©ThiscopyofthethesishasbeensuppliedonconditionthatanyonewhoconsultsitisunderstoodtorecognisethatitscopyrightrestswiththeauthorandthatuseofanyinformationderivedtherefrommustbeinaccordancewithcurrentUKCopyrightLaw.Inaddition,anyquotationorextractmustincludefullattribution.

NewapplicationsofcontinuousatmosphericO2measurements:

meridionaltransectsacrosstheAtlanticOcean,andimproved

quantificationoffossilfuel‐derivedCO2

 

By

PENELOPEPICKERS

Athesissubmittedtothe

SchoolofEnvironmentalSciencesofthe

UniversityofEastAngliainpartial

fulfilmentoftherequirementsforthe

degreeofDoctorofPhilosophy

SchoolofEnvironmentalSciences

UNIVERSITYOFEASTANGLIA

2016

 

 

 

Chapter5

QuantifyingfossilfuelCO2usingAPO:anovelapproach

1925.1Introduction

Anthropogenicgreenhousegasemissionsfromfossilfuelburningarethedominant

driverofcurrentclimatechange.Inordertomitigateadverseconsequencesofanthropogenic

climatechange,emissionsofanthropogenicCO2andnon‐CO2long‐livedgreenhousegases,

suchasmethane(CH4)andnitrousoxide(N2O),requiresignificantreduction,whichhasledto

widespreadnationalandinternationalregulationofsomeanthropogenicgreenhousegas

emissionsinrecentyears(WeissandPrinn,2011).Althoughonaglobalscale,annual

anthropogenicgreenhousegasemissionsarerelativelywellknown,thereissignificant

uncertaintyassociatedwithregionalandcountry‐scaleannualemissions,aswellastheintra‐

annualvariabilityofemissions(Peylinetal.,2011).

Thesourceofuncertaintyinanthropogenicgreenhousegasemissionslargelystems

fromtheso‐called‘bottom‐up’methodologiesemployed;typically,greenhousegasemissions

arecalculatedusingabook‐keepingorinventoryapproach,wherebyemissionfactorsare

appliedtoparticulareconomicactivities,whicharethenscaled‐uptoregionalandcountry‐

levelspatialscalesusingland‐useandeconomicdatabases,withuncertaintiesthatareoften

eitherstatedas‘unknown’orarequotedtounrealisticallyhighprecision(NisbetandWeiss,

2010;WeissandPrinn,2011).Suchbottom‐upmethodsarevulnerabletolargeuncertainties

andbiasesbecausetheyarebasedonemissionfactorsassociatedwiththerawmaterialsused

forvariouseconomicactivities,ratherthantheactualemissionsthataregeneratedbysuch

economicactivities,whichcanbeveryvariable,dependingontheefficiencyofindividual

processesandonthequalityofthefuel,forexample.Asstatedby(NisbetandWeiss,2010),

relyingonbottom‐upmethodologiesforquantifyingandsubsequentlymitigating

anthropogenicgreenhousegasemissionsisanalogousto“dietingwithoutweighingoneself”,or

inotherwords,relyingoncaloriecountingalone.

Accurateandprecisequantificationofanthropogenicgreenhousegasemissionsmay

benecessaryinordertofacilitatealegallybindinginternationalagreementonclimatechange,

withtrulyeffectiveemissionsreductions.Inaddition,well‐knownanthropogenicgreenhouse

gasemissionsarerequiredinordertoprovidestabilitytothecarbonemissionstrading

markets,whicharecurrentlyworthaboutUS$350billionperyearglobally(Kossoyetal.,

2015).Thereisalsoastrongneedfromthescientificcommunityforaccurateanthropogenic

greenhousegasquantification,owingtothefactthatmanygreenhousegases(suchasCO2and

CH4)haveanthropogenicandnaturalsources.Inversemodellingstudiesaimingtoquantify

naturalgreenhousegassourcesandsinksoftenassumethatanthropogenicgreenhousegas

emissionsareaccurateandprecise,whichcanleadtosignificantbiasesinnaturalgreenhouse

193gasfluxes,particularlyasthespatialandtemporalresolutionofatmospherictransportmodels

increases(Gurneyetal.,2005;Peylinetal.,2011).

Usingatmosphericmeasurementsandinversemodellingtoverifyanthropogenic

greenhousegasemissions,knownasa‘top‐down’approach,canprovideanindependent

methodforverifyinganthropogenicgreenhousegasemissions.Recentimprovementsin

atmosphericgreenhousegasmeasurementtechnologies,theexpansionofmeasurement

networks,anddevelopmentsininversemodellingtechniquesnowenablecountry‐scaletop‐

downverificationofsomeanthropogenicgreenhousegasemissionsindevelopedregions,such

asNorthAmericaandEurope(e.g.Bergamaschietal.,2005;Levinetal.,2011),with

uncertaintiesthatareatleastcomparabletorealisticbottomupinventoryestimates(Nisbet

andWeiss,2010;WeissandPrinn,2011).

QuantifyingfossilfuelCO2emissionsusingatmosphericmeasurementsrequiresthe

separationofnatural(mainlybiospheric)andanthropogenic(mainlyfossilfuel)influenceson

atmosphericCO2molefractions,inordertoisolatethefossilfuelcomponentofatmospheric

CO2(ffCO2).InversemodellingcanthenbeperformedusingatmosphericffCO2data(inppm)to

verifyfossilfuelCO2emissions.Thistop‐downseparationofbiosphericandfossilfuelderived

CO2andsubsequentquantificationofffCO2isnottrivial.Thecurrentmethodologyfor

quantifyingffCO2fromatmosphericCO2measurementsistousediscretemeasurementsof

radiocarbon(14C)contentinCO2(14CO2):14Chasahalf‐lifeofabout5730years,andtherefore

fossilfuelderivedCO2containsno14C(Manningetal.,1990;Turnbulletal.,2009;Zondervan

andMeijer,1996).Measurementsof14CO2are,expensive,however,andcannotbemade

continuously;hence,most14CO2timeseriesconsistofasinglemeasurementapproximately

onceortwiceeverytwoweeks.ffCO2iscalculatedfrom14CO2measurementsasfollows(Levin

etal.,2003;Turnbulletal.,2009):

∆ ∆

∆ ∆ (Eq.5.1)

whereCO2obsdenotestheatmosphericCO2molefraction,andΔobs,ΔbgandΔffdenotethe14C

contentofCO2(inpermilunits)oftheobservations,well‐mixedatmosphericbackground,and

fossilfuels(‐1000‰,whichisthevalueforzero14Ccontent),respectively.Inadditiontothe

termsshowninEquation5.1,asmallcorrectionisalsoappliedtoffCO2whichaccountsfor

otherminorsourcesof14C,includingheterotrophicrespirationandnuclearindustrysources

(Turnbulletal.,2009).

194 InordertoobtainhighertemporalresolutionffCO2quantification(i.e.dailyorhourly,

asopposedtoweeklyorfortnightly),continuousatmosphericmeasurementsofcarbon

monoxide(CO)canbeusedaccordingtoEquation5.2,becauseCOisco‐emittedwithCO2

whenfossilfuelsarecombusted(Gamnitzeretal.,2006;Turnbulletal.,2006;vanderLaanet

al.,2010).

:

(Eq.5.2)

whereCOobsandCObgaretheCOmolefractionsoftheobservationsandofthewell‐mixed

atmosphericbackgroundrespectively,andRCO:CO2istheCO:CO2combustionratioforfossilfuel

emissions,whichvariesbothtemporallyandspatiallyaccordingtochangesinfueltype.

