An evaluation of pH and NO3 sensor data from SOCCOM floats and

AnevaluationofpHandNO3sensordatafromSOCCOMfloatsandtheirutilizationto
developoceaninorganiccarbonproducts
AsummaryofdiscussionsandrecommendationsoftheCarbonWorkingGroup(CWG)of
theSouthernOceanCarbonandClimateObservationsandModelingproject(SOCCOM)
March2016
AnevaluationofpHandNO3sensordatafromSOCCOMfloatsandtheirutilizationto
developoceaninorganiccarbonproducts
R.Wanninkhof1,K.Johnson2,N.Williams3,J.Sarmiento4,S.Riser5,E.Briggs6,S.Bushinsky4,
B.Carter5,7,A.Dickson6,R.Feely7,A.Gray4,L.Juranek3,R.Key4,L.Talley6,J.Russel8,andA.
Verdy6
1.AtlanticOceanographicandMeteorologicalLaboratoryofNOAA,MiamiFL
2.MontereyBayAquariumResearchInstitute,MossLandingCA
3.OregonStateUniversity,CorvallisOR
4.PrincetonUniversity,PrincetonNJ
5.UniversityofWashington,Seattle,WA
6.ScrippsInstitutionofOceanography,UCSD,LaJollaCA
7.PacificMarineEnvironmentalLaboratoryofNOAA,SeattleWA
8.UniversityofArizona,TucsonAZ
Tableofcontent:
Page
Recommendationsandfindings
3
1.Introduction
4
2.Biogeochemicalsensorsonfloats
5
Typesoffloats
6
DetailsofsensorsonSOCCOMfloats
7
Frequencyofprofilingmeasurements
10
3.Performanceofsensors
12
4.Sensordeployments,calibration,validationandchecks
12
5.Recommendedconstantsforinorganiccarbonsystemparameters 13
6.pHcomparisonsbetweenbottleandfloatdata
16
7.UtilizingmultiplelinearregressionstoadjustpHandNO3sensorsonfloats18
8.Datamanagementandqualitycontrol/adjustments 20
9.Productdevelopment
21
10.Outlook
23
AppendixA.Chargeforthecarbonworkinggroup(CWG)ofSOCCOM 25
References
28
2
AnevaluationofpHandNO3sensordatafromSOCCOMfloatsandtheirutilizationto
developoceaninorganiccarbonproducts
Recommendationsandfindings:
TheCarbonWorkingGroup(CWG)oftheSouthernOceanCarbonandClimate
ObservationsandModeling(SOCCOM)projectistaskedwithprovidingrecommendations
foracquisitionandqualitycontrolofdatafrombiogeochemicalsensorsonSOCCOMfloats:
CWGFindings
Ø TheNO3andpHsensorsonfloatsareindevelopmentstagebutrapidlyreachingthe
pointofprovidingqualitydatatostudykeybiogeochemicalprocessesinthe
SouthernOcean.
Ø Thereisanimprovedunderstandingandremediesofissuescausingdriftinthe
biogeochemicalsensors.
Ø ThetransitionfromanadhocadjustmentprocedureofpHandNO3toasystematic
approachhandledbydatacentersiswelllaidoutandprogressing.
Ø TheaccuraciesoffloatO2,NO3andpHdataafterappropriateadjustmentsare2
µmolkg-1,0.5µmolkg-1,and0.01,respectively.
Ø NO3andpHdatafromtheprofilingfloatsinSOCCOMcanbeadjustedasneeded
usingMultiLinearRegression(MLR)algorithmsbasedonqualitybottledatafrom
1000-2100dbarintheregion(seeeqns.3and4)towithinthelevelofcurrent
instrumentaluncertainties.
CWGRecommendations
Ø Allsamplesbroughtbacktoshorefromfloatdeploymentvalidationcastsshouldbe
analyzedfortotaldissolvedinorganiccarbon,DIC,inadditiontopHandtotal
alkalinity,TAlk.
Ø Continuedacquisitionandassemblyofhighqualityinorganiccarbondatafrom
CTD/bottlecastsiscriticalforqualitycontroloffloatsensors,modelvalidationand
algorithmdevelopment/improvement.
Ø Aconsistentsetofdissociationconstantsandachemicalmodelshouldbeusedfor
calculatinginorganiccarbonsystemparametersasdescribedinTable4.
Ø ItisimperativethatthetemperatureandpressuredependenceofpHsensors,and
theircorrespondencetotheappropriatepHscalesisfurtherstudiedindetail.
Ø Continuedstudyandimprovementofadjustmentapproachesandincorporationof
theseeffortsintoaroutineapproachfollowingArgoguidelinesisneeded.
Ø Continuedstudyandimprovementindeploymentprocedures,physical
configurationofsensors,sensordesignandqualitycontrolisneeded.
Ø AccommodationisneededforupdatedprotocolsinArgotoprocessthe
biogeochemicalandbiologicaldatausingseparatefilesanddataacquisitioncenters
priortomergingofphysicalandbiogeochemicaldata.
3
1.Introduction
i.Purposeofdocument
TheSouthernOceanCarbonandClimateObservationsandModeling(SOCCOM)projectis
revolutionizingoureffortstoinvestigatetheSouthernOcean.Thenewapproaches,in
particulartheutilizationofbiogeochemicalsensorsonfloats,requireacarefuluncertainty
analysisandclearprocedurestocreateusefulproducts.Thecarbonsystemworkinggroup
(CWG)ofSOCCOMischargedtoaddresstheseissueswithregardstothemeasuredand
derivedinorganiccarbonsystemparameters:totalalkalinity(TAlk),totaldissolved
inorganiccarbon,(DIC),partialpressureofCO2(pCO2),andpH.Theonlyinorganiccarbon
systemparameteracquiredfromtheSOCCOMfloatsispH,theothersarecalculated
throughdifferentmeansasdescribedbelow.Thisdocumentisanauthoritativeoverviewof
procedurestoadjustsensorsforoffsetsanddrift,andtouseofthefloatdatatocreate
products,suchasregionalfieldsofinorganiccarbonparameters.
ii.TheCWGandSOCCOMobjectives
ThecarbonworkinggroupactivitiesarepartoftheoverallobjectivesofSOCCOM.SOCCOM
isfocusedonunlockingthemysteriesoftheSouthernOceananditsinfluenceonclimate.
SOCCOMisawell-structuredprogramwiththreemajorthemesofobservation,modeling
andbroaderimpacts.TheCWGaddressestheobservationthemewithfocusoncreating
fieldsofcarbonsystemparameters,suchascarbonatemineralsaturationstates.Afull
descriptionofthechargeoftheCWGisprovidedinAppendixA.Therecommendationsand
implementationwillalsobenefitthecarboncyclemodelsoftheSouthernOceanthrough
improvedconstraints.TheCWGwillhavesignificantbroaderimpactsastheprocedures
andapproachesdescribedcanbeutilizedtoimprovethequalityandproductsresulting
fromfloatsequippedwithbiogeochemicalsensorsocean-wide.
iii.Outlineofdocument
Thisdocumentprovidesrecommendationsandguidelinestoadjustandcheckdatafrom
biogeochemicalsensorsonprofilingfloats,inparticularpHandNO3sensorsbasedon
empiricalapproachesandmodels.Theadjusteddataalongwithmodelsofdifferentlevels
ofsophisticationareusedtodeterminethelargerscalefieldsofthebiogeochemical
parametersofinterest.
Thefirstsectionofthedocumentdescribesthesensorsonfloatsandtheirpre-cruise
calibrationandtypicalbehaviorwhenfirstdeployedwithfocusontheNO3andpHsensors.
ThisisfollowedbyadescriptionofthepHmeasurementandthederivationofcarbon
systemparametersandtheiruncertainties.AdjustmentproceduresoftheNO3andpH
sensorsaredescribedbasedonmultiplelinearregression(MLR)algorithmsderivedfrom
thevalidationcruisesandotherhighqualitybottledata.Thefinalsectiontouchesuponthe
carbonproductsthatwillbeproducedaspartoftheSOCCOMprojectusingdifferent
empiricalapproaches.
