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1Longopen-path,TDLbasedsystemformonitoringmethanebackgroundconcentrationfordeploymentatJungfraujochHighAltitudeResearchStation-SwitzerlandValentinSimeonov,HubertvandenBergh,andMarcParlangeEPFL-ENAC-EFLUMStation2,CH1015Lausanne,Switzerland,Tel.
+41(0)216936185,Fax.
+41(0)216936390email(valentin.
simeonov@epfl.
ch)Anew,longopen-pathinstrumentformonitoringofthebackgroundmethaneconcentrationpath-averagedover1000mwillbepresented.
Theinstrumentallowsmonitoringofwatervaporconcentrationaswell.
Theinstrumentisbuiltonthemonostaticscheme(transceiver–distantretroreflector).
AVCSELtunablediodelaser(TDL)withacentralwavelengthof1654nmisusedasalightsource.
Thereceiverisbuiltarounda20cmNewtoniantelescope.
Toavoiddistortionsintheshapeofamethaneline,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
FastInGaAsphotodiodesanda200MHz,14bitADCareusedtoachievethisscanningrate.
Theexpectedconcentrationresolutionfortheabovementionedpath-lengthsisoftheorderof1ppbwithaccuracybetterthan5%.
TheinstrumentisdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)SwitzerlandandwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbeinstalledin2012attheHighAltitudeResearchStationJungfraujoch(HARSJ).
TheHARSJislocatedat3580mASLandisoneofthe24globalGAWstationsandcarriesoncontinuousobservationsofanumberoftracegasses,includingmethane.
Oneofthegoalsoftheprojectistocomparepath-averagedtoongoingpointmeasurementsofmethaneinordertoidentifypossibleinfluenceofthestation.
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
1.
IntroductionAnumberofgasesareinvolvedintheanthropogenicenhancementofthegreenhouseeffect.
Themostimportantofthesegreenhousegases(GHGs)arecarbondioxide(CO2);methane(CH4);nitrousoxide(N2O);halocarbons(HC);andtroposphericozone(O3).
Methanehasaspecialplaceamongthesegasessinceitsglobalwarmingpotentialis25timesoreven72timeslargerthanthepotentialofCO2in100yr.
and20yr.
timehorizonsrespectively[1].
Besidethedirectradiativeforcing(RF),estimatedto+0.
48Wm-2methanehasimportantindirecteffectbecauseofitschemicalreactivityresultinginatotalRFof+0.
7Wm-2.
TheindirectradiativeeffectsofmethaneresultmostlyfromreactionwithatmosphericOH.
Thisreaction:reducestheOHconcentration,leadingviaapositivefeedbacktoanenhancementofmethaneandsomeHClifetimes;producesadditionalCO2;enhancesstratosphericwatervapor;andincreasestroposphericozoneconcentrationthroughthemethaneproductionchain.
BecauseofconcentrationslowerthanthoseofCO2,atpresentmethanehasthesecond-largestradiativeforcing(RF)effectafterCO2.
Methaneisemittedbynaturalandanthropogenicsourceswithanthropogenicsourcesaccountingfor60%ofthetotalCH4budgetatpresent.
Themainnaturalsourcesarewetlands,termites,oceans,geologicalsources,methanehydrates,andwildfire.
Themostimportantanthropogenicsourcesareenergyproduction,(includingmining,fossilfuelproduction,distributionanduse),riceagriculture,ruminantanimals,landfills,wastetreatmentandbiomassburning.
ThemainmethanesinksaretheoxidationbytroposphericOH,oxidationinsoilsandlosstostratosphere.
Methaneconcentrationshaveincreasedalmost2.
5timessince1750reaching1774ppbin2005.
Systematicmeasurementsoverthelast25yearsshowa30%increaseinmethaneconcentrationsduringthatperiod.
Thisincreaseisnotmonotonic,withgreaterthan1%peryeargrowthratesinthelate1970andearly1980,slowingdowntozeroornegativevaluesduring1999-2005withsubstantialinterannualvariationsintheperiod1988-2005.
Recentdatashowrenewedgrowthofatmosphericmethane[2]duringlastyears.
ThereasonsforthedecreaseandvariabilityintheCH4growthrateandtheimplicationsforfuturechangesarenotunderstoodalthoughanumberofhypothesesweresuggested[1].
Inresponsetoanincreaseinglobalaveragetemperature,largequantitiesofmethanecanbereleasedinrelativelyshorttimescalesfromgeologicalstorage,suchasmethanehydratesandpermafrostandthroughbiogenicprocesses.
Theglobaltemperatureincreasecancausesignificantincreaseinmethaneconcentrationssince70%oftheatmosphericmethaneoriginatesfrombiogenicsources,whicharehighlysensitivetoclimatevariables.
Recentstudieshaveshownthatthepermafrostcontainsapprox.
500-900Gtcarbon,upto30%ofwhichcanbeconvertedinmethanebymicroorganismsduringthawing[3,4,5].
Inresponsetoclimatewarming,permafrosthasalreadybeguntothaw,withextremeprojectionsthatbytheendofthecenturyitwillhavethawedalmostcompletely.
Climatechangesalsoaffectthestabilityofmethanehydratesbeneaththeoceanwhere~4Ttarestored[6].
ThelatestobservationsoftheSiberianArcticshelfsuggestthat900Gtmethanearestoredthereinmethanehydratedepositsandasafreegasbelowthehydratedeposits.
Thereishighprobabilitythat50Gtmethaneofthisstoragecanbereleasedabruptlyatanytimedueto2thechangesinseawatertemperatures,relatedtothemeltingofArcticice,orasaresultofgeologicalevents[7,8,9].
DatafromSeptember2005showhigh(12000%)CH4supersaturationofsurfacewater,andhigh(upto8ppm)CH4concentrationintheatmosphericlayerabovetheseasurfaceovertheEastSiberianShelf[7].
Currentlyseveralnetworksandgovernmentalorganizationssystematicallymeasuremethaneconcentrationinsurfaceair[1].
NOAA/GMDisthemostgeographicallyextensivenetworkoperating40surfaceairflask-samplingsitesandacquiringdataalmostweeklysince1983[10].
GAGE/AGAGEnetworkoperates5+3sitesequippedwithautomaticsystemswithsamplingratesupto36samples/24hsincelate1980[11].
Allnetworksuseexclusivelythegaschromatograph(GC)techniquewithaflameionizationdetector(FID)formeasuringCH4concentration.