AlthoughitisalotcheapertomakecontinuousCOmeasurementsthandiscrete14CO2

measurements,itisnotpossibletouseCOaloneasareliabletracerforffCO2,owingtothelarge

uncertaintyandspatialandtemporalvariabilityassociatedwithRCO:CO2(Gamnitzeretal.,2006;

Vogeletal.,2010).Inaddition,thereislargeuncertaintyassociatedwithnon‐fossilfuelrelated

COsources(e.g.biomassburning,soils,andatmosphericmethaneoxidation)andsinks(e.g.

fromhydroxylradicalreactions,anduptakebysoils)(Gamnitzeretal.,2006).ffCO2from

continuousCOmeasurementscan,however,becalibratedbyco‐located14CO2measurements,

whichcanbeusedtodetermineaccurateRCO:CO2values(Vogeletal.,2010).Therefore,

continuousCOmeasurementscombinedwithdiscrete14CO2measurementscanbeusedto

quantifyffCO2withhightemporalresolution,butthismethodstillassumesthatanynatural

influencesonCOarenegligible.

Thereareseveralkeylimitationstousing14CO2andCOmeasurementsinorderto

quantifyffCO2.Firstly,RCO:CO2ishighlyvariable,andisknowntovaryondiurnalandsub‐

diurnaltimescales.Thus,using14CO2tocalibrateffCO2fromCOmeasurementsonceperweek

oronceperfortnightwillonlyguaranteeaccurateffCO2atthetimeofthe14CO2measurements.

Secondly,itisnotpossibletodistinguishbetweenfossilfuelsourcesandbioenergysources

usingatmosphericCOdata;hence,calculatingffCO2usingCOmayresultinerroneously

allocatingbioenergy‐derivedCO2asffCO2.Althoughbioenergycurrentlyaccountsforasmall

proportionoftotalanthropogenicfuelsources(approximately10%ofglobalprimaryenergy

supply;IEA,2012b),itispredictedtobecomemuchmoreprevalentinthecomingdecades,

whichmayrenderCOmeasurementsredundantasamethodforquantifyingffCO2inthe

future.Thirdly,itisnotpossibletoaccuratelyquantifyffCO2from14CO2measurementsinsome

195regions,owingtointerferencefromcertainnuclearpowerplant14Cemissions(Gravenand

Gruber,2011;Vogeletal.,2013).ThisisparticularlyaproblemintheUK,wheretheprevailing

south‐westerlywindsoftenpreventaccurateffCO2from14CO2quantification,owingtothe

abundanceofgas‐coolednuclearpowerplantsinsouthernEngland.Thus,intheUK,theonly

top‐downmethodforCO2emissionsverificationthatiscurrentlyavailableinvolves

performingatmosphericinversionsusingwinter‐timeonlytotalCO2atmospheric

measurements(AlistairManning,personalcommunication,2015),whichareverylikelytobe

significantlyinfluencedbywinter‐timebiosphericrespiration.ForParis,thecurrentapproach

istoquantifyffCO2fromdown‐windgradientsinCO2data,incombinationwithbiogenicCO2

fluxesfromlandsurfacemodels,althoughthismethodresultsinverydrasticdataflaggingand

posteriorfluxestimatesthatareheavilyreliantonthepriorinventoryestimates(Breonetal.,

2015;Stauferetal.,2016).

Inadditiontothenuclearpowerplantemissionsissue,(Graven,2015)suggeststhat

thesensitivityof14CO2tofossilfuelderivedCO2iscurrentlydecreasing,owingtotheglobal

increaseinanthropogenicCO2intheatmosphere,andthat14CO2measurementprecisionwill

needtoimprovebyafactorof2overthenextfewdecades,inordertomaintaintoday’s

detectioncapabilityof14CO2toffCO2.ThedevelopmentofanewtracertoquantifyffCO2,which

ismorepreciseandmoreaccuratethanCO,andwhichcanalsobeusedindependentlyfrom

14CO2measurements,wouldthereforebeahighlyvaluedtoolforatmosphericverificationof

fossilfuelCO2inventoryestimates;suchatoolwouldbeextremelyusefultoday,inregionsthat

areseverelyaffectedbygas‐coolednuclearpowerplant14Cinfluences,andalsointhecoming

decades,asthesensitivityof14CO2measurementstoffCO2declines.

5.1.1Outlineofthischapter

InSection5.2,IpresentatmosphericO2,CO2andAPOdatameasuredfromtheroofof

theEnvironmentalSciencesbuildingattheUniversityofEastAnglia(UEA),duringthesummer

of2014.Ithencomparetheshorttermvariabilityinthesedatatotwoothermeasurement

sitesinNorfolk,UK:theTacolnestontalltower(TAC)andWeybourneAtmospheric

Observatory(WAO).InSection5.3,IpresentanewmethodologyforcalculatingffCO2from

APOdata,andcomparetheresultstoffCO2calculatedusingCOand14CO2measurementsand

tomodelledffCO2usingbottomupinventorydata.Lastly,inSection5.4,Isummarisethe

resultsfromthischapter,andoutlineanewpotentialforurbanatmosphericO2andCO2

measurements.

1965.2AtmosphericO2andCO2measuredfromtheEnvironmentalSciences

building,UniversityofEastAnglia

PriortofinaldeploymentonboardtheCapSanLorenzocontainership,the

atmosphericO2andCO2measurementsystemwastestedintheCRAMLaboratoryatUEA

(52.62°N,1.24°E;seeFigure5.1),andairwassampledfromtheroofoftheEnvironmental

Sciencesbuilding(~25mabovetheground)usingaspiratedairinlets,from09Jul‐03Sep2014.

Theaspiratedairinletsweremountedatthehighestpointoftheenvironmentalsciences

building,andthereforewerenotobstructedbyanyotherbuildings,andwerenotclosetoany

ofthebuildingvents.FortechnicaldetailsrelatingtotheatmosphericO2andCO2

measurementsystem,seeChapter2.

Figure5.1.MapshowingthelocationoftheUniversityofEastAnglia(UEA),andalsotheTacolnestontalltower(TAC)andWeybourneAtmosphericObservatory(WAO).

Figure5.2presentstheUEAatmosphericO2,CO2,andAPOdata,aswellasmodel‐

derivedmeteorologicaldata(atmospherictemperature,relativehumidity,atmospheric

pressure,winddirection,andwindspeed),whicharefromtheUSANationalOceanicand

197AtmosphericAdministration(NOAA)GlobalDataAssimilationSystem(GDAS)database.APO

iscalculatedfromtheUEAatmosphericO2andCO2datausingEquation5.3:

.

. 350 (Eq.5.3)

whereO2andCO2aretheatmosphericO2andCO2measurementsinpermegandppmunits,

respectively,‐1.1istheO2:CO2ratioofglobalterrestrialbiosphere‐atmosphereexchange,

0.2095isthemolefractionofO2moleculesindryair,and350isanarbitraryreference.

MultiplyingCO2by‐1.1anddividingby0.2095convertstheCO2datafromppmtopermeg

units.