4
2.Biogeochemicalsensorsonfloats
TheArgoprofilingfloatprogramstartedintheearly1990'swithanambitiousgoalof
deployingaglobalarrayoffloatsequippedwithtemperature(T),salinity(S),andpressure
(P)sensors.Itsprimaryobjectivewastodeterminechangesinoceanheatcontentinthe
upper1500m.TheoverallvisionofArgoistogreatlyexpandourknowledgeofchangesin
theoceanwith"greatlyimprovedcollectionofobservationscapabilitiesinsidetheocean
throughincreasedsamplingofoldandnewquantitiesandincreasedcoverageintermsof
timeandarea"1.Thecurrentprogramhaswell-definedobjectivesalongwithgood
dissemination,qualitycontrol,anddatamanagementprotocols,anddatafromtheArgo
programisnowusedinoverscientific200publicationsperyear.ThegreatsuccessofArgo,
theurgentneedtostudytrendsinothervariablesinachangingocean,andrapidadvances
insensortechnologyhaveinspiredresearcherstoincorporatenovelsensorsontothe
profilingfloatplatforms.Oxygensensorswerethefirsttobeusedextensively(Körtzinger
etal.2004)andarenowdeployedonabout5%oftheglobalfloatarray(seeFigure1for
floatpositions).Thesensorsarewellbehavedandwithproperprecautionsyielddata
believedtobeaccuratetowithin1%(≈2µmolkg-1).Oxygendataprocessingprotocolsare
closetoroutinebutArgobiogeochemicalsensordatastillfollowadifferentqualitycontrol
paththancoreArgodata.
ThebiogeochemicalsensorsofprimaryinteresttothisdiscussionareNO3andpH.Wealso
considerpropertiesthatareestimatedorcalculatedfromtheseandotherArgo
measurements(e.g.aragonitesaturationΩA,TAlk,andDIC).TheO2sensorsarealso
discussedasO2dataareusedextensivelytoestimatethecarbonsystemparameters.The
O2sensorsserveasanexampleofacomparativelymatureefforttodeploynovel
biogeochemicalsensorsontheArgoplatform.
1http://www.argo.ucsd.edu
5
Figure1.LocationsofallfloatsreportingthroughtheWorldMeteorologicalOrganization’s
GlobalTelecommunicationsSystem(GMS)forJanuary20162(top)andfloatsequipped
biogeochemicalsensorsasofSeptember20153(bottom)
Typesoffloats
FivetypesoffloatsarecommonlydeployedintheArgoprogram(APEX,Navis,SOLOII,
Provor,andNOVA).SOCCOMexclusivelyusesAPEXandSea-BirdNAVISfloatsattimeof
writing.TheNAVISfloatswithbiogeochemicalsensorsareastandardproductofSeabird
Electronicsandcanbepurchasedwithuserspecifiedconfigurations.Theyareinthelate
stagesofdevelopmentandnotyetconsideredfullymature(asoftheendof2015).The
TeledyneWebbResearch(TWR)APEXfloatsaredeployedbytheUniversityofWashington
(UW)andaretheprimaryplatformfordeployingthebiogeochemicalsensorsdescribed
below.However,theAPEXfloatwiththeBGCsensorsusedinSOCCOMisnotastandard
productofTWRandisnotcommerciallyavailable.SOLOIIfloatsareextensivelydeployed
byWHOIandSIOforthecoreArgoarray,buttheyhavenotyetbeenadaptedtocarrya
varietyofBGCsensors.Theplatformshavenoappreciableeffectonthedatafromthe
biogeochemicalsensors,otherthanthatsomefloatshavepH,NitrateandO2sensorsina
pumpedwatercircuit4.Thismayimpactsensoroperation.Oxygensensorsinthepumped
streamprecludeO2aircalibrationsandanecdotalevidencesuggestsmorefoulingissues
withthenitratesensorswhentheyareinapumpedloop.
2http://www.argo.ucsd.edu/statusbig.gif
3http://argo.jcommops.org/maps.html
4Theplacementinapumpedwatercircuitistominimizefouling.However,someresults
suggestthat,tothecontrary,thisisnotoptimalforthepHandNO3sensors.Theyarebeing
repositionedtohavedirectexposuretoseawater.
6
DetailsofsensorsonSOCCOMfloats5
Temperature(T),Pressure(P),andconductivity/"salinity"(C/S)sensorsarethecore
sensorsontheArgoprofilingfloatsandhavehighaccuracy.AlloftheArgoandSOCCOM
floatscarrySea-Bird(SBE)pressure,temperatureandconductivitysensors.Conductivity
sensorssometimesexperienceslowdriftratesthatarecorrectedforinthedelayedquality
control(QC)processusingsalinityclimatologies6.
ThetemperaturesintheArgoprofilesareaccurateto±0.002°C.Pressuresarenominally
accurateto±2.4dbar,buttheymayalsodrift.Pressurevaluesareadjusted,ifneeded,in
nearreal-timebasedontheknownpressurewhenthesensorisatthesurface.Forsalinity
theaccuracyis0.005basedonpropagationofuncertaintiesinconductivityand
temperature.ThefactoryspecificationsoftheSBEsensorsareprovidedinTable1.
---------------------------------------------------------------------------------------------------------------------
Table1.Accuracyanddriftofthetemperature,Pressure,andconductivitysensorsas
providedbythemanufacturer(Sea-BirdSBE)a.
Sensor
CalibrationStandard Initialaccuracy
Typicaldrift
T(˚C)
ITS-90
0.002
0.0002˚Cy-1
C(S/m)
IAPSO/OSIL
0.0005
0.0002(S/m)y-1
P(dbar)
Deadweight&Pref
2
0.8dbary-1
a:basedonapresentationbySRisertotheCWGon9/15/2015.
---------------------------------------------------------------------------------------------------------------------
ThreetypesofO2sensorsarecurrentlydeployedonSOCCOMfloatsthatallworkonthe
sameprinciplereferredtoasoptodes.TheseopticalO2sensorsconsistofasemi-permeable
membrane,sensingelement,light-emittingdiode(LED)andphotodetector.Thesensing
elementcontainsaluminescentdyethatisimmobilizedinagelmatrix.Whenexposedto
bluelightfromtheLED,thedyeeitherluminescesorisquenchedbyinteractionwithO2.
Somesensorsalsoemitaredlightasareferencetoenhancestability.Thisredlightis
simplyreflectedbackbythedye.Theintensityorlifetimeofthereturnedluminescenceis
measuredbyaphoto-detector.Becausetheintensityofresponsedriftsrelativelyquickly
SBE63sandAanderaaoptodesonlyusethedecaylifetimetocalculatethedissolved
oxygenconcentration.ThequantitymeasurediscloselyrelatedtothepartialpressureofO2
andthesensorcanmeasuretheO2inairaswell,thereforofferingauniquewaytocalibrate
thesensorwhenthefloatsurfaces(Johnsonetal.,2015).
5Fromhttp://sio-argo.ucsd.edu/RG_Climatology.html)and
http://www.argo.ucsd.edu/Data_FAQ.html
6http://www.jcommops.org/Apps/WebObjects/Argo.woa/wo/
OR63C76oBdZD8I3GtmaI1g/1.0.0.41.1.1
7
ForthefloatsdeployedduringSOCCOMtheSBE63andAanderaaOptode38307and4330
modelshavebeenused.AsshowninTable2belowtheyhavesimilarcharacteristics.The
importantdifferenceisthattheSBE63isplumbedintothewaterloopthatalsohousesthe
conductivitysensorandthereforcannotmakeoxygenmeasurementsinairwhenit
surfaces.