Withapropercalibration,thistechniquecansupplymeasurementswithrelativelygoodsensitivityandprecisionof0.
2-1%.
Sincethemeasurementprocessdependsonanumberofexternalparameters,toachievesufficientcorrelationamongmeasurementstakenwithdifferentinstrumentsandbetweenconsecutiverunstakenwiththesameinstrument,theGCrequirescalibrationwithaprecisestandardgasmixturebeforeeverysamplemeasurement.
Driftsandshiftsinthestandardscale[12]mayrequirereassessmentofthewholedataseries.
Inaddition,GCinstrumentshaverelativelyhighinitialpriceandoperationalcostsbecauseoftheneedofexpensiveconsumables(highandvery-highpuritygases),theneedofspecialenvironment(housing),andregularmaintenancebyqualifiedpersonnel.
Sincethesamplingisdoneatafixedpoint,themeasurementscanbeeasilyalteredbysampletakingandcanbeaffectedbylocalsources,sinks,orlocaltransport,particularlyinpoorlymixedatmospheresasinthecaseofwetlandswhereconcentrationcanvarybyordersofmagnitudefordistancesofseveralmeters.
Therefore,pointmeasurementslackthespatialrepresentativenessneededformodelingpurposes.
BecauseofallthesefactorsGCarenotsuitablefortheenlargementoftheexistingCH4networksasenvisagedintheGAW2008-2015strategicplanespeciallyinArcticandtropicalregions.
Infrared(IR)spectralanalysistechniquessuchasFourierTransformInfrared(FTIR)orTunableLaserAbsorptionSpectroscopy(TLAS)arewidelyusedforaccuratequantitativemeasurementsoftracegasconcentrations[13].
TheconcentrationisderivedfromIRabsorptionmeasuredoveranopticalpath.
Thesensitivityandtheprecisionofthesemeasurementsdependstronglyonthepath-length.
Toachievesufficientsensitivityandprecisionfortraceconcentrationmeasurements,thepathlengthisextendedeitherbyusingmultiple-pathcellsormeasuringoverlong-pathinopenair(referredtohereinas"open-path"orOP).
Themultiple-passtechniqueallowscompactinstrumentaldesignandrelativelyeasycalibrationbutbeingapointmeasurementtechniquesuffersfromthesamedrawbacksastheGCtechniquementionedabove.
IntheOPtechniquetheconcentrationmeasurementsareaveragedoveranextendedpath,andthereforearemuchlessaffectedbylocalunrepresentativefluctuationsingasconcentrationthanmeasurementstakenwithpointsensors.
Thepassiveopen-pathFTIRmethodusestheSunasalightsourceandhasbeenusedtoproducecontinuous,longtimeseriesofhighquality,totalcolumnGHGsdata[14].
TheFTIRmethod,however,islimitedtoonlydaytimeandclearweatherconditions.
TheactiveOP-FTIRcanbeoperatedaroundtheclockbuttheachievableabsorptionpathsareusuallyshort(<1000m)becauseoftheuseofnon-coherentlightsources,whichleadstolowsensitivity.
BothOP-FTIRmethodsrequiresignificantresourcesandexpertknowledgetoensureproperdeployment,operation,andfinaldataproduction.
Furthermore,FTIRinstrumentsaredelicateandhavemovingparts,whichmakesthemdifficulttodeployinfieldconditions.
Theadvancesinsemiconductorlaserscienceandtechnologyhasmadeavailabletunable,sourcesforreal-timeTLASmonitoringofalargenumberofmolecularspeciesintheIR.
TunableDiodelaser(TDL)andQuantumCascadeLaser(QCL)basedpointmeasurementsofanumberoftracegasesincludingCO2,CH4,O3N2Oandotherhavebeensuccessfullydemonstratedinrecentyears[15,16,17,18,19,20]andcommercialinstruments[21,22,23]havebecomeavailablerecently.
Themajorityoftheseexperimentsandmostcommercialinstrumentsarehowever,designedforpointobservations.
SomesuccessfulOPexperimentshavebeencarriedoutinthelate1970,butbecauseofthelackofsuitablelasersources,thesetypemeasurementsdidnotfindwideapplications.
Recentlywiththeappearanceofnewlasersources,OPmeasurementsofmethanehavebeenreportedandcommercialinstruments[24]areavailable.
Theseinstrumentshoweveraredesignedmostlyfordetectinggasleaksormeasuringhigherthanbackgroundconcentrations.
Inthispaperwedescribeanew,TDLbasedOPinstrumentformonitoringofthebackgroundmethaneconcentration.
TheinstrumentisdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)SwitzerlandandwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbeinstalledin2012attheHighAltitudeResearchStationJungfraujoch(HARSJ).
TheHARSJislocatedat3580mASLandisoneofthe24globalGAWstationsandcarriesoncontinuousobservationsofanumberoftracegasses,includingmethane.
Oneofthegoalsoftheprojectistocomparepath-averagedtoongoingpointmeasurementsofmethaneinordertoidentifypossibleinfluenceofthestation.
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
2.
TheoryofoperationTheOPTDLtechniqueemploystheabsorptionspectroscopyprincipletoobtainspeciesconcentration.
TheconcentrationCisderivedfromthemeasuredoverthesamplelengthLlighttransmittance)(νTasLTC)()(lnνσν=3where)(νσiswavelength)(νdependantabsorptioncrosssection,specificforthedetectedsubstance.
Thetransmittanceismeasuredbysweepingrepeatedlythelaserwavelengthacrossanabsorptionlineofthespeciesbeingdetected.
Thetransmittancemeasuredoutsideoftheabsorptionlineisusedtocorrectforlightlossesotherthanspeciesabsorption.
Toachievesufficientsensitivity(oftheorderofppbv),theopen-pathmonitoringuseslongopticalpathsthroughtheatmosphere.
Thisgivesapath-averagedvalueofthespeciesconcentration.
ThemainfeaturesoftheOPTDLmethod,whichmakeitavaluabletechniqueforatmosphericmeasurements,canbesummarizedas:-Asahigh-resolutionspectroscopictechniqueitisvirtuallyimmunetointerferencesbyotherspecies,aproblemthatplaguesmostcompetingmethods.
-Asadifferentialspectroscopictechniquethemethodallowsstraightforwardcalibrationandcancelationofbackgroundabsorption-Concentrationmeasurementsareaveragedoveranextendedpath,andthereforearemuchlessaffectedbylocalunrepresentativefluctuationsingasconcentrationthanmeasurementstakenwithpointsensors.
