +(1+µσ)(ρ¯)w T ,¯c/ρ¯a)w ρv¯c/TFcc=w ρc(2)

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