Theseairmeasurementsofferanaccuratecalibrationvalueifatmosphericpressureand
watervaporpressureareknownattimeofsurfacing.Airimmediatelyabovetheoceanis
generallyat100%relativehumidity,sothatthewatervaporpressurecanbecalculated
fromairtemperature,salinity,andatmosphericpressure.Theseopportunisticcalibrations
inairwhenthefloatsurfacesofferameanstoadjustforsmalldrifts(Bushinskyetal.,
2016).Theaircalibrationshaveimprovedaccuraciesto2µmolkg-1comparedtothe
factoryspecificationsinTable2.Theaccuracyislimitedprimarilybyknowledgeofthe
barometricpressureatthelocationthefloatreachesthesurface.
---------------------------------------------------------------------------------------------------------------------
Table2.SummaryoffloatO2sensorperformancebasedprimarilyonmanufacturers'
specificationsa.
a:FromapresentationofS.RisertotheCWG(9/15/2015)
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TheNO3sensorsonAPEXfloatsareInSituUltravioletSpectroscopy(ISUS)sensorsbuilt
andcalibratedatMBARI.TheNAVISfloatshaveSUNAsensorsbuiltandcalibratedby
Satlantic.TheSUNAV2(SubmersibleUltravioletNitrateAnalyzer)isachemical-freeUV
nitratesensorbasedontheISUSnitratemeasurementtechnologydevelopedatMBARI.
ISUSandSUNAhavethesamemainopticalcomponents,butSUNAhastheopticalpath
configureddifferently.ThedescriptionbelowisadoptedfromJohnsonetal.(2013).Nitrate
absorbslightintheultravioletwithpeakabsorptionmaximumnear200nmwitha
7The3830sensorswereusedinthe12"pre-SOCCOM"floatsdeployedduringP16Sin
2014.TheyarenolongerusedinSOCCOMfloats
8
moderatelystrongmolarabsorptivity.ThedeepUVabsorptionspectrumofseawateris
dominatedbythecombinedsignalfromnitrateandbromide,withamuchsmaller
contributionfromdissolvedorganicmatterthatisseparatedbymeasurementsatmultiple
wavelengths.TheISUSmakesasingleUVspectralscanwiththelampon(lightscan)and
lampoff(darkscan).Thelightanddarkscansareused,alongwithreferencespectral
intensitiesfromsimilarscansofdeionizedwatermadeinthelaboratory,tocomputethe
absorbancespectrumfrom200to400nm.Theabsorbancespectrumisusedwiththe
concurrenttemperatureandsalinitytocomputenitrateusingthetemperature
compensated,salinitysubtractedalgorithm(SWwithT/Scorr.).Theobservedsalinityand
temperatureareusedtopredicttheUVspectrumduetobromide.Thecorrectedspectrum
thencontainsonlycomponentsduetonitrateandanapproximatelylinearbaselinedueto
organicmatterandinstrumentaldrift.Allspectraldataistelemeteredtoshoresuchthat
nitrateconcentrationscanberecomputedasnecessary.Theaccuracyofthesensorisabout
2μM,andthiscanbeimprovedthroughinsituadjustments.Errorsinnitrateconcentration
areconstantoffsetsovertheentireverticalprofilesuchthattheerroratthereference
depthcanbeaddedbacktotheentireprofileandamuchhigher-qualitysetofnitratedata
canbeobtained.Theadjusteddatahaveaccuraciesontheorderof0.5μM,relativetothe
referencevalue.
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Table3.AccuracyoftheSUNANitratesensorinseawatera,b
withT/S withoutT/Scorrection
b
corr. Detectionlimit
0.5μM 2.0μM
Accuracy(greaterof)
±2μM or±10%ofreading Precision(shortterm)
0.3μM 1.0μM
Drift(perhourlamptime)c
0.3μM 2.4μM
a:Fromhttp://satlantic.com/sites/default/files/documents/2015_datasheet_SUNAV2.pdf
b:Thesevaluesarewithappropriatecorrectionsforsalinityandtemperature
c:Duringatypicalprofilethelampisactivated70secondsoratotalof5.5hoursfora5-
yeardeployment
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The(SBE/Satlantic)pHsensorisanewbiogeochemicalsensoranditisthefirstinorganic
carbonsystemparameterroutinelydeployedonprofilingfloats.TodatethepHsensorson
theSOCCOMAPEXfloatshaveallbeenbuiltatMBARIaspartofatechnologytransfer
agreement.ToensureasmoothtransitionfromsensorsconstructedatMBARIto
commercialversions,theMBARIsensorswillbegraduallysubstitutedwithcommercially
builtsensorsofsimilardesign,beginningwiththedeploymentofSBEpHsensorsonNavis
floats.
MeasurementofpHusingIonSensitiveFieldEffectTransistor(ISFET)technologyis
summarizedinMartzetal.(2010)andbrieflydescribedhere.TheISFETisametaloxide
semiconductorfieldeffecttransistor(MOSFET).Theconductionchanneliscoveredbya
9
thininsulatinglayerofamphotericmaterial.ThepHofthesolutionatthe
insulator/solutioninterfacecontrolsthesite-bindingprotonation/deprotonationstateof
theinsulatormaterialand,hence,thesurfacechargeattheinterface.Theinterfacialcharge
determinesthestrengthoftheelectricfieldintheconductionchanneloftheFET,located
betweenthesourceanddrain.TheISFETsusedinSOCCOMareoperatedbyapplyinga
constantdrainsourcecurrent.Aconventionalreferenceelectrodeisusedinconjunction
withtheISFET.Itisasolid-statechlorideion-selectiveelectrode(Cl-ISE)withverylittle
pressurehysteresis.TheapproximatesalinitysensitivityfortheFET|CI-ISEis0.013pH
salinity–1(Martzetal.,2010).
TheISFETsensordeployedonprofilingfloatsiscalledtheDeep-SeaDuraFETpHsensor.
ThepHisreportedonthetotalprotonscale(pHT).Itcanoperateatpressuresto2000m
depthintheocean.ThereforethesystemmustbecalibratedforpHmeasurementsthrough
largetemperatureandpressuregradients.TheintegratedDeep-SeaDuraFETincludesa
pressuretolerant,solid-stateAgClreferenceelectrodeCI-ISEdescribedabove.
Theoutputofthesensorisavoltage,VRSthatisrelatedtoH+ionactivity,temperatureand
pressureby:
VRS=k0+k2(T-273.15)+f(P,T)-RT/Fln(aH+aCl-)T,P
=k0+k2(T-273.15)+f(P,T)-RT/F[ln(mH,fmCl)+ln(γHγCl)T,1+V$ HClP/RT]
(1)
(2)
k0isthereferencepotential.k2isthetemperaturedependenceofVRS.f(P)isthepressure
coefficientofthesensor.Unlikeaconventionalelectrode,thek0andk2valuesoftheISFET
containtermsassociatedwithFETdesign,semiconductorprocessingandpower
applicationvalues.Thereforethek0andk2valuesmustbedeterminedforeachindividual
sensor.
Giventheuncertaintyofcoefficientsineqn.(2)overtherangeofTandPintheoceanthe
uncertaintycantranslatetoa0.01pHerrorduetopressureat2000dbarpressureand
0.015duetotemperature.Asaresult,theestimatedpHTvaluescanhavebiasesonthe
orderof0.02atpressuresnear2000dbar,basedonpropagationofpossibleerrorsdueto
uncertaintiesinTandPdependence.
InitialdeploymentsoffloatswithpHshowedameanrateofdrift,diagnosedasthechange
betweenpHobservedbelow1000mdepthandtheexpectedpHatthisdepth,of-0.036pH
yr-1.However,withappropriatesensorconditioning,inparticularequilibrationoftheAgCl
referenceelectrode(CI-ICE)withbromideinseawater,thisdriftcanbedecreasedby5-fold.
(K.Johnson,pers.com.).
Frequencyofprofilingmeasurements
ThepHandNO3sensorsareloggedontheupcastofthefloatwithadefault10-daycycle
Approximately65measurementsaretakenontheupwardprofilewiththeAPEXfloats.