-Itofferscontinuousmeasurementsattime-constantsoftensofsecondsorsowithppborsub-ppblowdetectionlimit.
Thetime-constantofthetechniquecanbetradedoffagainstsensitivityandthiscanallowfluxmeasurementsofrelativelyabundantspeciesbytheeddy-fluxcorrelationtechnique.
-Measurementscanbemadeinregionsofdifficultaccess,especiallyabovegroundlevel.
-Thereisnomaterialcontactbetweengasandsensorandthusthereisnodegradationofthegasbeingmeasuredor"poisoning"ofthesensor.
-Severalspeciescanbemeasuredsimultaneouslywithasinglelaser-Itisageneraltechnique.
Thesameinstrumentcaneasilybeconvertedfromonespeciestoanotherbychangingthelaserorthelasertemperature.
Furthermore,thenumberofsimultaneouslymeasuredspeciescanbeextendedbymultiplexingtheoutputsofseverallasers.
3.
InstrumentdesignanddeploymentTheinstrumentconsistsofatransmitter-receiveranddistantretroreflectoroperatedinmonostaticconfigurationasshowninFig.
1.
Thetransmitter-receiverisdesignedasacompactblockbuiltaroundthereceivingtelescope.
ThetransmitterusesaVerticalCavitySurfaceEmittingLaser(VCSEL)withacentralwavelength1.
654μm,current-tunedover3nm.
Therelativelywidetuningrangeofthelaserallowssimultaneousmeasurementsofwatervaporusingawatervaporabsorptionbandcenteredatapprox.
6047.
8cm-1.
Thelaserradiationiscollimatedbyanoff-axisparabolicmirroranddirectedtotheretroreflectoralongthetelescopeopticalaxis.
ForalignmentpurposesagreentracinglaserbeamistransmittedcoaxiallytotheIRlaserbeam.
ThetelescopeisNewtoniantypewith20cmdiameterofthemainmirror.
PeltiercooledInGaAsphotodiodeandalow-noisetransimpedanceamplifierareusedinthereceiver.
Toavoiddistortionsinthelineshape,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
Theacquisitioniscarriedoutbyafast14bit,200MHzADCcardinstalledinaPC.
Thelaser,thecollimatingoptics,andthereceiverdetectorarefixedonthetelescope.
Aretroreflectorwith15cmclearaperturewasassembledfromflatmirrorsusingthetechnologyforbuildingandalignmentdevelopedatEPFL.
TheinstrumentiscontrolledviaLabViewbasedsoftware.
Toensurebetterthan1ppbaccuracythedatatreatmentsoftwarewilltakeintoaccounttheactualatmosphericpressureandtemperaturemeasuredatthetwoendsoftheopticalpathandlaserpowervariations.
Fig.
1.
Opticalschemaoftheinstrument.
Theenclosuresofthetransceiverandtheretroreflectorarenotshownhereforsimplicity.
4.
Fig.
2LeftAphotographoftheopen-pathinstrumentformonitoringbackgroundmethaneconcentration.
Rightpane;lInstrumenttransceiver.
Leftpanel.
ThemeasuringsiteatEPFLshowingthepositionoftheretroreflector.
Theinsetshowsaclosepictureoftheretroreflector.
Theinstrumenthasalreadybeenbuiltandextensivetestsmeasurementsover1000mopticalpathhasbeencarriedout.
ApictureoftheinstrumentisshownontheleftpanelofFig.
2.
TherightpanelofthesamefigureshowsthedeployedattheEPFLcampusindicatingthepositionoftheretroreflector.
Acomparisonbetweenasimulated(usingHITRANdatabase)andexperimentallymeasuredatmosphericabsorptiontakenduringthetestsisshowninFig.
3.
Well-expressedmethaneandwatervaporspectralfeaturesareclearlyseeninthefigure.
ThesignallevelandsignaltonoiseratioallowustoestimatethatthemeasurementofambientCH4concentrationswithaccuracyandprecisionbetterthan1ppbisfeasiblewiththecurrentconfigurationforacquisitiontimesoftheorderoftensofseconds.
Thelowerdetectionlimitforwatervaporisexpectedtobeoftheorderoftensofppb.
Laser(VCSEL)ColimationopticsTracinggreenlaserTelescopeDetectorFTAmGcpaTFig.
3.
Atmospsimulation;1.
7ThesignalisaAftercAltitudeReseamostofthetimGAWstationschromatographpossiblepositisurroundingsoaltitudeof380TheMnchsjoFig.
4.
Pstphericabsorpti79ppmCH4,4averagedover2completingthearchStationJunmeinthefreesandcarrieshandanFTIRionsoftheretofMnchsjohh00mASLandhhüttehutisloPossibleopticPTTtationionspectrumi4%H2O,10002000laserpulsongoinginitingfraujoch(HAtroposphereaoncontinuousRsystem.
Thetroreflectorarehüttehutataphassuitableinocatedat3627alpathsfromH1.
1kmHARSJinthevicinitymopticalpathses(0.
5s),timealtestsattheARSJ).
Duetoallowingbackgsobservationstransceiverwieenvisaged;approx2.
3kmnfrastructurealmASLandisHARSJ2yof6047cm-h.
LowerpaneeforscanningEPFL,theinoitshigh-altitudgroundtracegofanumberillbeinstalledaPTTstationNEfromHAllowingeasyinaccessibleparMhu2.
3km1CH4line.
Upl,MeasuredovtheCH4line1nstrumentwilldelocation(35gasmonitoringroftracegasdintheSphinxatapprox.
1.
ARSJ(Fig.
5).
nstallationandrtoftheyear.
MnchsjohhüutUpperpanel,Hver1000mpaμs.
beinstalledi580mASL)th.
HARSJisoses,includingxobservatoryo1kmwestfroThePTTstatdmaintenanceütteHITRAN[25]athlengthspecn2012attheheHARSJissitoneofthe24gmethanebyofHARSJanomHARSJanionislocatedoftheretroreflbasedctrum.
Hightuatedglobalagasndtwondtheatanlector.
6Thenextphaseoftheprojectisdueforexperimentalmeasurements,comparisonwiththeoperationalatJungfraujochpointandremotemeasurementsandanalysisoftheresults.
Todefinetheweatherdependencemeasurementswillbecarriedoutindifferentweatherconditions.