LargegapsintheatmosphericO2,CO2andAPOdataareduetoperiodsofexperimental

testingofthemeasurementsystem(forexample,whencheckingthemeasurementsystemfor

leaks),whichmeantthatitwasnotpossibletosampleoutsideair.Shortgaps(1‐3hours)are

mostlycausedbyWSS,ZT,andTTcalibrationroutinesbeingcarriedout(seeChapter2,

Section2.3fordetails).AsshowninFigure5.2,theCO2andO2dataarestronglyanti‐correlated,

owingtothedominanceofterrestrialprocessesonthedata.Strongdiurnalvariabilityis

apparentinbothspecies,withhigherCO2andlowerO2generallyoccurringatnight‐time.This

diurnalvariabilityislikelytobestronglyinfluencedbythediurnalrectifiereffect,whereby

atmosphericCO2andO2isdilutedduringtheday,owingtoawell‐mixedboundarylayer,and

relativelyhighboundarylayerheight,andbothspeciesareconcentratedatnight,whenthe

boundarylayerisstableandtheboundarylayerheightisrelativelylow.Inaddition,owingto

thetimeofyearandrelativelyrurallocation,photosynthesiswilllikelybedominatingthe

atmosphericCO2andO2signalsduringtheday,causingadrawdownofCO2andreleaseofO2,

whereasatnight,respirationwillbethedominantbiosphericprocess,resultingintherelease

ofCO2anduptakeofO2.

Thus,inthesummer,diurnalvariabilityinatmosphericCO2andO2iscausedbytwo

reinforcingeffects:diurnalvariabilityinatmosphericmixing,anddiurnalvariabilityin

biosphericO2andCO2fluxes.IncontrastwiththeatmosphericO2andCO2datafromUEA,the

APOdatashowverylittlevariability,andingeneral,donotexhibitastrongdiurnalpattern.

ThisisbecauseAPOisinvarianttolandbiosphericinfluences,andlargelyreflectsonlyfossil

fuelinfluencesonshort‐timescales,andoceaninfluencesonseasonalandlong‐termtime

scales.

198

Figure5.2.Hourly‐averagedatmosphericCO2(toppanel),δ(O2/N2)(2ndpanel)andAPO(3rdpanel)measuredfromtheroofoftheEnvironmentalSciencesbuildingatUEA.Notethatthey‐axesforδ(O2/N2)andAPOhavebeenscaledtobevisuallycomparabletotheCO2y‐axisonamolepermolebasis,and‘bad’datacausedbytechnicalproblemshavebeenexcludedpriortoaveraging.Alsoshownare3‐hourlymodel‐derivedGDASmeteorologicaldata(NOAA):atmospherictemperature(4thpanel:darkredsolidline),relativehumidity(4thpanel:cyanshort‐dashedline),atmosphericpressure(4thpanel:pinkdottedline),winddirection(bottompanel:darknavylong‐dashedline),andwindspeed(bottompanel:greydashed/dottedline).

199

Figure5.3.O2:CO2ratioofhourly‐averageddatameasuredatUEAduringthesummerof2014.δ(O2/N2)isgiveninppmequivalentunitstobecomparabletoCO2onamolepermolebasis.Thesolidredlineindicatesthemajoraxisregressionline,whichisweightedaccordingtothedifferenceinmeasurementprecision(andthereforeuncertainty)associatedwiththeδ(O2/N2)andCO2data,andhasaslopeof‐1.10.ThenegativevalueoftheO2:CO2ratioindicatesthatthetwospeciesareanti‐correlated.

Asmentionedabove,theUEACO2andO2variabilityshowninFig.5.2isdominatedby

terrestrialprocesses,ratherthanfossilfuelburning.ThisisalsodemonstratedinFigure5.3,

whichshowsthatthemeanO2:CO2molarratioforthedatasetis‐1.10;avaluethatisindicative

ofterrestrialbiosphereO2andCO2exchange(Severinghaus,1995).Thereisasmallamountof

scatteraroundthemajoraxisregressionlineshowninFig.5.3,whichsuggeststhatthereis

sometemporalvariabilityintheO2:CO2ratioduringthisperiod.

200

Figure5.4.Hourly‐averagedCO2(toppanel)andδ(O2/N2)(bottompanel)withselecteddiurnaleventscolouredaccordingtotheO2:CO2ratio(seelegendinfigure).They‐axeshavebeenscaledsothattheδ(O2/N2)andCO2panelsarevisuallycomparableonamolepermolebasis.

InordertoinvestigatethisO2:CO2temporalvariability,IcalculatedtheO2:CO2ratiofor

someofthelargest(inmagnitude)individualdiurnalO2andCO2events,andthencategorised

theseeventsintothreegroups,accordingtotheO2:CO2ratiovalues.Figure5.4showsthat

thereisnocorrelationbetweenthemagnitudeandtheO2:CO2ratioofthediurnalevents,

whichindicatesthatthelargesteventsarenotcausedbyacommonsource,andsuggeststhat

atmospherictransporteffectsmayhaveasignificantimpactonthemagnitudeofthediurnal

variabilityatUEA.TherangeofO2:CO2ratiosforthediurnaleventsis‐1.03to‐1.14,which

suggeststhatmanyoftheeventswithmorenegativeO2:CO2ratiosarecausedbya

combinationofbiosphericandfossilfuelCO2.Sincetheterrestrialbiosphereisdominatingthe

O2andCO2variabilitysostrongly,itisdifficulttoidentifywhicheventsarelikelytobe

influencedbyfossilfuelprocesses,andwhicharenot.Thisdifficultyisinpartcausedby

uncertaintyintheO2:CO2ratioofthelocalterrestrialbiosphere.Althoughonaglobalscale,

terrestrialprocesseshaveanoxidativeratioofapproximately‐1.1,onalocalscale,thisvalue

canbeeitherlowerorhigher,dependingonthelocaltypesofvegetationandsoil.Thedata

showninFig.5.4seemtoindicatethatinNorfolk,theO2:CO2ratioofthelocalterrestrial

201biospheremaybeslightlyhigher(lessnegative)than‐1.1,althoughanexactvaluecannotbe

determinedfromtheatmosphericO2andCO2dataalonewithoutalsohavingindependent

quantitativeknowledgeoftheimpactoffossilfuelcombustionontheatmosphericCO2data,or

conductinganelementalanalysisoftheO2:CO2ratioofvarioussoilsandvegetation

representativeoftheNorfolkregion.

Figure5.5.Apolarplotofthevariabilityin2‐minuteO2:CO2ratioswithwindspeed(ms‐1)andwinddirection.MeteorologicaldataarefromtheNOAAGDASproduct.ThepolarplotwascreatedinRusingthe‘polarPlot’functionfromthe‘Openair’package(CarslawandRopkins,2012).

Byusingthehigh‐resolution,2‐minuteO2andCO2datatocalculate2‐minuteO2:CO2

ratios,itispossibletocreateapolarplot,asshowninFigure5.5,toexaminetheoriginof

oxidativeratiosthatareindicativeoffossilfuelinfluences,andthosethatareindicativeof

biosphericinfluences.Thelowest(mostnegative)O2:CO2ratios(i.e.thosethatareindicativeof

fossilfuelcombustion)originatefromtheeast,whichindicatesthatthereisastrongfossilfuel

influencefromNorwich.Thereisalsoanoticeablefossilfueloxidativeratiosignalfromthe

south‐west,whichissuggestiveoffossilfuelinfluencesfromLondon,andpossiblyalsofrom

thenearbyA47andA11majorroadstothesouth‐west.TheUEAcampusisover1.2km2in

area,andischaracterisedbywoodland,marshland,andopengreenareas.Thecampusis

202surroundedbyfieldsandfarmland,withafewsmallvillagestothenorth,southandwest,and

thesuburbsofNorwichcitytotheeast.TheabundanceofvegetationontheUEAcampusand

inthesurroundingarealikelyexplainswhytheO2:CO2ratiosareclosetotheexpectedvaluefor

terrestrialbiosphereprocesseswhenthewindspeedislow(<5ms‐1),withtheexceptionof

windsthatoriginatefromthenorth‐east,forwhichtheO2:CO2ratiosaremorenegative.There

isalsoasmallamountofdatathatdisplaysquitehigh(lessnegative)O2:CO2ratios,which

occurswhenthewindspeedishigh(>15ms‐1)andthewinddirectionisfromthenorth‐west.