ThisrateisdeterminedbypowerrequirementsoftheNO3sensorandlimitationsofthe
electronics.Themeasurementspacingisvariabletocapturerelevantfeatureswith
10
nominally100-mintervalsfrom1500to1000m;50mfrom1000to400m;20mfrom400
to360m;10mfrom360to100m;and5-mintervalsfrom100mto5m.TheassociatedT,
S,andO2dataareprovidedatthesamedepthsinthedatafiles.TheNAVISfloats,withmore
modernelectronics,arecapableofmuchhigherverticalresolution.TheyprovidepH,
oxygenandbio-opticalmeasurementsat2-meterspacingintheupper1000m.Nitrate
remainsatthesameresolutionobtainedwithAPEXfloats.ExamplesoftwoprofilesofO2
andpHfromSOCCOMAPEXfloat9254areprovidedinFigure2.
Figure2.Profilesofoxygen(blue)andpHT(red)fromfloat9254at39˚S,155˚W,for
11/21/2015(opensymbols)and12/01/2015(closedsymbols),illustratingthedepth
resolution,andtherapidbutcoherentchangesinbiogeochemicalparametersintheupper
11
watercolumnduringtheAustralSpring/Summer(dataobtainedon12/05/2015from
http://soccom.princeton.edu/soccomviz.php).
3.Performanceofsensors
Basedoninformationonthesensorsprovidedaboveandthemeasurementsinthefieldto
datewecanprovideanassessmentoftheperformanceofthesensors.Thisisnotexact,as
onlylimitedinsituvalidationispossible.Moreover,sensorsandtheshoresidepreparations
andcalibrationroutinesarestillbeingimproved.Table4providesanassessmentof
currentestimatesofperformanceofthesensorsbasedonmanufacturersspecificationsand
calibrationsinlaboratoryandothervenues.
---------------------------------------------------------------------------------------------------------------------
Table4.Sensorresponseandcurrentestimatesofaccuracyofsensorsduringdeployment
Parameter
Accuracy Stability(drift)
P
2dbar
0.8yr-1
T
0.002˚C
0.0002yr-1
S
0.005
0.01yr-1
O2 2µmol/kg
0.6µmolyr-1
-1(a)
NO3
0.5µmol
0.3μMyr
pH
.01
-0.036yr-1(b)
(a):Basedona1-hourtotallampusageoverayear(≈52profiles)(seeTable3).
(b):FromMartzetal.(2010),newconditioningapproacheshavedecreasedthisdrifttoless
than0.007yr-1
---------------------------------------------------------------------------------------------------------------------
Thereareslightdifferencesinsetupsandtypesoftheinstrumentsdeployedbutoverall
theyshouldhavesimilarperformance.Animportantdifferenceisifthesensorsare
plumbedintoawatercirculationsystemorexposeddirectlytotheoceanwater.The
circulationsystemprecludestheaircalibrationoftheO2sensor,butshouldimprovethe
responsetimeofthesensor.Thecirculationloopshaveananti-foulingagentbuttheNitrate
(opticalsensor)appearstohaveotherfoulingissuesintheloop.Currentlyallthe
biogeochemicalsensorsontheAPEXfloatsarebeingtestedwithsensorsdirectlyexposed
toseawater.
4.Sensordeployments,calibration,validationandchecks
Laboratorycalibrationofsensorshasbeenchallengingduetodifficultiesrecreatingthe
environmentatseawhenthesensorsaredeployed.O2sensorsareroutinelycheckedinthe
laboratory.FortheAPEXfloatstheO2sensorsarecomparedside-by-sidewithacalibrated
laboratorysensorofthesamemakeandmodel.pHsensorsarecheckedpriorto
deploymentinartificialseawaterbutduetodifferencesinchemicalconstituentsbetween
artificialseawaterandrealseawater,appreciableadjustmentsarenecessary.NO3sensors
arecheckedinthelaboratoryaswellbutwhileoverallresponseandgeneralbehaviorof
thesensorcanbechecked,mostdatarequireanadjustmentoncedeployed.
12
ThevalidationCTD/bottlecaststhatarepartoftheusualSOCCOMprotocolatthetimeof
floatdeploymentareanimportantmeanstocheckthesensorswhentheyaredeployedand
todetermineappropriateadjustments.Thefirstprofilefromthefloatoftendoesnot
correspondwellwiththebottledatafromthecastbecausethesensorssometimes
experiencedriftduringtheirinitialprofile,notablytheNO3sensor.TheO2sensors
generallydonotundergoacheckagainstthecalibrationcast,inpartbecausethesensors
showgoodagreementbetweenlabbasedcalibrationvaluesandthedeploymentvalues.
However,forthesakeofconsistency,similarchecksandadjustmentsshouldbemadefor
O2,pHandNO3sensors.Thedeploymentcastisagoodqualitativecheckfordriftand
offsetsbutcurrentlyadjustmentstopHandNO3sensorvaluesareperformedbasedonthe
MLRsdevelopedfromdeploymentcastsandotherhighqualitybottledataintheSouthern
Ocean(seesection7).
ThepHandNO3sensorsroutinelyneeda"soakingtime"ofafewprofilesduringwhich
thereisappreciabledrift.Thedriftisattributedtoseveralfactors.ForNO3itappearsthat
theopticalwindowcouldbedirtiedbyorganicsduringstorageandtransport.ForpHthe
AgClinthereferenceelectrodereactswithBrinseawateruntilequilibriumbetweenAgCl
andAgBrintheelectrodeisobtained.Cleaningproceduresofopticalwindowsandlonger
soakingtimesofpHsensorsinrealseawaterinthelaboratoryarecurrentlybeing
investigatedasaviablemeanstodecreaseinitialdrift.
OncetheNO3sensorstabilizesfromlargedriftthereisasmallerresidualdrift(seeTable
3).ThustheNO3sensoradjustmentofteninvolvesanadjustmentforoffsetsoonafter
deploymentfollowed,atalatertime,bycorrectionforalongertimedrift.Theadjustments
aredoneinanopportunisticfashionbasedonknowledgeofclimatologiesand
stoichiometriesbetweenparameters.Theprimaryadjustmentsaredoneatdepthbasedon
theWorldOceanAtlasWOA,otherclimatology,orasrecommendedforSOCCOMbyaMLR
algorithm(section7).ForNO3sensorsdeployedinsubtropicalregions(e.g.,nearBATS)
wheresummertimenitratevaluesinthesurfacearezero,asecondcalibrationcheckis
basedonthezeroconcentrationinthemixedlayer.Driftovertimeisdeterminedand
correctedbasedoncomparisonoffloatandMLRalgorithmatdepth(1000-2100m)where
nitrateislinearlyregressedagainsttemperature,salinity,pressureandoxygen:NO3=
a+bT+cS+dP+eO2,wherethecoefficientsaredeterminedfromcruisedatainthearea.
5.Recommendedconstantsforinorganiccarbonsystemparameters
AdjustmentofpHsensorsissignificantlymorechallengingthanforothersensorsand
requiresinformationonthedissociationoftheinorganiccarbonspeciesinseawater.This
CO2systeminformationisalsonecessarytocreateinorganiccarbonproductsfromthepH
floatdata.pHsensorsonthefloatsmeasurethepHatinsitutemperatureandpressure
whichisdifferentfromthecalibrationandvalidationsampleswhichareanalyzedat
constanttemperatureand1atm.ThesensorsmeasurethefreeH+ionsanditismost
appropriatetoexpressthepHonthefreescale(K.Johnson,pers.com.).ThepHsamples
fromCTD/Bottlecastsaremeasuredatconstanttemperature(usually20or25˚C)onthe
totalscaleinaship'slaboratory.OnselectoccasionspHisnotmeasuredonboardbut
ratherdeterminedalongwithTAlkonbottlesamplesshippedtotheshoresidelaboratory
13
ofA.DicksonofSIO.Tocomparethebottledatawiththesensordataandapply
adjustmentstothesensordata,carbonatedissociationconstantsandachemicalmodel
needtobeapplied.Detailsofthebasicinorganiccarbonchemistryandrecommended
analysisprocedurescanbefoundinDicksonetal.(2007).