Becauseoftheshortacquisitiontimeoftheopen-pathmidIRsystem,measurementswillbepossibleinscatteredcloudsinshorttimeintervals.
ThecomparisonwiththeregularpointgaschromatographandTDLmulti-passcellmeasurementsandwiththeFTIRspace-averagedmeasurementswillbecarriedout.
ThegoaloftheintercomparisonisnotonlytoverifytheTDL-open-pathdatabutalsotoidentifypossibleinfluenceoftheemissionsfromtheJungfraujochstationandtouristsitesonthepointGHGsmeasurements.
TheJungfraujochisaverybusytouristsitewithahighlysophisticatedinfrastructureandupto8'000visitorsperday;thereforetherealwaysexiststhepossibilitythatemissionsfromtheJungfraujochtouristfacilitiescanaffectthepointmeasurementsofCH4andwatervaportoamuchlargerextentthanthelongopenpathmeasurements.
SincetheTDLinstrumentwillsupplydataaveragedoverone(two)kilometersthiscouldpossiblyallowtheidentificationoftheJungfraujochstation'sinfluenceontheGHGsmeasurementsbystudyingthedifferencesbetweenthepointandspatiallyaverageddata,andvisitorstatisticsandmeteorologicalconditions.
4.
ConclusionAnew,longopen-pathinstrumentformonitoringofatmosphericwatervaporandbackgroundmethaneconcentrationpath-averagedover1000mwasdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)Switzerland.
Theinstrumentisbuiltonthemonostaticscheme(transceiver–distantretroreflector).
UsinganIRVerticalCavitySurfaceEmittingLaserinthetransmitter.
Thereceiverisbuiltarounda20cmNewtoniantelescope.
Toavoiddistortionsintheshapeofamethaneline,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
FastInGaAsphotodiodesanda200MHz,14bitADCareusedtoachievethisscanningrate.
TheinstrumentwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbedeployedin2012attheHighAltitudeResearchStationJungfraujoch(3580mASL).
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
References:1.
IPCCFourthassessmentreport,WorkingGroupIReport"ThePhysicalScienceBasis",2007,Ch2andCh7,availablefromhttp://www.
ipcc.
ch/ipccreports/ar4-wg1.
htm2.
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Zymovetal.
"PermafrostandtheGlobalCarbonBudget",ScienceV312,pp.
1612-1613,20063.
M.
Rigbyatal,Renewedgrowthofatmosphericmethane,Geophys.
Res.
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Walter,MethanebubblingfromSiberianthawlakesasapositivefeedbacktoclimatewarming,NatureVol443|7September2006|doi:10.
1038/nature05040,pp.
71-75,20065.
M.
Mastepanovatal,Largetundramethaneburstduringonsetoffreezing,NatureLett.
Vol456|4December20086.
Buffett,B.
,andD.
Archer,2004:Globalinventoryofmethaneclathrate:sensitivitytochangesinthedeepocean.
EarthPlanet.
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AnomaliesofmethaneintheatmosphereovertheEastSiberianshelf:IsthereanysignofmethaneleakagefromshallowshelfhydratesGeophysicalResearchAbstractsVol.
10,EGU2008-A-01526,20088.
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MethanereleaseandcoastalenvironmentintheEastSiberianArcticshelf,JournalofMarineSystemsv.
66,pp.
227–243,20079.
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Mascarelli,"Asleepinggiant",NaturereportsclimatechangeV.
3April,200910.
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Dlugokenckyatal,"Thegrowthrateanddistributionofatmosphericmethane",JGRV,.
99,NO.
D8,pp17,021-17,043A,199411.
D.
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Cunnoldatal,InsitumeasurementsofatmosphericmethaneatGAGE/AGAGEsitesduring1985–2000andresultingsourceinferences",JGRV.
107,NO.
D14,pp.
ACH20-1-ACH20-18,200212.
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Dlugokencky,ConversionofNOAAatmosphericdryairCH4molefractionstoagravimetricallypreparedstandardscale,JGR,V.
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1029/2005JD006035,200513.
Airmonitoringbyspectroscopictechniquesed.
MSigrist,JohnWiley&sons.
Inc.
NeyYork,199414.
L.
Delbouille,andG.
Roland,High-resolutionsolarandatmosphericspectroscopyfromtheJungfraujochhigh-altitudestation,Opt.
Eng,34,pp.
2736-2739,199515.
F.
Titel,etal,Mid-InfraredLaserApplicationsinSpectroscopy,inSolid-StateMid-InfraredLaserSources,I.
T.
SorokinaandK.
L.
Vodopyanov,2003,Springer,Verlag:BerlinHeidelberg.
p.
445-51016.
P.
Werle,Near-andmid-infraredlaser-opticalsensorsforgasanalysis,OptandLasersEng.
,37,101-114,200217.
M.
Taslakov,V.
Simeonov,andH.
vandenBergh,"Open-pathozonedetectionbyQuantumCascadeLaser",AppliedPhysicsB,82,501-506,(2006)718.
RJimenez,M.
Taslakov,V.
Simeonov,B.
Calpini,F.
Jeanneret,D.
Hofstetter,M.
Beck,J.
Faist,andH.
vandenBergh,Ozonedetectionbydifferentialabsorptionspectroscopyatambientpressurewitha9.
6mpulsedquantum-cascadelaser,Appl.
Phys.
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249-256,(2003)19.
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Werle,Areviewofrecentadvancesinsemiconductorlaserbasedgasmonitors,SpectrochimicaActaPartA54,pp197–236,199820.
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Wainner,"Handheld,battery-powerednear-IRTDLsensorforstand-offdetectionofgasandvaporplumes",Appl.
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http://www.
aerodyne.
com/Tunablediodelasertracegasdetectors,andQuantumcascadelasertracegasdetectors.
22.
http://www.
lgrinc.
com/index.
aspsubid=ps&ProductCategoryID=1523.
http://www.
picarro.
com/markets/greenhouse/24.
http://www.
boreal-laser.
comTheannounced(webpage)maximumpathis1000m.
Accordingtoacompanyengineertherealmaximumpathis400mwithapossibilitytoextendthepathlengthto1000musingamulti-retroreflectorarraythathastobedevelopedspecially.
Ourexperienceofusingmulti-reflectorarrayswithQCLandTDLsystemsshowsthatmulti-reflectorconfigurationwhenusedwithcoherentsourcesproducesdynamicinterferencefringeswhichcompromisethemeasurement.