Thisdatamayberepresentativeofoceanicinfluence,whichcancauseO2:CO2ratiostobeclose

toorlessnegativethan‐1.0,long‐rangetransportofairfromahigherlatitude,oran

undiagnosedtechnicalproblemwiththemeasurementsystem.

ItisusefultocomparetheatmosphericO2andCO2datafromUEAtoothernearby

atmosphericmeasurementsofeachspecies,inordertogaingreaterunderstandingofthe

spatialvariabilityofatmosphericO2andCO2.TheTacolnestontalltower(TAC)issituated

about12kmsouth‐westofUEA(seeFig.5.1),andisfundedbytheUKGovernment

DepartmentofEnergyandClimateChange(DECC)tomeasurearangeofatmosphericspecies,

includingCO2(fromthreeheights:54m,100m,and185m)andCO(fromasingleheight:100

m).WeybourneAtmosphericObservatory(WAO)issituatedabout35kmnorthofUEAonthe

northNorfolkcoast(seeFig.5.1).WAOismanagedbytheUniversityofEastAngliaandisalso

supportedbyNCAS(NationalCentreforAtmosphericScience),tomakemeasurementsof

atmosphericgreenhousegasesandrelatedspecies,includingatmosphericO2,CO2,andCO(all

from~15mheight).

Figure5.6comparesatmosphericCO2atUEAandTAC,andatmosphericCO2andO2at

UEAandWAO.Ingeneral,thethreemeasurementlocationsexhibitverysimilardiurnal

variabilityinCO2(andO2forUEAandWAO),withonlyafewrareexceptions,suchasthe

differencesinO2andCO2betweenWAOandUEAon26‐27August.Althoughthediurnal

patternintheatmosphericCO2andO2isverysimilarbetweenthemeasurementsites,the

magnitudeofthevariabilitydifferssignificantly.CO2measuredatUEAisalmostalwayshigher

atnight‐timethanCO2measuredatTACandWAO.Similarly,night‐timeO2atUEAisalmost

alwayslowerthanO2measuredatWAO.Themostlikelyreasonforthesedifferencesin

magnitudebetweenUEAandTACisthatthemeasurementheightatUEA(~25m)ismuch

lowerthanallthreeofthemeasurementheightsatTAC(lowestheightof54m).CO2

measurementsthataremadeclosertothegroundareusuallyhigherinCO2molefractionthan

thosethataremeasuredfurtherupintheatmosphere,partlybecauseCO2sourcesaremainly

atgroundlevel,andpartlybecausetheentrainmentof‘backgroundair’(lowerCO2mole

fractions)fromabovetheboundarylayerwillaffectCO2measurementsmadehigherupmore

203thanthosemadeclosetotheground.Thus,CO2measurementsmadefrom~25mheightabove

thegroundwilllargelyreflectlocalinfluencesonCO2,whereasCO2measurementsmadeat

185mabovethegroundwillreflectCO2influencesfromanentireregion,coveringatleast

severalhundredsquarekilometres.

Figure5.6.ComparisonofatmosphericCO2atUEAandTAC(toppanel),andcomparisonofatmosphericCO2andδ(O2/N2)atUEAandWAO(middlepanelandbottompanel).Y‐axeshavebeenscaledsothattheδ(O2/N2)andCO2panelsarevisuallycomparableonamolepermolebasis.

Somewhatcontradictorytothisexplanation,isthefactthatUEAconsistentlyexhibits

higherCO2thanWAOatnight,whenthemeasurementsatWAOaremade~10mclosertothe

groundthanthoseatUEA.ThereasonwhyO2andCO2variabilityatWAOisattenuatedin

magnitudecomparedtoO2andCO2variabilityatUEA,isthatWAOissituatedonthecoast,and

soanyterrestrialsourcesorsinksofO2andCO2willbedilutedwithcoastalandopenoceanair,

whichwillusuallyexhibitO2andCO2molefractionsclosetothoseofwell‐mixed‘background

204air’.Thisalsoexplainswhy,duringtheday‐time,atmosphericCO2molefractionsareoften

loweratUEAthanatWAO,andatmosphericO2molefractionsareoftenhigheratUEAthan

WAO(e.g.26July),whereastheatmosphericCO2molefractionatTACdoesnoteverdrop

significantlybelowtheatmosphericCO2molefractionatUEA.Duringthesummer,the

biosphericphotosynthesisduringthedaytimewilltakeupCO2andproduceO2;thisbiospheric

signalwillmanifestmuchmorestronglyatarural,in‐landmeasurementlocation,suchasUEA,

thanatacoastalmeasurementsite,suchasWAO.

Therearealsosomesignificantdifferencesintheanthropogenicsignalsinatmospheric

speciesbetweenTAC,UEAandWAO.Figure5.7comparesshort‐termvariabilityinAPOfrom

UEAandCOfromTAC(100mheight),aswellasAPOandCOfromWAO.Itisclearthatthereis

oftensignificantanti‐correlationintheAPOandCOshort‐termvariability,whichislikely

attributabletothefactthatbothspeciesarepredominantlyaffectedbyanthropogenicsources.

AlthoughtheUEAAPOandTACCOdataarenotco‐located,itisassumedthattheyaresituated

closeenoughthatthepatternsofvariabilityseenateachlocationwilllargelybesimilar.Hence,

periodswhentheAPOandCOdatadonotdisplayanti‐correlatedsignalsmaybeduetothefact

thatthemeasurementsarenotco‐locatedandaresampledfromdifferentheightsabovethe

ground,butalsomaybecausedbythesignificantnaturalsourcesandsinksthatexistforCO,

suchassoilsandtroposphericphotochemicalreactions(Bergamaschietal.,2000;Moxleyand

Cape,1997),whereasthemainnaturalinfluenceonAPOisfromtheoceans,whichisnot

expectedtohaveasignificanteffectonAPOonshorttimescales(seeChapter4,Section4.2for

details).

ThemiddlepanelofFig.5.7showsco‐locatedAPOandCOmeasuredatWAO,fromthe

samesamplingheight.AswiththeUEAandTACdata,thereissubstantialanti‐correlation

betweenthetwospecies,aswellassomeperiodswherethevariabilityisnotanti‐correlated.

BasedonvisuallyinspectionofFig.5.7alone,thereisasimilaramountofanti‐correlation

betweentheWAOCOandAPOdataasthereisbetweentheUEAAPOandTACCOdata,where

thetwospeciesarenotco‐located.Thisfindingsuggeststhatperiodsofdatathatdonotshow

anti‐correlationbetweenAPOandCOmaybedominatedbydifferencesintheCOandAPO

sourcesandsinks,andnotbywhetherthemeasurementsareco‐locatedornot.Thebottom

panelofFig.5.7,showingwinddirectionmeasuredatWAO,showsthattheperiodsof

strongestanti‐correlationbetweenWAOCOandAPOmostlycoincidewithsouth‐westerly

winddirections(i.e.fromtheland),andperiodsshowinglittleornoanti‐correlationbetween

COandAPOoftencoincidewithnortherlyandeasterlywinddirections(i.e.fromthesea),

althoughthelinkbetweenCOandAPOcorrelation/anti‐correlationandwinddirectionat

WAOdoesnotalwaysholdtrue.