Herewefocusonusingdiscretebottlesamplesatfixedtemperatureand1atmpressureto
adjustinsitupHsensordata.Excel®andMATLAB®routinesarereadilyavailableto
performtheconversions.Thetemperatureandspeciesconversionsrequireachemical
modelandtwoinorganiccarbonsystemspecies(DIC,TAlk,pH,orpCO2)(e.g.Pierrotetal.
2006).TherecommendedconstantsforSOCCOMeffortsareprovideinTable5.Theyare
consideredthemostconsistentbasedoncurrentknowledge,butthemainpurposeofthe
recommendationisforconsistency.Thatis,allSOCCOMproductsshouldbebasedonthe
sameconstantssuchthatdisagreementsinresultsorinterpretationsarenotcausedby
productsbeingdevelopedusingdifferentconstants.
---------------------------------------------------------------------------------------------------------------------
Table5.Recommendedconstantsforcalculatinginorganiccarbonsystemparametersin
SOCCOM,includingtemperatureandpressurenormalizations.
Carbonatedissociationconstants(1)
pHT(2)
Totalscale(molkg-1seawater)
pK0(3):
SolubilityWeiss&Price(1980)
pK1andpK2(4)Luekeretal.(2000)usingpHT
Associatedconstants(5)
pK(B)
DissociationconstantboricacidDickson,(1990)
pK(HF)
DissociationconstantforhydrofluoricacidofPerez&Fraga(1987)
(6)
pK2P,pK3P 2ndand3rddissociationconstantsphosphoricacidDicksonetal.,(2007)
Pressure(7) Pressurecorrectionsforvariousacid-basedissociationconstantsas
implementedinCO2SYS
TB
TotalboroninseawaterparameterizedwithsalinityLeeetal.,(2010)
(8)
KHSO4 Dickson(1990)
ΩAr,ΩCa(9)
AragoniteandCalcitesaturationstatesasprovidedinCO2SYS.
(1)TheCO2SYSprogramhasbeenadjustedbyD.Pierrottoaccommodatetheseconstants.
(CO2SYSv2.2).ThefootnotesnotesbelowareadaptedfromA.Dickson,pers.com.
(2)Spectrophotometricmeasurementusingpurifiedm-cresolpurpleat25˚Casreference
(Liuetal.,2011).Thesehavebeenverifiedinthelaboratory(at25˚CandS=35)ofA.
Dickson,SIOandtheyagreewithLiuetal.to0.002inpH.However,theresultingpHis
notnecessarilyinperfectagreementwiththewaypHisdefinedinthedefinitionofthe
variousacid-dissociationconstants.Preliminaryworksuggestsadiscrepancyofabout
0.006inpH.IfthisdiscrepancyisaccountedforthenpH,TAlk,andDICareinternally
consistentwithinlikelyuncertainties.
(3)pK0basedonWeiss(1974),asimplementedintheappendixofWeiss&Price(1980)
14
(4)pK1andpK2basedondataofMehrbachetal.(1973),convertedtototalhydrogenion
concentrationscale(Luekeretal.,2000).
(5)Toconverttotalalkalinitytocarbonalkalinityneededincalculations,andtoestimate
thehydrogenionactivityfromnon-carbonatespeciesinseawater.Valuesarevalidfor
S≈20-38,T≈-2to40˚C)
(6)pK2P,pK3PforH3PO4dissociationarebasedonMillero(1995)correctedtopHTscale
(seeDicksonetal.,2007)
(7)Pressurecorrectionsforacid-basedissociationconstantsasimplementedinCO2SYS,
ThispressuredependenceisbasedonoriginalworkbyCulbersonetal.(1968)onthe
equivalentofthefreepHscalewithsubsequentapproximationsofthe∆VofHCO3-
basedonthatofHSO4-(K.Johnson,pers.com.).
(8)WatersandMillero(2013)andKhooetal.(1977)showvaluesofKHSO4thataretwiceas
highastherecommendedKHSO4ofDickson(1990).Theeffectsofpressureand
temperaturearedifferentforthevariousKHSO4valuesthatwillleadtodifferencesin
pHof≈0.01forthedifferentKHSO4valuesonthefreescale
(9)Note,somecurrentbiogeochemicalmodels(e.g.TOPAZ)donotreproducecalculated
valuesfromCO2SYSusingTAlkandDIC.ThisappearstobeanissuehowCa2+is
determined.InCO2SYS,Ca2+=293.84*S.
---------------------------------------------------------------------------------------------------------------------
ForchecksofotherCO2systemcodesthefollowingtwoexamplesareprovided.
TestcasesusingCO2SYSV2.2andconstantsinTable5:
Sample400217Station40,P16S(2014)
Inputvalues
P=1000.3dbar
S=34.319
T(insitu)=5.720˚C
TAlk=2286.4µmolkg-1
DIC=2169.1µmolkg-1
pHT(20,0)(a)=7.7366
PO4=1.82µmolkg-1
SiO3=17.8µmolkg-1
outputusingTAlkandpHT(20,0)
pHT(5.72,1000)=7.9104
pHF(5.72,1000))(b)=7.9593
DIC=2167.8µmolkg-1
fCO2(5.72,1000)=493.4µatm
ΩAr(5.72,1000)=1.14
ΩCa(5.72,1000)=1.79
Sample400210Station40,P16S(2014)
P=1999.5dbar
S=34.581
T(insitu)=2.626˚C
TAlk=2350.6µmolkg-1
DIC=2264.3µmolkg-1
15
pHT(20,0)=7.632
PO4=2.37µmolkg-1
SiO3=82.4µmolkg-1
outputusingTAlkandpHT(20,0)
pHT(2.62,2000)=7.8062
pHF(2.62,2000)=7.8459
DIC=2265.9µmolkg-1
fCO2(2.63,2000)=576.5µatm
ΩAr(2.63,2000)=0.74
ΩCa(2.63,2000)=1.15
(a):pH (20,0)isthepHonthetotalscaleattemperatureof20˚Candpressureof1atm.
T
(b):pH (5.72,1000)isthepHonfreescaleattemperatureof5.72˚Candpressureof1000
F
dbar(98.7atm).pHFiscalculatedfromDICandTAlkwhereDICinturnisdetermined
fromTAlkandpHT(20,0).
6.pHcomparisonsbetweenbottleandfloatdata
FortheSOCCOMeffort,useofthetotalpH(pHT)scaleisrecommended.Theotherscales
commonlyusedinseawateraretheseawaterscale(pHsw)andfreescale(pHf)8.A
comprehensivedescriptionofthemetrologyofpHcanbefoundinDicksonetal.(2016).
TheDeep-SeaDuraFETpHsensormeasuresthefreehydratedhydrogenionsandpHf
wouldbethebestscaleforthissensorinparticularbecausethepressuredependenceof
bisulfateioninseawaterisnotwellknowncausinganuncertaintyinthecalibrationofthe
pHdataatdepthcomparedtothesurfacereference.However,theDuraFETsensorsare
calibratedandvalidatedonthepHTscaleasthisisthescaleofchoicefor
spectrophotometricbottlepHmeasurementsinseawater.
Table6providesanoverviewofthecalculatedpHat25and0˚Candatboththesurface
and2000dbarusingdifferentpHscales.ThistableshowsthatthenumericalvaluesofpHT
andpHswarequitesimilar,duetothefactthathydrofluoricacidhasasmallcontributionto
hydrogenionsinseawater,butthedifferencewiththefreescaleisappreciable.Of
particularnoteisthatthetemperatureandpressuredependenceofthepHfandpHTscales
differwhichisacauseofambiguityintheDuraFETpHvaluesreported.Withtheadvances
inpHsensortechnologyacarefulevaluationofthetemperatureandpressuredependence
ofpHinseawaterisurgentlyneeded.