Thefringesarecausedbytheinterferenceofthemodulatedbyatmosphericturbulencebeamsthatoriginatefromindividualretroreflectors.
Paperonthissubjectisonpreparation.
+41(0)216936185,Fax.
+41(0)216936390email(valentin.
simeonov@epfl.
ch)Anew,longopen-pathinstrumentformonitoringofthebackgroundmethaneconcentrationpath-averagedover1000mwillbepresented.
Theinstrumentallowsmonitoringofwatervaporconcentrationaswell.
Theinstrumentisbuiltonthemonostaticscheme(transceiver–distantretroreflector).
AVCSELtunablediodelaser(TDL)withacentralwavelengthof1654nmisusedasalightsource.
Thereceiverisbuiltarounda20cmNewtoniantelescope.
Toavoiddistortionsintheshapeofamethaneline,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
FastInGaAsphotodiodesanda200MHz,14bitADCareusedtoachievethisscanningrate.
Theexpectedconcentrationresolutionfortheabovementionedpath-lengthsisoftheorderof1ppbwithaccuracybetterthan5%.
TheinstrumentisdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)SwitzerlandandwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbeinstalledin2012attheHighAltitudeResearchStationJungfraujoch(HARSJ).
TheHARSJislocatedat3580mASLandisoneofthe24globalGAWstationsandcarriesoncontinuousobservationsofanumberoftracegasses,includingmethane.
Oneofthegoalsoftheprojectistocomparepath-averagedtoongoingpointmeasurementsofmethaneinordertoidentifypossibleinfluenceofthestation.
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
1.
IntroductionAnumberofgasesareinvolvedintheanthropogenicenhancementofthegreenhouseeffect.
Themostimportantofthesegreenhousegases(GHGs)arecarbondioxide(CO2);methane(CH4);nitrousoxide(N2O);halocarbons(HC);andtroposphericozone(O3).
Methanehasaspecialplaceamongthesegasessinceitsglobalwarmingpotentialis25timesoreven72timeslargerthanthepotentialofCO2in100yr.
and20yr.
timehorizonsrespectively[1].
Besidethedirectradiativeforcing(RF),estimatedto+0.
48Wm-2methanehasimportantindirecteffectbecauseofitschemicalreactivityresultinginatotalRFof+0.
7Wm-2.
TheindirectradiativeeffectsofmethaneresultmostlyfromreactionwithatmosphericOH.
Thisreaction:reducestheOHconcentration,leadingviaapositivefeedbacktoanenhancementofmethaneandsomeHClifetimes;producesadditionalCO2;enhancesstratosphericwatervapor;andincreasestroposphericozoneconcentrationthroughthemethaneproductionchain.
BecauseofconcentrationslowerthanthoseofCO2,atpresentmethanehasthesecond-largestradiativeforcing(RF)effectafterCO2.
Methaneisemittedbynaturalandanthropogenicsourceswithanthropogenicsourcesaccountingfor60%ofthetotalCH4budgetatpresent.
Themainnaturalsourcesarewetlands,termites,oceans,geologicalsources,methanehydrates,andwildfire.
Themostimportantanthropogenicsourcesareenergyproduction,(includingmining,fossilfuelproduction,distributionanduse),riceagriculture,ruminantanimals,landfills,wastetreatmentandbiomassburning.
ThemainmethanesinksaretheoxidationbytroposphericOH,oxidationinsoilsandlosstostratosphere.
Methaneconcentrationshaveincreasedalmost2.
5timessince1750reaching1774ppbin2005.
Systematicmeasurementsoverthelast25yearsshowa30%increaseinmethaneconcentrationsduringthatperiod.
Thisincreaseisnotmonotonic,withgreaterthan1%peryeargrowthratesinthelate1970andearly1980,slowingdowntozeroornegativevaluesduring1999-2005withsubstantialinterannualvariationsintheperiod1988-2005.
Recentdatashowrenewedgrowthofatmosphericmethane[2]duringlastyears.
ThereasonsforthedecreaseandvariabilityintheCH4growthrateandtheimplicationsforfuturechangesarenotunderstoodalthoughanumberofhypothesesweresuggested[1].
Inresponsetoanincreaseinglobalaveragetemperature,largequantitiesofmethanecanbereleasedinrelativelyshorttimescalesfromgeologicalstorage,suchasmethanehydratesandpermafrostandthroughbiogenicprocesses.
Theglobaltemperatureincreasecancausesignificantincreaseinmethaneconcentrationssince70%oftheatmosphericmethaneoriginatesfrombiogenicsources,whicharehighlysensitivetoclimatevariables.
Recentstudieshaveshownthatthepermafrostcontainsapprox.
500-900Gtcarbon,upto30%ofwhichcanbeconvertedinmethanebymicroorganismsduringthawing[3,4,5].
Inresponsetoclimatewarming,permafrosthasalreadybeguntothaw,withextremeprojectionsthatbytheendofthecenturyitwillhavethawedalmostcompletely.
Climatechangesalsoaffectthestabilityofmethanehydratesbeneaththeoceanwhere~4Ttarestored[6].
ThelatestobservationsoftheSiberianArcticshelfsuggestthat900Gtmethanearestoredthereinmethanehydratedepositsandasafreegasbelowthehydratedeposits.
Thereishighprobabilitythat50Gtmethaneofthisstoragecanbereleasedabruptlyatanytimedueto2thechangesinseawatertemperatures,relatedtothemeltingofArcticice,orasaresultofgeologicalevents[7,8,9].
DatafromSeptember2005showhigh(12000%)CH4supersaturationofsurfacewater,andhigh(upto8ppm)CH4concentrationintheatmosphericlayerabovetheseasurfaceovertheEastSiberianShelf[7].
Currentlyseveralnetworksandgovernmentalorganizationssystematicallymeasuremethaneconcentrationinsurfaceair[1].
NOAA/GMDisthemostgeographicallyextensivenetworkoperating40surfaceairflask-samplingsitesandacquiringdataalmostweeklysince1983[10].
GAGE/AGAGEnetworkoperates5+3sitesequippedwithautomaticsystemswithsamplingratesupto36samples/24hsincelate1980[11].
Allnetworksuseexclusivelythegaschromatograph(GC)techniquewithaflameionizationdetector(FID)formeasuringCH4concentration.
Withapropercalibration,thistechniquecansupplymeasurementswithrelativelygoodsensitivityandprecisionof0.
2-1%.