205

Figure5.7.Comparisonofhourly‐averagedTACCOandUEAAPOdata(toppanel)andhourly‐averagedWAOCOandAPOdata(middlepanel),illustratingthatalotoftheshort‐termvariabilityinCOandAPOisanti‐correlated.AlsoshowniswinddirectionmeasuredatWAO(bottompanel).TheCOmeasurementsatTACaresampledfromthe100mtowerinlet.ItshouldbenotedthattheTACCOdatashownabovearenotthefinalised,qualitycontrolleddata,duetoanon‐goingcalibrationissuethatisaffectingtheaccuracyofthehighCOvalues.

5.3FossilfuelCO2quantificationusingAPOfromsitesinNorfolk,UK

5.3.1.Using‘fixed’fossilfuelemissionratios

Inthissection,IpresentanewmethodologyforquantifyingffCO2usingAPOdatafrom

UEAandWAO.Asmentionedpreviously,thereareseverallimitationsassociatedwithusingCO

asatracerforquantifyingffCO2,includinglargeuncertaintyinthenaturalsourcesandsinks,

206largeuncertaintyintheCO:CO2emissionratiosforfossilfuels,andtheCOtracermethodis

unabletodistinguishbetweenCO2producedbyrenewablebioenergysourcesandffCO2.In

contrast,theonlysignificantnaturalsource/sinkaffectingAPOistheocean,whichisnot

expectedtohaveanimpactonshorttimescales.Inaddition,anyshort‐termoceanicinfluences

onAPOshouldbeeasytoidentify,becauseoceanicairmassesarecharacterisedbyinvariant

CO2(owingtothelong‐equilibrationtimeofair‐seaCO2fluxescomparedtotherateof

atmosphericmixing).APOisalsoassociatedwithamuchsmallerrangeofpossibleO2:CO2

emissionratiosforfossilfuels(from~‐1.2to~‐1.95,buttypicallyintherangeof‐1.3to‐1.4)

comparedtoCO:CO2emissionratios(from<5to>100,buttypicallyintherangeof5to25),

whichtranslatesintoloweruncertaintyinthedenominatorofEquation5.4(seebelow)

comparedtothedenominatorofEq.5.2.

Finally,althoughAPOcannotdistinguishbetweenbiodieselandbiogasemissionsand

theirfossilfuelcounterparts,owingtothefactthattheoxidativeratiosforbiodieselandbiogas

areverysimilartothoseforliquidandgaseousfossilfuels,APOisabletodistinguishbetween

biomassburningemissions,whichhaveanoxidativeratioofapproximately‐1.1,andfossilfuel

emissions,whichhaveoxidativeratiosintherangeof~‐1.2to~‐1.95.Thispotentiallyenables

APOtobeusedasatracerofffCO2incitiesindevelopingcountries,suchasIndia,whichstill

heavilyrelyonbiomassburningasamajorsourceofenergyindomesticsettings,andalsoin

citiesindevelopedcountriesthatarefrequentlyaffectedbylocalforestfires,suchasinVictoria,

Australia,andCalifornia,USA.

ffCO2canbecalculatedfromAPOdatausingEquation5.4,whichisanalogous

toEq.5.2forcalculatingffCO2fromCO:

: (Eq.5.4)

whereAPOistheatmosphericvaluecalculatedfromhigh‐precisionatmosphericO2andCO2

data,APObgistheAPObackground,orbaselinevalue,whichisdeterminedusingastatistical

baselinefittingmethod,andRAPO:CO2istheAPO:CO2combustionratioforfossilfuelemissions.

IhaveusedEq.5.4tocalculateffCO2fromAPOdataatUEAandWAO,andhave

comparedtheresultstoffCO2fromCOdataatTACandWAO,calculatedusingEq.5.2(see

Figure5.8).NotethatasmallamountofAPOdatawasexcludedfromtheffCO2calculationasit

wasnotdeemedtoberelatedtofossilfuelvariability(owingtolittleornovariabilityinCO2),

andismostlikelycausedbytechnicalproblems.Therearetwoimportantunknown

207parametersthatmustbedeterminedinEq.5.2and5.4:theCOandAPObaselines,andthe

CO:CO2andAPO:CO2emissionratios.Fornow,Ihaveusedtime‐invariantvaluesof5ppbppm‐

1fortheCO:CO2emissionratioatTACandWAO(atypicalvaluefortrafficemissions),and‐0.3

molmol‐1fortheAPO:CO2emissionratioatUEAandWAO(atypicalvalueforliquidfossilfuel

emissions,giventhatAPO:CO2ratio=O2:CO2ratio+1.1).Amoresophisticatedmethodfor

calculatingtime‐varyingCO:CO2andAPO:CO2emissionratioswillbediscussedandpresented

laterinthissection.ItshouldbenotedthattheequationforcalculatingAPOfromO2andCO2

measurementsthatIhaveusedthroughoutthisthesisisactuallyasimplificationofthefullAPO

equationgivenin(Stephensetal.,1998),whichalsotakesintoaccounttheeffectsofCH4and

COoxidationonO2,althoughtheseeffectsarenegligibleformostapplications.Ihavenotused

thefullAPOequationinthischapterbecauseCH4andCOemissionsinNorfolkarerelatively

low,andIcalculatedthattheywouldnotsignificantlyaffectAPO.ForurbanAPO

measurements,however,itmaybeadvisabletousethefullAPOequationthatisconservative

withrespecttoCH4andCOoxidationinadditiontoterrestrialbiosphereprocesses,because

CH4andCOfluxesaremuchlargerinurbanenvironments.

Fig.5.8showsffCO2calculatedusingCOand14CO2fromTACandAPOfromUEA(top

panel),aswellasffCO2calculatedusingCOandAPOfromWAO(bottompanel).Althoughthe

COandAPOdataatTACandUEAarenotco‐located,theffCO2calculatedusingthetwotracers

appearsverysimilar,withthemaindifferencespresentingasdifferencesinthemagnitudeof

theffCO2peaks(e.g.14‐15Aug),ratherthandifferencesinthepatternsofffCO2variability(e.g.

3Aug).TheffCO2from14CO2dataatTACwereprovidedbyAngelinaWenger,Universityof

Bristol.Approximately40%oftheffCO2from14CO2dataatTACcollectedfromJul‐Sep2014

weredeemedtobeunreliable,eitherowingtonegativeffCO2values,whicharecausedby

strongnuclearpowerplantemissionscancellingoutanyffCO2signalin14CO2,orbecause

NAMEmodelbacktrajectoriesindicatedtheairmassesarrivingatTAChadoriginatedfrom

thesouth‐west,andffCO2from14CO2wasthereforelikelytobebiasedbynuclearpowerplant

influences,eventhoughthevalueswerenotnegative.Ingeneral,theffCO2from14CO2agrees

wellwiththeffCO2calculatedfromtheCOandAPOdata,although,owingtodatagaps,there

areonlytwo14CO2datapointsthatcoincidewithperiodsofAPOdata.Allofthe14CO2data

pointsalsohappentocoincidewithperiodsofrelativelylowffCO2,becausethe14CO2flask

Figure5.8.ffCO2fromCOatTACandAPOatUEA(toppanel)andffCO2fromCOandAPOatWAO(bottompanel).AlsoshownisffCO2from14CO2atTAC(toppanel,blackdots).