---------------------------------------------------------------------------------------------------------------------
Table6.ComparisonofpHscalesfordifferenttemperaturesandpressuresa
S
T(˚C)
P(dbar)
pHsw
pHT
pHf
35
25
0
7.514
7.523
7.635
8
pHf=-log{H+},pHT=-log({H+}+{HSO4-}),andpHsw=-log({H+}+{HSO4-}+{HF0},where
{H+}indicatestheactivityoffreehydratedhydrogenions.
16
35
0
0
7.876
7.883
7.824
35
25
2000
7.443
7.452
7.552
35
0
2000
7.793
7.799
7.839
a:calculatedusingtheCO2SYSV2.2programwhereTAlk=2300µmolkg-1;DIC=2230µmol
kg-1;PO4=2µmolkg-1;andSi=50µmolkg-1
---------------------------------------------------------------------------------------------------------------------
AnexampleofthemagnitudeofcorrectionsofpHforadepthprofileofacalibration
station,Station40onP16S,2014whereaSOCCOMfloatswasdeployed,isshowninFigure
4.TheredlineisthepHTmeasuredfromCTD/Bottlesamples(20˚C,1atm)andthegreen
lineisthecalculatedpHTatinsitutemperatureandpressurewhichwouldbesimilartoa
pHprofileobtainedfromaSOCCOMpHsensor.Atapressureof1600dbarandinsitu
temperatureof3.15˚Cthedifferencebetweenthevalidationdataat20˚C,1atmandinsitu
pHdatais0.188ofwhich0.242isassociatedwiththetemperaturedifferenceand-0.065is
thepressureeffectonpH.Incomparison,thedifferencebetweenmeasuredpHT(20,0)and
calculatedpHT(20,0)fromTAlkandDICfortheshipboardmeasurementis
-0.005.
Figure4.ExampleofpressureandtemperatureeffectonpHbasedonaCTD/bottlecaston
cruiseP16.TheredlineistheinterpolationofthepHTmeasuredbyspectrophotometry(open
redcircles).TheblueopensquaresarethepHTvaluescalculatedfromTAlkandDIC.The
greenlineisthecalculatedpHTatinsitutemperatureandpressuremimickingafloatprofile.
17
Thedashedlinewithcrossesistheeffectofpressurerelativetothemeasureddata,andthe
dashedlinewithplussymbolsistheeffectoftemperaturerelativeto20˚C.
Bothtemperatureandpressureareappreciablecorrectionsthataresensitivetothe
dissociationconstantsused.TocalculatetheeffectoftemperatureonpHrequiresDICor
TAlkasasecondinorganiccarbonparameterbuttheaccuracyofthemeasurementofthe
inorganiccarbonparameterhaslittleeffectontheestimate.Forinstance,a1%uncertainty
intheICparameter(DICorTAlk)willtranslatetoanegligibleuncertaintyof0.0005inthe
pHforaconversionfrom20˚Cto0˚C.Thusforvalidationsamplestakentoshoreforthe
purposeofcomparingpH,theTAlkmeasurementneednotbeofhighaccuracy.The
temperatureeffectonpHisabout0.2%˚C-1onlogscale.Thepressurecorrectionisabout0.04/1000dbar.Onlyonesetofconstantsisavailableforthiscorrection,andits
uncertaintyisunknown.Sincethereareuncertaintiesintheconstantsandconversions
frominsitupHfromthefloattospectrophotometricpHmeasurementsitisprudentto
attainthehighestqualityDICandTAlkmeasurementsasthesemeasurementscanprovide
anindependentestimateofpH.
7.UtilizingmultiplelinearregressionstoadjustpHandNO3sensorsonSOCCOM
floats
SeveralapproachescanbeappliedtoadjustthepHandNO3datafromsensorsonthefloats
butrobustmeanstodosoinsituarelimited.DuringthefirstyearsofSOCCOMitwas
realizedthatapplyingmultiplelinearregressions(MLR)developedfromhighquality
shipboarddataintheregionisapowerfulmeanstodiscernoffsetsinfloatsensorsat
depth.ThepHandNO3sensorsappeartovaryinaconsistentmannerwithinthepressure
andtemperaturerangesoverwhichtheyprofile.Thismakesitpossibletoadjusttheprofile
basedonsensor-offsetsobtainedbycomparingthesensormeasurementsatdepthwith
MLRscreatedfordepthsof1000-2100dbar.
TheMLRsforNO3andpHarecreatedwiththesameindependent/predictorvariables:P,T,
S,andO2asthesevariablesaremeasuredoneachofthefloatsthatcarrythepHandNO3
sensors.Therefore,eachfloatcontainstheinformationforchecksoftheNO3andpH
sensorsonboard.TheoverallapproachtocreatetheMLRsisdescribedinJuraneketal.
(2009)withconsiderationofcrosscorrelationandoverfittingissuesthatcanplagueMLR
approaches.
Theempiricalmultiplelinearregressionrelationshipsaredevelopedfromcruisedatain
2011(S04P)and2014(P16S)intheSouthPacificsouthof45˚Sfordepthfrom1000to
2100dbar.Thedepthrangeandassociatednarrowparameterspaceincreasesthe
goodnessoffitandavoidsspuriousresultsthatcanoccurwhenindependent/predictor
variablesarerelatedasoftenisthecaseforbiogeochemicalandphysicalparametersinthe
ocean.
TheMLRalgorithmsdeterminedoverthe1000to2100mrangeare:
18
𝑝𝐻! 𝑇, 𝑃 = 1.380 + 1.8020×10!! 𝑂! + 0.17859 𝑆 + 7.4820×10!! 𝑇 − 3.966×10!! 𝑃, 𝑟 !
= 0.98, 𝑠. 𝑑. = 0.004 (3)
𝑁𝑂! = 544 − 0.108 𝑂! − 11.4 𝜎! − 4.92 𝑆 − 2.69 𝑇 − 3.14×10!! 𝑃, 𝑟 ! = 0.85,
𝑠. 𝑑. = 0.3 (4)
DetailsonthealgorithmscanbefoundinthesupportinginformationofWilliamsetal.
(2016).AlgorithmsforpHTutilizingNO3insteadofO2havebeendevelopedaswellwith
similaruncertaintiesbutsincetheO2sensorsonthefloatsaremoreplentifulandmore
accurate,thealgorithmswithO2willbeusedexclusivelyforthepurposeoffloatpH
adjustment.ForthepHalgorithmsthediscretepHmeasurementsat20or25˚Catthe
surfacewerecorrectedtoinsituvaluesusingthedissociationconstantsinTable4.The
outputsfromalgorithmshavebeencomparedwithindependentcruisedataintheSouth
Pacific(P18S-2007),SouthernIndianOcean(S04I-2012),andAtlanticOcean(A16S-2014)
anddifferencesaregenerallywithinthelimitsof0.01forpHand0.5µmolkg-1forNO3
(Figure5).Atthegeographiclimitsandinareasofdeep-waterformationthedifferences
tendtoincreaseandthisshouldbetakenintoconsiderationwhenfloatdataareadjustedin
theseregions.
19
Figure5.ComparisonoftheMLRoutputforpHT(T,P)withmeasuredvaluesnotusedinthe
MLRdevelopmentintheSouthernOcean.A16SisameridionallineintheSouthAtlanticand
P14Sisazonalline(≈62˚S)SouthofAustralia.Thecirclesdepictthesamplingdepthand
thosewithcolorlesscentersindicatethattheMLRoutputiswithintheuncertaintylimitsof
0.01oftheanalysis.
ForSOCCOMallpHandNO3sensordataareadjustedbasedonthealgorithmlistedabove.
AdjustmentsareperformedbasedonpHandNO3floatdataacquiredatdepth.ForpHan
additiveadjustmentismadetothereferencepotentialk0(seeeqn.1)toreach
correspondencewiththealgorithmsvalues.ForNO3theadjustmentisadditivetoits
reportedconcentrationvalue.