Sincethemeasurementprocessdependsonanumberofexternalparameters,toachievesufficientcorrelationamongmeasurementstakenwithdifferentinstrumentsandbetweenconsecutiverunstakenwiththesameinstrument,theGCrequirescalibrationwithaprecisestandardgasmixturebeforeeverysamplemeasurement.
Driftsandshiftsinthestandardscale[12]mayrequirereassessmentofthewholedataseries.
Inaddition,GCinstrumentshaverelativelyhighinitialpriceandoperationalcostsbecauseoftheneedofexpensiveconsumables(highandvery-highpuritygases),theneedofspecialenvironment(housing),andregularmaintenancebyqualifiedpersonnel.
Sincethesamplingisdoneatafixedpoint,themeasurementscanbeeasilyalteredbysampletakingandcanbeaffectedbylocalsources,sinks,orlocaltransport,particularlyinpoorlymixedatmospheresasinthecaseofwetlandswhereconcentrationcanvarybyordersofmagnitudefordistancesofseveralmeters.
Therefore,pointmeasurementslackthespatialrepresentativenessneededformodelingpurposes.
BecauseofallthesefactorsGCarenotsuitablefortheenlargementoftheexistingCH4networksasenvisagedintheGAW2008-2015strategicplanespeciallyinArcticandtropicalregions.
Infrared(IR)spectralanalysistechniquessuchasFourierTransformInfrared(FTIR)orTunableLaserAbsorptionSpectroscopy(TLAS)arewidelyusedforaccuratequantitativemeasurementsoftracegasconcentrations[13].
TheconcentrationisderivedfromIRabsorptionmeasuredoveranopticalpath.
Thesensitivityandtheprecisionofthesemeasurementsdependstronglyonthepath-length.
Toachievesufficientsensitivityandprecisionfortraceconcentrationmeasurements,thepathlengthisextendedeitherbyusingmultiple-pathcellsormeasuringoverlong-pathinopenair(referredtohereinas"open-path"orOP).
Themultiple-passtechniqueallowscompactinstrumentaldesignandrelativelyeasycalibrationbutbeingapointmeasurementtechniquesuffersfromthesamedrawbacksastheGCtechniquementionedabove.
IntheOPtechniquetheconcentrationmeasurementsareaveragedoveranextendedpath,andthereforearemuchlessaffectedbylocalunrepresentativefluctuationsingasconcentrationthanmeasurementstakenwithpointsensors.
Thepassiveopen-pathFTIRmethodusestheSunasalightsourceandhasbeenusedtoproducecontinuous,longtimeseriesofhighquality,totalcolumnGHGsdata[14].
TheFTIRmethod,however,islimitedtoonlydaytimeandclearweatherconditions.
TheactiveOP-FTIRcanbeoperatedaroundtheclockbuttheachievableabsorptionpathsareusuallyshort(<1000m)becauseoftheuseofnon-coherentlightsources,whichleadstolowsensitivity.
BothOP-FTIRmethodsrequiresignificantresourcesandexpertknowledgetoensureproperdeployment,operation,andfinaldataproduction.
Furthermore,FTIRinstrumentsaredelicateandhavemovingparts,whichmakesthemdifficulttodeployinfieldconditions.
Theadvancesinsemiconductorlaserscienceandtechnologyhasmadeavailabletunable,sourcesforreal-timeTLASmonitoringofalargenumberofmolecularspeciesintheIR.
TunableDiodelaser(TDL)andQuantumCascadeLaser(QCL)basedpointmeasurementsofanumberoftracegasesincludingCO2,CH4,O3N2Oandotherhavebeensuccessfullydemonstratedinrecentyears[15,16,17,18,19,20]andcommercialinstruments[21,22,23]havebecomeavailablerecently.
Themajorityoftheseexperimentsandmostcommercialinstrumentsarehowever,designedforpointobservations.
SomesuccessfulOPexperimentshavebeencarriedoutinthelate1970,butbecauseofthelackofsuitablelasersources,thesetypemeasurementsdidnotfindwideapplications.
Recentlywiththeappearanceofnewlasersources,OPmeasurementsofmethanehavebeenreportedandcommercialinstruments[24]areavailable.
Theseinstrumentshoweveraredesignedmostlyfordetectinggasleaksormeasuringhigherthanbackgroundconcentrations.
Inthispaperwedescribeanew,TDLbasedOPinstrumentformonitoringofthebackgroundmethaneconcentration.
TheinstrumentisdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)SwitzerlandandwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbeinstalledin2012attheHighAltitudeResearchStationJungfraujoch(HARSJ).
TheHARSJislocatedat3580mASLandisoneofthe24globalGAWstationsandcarriesoncontinuousobservationsofanumberoftracegasses,includingmethane.
Oneofthegoalsoftheprojectistocomparepath-averagedtoongoingpointmeasurementsofmethaneinordertoidentifypossibleinfluenceofthestation.
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
2.
TheoryofoperationTheOPTDLtechniqueemploystheabsorptionspectroscopyprincipletoobtainspeciesconcentration.
TheconcentrationCisderivedfromthemeasuredoverthesamplelengthLlighttransmittance)(νTasLTC)()(lnνσν=3where)(νσiswavelength)(νdependantabsorptioncrosssection,specificforthedetectedsubstance.
Thetransmittanceismeasuredbysweepingrepeatedlythelaserwavelengthacrossanabsorptionlineofthespeciesbeingdetected.
Thetransmittancemeasuredoutsideoftheabsorptionlineisusedtocorrectforlightlossesotherthanspeciesabsorption.
Toachievesufficientsensitivity(oftheorderofppbv),theopen-pathmonitoringuseslongopticalpathsthroughtheatmosphere.
Thisgivesapath-averagedvalueofthespeciesconcentration.
ThemainfeaturesoftheOPTDLmethod,whichmakeitavaluabletechniqueforatmosphericmeasurements,canbesummarizedas:-Asahigh-resolutionspectroscopictechniqueitisvirtuallyimmunetointerferencesbyotherspecies,aproblemthatplaguesmostcompetingmethods.
-Asadifferentialspectroscopictechniquethemethodallowsstraightforwardcalibrationandcancelationofbackgroundabsorption-Concentrationmeasurementsareaveragedoveranextendedpath,andthereforearemuchlessaffectedbylocalunrepresentativefluctuationsingasconcentrationthanmeasurementstakenwithpointsensors.