209

samplesaredeliberatelycollectedduring‘cleanair’conditions,totryandavoidnuclearpower

plantinfluences.WhenadifferenceinffCO2betweenthe14CO2methodandtheothertwo

methodsdoesoccur,andtheffCO2from14CO2valueislowerthantheffCO2fromCOorAPO

(e.g.29Aug),itisdifficulttoascertainwhichffCO2valueiscorrect,becausenuclearpowerplant

influenceswillcausetheffCO2from14CO2tobebiasedlow,anditisthereforedifficulttohave

confidenceintheaccuracyofthe14CO2datainsuchinstances.

AtWAO,theffCO2agreementbetweentheAPOandCOtracersissimilartothatatUEA

andTAC,withperiodswhentheffCO2fromthetwotracersagreewell(e.g.7‐11Aug),andother

timeswhentheffCO2patternofvariabilityisverysimilar,butthemagnitudesoftheffCO2

signalsdiffer(e.g.31Jul‐2Aug).Overall,theffCO2observedatWAOislessthanthatobservedat

UEAandTAC,whichisexpected,giventhedilutionofterrestrialsignalsthatoccuratWAO,due

toitscoastallocation,aswellasthefactthatWAOislocatedfurtherfromthemainlocalffCO2

hotspots,suchasNorwich,andtheA11andA47mainroads.

TheCOandAPObaselineshavebeencalculatedusingthe‘rfbaseline’functionfromthe

‘IDPmisc’packageinR.‘rfbaseline’isastatisticalmethodforcalculatingabaselinefrom

atmosphericdatabasedonrobustlocalregression,andemploysasymmetricalweightingto

theresidualsofthefit,inordertopreventthebaselinefrombeingbiasedbyuni‐directional

pollutionevents,whichisacommoncharacteristicofmanyatmosphericspecies(Ruckstuhlet

al.,2012).ThisasymmetricalweightingisimportantinthebaselinefittingofbothAPOandCO,

becauseallofthefossilfuelrelatedvariabilityinAPOpresentsasnegativeexcursions(because

O2isconsumedduringfossilfuelcombustion),whilethefossilfuelrelatedvariabilityinCO

presentsaspositiveexcursions(becauseCOisproducedduringfossilfuelcombustion),as

illustratedinFig.5.7.

5.3.2.Baselineandmeasurementuncertaintyanalysis

InordertodeterminetheuncertaintyoftheffCO2calculatedusingAPOorCO,onemust

determinetheuncertaintyassociatedwiththethreecomponentsofEqs.5.2and5.4:theAPO

orCOmeasurementuncertainty,theuncertaintyassociatedwiththebaselinefitting,andthe

uncertaintyassociatedwiththefossilfuelemissionratios(RCO:CO2orRAPO:CO2).Thebaseline

uncertaintycanbequantifiedbyassessingthevariabilityintheffCO2whendifferentbaselines

areused.InFig.5.8,IusedAPOandCObaselinesofmoderateflexibilitytocalculateffCO2.In

Figure5.9,IpresentffCO2forbothAPOandCOatUEAandTACusingthebaselinesemployed

forFig.5.8,aswellasveryflexiblebaselines,wherealotmoreoftheshort‐termvariabilityin

Figure5.9.ffCO2calculatedfromCoatTAC(toppanel)andAPOatUEA(bottompanel)usingthemoderatelyflexiblebaselinefitsusedinFig.5.8,aswellasinflexiblebaselinefits(dashedpinkandorangelines)andflexiblebaselinefits(dotted‐dasheddarkpurpleanddarkredlines).

211

APOandCOisassignedas‘backgroundair’variability,andalsoveryinflexiblebaselines,which

hardlyvaryatall,thusalmostalloftheshort‐termvariabilityinAPOandCOisexcludedfrom

thebaseline.

Fig.5.9demonstratesthatattimes,thereissignificantuncertaintyassociatedwiththe

statisticalbaselinefittingprocedurefortheCOandAPOmethods,asthemagnitudeofffCO2is

oftendependentonthechoiceofbaselinefit.ItshouldbenotedthatthevariabilityinffCO2is

notdependentonthechoiceofbaselinefit.Figure5.10demonstratesthedifferencesinthe

baselinefitsusedtocalculatetheffCO2fromCOandAPOthatisshowninFig.5.9.Sincethe

numeratortermsinEqs.5.2and5.4aredeterminedfromthedifferencebetweenthe

measurementsandthebaselineforeachspecies,theflexiblebaselinefitstendtoproduce

smallerffCO2values,andtheinflexiblebaselinefitstendtoproducelargerffCO2values,with

themoderatelyflexiblebaselinefitsproducingintermediateffCO2values.

Figure5.10.Moderatelyflexible,inflexible,andflexiblebaselinefitstoCOfromTAC(toppanel)andAPOfromUEA(bottompanel).

ThemeanuncertaintyinffCO2associatedwiththechoiceofbaselinefitiscalculatedto

be±17.5%and±27.5%fortheCOdataandAPOdatarespectively(basedontheffCO2

differencesusingdifferentbaselineflexibilities),withnosignificantdifferencesinthebaseline

uncertaintiesateachmeasurementsite.Theseuncertaintyestimatesarebasedonthefactthat

theflexiblebaselinefitsareprobablynotfitforpurpose,giventhattheygenerallycauseffCO2

212tobeunderestimated,andthatthemostappropriatebaselinefitliesbetweenthestandardfit

andtheinflexiblefit:thus,theflexiblebaselinefithasnotbeenaccountedforinthebaseline

uncertaintyestimates.Inreality,theinflexiblefitislikelytobethemostappropriatebaselinefit,

assumingthatffCO2‘events’maybepresentinatmospherictimeseriesdataforperiodsof

severaldaysuptoaboutaweek,ratherthanforperiodsofonlyseveralhoursuptoadayorso;

hence,theuncertaintyestimatesstatedaboveareconservative.

TheuncertaintyassociatedwiththeCOandAPOdataisquantifiedfromthe±1σ

standarddeviationofthehourly‐averagedatmosphericmeasurementsduringaperiodwhen

theatmosphericvariabilityineachspeciesislow,andthusincludesboththeuncertaintyofthe

measurementtechnique,andtheuncertaintyassociatedwithsomenaturalatmospheric

variability.ForCO,themeasurementuncertaintyis±5.54ppbatTACand±1.58ppbatWAO.

ThelargermeasurementuncertaintyatTACisprimarilyduetogreaterimprecisioninthe

measurementtechniqueemployedatTACcomparedtothatusedatWAO,butisalsopartly

duetotheslightlygreaterCOvariabilityobservedatTACcomparedtoWAO.

ForAPO,themeasurementuncertaintyisdeterminedfromthe±1σstandarddeviation

inboththehourlyCO2andO2measurements,sinceAPO=O2+(‐1.1×CO2),where‐1.1isthe

oxidativeratiooftheglobalterrestrialbiosphere.Sincetheoxidativeratiooftheterrestrial

biospherecanvaryregionally,anuncertaintyof±0.05isassigned,whichisthensummedin

quadraturewiththeuncertaintiesoftheO2andCO2measurementstoobtainanoverall

uncertaintyestimatefortheAPOdata,whichis±13.80permegatUEAand±12.35permegat

WAO.TheO2andCO2measurementuncertaintiesatUEAareactuallysmallerthanthoseat

WAO;however,theAPOuncertaintyatUEAislargerthanthatatWAOowingtothelargerAPO

variabilityobservedatUEAcomparedtoWAO.Aspercentages,themeasurement

uncertaintiesare±4.29%forCOatTACand±1.28%forCOatWAO,and±4.63%forAPOat

UEAand±4.14%forAPOatWAO;thus,allofthemeasurementuncertaintiesaresignificantly

smallerthantheuncertaintiesassociatedwiththechoiceofCOandAPObaselinefits.