8.Datamanagement,qualitycontrolandadjustments
SuccessfulfullimplementationoftheSOCCOMfloatprogramrequiresimprovedefficiencies
andprotocolsindataacquisitionandmanagement.ContinuedinteractionswithArgodata
acquisitioncenters(DAC)areamusttoassurefullincorporationofbiogeochemicalsensors
andbiologicalsensorsintotheArgodatasuite.ThisisparticularlytrueiftheSOCCOM
effortisconsideredaprecursorofaglobalbiogeochemicalandbiologicalsensorarray.The
challengesofmakingadevelopmentaleffortlikeSOCCOM,whichrequiresflexibilityand
adaptationsbutalsothemorerigidrequirementsofthemorematurearrayaresignificant.
Atthesametimepiggy-backingontheArgoarrayisagreatbenefitforlessonslearnedand
establishedinfrastructure.
TheSOCCOMbiogeochemicalparametersmeasuredonfloatsO2,NO3andpHwillbe
transitionedfrominvestigatorbasedmanagement(K.JohnsonandS.Riserlead)toa
streamlinedandconsistentapproachmanagedbytheUniversityofWashington(S.Riser).
ThebiogeochemicaldatawillbeprocessedinparallelwiththecoreArgodatafollowing
agreementswiththeArgosteeringgroup.
20
TheArgoprocessingisasfollows9.Therearethreerelevantfilesthatarecreatedforeach
float:acore-Argo,B-ArgoandM-Argoprofileandtrajectoryfiles.Thecore-Argoprofileand
trajectoryfilescontainonlytheCTDdata.Theparametersincludedinacore-Argofileare
pressure,temperature,salinity,andconductivity.TheB-files,whereBstandsfor
biogeochemistryorbiology,willincludeallotherparametersexcepttemperature,salinity,
andconductivity.Theywillincludeallintermediateparametersthatarenecessaryfor
calculatingparametersofinterestsuchastheUVspectraforNO3andluminescencedecay
timefortheO2optodes.ThecoreandBfilesarecreatedprimarilyforeaseofcalculating
and,whennecessary,adjustingparameters.ThecoreandBfilesarethenmergedintoMfileswhichwillbeofmostinteresttoinvestigators.Theywillcontainalloceanvariables
thatthefloatmeasures.Thisfilewillcontaintheconcentrationofinterestforthecore
parametersandparametersfoundintheB-filebutnottheintermediateparameters.The
transitiontothisnewdataprocessingscheme(calledversion3.1)willbechallengingand
possiblyslowdownthereleaseofbiogeochemicaldatafromfloatswhiletheprotocolsare
beingimplemented.
9.Productdevelopment
ApriorityforSOCCOMistoutilizethelargeincreaseinhigh-resolutiondatainthe
SouthernOceantoestimatethestateofbiogeochemicalproperties,andtheprocessesand
ratesinvolved.Inparticular,themagnitudeandcausesofseasonalvariabilitywillbe
establishedforthefirsttime.Theyear-roundpresenceoftheprofilingfloatsandabilityto
sampleundericewillofferseasonallyandspatiallyunbiasedresults.
Twoapproachescanbefollowedfordeterminingfieldsandprocessesinvolvinginorganic
carbonparametersbasedonthefloatandadditionaldata.Thefirstapproachisto
determineTAlkempiricallyfromfloatdataorothermeans.ThederivedTAlkand
measuredpHfromthefloatcanthenbeusedwiththethermodynamicrelationshipsand
constants(Table4)todetermineanyoftheothercarbonsystemparametersofinterest
suchasDIC,pCO2,ΩArorΩCa.RobustrelationshipsofTAlkandsalinityandtemperature
havebeenderivedforsurfacewater(Leeetal.,2006;Takahashietal.,2014)basedonthe
conservativebehaviorofTAlkonregionalscales.Forsubsurface,remineralizationand
mixingprecludessimplerelationshipsandmoresophisticatedalgorithmsareestablished
basedontheglobaloceanwatercolumncarbondataset(GLODAP-2,Keyetal.,2004).
Carteretal.(2016)developedanalgorithm(andassociatedMATLABcode)toestimate
alkalinityglobally,referredtoas“locallyinterpolatedalkalinityregression.”Specific
algorithmswillalsobedevelopedfortheSouthernOceanaspartofSOCCOM(Williams,
pers.com.)
9www.argo.ucsd.edu/Data_FAQ.html
21
Figure6.Diagramofpathwaysfordataflowandqualitychecks.Thebluelinesindicatethe
datastreamsusedforinterpretation,thegreenlinesarethepathwaysusedtoadjustthefloat
dataandtheredlinesarethepathwaysofqualitycontrolleddataflow.Thedoublearrows
indicatethatthedatarepositoriesbothingestandcontributetothequalitycontrol.The
proceduresinthedashedrectanglearethefocusofthisreport.Notethatmostdepositories
havemultiplefunctions.
Asecondapproachistoestablishempiricalrelationshipswiththeparameterofinterest
andpredictorvariablesthatcanbemeasuredonfloats,ordeterminedfrombottledata
fromcruisesandappliedtofloatdata.ThisapproachissimilartotheMLRalgorithmsused
toadjustthepHandNO3dataatdepthforfloats.Thismethoddoesnotnecessarilydepend
onpHdataorTAlkestimates.However,itdoesassumethattheMLRwithpredictor
variablesthatarenotinorganiccarbonsystemparameterscanfaithfullypredictthe
inorganiccarbonsystemparametersofinterestonscalesthatarerelevanttothecarbon
cycle,fromsub-mesocaletomesoscaleandbeyond;andtimescales,fromseasonalthrough
interannualtodecadalandbeyond.Anyprocessthatchangesthecarbonsystem
parameterswithrespecttothosepropertiesusedintheMLRwilldegradetheestimatesof
22
theevolvingcarbonsystemparameters.Forexample,inthesurfacemixedlayerfunctional
dependenciescanchangerapidlyduetogastransferandbiologicalproductivityandoften
cannotbewellrepresentedbyMLRs.OndecadalscalesanthropogenicCO2increaseswill
invalidateMLRspredictingDIC,pCO2orpH.Whilebothapproachesneedtobeinvestigated
infull,itisprobablethatparametersthatcanbebetterpredictedinthesurfacelayersuch
asTAlkalongwithmeasuredfloatpHwillhavesomeadvantagesoverlarge-scaleMLR
approachesnotusinginorganiccarbonsystemparametersinthesurfacemixedlayer.
10.Outlook
TheSOCCOMeffortisinitsinfancyandmakingthenecessarypreparationstofullyutilize
theobservationsandmodelswhenfullyinplace.AsofFebruary2016,37,or18%ofthe
planned200total,floatsaredeployed;thisnumberwillbecloseto50,or25%,bytheend
ofMay2016.Implementationofrobustdatamanagementandbiogeochemicaldata
adjustmentschemesisunderwaybutwillrequirecontinuedcloseinteractionwithArgo
andsignificantresourcesfromSOCCOM.Asdescribedabove,creativeandrobust
approachesusingmultiplelinearregressionsfromCTD/bottledataareusedtocorrectNO3
andpHdatatowithin0.5µMand0.01,respectively,whichiswithincurrentinstrument
specifications.ContinuedchecksandimprovementsofthealgorithmsutilizingCTD/bottle
dataandbottlesynthesisproductsthatwillbedeliveredduringtheSOCCOMprojectare
advisable.Further,theprogrammustrigorouslyassesssensorperformanceandfailuresto
ensurethatsensorreliability,accuracyandprecisionarecontinuouslyimproved.Thiswill
minimizethedependenceonadjustmentproceduresandresultindataofevergreater
utility.