-Itofferscontinuousmeasurementsattime-constantsoftensofsecondsorsowithppborsub-ppblowdetectionlimit.
Thetime-constantofthetechniquecanbetradedoffagainstsensitivityandthiscanallowfluxmeasurementsofrelativelyabundantspeciesbytheeddy-fluxcorrelationtechnique.
-Measurementscanbemadeinregionsofdifficultaccess,especiallyabovegroundlevel.
-Thereisnomaterialcontactbetweengasandsensorandthusthereisnodegradationofthegasbeingmeasuredor"poisoning"ofthesensor.
-Severalspeciescanbemeasuredsimultaneouslywithasinglelaser-Itisageneraltechnique.
Thesameinstrumentcaneasilybeconvertedfromonespeciestoanotherbychangingthelaserorthelasertemperature.
Furthermore,thenumberofsimultaneouslymeasuredspeciescanbeextendedbymultiplexingtheoutputsofseverallasers.
3.
InstrumentdesignanddeploymentTheinstrumentconsistsofatransmitter-receiveranddistantretroreflectoroperatedinmonostaticconfigurationasshowninFig.
1.
Thetransmitter-receiverisdesignedasacompactblockbuiltaroundthereceivingtelescope.
ThetransmitterusesaVerticalCavitySurfaceEmittingLaser(VCSEL)withacentralwavelength1.
654μm,current-tunedover3nm.
Therelativelywidetuningrangeofthelaserallowssimultaneousmeasurementsofwatervaporusingawatervaporabsorptionbandcenteredatapprox.
6047.
8cm-1.
Thelaserradiationiscollimatedbyanoff-axisparabolicmirroranddirectedtotheretroreflectoralongthetelescopeopticalaxis.
ForalignmentpurposesagreentracinglaserbeamistransmittedcoaxiallytotheIRlaserbeam.
ThetelescopeisNewtoniantypewith20cmdiameterofthemainmirror.
PeltiercooledInGaAsphotodiodeandalow-noisetransimpedanceamplifierareusedinthereceiver.
Toavoiddistortionsinthelineshape,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
Theacquisitioniscarriedoutbyafast14bit,200MHzADCcardinstalledinaPC.
Thelaser,thecollimatingoptics,andthereceiverdetectorarefixedonthetelescope.
Aretroreflectorwith15cmclearaperturewasassembledfromflatmirrorsusingthetechnologyforbuildingandalignmentdevelopedatEPFL.
TheinstrumentiscontrolledviaLabViewbasedsoftware.
Toensurebetterthan1ppbaccuracythedatatreatmentsoftwarewilltakeintoaccounttheactualatmosphericpressureandtemperaturemeasuredatthetwoendsoftheopticalpathandlaserpowervariations.
Fig.
1.
Opticalschemaoftheinstrument.
Theenclosuresofthetransceiverandtheretroreflectorarenotshownhereforsimplicity.
4.
Fig.
2LeftAphotographoftheopen-pathinstrumentformonitoringbackgroundmethaneconcentration.
Rightpane;lInstrumenttransceiver.
Leftpanel.
ThemeasuringsiteatEPFLshowingthepositionoftheretroreflector.
Theinsetshowsaclosepictureoftheretroreflector.
Theinstrumenthasalreadybeenbuiltandextensivetestsmeasurementsover1000mopticalpathhasbeencarriedout.
ApictureoftheinstrumentisshownontheleftpanelofFig.
2.
TherightpanelofthesamefigureshowsthedeployedattheEPFLcampusindicatingthepositionoftheretroreflector.
Acomparisonbetweenasimulated(usingHITRANdatabase)andexperimentallymeasuredatmosphericabsorptiontakenduringthetestsisshowninFig.
3.
Well-expressedmethaneandwatervaporspectralfeaturesareclearlyseeninthefigure.
ThesignallevelandsignaltonoiseratioallowustoestimatethatthemeasurementofambientCH4concentrationswithaccuracyandprecisionbetterthan1ppbisfeasiblewiththecurrentconfigurationforacquisitiontimesoftheorderoftensofseconds.
Thelowerdetectionlimitforwatervaporisexpectedtobeoftheorderoftensofppb.
Laser(VCSEL)ColimationopticsTracinggreenlaserTelescopeDetectorFTAmGcpaTFig.
3.
Atmospsimulation;1.
7ThesignalisaAftercAltitudeReseamostofthetimGAWstationschromatographpossiblepositisurroundingsoaltitudeof380TheMnchsjoFig.
4.
Pstphericabsorpti79ppmCH4,4averagedover2completingthearchStationJunmeinthefreesandcarrieshandanFTIRionsoftheretofMnchsjohh00mASLandhhüttehutisloPossibleopticPTTtationionspectrumi4%H2O,10002000laserpulsongoinginitingfraujoch(HAtroposphereaoncontinuousRsystem.
Thetroreflectorarehüttehutataphassuitableinocatedat3627alpathsfromH1.
1kmHARSJinthevicinitymopticalpathses(0.
5s),timealtestsattheARSJ).
Duetoallowingbackgsobservationstransceiverwieenvisaged;approx2.
3kmnfrastructurealmASLandisHARSJ2yof6047cm-h.
LowerpaneeforscanningEPFL,theinoitshigh-altitudgroundtracegofanumberillbeinstalledaPTTstationNEfromHAllowingeasyinaccessibleparMhu2.
3km1CH4line.
Upl,MeasuredovtheCH4line1nstrumentwilldelocation(35gasmonitoringroftracegasdintheSphinxatapprox.
1.
ARSJ(Fig.
5).
nstallationandrtoftheyear.
MnchsjohhüutUpperpanel,Hver1000mpaμs.
beinstalledi580mASL)th.
HARSJisoses,includingxobservatoryo1kmwestfroThePTTstatdmaintenanceütteHITRAN[25]athlengthspecn2012attheheHARSJissitoneofthe24gmethanebyofHARSJanomHARSJanionislocatedoftheretroreflbasedctrum.
Hightuatedglobalagasndtwondtheatanlector.
6Thenextphaseoftheprojectisdueforexperimentalmeasurements,comparisonwiththeoperationalatJungfraujochpointandremotemeasurementsandanalysisoftheresults.
Todefinetheweatherdependencemeasurementswillbecarriedoutindifferentweatherconditions.
Becauseoftheshortacquisitiontimeoftheopen-pathmidIRsystem,measurementswillbepossibleinscatteredcloudsinshorttimeintervals.