5.3.3.Using‘time‐varying’fossilfuelemissionratios

InFig.5.8,IpresentedffCO2fromCOandAPOdatausingfixedvaluesforthefossilfuel

emissionratios.Inreality,thefossilfuelemissionratiosobservedatameasurementsitecan

varysignificantly,owingtochangesintheemissionratiosthemselvespriortotransportation

tothemeasurementsite,aswellaschangesintheatmosphericfootprintofthemeasurement

site.Hence,amuchmoreappropriatewaytocalculatedffCO2fromCOandAPOdataistouse

time‐varyingfossilfuelemissionratios,whichcanbedeterminedbycombiningfossilfuel

213

emissionratiosfromgriddeddatabaseswithatmospherictransportmodelfootprints,as

showninEquation5.5:

∑ (Eq.5.5)

whereRtisthetime‐varyingfossilfuelemissionratioatthemeasurementsitefromtimest1to

tn,b1tobnrepresenttheindividualgridboxesoftheatmospherictransportmodelfootprint,E

isthefossilfuelemissionratioforeachgridboxoftheatmospherictransportmodel,Pisthe

numberofatmospherictransportmodelparticlesinthegridbox,andTPisthetotalnumberof

particlesinthewholeatmosphericfootprint.

InordertocalculateRtinEq.5.5,IusedtheUKMetOfficeNAME(Numerical

Atmospheric‐dispersionModellingEnvironment)model(Jonesetal.,2007)toproduce2‐day,

backwardsrunatmosphericfootprintsevery3hours,consistingof10,000inertparticles,that

weremonitoredfrom0‐200mabovetheground.TheNAMErunsweredrivenbytheMet

OfficeUnifiedModelmeteorology,whichhasaspatialresolutionof17kmby17km.ForE,the

fossilfuelemissionratios,IusedgriddedO2:CO2ratiosfromtheCOFFEE(CO2releaseand

OxygenuptakefromFossilFuelEmissionsEstimate)database(Steinbachetal.,2011)forthe

APOmethod,whichwereconvertedtoAPO:CO2ratiosbysubtractingtheO2:CO2ratioofglobal

terrestrialbiosphere‐atmosphereexchange(‐1.1)fromallthevalues,andgriddedCO:CO2

ratiosfromtheEDGAR(EmissionsDatabaseforGlobalAtmosphericResearch)databasefor

theCOmethod.

TheEDGARCO:CO2ratiosareonlyavailablewithannualtimeresolution(andarealso

onlyavailableupto2010,not2014),andthereforethetime‐varyingCO:CO2ratioscalculatedat

TACandWAOonlyincludevariabilityfromthechangingNAMEfootprints(i.e.spatial

variability).TheCOFFEE‐derivedAPO:CO2ratiosareavailableonhourlytimeresolution,and

wereconvertedinto3‐hourlyaveragesinordertomatchthetimeintervaloftheNAME

footprints.Originally,theCOFFEEdatabasewasonlyavailableupto2010,however,COFFEE

hasrecentlybeenupdatedto2014byChristophGerbig(MaxPlanckInstituteof

Biogeochemistry,Jena,Germany),andnowincludesanupdatedsetofO2:CO2ratiosfor

differentfueltypes(includingbetterdifferentiationoflightoilversusheavyoilratios,and

differentratiosfordifferenttypesofbioenergy),whichIcalculated.Boththetime‐varying

CO:CO2andAPO:CO2emissionratioswerecalculatedon3‐hourlytimeintervalstobe

compatiblewiththeNAMEfootprints,whichweretheninterpolatedtohourlytimeresolution

tobecompatiblewiththehourly‐averagedAPOandCOatmosphericdata.

214 Theuncertaintyofthetime‐varyingemissionratiosisdifficulttocalculate,sinceneither

theEDGARorCOFFEEdatabasesassignuncertaintiestothefossilfuelemissionsestimates.

Therefore,aproxyfortheuncertaintyofthetime‐varyingemissionratioswasdeterminedby

dividingRtbythe±1σstandarddeviationofalloftheemissionratiosinthefootprint.Themean

uncertaintiesofthetime‐varyingCO:CO2emissionratiosatTACandWAOare±78.3%and

±72.9%,respectively,andthemeanuncertaintiesofthetime‐varyingAPO:CO2emissionratios

atUEAandWAOareboth±21.8%.ThelargedifferencebetweentheCOandAPOfossilfuel

emissionratiouncertaintiesreflectsthemuchlargerspatialvariabilityintheCO:CO2ratio

values(sincethereisnotemporalvariabilityavailableintheEDGARgriddeddatabases),

comparedtoboththespatialandtemporalvariabilityoftheAPO:CO2ratiovaluesfromthe

COFFEEdatabase.

5.3.4.ComparisonofCOandAPOfossilfuelquantificationmethods

ThetotalffCO2uncertaintyforboththeCOandAPOmethodscanbecalculatedby

summinginquadraturethemeasurement,baseline,andemissionratiouncertainties.This

producesmeantotalffCO2(CO)uncertaintiesof±87.5%atTACand±78.4%atWAO,andmean

ffCO2(APO)uncertaintiesof±35.8%atUEAand±35.6%atWAO.Atbothlocations,themean

ffCO2(CO)uncertaintyismuchlargerthanthemeanffCO2(APO)uncertainty(bymorethana

factorof2).ThisispredominantlyduetothemuchlargeruncertaintyintheCO:CO2emission

ratioscomparedtotheAPO:CO2emissionratios.TheffCO2uncertaintiesatWAOarelower

thanthoseatTACandUEAforboththeCOandAPOmethods,owingtothesmallerffCO2

signalsthatareobservedatWAOinbothspecies.Table5.1summarisesthedifferencesin

uncertaintybetweenffCO2(CO)andffCO2(APO)ateachmeasurementsite.

Table5.1.ComponentandtotaluncertaintiesfortheCOandAPOffCO2quantificationmethodsatTAC,WAOandUEA,givento2significantfiguresforeasiercomparison.

ffCO2(CO) ffCO2(APO)

TAC WAO UEA WAO

Baselineuncertainty ±18% ±18% ±28% ±28%

Measurementuncertainty ±4.3% ±1.3% ±4.6% ±4.1%

Emissionratiouncertainty ±78% ±73% ±22% ±22%

Totaluncertainty ±88% ±78% ±36% ±36%

215

AsshowninTable5.1,fortheCOmethod,thetotalffCO2uncertaintyatbothlocations

isdominatedbytheCO:CO2emissionratiouncertainty,withtheCObaselineuncertainty

contributingfarless,andtheCOmeasurementuncertaintycontributingtheleast.Incontrast,

theAPOmethodtotalffCO2uncertaintyismoststronglyinfluencedbytheAPObaseline

uncertainty,closelyfollowedbytheAPO:CO2emissionratiouncertainty,withtheAPO

measurementuncertaintycontributingtheleast.ItisclearthattheCOmethodisfarless

precisethantheAPOmethod,owingtothelargeuncertaintyassociatedwiththeCO:CO2

emissionratios.ItshouldbenotedthatthetotalffCO2(CO)uncertaintiesdonotincludethe

uncertaintyassociatedwithpotentialnaturalCOsourcesandsinks,whichwouldbevery

difficulttoquantify.Additionally,neitherthetotalffCO2(CO)northetotalffCO2(APO)

uncertaintiesincludetheuncertaintyassociatedwithpotentialbioenergyinfluences,which

wouldalsobedifficulttoquantify,andwillhaveagreaterinfluenceontheCOmethodthanthe

APOmethod,becausetheAPOmethodisconservativewithrespecttosolidbioenergyand

biomassburning.