Biologicaldatafromtheprofilingfloats,"bio-optics",arenotaddressedinthisreportbut
willrequireasimilarscrutinyonsensorperformanceandapproachestoadjustsensor
values.Asmanyofthebio-opticalparametersarenotstatevariablesandsometimesare
operationallydefinedthiswillbeamuchgreaterchallenge.Uncertaintyestimateswillalso
begreaterandapplicabilitytosomeofthelarge-scalequestionsraisedinSOCCOMwillbe
limited.However,theiruseinstudyofseasonalityofthebiologicalcycleandspatial
heterogeneity(bothinthehorizontalandvertical)ofbiologicalparameters,andtheir
influenceontheinorganiccarboncyclewillbeinvaluable.Moreover,thebio-optical
parameterslinkcloselytoseveralofthespacebornemeasurementsofferingthepossibility
toexpandthe2-Dviewoftheoceansurfacefromspacetoa3-Doreven4-Dviewthat
includestimeanddepthdimensions.
AmajorfocusofSOCCOMaspartofunravelingthemysteriesoftheSouthernOceanisto
vastlyimproveourunderstandingofbiogeochemicalprocessesandassociatedestimatesof
biogeochemicalfieldsintheSouthernOceanasawhole.Themeasurementsandtoolsto
tacklethislargescaleproblemarebeingdevelopedanddeployedoverthefull6yeartime
scaleofthegrant,butitiscriticalthatinitialestimatesbemadeasearlyaspossiblewitha
smallsubsetsofthearrayinordertoidentifyandaddressasearlyaspossibleimportant
issuessuchasthoseaddressedinthisreport.Similarquestionshavetobeaddressedwith
respecttomodelsthatwillimprovethroughouttheSOCCOMperiod.
23
Evenwithlimitedfloatdeploymenttodateseveralnuggetsor"firsts"canbeobtainedfrom
thequality-controlleddata.Thispertainsparticularlytosomeofthefirstbiogeochemical
observationsinwintertimewhenconditionsaresuchthatship-basedobservationsare
challengingandabilitytosampleunderice.Undericeprocessesaspertainingto
biogeochemistryinseasonalsea-iceregionscannowbedetermined.Biogeochemicalstate
estimatescanbevalidatedbyfloatmeasurementsparticularlyforthewintertime.
ObservingSystemSimulationExperiments(OSSEs)fornetworkdesigncanbecheckedwith
thefloatscurrentlydeployedandthevalidityoftheassumptionusedintheOSSEscanbe
verified.
ThenextmajordevelopmentforbiogeochemicalinvestigationinSOCCOMwillbethe
developmentofso-callcarbonproducts,orfieldsofcarbonparameterssuchasTAlk,DIC,
ΩArandΩCausingavarietyofapproachesrangingfrominterpolationoffloatdata,to
regressionanalysesandnumericalmodels.Byproducingthesefieldsthroughtimekey
processessuchasnetcommunityproduction,calcification,andrespirationcanbeinferred.
Moreover,thetemporalevolutionofpCO2patternsandsaturationhorizonscanbestudied
andtheroleoftheSouthernOceanasamajoranthropogenicCO2sink.
24
AppendixA.Chargeforthecarbonworkinggroup(CWG)ofSOCCOM
CHARGEFORTHESOCCOMINORGANICCARBONSYSTEMWORKING(CSW)GROUP
[OriginalJuly11,2015;EditedDecember2,2015]
RATIONALE-
AmajorobjectiveofSOCCOMistoassessthechangingcarbonsinkandassociatedchanges
ininorganiccarbonintheSouthernoceanthroughutilizationofnewautonomous
technology.Formanyproperties,suchasaragonitesaturationordissolvedinorganic
carbon,theautonomoussensorsdonotdirectlymeasuretheparametersofinterest.In
particular,fortheinorganiccarbonsystem,SOCCOMcurrentlymeasuresonlypH.In
additiontotheobservedpH,asecondcarbonparametermustbeobtainedinordertosolve
thefullinorganiccarbonsystem.Thiswillbeaccomplishedbyusingstoichiometricand/or
statisticalrelationshipsderivedfromthebottledatasetsavailable.Thesealgorithmswill
thenbeappliedatthelocationofthefloatmeasurementsusingtheinsitu[float]data.Such
relationshipsmayalsobeusedtogeneratepHandothertracerestimatesthatcanbeused
toqualitycontrolandadjustsensordata.Theoverallapproachwillprovideadatasetwitha
resolutionofthatofthefloatobservations.Timeresolutionwillbeontheorderofaweek
andspatialresolutionwillbe100’sofkminthehorizontalandmetersintheverticaldown
to2000m.
SeveralgroupswithinSOCCOMaredevelopingandapplyingthesenovelempirical
approachestodeterminetheinorganiccarbonsystemparametersofinterestfromthe
autonomoussensors.AworkinggroupwithinSOCCOMisdesiredtoexchangeideas;to
provideamechanismtoshareresults;andtoapplytheapproachesinnearreal-time
utilizingtheautonomoussensors.Theworkinggroupaimstofacilitaterapiddissemination
ofresultswithsoundinterpretation,providingclearconfidenceintervalsandspecifying
appropriatecaveatsandlimitations.
GOALS-
Theworkinggroupwilladdressseveralspecificquestionswiththeprincipalgoaltoobtain
ameasurement-basedestimateofthefullinorganiccarbonsystemincludingpH(measured
andcalculated),DIC,TAlk,pCO2,CO32-,andAragoniteandCalcitesaturationindexes.
Theworkinggroupwillengageinthefollowingactivities:
1.Assessqualityandissueswiththeindependent[floatbased]variablesincludingdriftand
hysteresis:
-Nitrate,
-Oxygen
-pH
25
-Temperature
-Pressure
Prioritytask:createaninterpolatedestimateoftheoceanicpHusingalgorithmsfromhigh
qualityshipboard/laboratoryinorganiccarbonsystemmeasurementsandrelated
parametersthatcanbeusedtotestthebehaviorofthepHsensor.
2.DevelopastreamlinedapproachforinitialQCandadjustmentoffloat-basedvariables
andadocumentedapproachforuserstosuggestadjustmentsandflags.
3.Documentuncertaintyandvalidity(time,distance,depthrange)ofalgorithms.A
particularfocuswillbeondistributionoferror/uncertaintyintimeandspace.
Prioritytask:ProvideaninterpolatedestimateofalkalinitythatcanbeusedwiththepH
sensorsdatatoproduceafirstsetofpapersonthecarbonsystemasdeterminedusingthe
floats(includingestimateofuncertainty)
Longer-termobjective:Examineallaspectsofthecarbonsystemmeasurementsand
calculationssoastoproducethehighestqualityestimatesofDIC,pCO2andpH,
4.Createabestpracticesmanualforad-hocfloatsensorvalidationsandchecks
*[5.ProvideadvicetotheSOCCOMexeconthecoordinationofmanuscriptsand
presentations]
*[6.Assessqualityandissueswiththevariablesfromopticalsensorsonfloatstomeasure
chlorophyllandbackscatter.]
*Possibleactivitiesatalaterpoint
SOCCOMCarbonWorkingGroupmembers:
Briggs,Ellen:surfacewaterpCO2,iceeffects
Bushinsky,Seth:floatsensors(oxygen)
Carter,Brendan:Alkalinityalgorithmdevelopmentandapplication
Dickson,Andrew:shorebaseddiscreteinorganiccarbonsystemmeasurements
Feely,Richard:P16Sinorganiccarbondata
Gray,Alison:algorithmdevelopmentandapplication
Johnson,Ken:floatdata
Juranek,Laurie:algorithmdevelopmentandapplication
Key,Bob:Bottledataassembly,includinghistoricaldata;hydrography
Riser,Steve:floatdata
Talley,Lynne:P16Smeasurements,CTDhydrography
Williams,Nancy:algorithmdevelopmentandapplication
Sarmiento,Jorge:modelapplication
Russell,Joelle:modelapplication
Verdy,Ariane:BiogeochemicalSouthernOceanStateEstimate(SOSE)
26
Wanninkhof:Coordinator
APPROACH:
-Bi-Weeklyteleconferenceswithassignmentsoftasks&reports
-Protectedsiteforsharingdataalgorithmsandpreliminaryfindings
-Inquiryto[outside]expertsonregimes,approaches,methods,&analyses
27
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