ThecomparisonwiththeregularpointgaschromatographandTDLmulti-passcellmeasurementsandwiththeFTIRspace-averagedmeasurementswillbecarriedout.
ThegoaloftheintercomparisonisnotonlytoverifytheTDL-open-pathdatabutalsotoidentifypossibleinfluenceoftheemissionsfromtheJungfraujochstationandtouristsitesonthepointGHGsmeasurements.
TheJungfraujochisaverybusytouristsitewithahighlysophisticatedinfrastructureandupto8'000visitorsperday;thereforetherealwaysexiststhepossibilitythatemissionsfromtheJungfraujochtouristfacilitiescanaffectthepointmeasurementsofCH4andwatervaportoamuchlargerextentthanthelongopenpathmeasurements.
SincetheTDLinstrumentwillsupplydataaveragedoverone(two)kilometersthiscouldpossiblyallowtheidentificationoftheJungfraujochstation'sinfluenceontheGHGsmeasurementsbystudyingthedifferencesbetweenthepointandspatiallyaverageddata,andvisitorstatisticsandmeteorologicalconditions.
4.
ConclusionAnew,longopen-pathinstrumentformonitoringofatmosphericwatervaporandbackgroundmethaneconcentrationpath-averagedover1000mwasdevelopedattheSwissFederalInstituteofTechnology–Lausanne(EPFL)Switzerland.
Theinstrumentisbuiltonthemonostaticscheme(transceiver–distantretroreflector).
UsinganIRVerticalCavitySurfaceEmittingLaserinthetransmitter.
Thereceiverisbuiltarounda20cmNewtoniantelescope.
Toavoiddistortionsintheshapeofamethaneline,causedbyatmosphericturbulences,themethanelineisscannedwithin1μs.
FastInGaAsphotodiodesanda200MHz,14bitADCareusedtoachievethisscanningrate.
TheinstrumentwillbeusedwithintheGAW+CHprogramforlong-termmonitoringofbackgroundmethaneconcentrationintheSwissAlps.
AftercompletingtheongoinginitialtestsattheEPFL,theinstrumentwillbedeployedin2012attheHighAltitudeResearchStationJungfraujoch(3580mASL).
FuturedeploymentsofacopyoftheinstrumentcouldincludetheColombianpartofAmazoniaandSiberianwetlands.
References:1.
IPCCFourthassessmentreport,WorkingGroupIReport"ThePhysicalScienceBasis",2007,Ch2andCh7,availablefromhttp://www.
ipcc.
ch/ipccreports/ar4-wg1.
htm2.
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Zymovetal.
"PermafrostandtheGlobalCarbonBudget",ScienceV312,pp.
1612-1613,20063.
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Rigbyatal,Renewedgrowthofatmosphericmethane,Geophys.
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Walter,MethanebubblingfromSiberianthawlakesasapositivefeedbacktoclimatewarming,NatureVol443|7September2006|doi:10.
1038/nature05040,pp.
71-75,20065.
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Mastepanovatal,Largetundramethaneburstduringonsetoffreezing,NatureLett.
Vol456|4December20086.
Buffett,B.
,andD.
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EarthPlanet.
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AnomaliesofmethaneintheatmosphereovertheEastSiberianshelf:IsthereanysignofmethaneleakagefromshallowshelfhydratesGeophysicalResearchAbstractsVol.
10,EGU2008-A-01526,20088.
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MethanereleaseandcoastalenvironmentintheEastSiberianArcticshelf,JournalofMarineSystemsv.
66,pp.
227–243,20079.
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Mascarelli,"Asleepinggiant",NaturereportsclimatechangeV.
3April,200910.
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Dlugokenckyatal,"Thegrowthrateanddistributionofatmosphericmethane",JGRV,.
99,NO.
D8,pp17,021-17,043A,199411.
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Cunnoldatal,InsitumeasurementsofatmosphericmethaneatGAGE/AGAGEsitesduring1985–2000andresultingsourceinferences",JGRV.
107,NO.
D14,pp.
ACH20-1-ACH20-18,200212.
E.
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Dlugokencky,ConversionofNOAAatmosphericdryairCH4molefractionstoagravimetricallypreparedstandardscale,JGR,V.
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Airmonitoringbyspectroscopictechniquesed.
MSigrist,JohnWiley&sons.
Inc.
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Delbouille,andG.
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Eng,34,pp.
2736-2739,199515.
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Titel,etal,Mid-InfraredLaserApplicationsinSpectroscopy,inSolid-StateMid-InfraredLaserSources,I.
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Vodopyanov,2003,Springer,Verlag:BerlinHeidelberg.
p.
445-51016.
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Werle,Near-andmid-infraredlaser-opticalsensorsforgasanalysis,OptandLasersEng.
,37,101-114,200217.
M.
Taslakov,V.
Simeonov,andH.
vandenBergh,"Open-pathozonedetectionbyQuantumCascadeLaser",AppliedPhysicsB,82,501-506,(2006)718.
RJimenez,M.
Taslakov,V.
Simeonov,B.
Calpini,F.
Jeanneret,D.
Hofstetter,M.
Beck,J.
Faist,andH.
vandenBergh,Ozonedetectionbydifferentialabsorptionspectroscopyatambientpressurewitha9.
6mpulsedquantum-cascadelaser,Appl.
Phys.
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249-256,(2003)19.
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Werle,Areviewofrecentadvancesinsemiconductorlaserbasedgasmonitors,SpectrochimicaActaPartA54,pp197–236,199820.
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http://www.
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com/Tunablediodelasertracegasdetectors,andQuantumcascadelasertracegasdetectors.
22.
http://www.
lgrinc.
com/index.
aspsubid=ps&ProductCategoryID=1523.
http://www.
picarro.
com/markets/greenhouse/24.
http://www.
boreal-laser.
comTheannounced(webpage)maximumpathis1000m.
Accordingtoacompanyengineertherealmaximumpathis400mwithapossibilitytoextendthepathlengthto1000musingamulti-retroreflectorarraythathastobedevelopedspecially.
Ourexperienceofusingmulti-reflectorarrayswithQCLandTDLsystemsshowsthatmulti-reflectorconfigurationwhenusedwithcoherentsourcesproducesdynamicinterferencefringeswhichcompromisethemeasurement.
Thefringesarecausedbytheinterferenceofthemodulatedbyatmosphericturbulencebeamsthatoriginatefromindividualretroreflectors.
Paperonthissubjectisonpreparation.
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