ffCO2(CO)fromTACandWAOandffCO2(APO)fromUEAandWAOcalculatedusing

time‐varyingfossilfuelemissionratios(usingEquation5.4)arepresentedinFigure5.11.In

contrasttoFig.5.8,theffCO2datainFig.5.11havebeencalculatedusinginflexiblebaselines,

ratherthanmoderatelyflexiblebaselines,asthelattercanleadtounderestimationoftheffCO2

variability,particularlyforffCO2eventslastingseveraldays,asshowninFig.5.10and

describedpreviously.TheffCO2uncertaintiesarerepresentedbytheshadedregions,andwere

calculatedbysummingthemeasurement,baselineandemissionratiouncertaintiesin

quadrature.AlsoshownistheffCO2calculatedfromdiscrete14CO2measurementsmadeat

TAC.Overall,theffCO2calculatedfromCOandAPOappeartoagreemorecloselyinFig.5.11

thanpreviously,inFig.5.8.Therearestillsomeperiodswherethetwocontinuousmethodsdo

notagreewithintheuncertaintiesofeachother,suchas31JulyatWAO,forexample.TheffCO2

from14CO2atTACisnormallyalsoinagreementwiththeffCO2fromCOandAPO,althoughas

before,theffCO2(14CO2)valuestendtobelowerthantheffCO2(APO)andffCO2(CO)values.Fig.

5.11illustratesthedifferenceinuncertaintybetweentheCOandAPOmethodsthatIhave

numericallypresentedinTable5.1,andshowsthattheAPOmethodissignificantlymore

precisethantheCOmethod.AnanalysisoftheairmasshistoryusingNAMEfootprintsreveals

thatmostoftheffCO2duringthesummer2014periodIhaveanalysedisfromthesouthofthe

UKandLondon,withsomefromthenorthoftheUK,andveryoccasionalffCO2fromFrance,

theNetherlandsandtheNorthSea(presumablyfromoilplatforms).Thereisnoapparent

connectionbetweentheagreementoftheCOandAPOffCO2quantificationmethodsandthe

Figure5.11.ffCO2(CO)andffCO2(APO)atTACandUEA,respectively(toppanel),andffCO2(CO)andffCO2(APO)atWAO(bottompanel),calculatedusingtime‐varyingemissionratiosandinflexiblebaselines.ShadedareasdenotetherespectiveuncertaintiesofthecalculatedffCO2.ffCO2from14CO2measurementsatTACaredenotedbytheblackcircles,ofwhichthesizerepresentstheuncertaintyoftheffCO2(14CO2)values.

217

originsoftheNAMEfootprints.Itisthereforelikelythatmostofthedisagreementbetweenthe

twomethodscanbeattributedtothefactthattheTACandUEAmeasurementsarenotco‐

located,aswellasundiagnosedtechnicalissuesanddifferencesinpotentialCOandAPO

influencesthatcannoteasilybequantified,suchasbiomassburning(forCO).

IncontrasttoFig.5.7,wheretheanti‐correlationinAPOandCOwassimilaratWAO

andatUEAandTAC,Fig.5.11indicatesthatffCO2agreementisactuallycloseratWAOthanat

UEAandTAC.Thisismostlikelyduetotheco‐locationoftheCOandAPOmeasurementsat

WAO,andaddsconfidencetobothffCO2quantificationmethods.Indeed,sinceffCO2hasbeen

calculatedusingtwoentirelyindependenttracers,periodsofstrongagreementinffCO2

betweenthetwomethodsareassociatedwithextremelyhighconfidenceintheffCO2accuracy

(e.g.21‐28Aug2014atWAO).

ItisalsoclearfromFig.5.11thattheCOmethodproducessignificantlyhigherffCO2

valuesthantheAPOmethod.ThisislargelyduetotheCO:CO2emissionratiosfromtheEDGAR

database,whicharelowerthanexpected,andcausethemagnitudeoftheffCO2fromCOtobe

high.Table5.2showstypicalffCO2valuesfromtheliterature,mostofwhichalsousetheCO

method,alongsidetheffCO2rangefromtheCOandAPOmethodsshownabove,and

demonstratesthattheffCO2fromCOatTACandWAOismuchhigherthanexpected,whenthe

valuesarecomparedtotypicalffCO2observedinurbanareas,suchasParis.Infact,itisnot

possibleforsomeofthelargestffCO2(CO)peaksatTACandWAOtobeaccurate,sincetheffCO2

valuesarelargerthantheCO2enhancementabovethebaseline,showninFig.5.6.This

suggeststhattheEDGARCOinventorydataareincorrect(toolow),sincetheCOFFEEAPO:CO2

ratiosarederivedfromEDGARCO2data(seeSteinbachetal.,2011fordetails),andthe

ffCO2(APO)valuesarewithintheexpectedrangeforarelativelyruralarea.Itshouldalsobe

notedthattheTACCOdataareknowntohaveanon‐goingcalibrationissuethatisaffectingthe

accuracyofthehighCOvalues.Itispossiblethatoncecorrected,thehighestffCO2(CO)valuesat

TACmayreducebyasmuchas30%(GrantForster,personalcommunication,2016),although

thiscorrectionwillnotaffectthepatternofvariability,northefactthattheCOmethodstill

produceshigherffCO2valuesoverallthantheAPOmethod,andunrealisticallyhighvaluesat

WAO,wheretheCOdatahavebeenqualitycontrolledandaredeemedaccurate.

218

Table5.2.TypicalffCO2rangesfromtheliterature,shownalongsidetheffCO2rangesforTAC,UEAandWAOpresentedinthiswork,calculatedusingCO,APOand14CO2atmosphericdata.Publication Location Speciesused TypicalffCO2

range

ffCO2uncertainty

vanderLaan

etal.(2010)

Lutjewad,The

Netherlands

14CO2andCO 0–30ppm ±2.5ppm

Lopezetal.

(2013)

Paris,France 14CO2,CO,NOx

and13CO2

0–40ppm Notgivenformost

species.±1.0ppm

for14CO2

Gravenetal.

(2009)

California,U.S.A. 14CO2andCO 0–10ppm ±1.6–2.9ppm

Turnbullet

al.(2006)

NewEngland

andColorado,

U.S.A.

14CO2,COand

SF6

0–15ppm ±2–4ppm

Thiswork Norfolk,U.K. CO(TAC)

CO(WAO)

APO(UEA)

APO(WAO)

14CO2(TAC)

0–70ppm

0–40ppm

0–20ppm

0–15ppm

1.2–2.5ppm

±5.8ppm

±4.5ppm

±1.2ppm

±1.1ppm

±1.6ppm

Fig.5.11suggeststhatusinginventorydatacombinedwithanatmospherictransport

modeltoestimatetheemissionratiosmayleadtoinaccurateffCO2,mainlyduetoinaccuracies

withtheinventorydata,butalsoduetopotentialatmospherictransportmodelinaccuracies.

Therefore,itisimportanttoconsiderothermethodsofdeterminingthefossilfuelemission

ratiosfortheCOandAPOmethods.Figure5.12showsffCO2fromUEAandTACcalculated

usingthetime‐varyingemissionratios(asshownFig.5.11,withuncertaintiesomittedfor

visualclarity),aswellasffCO2usingthepreviousfixedemissionratiosof0.3molmol‐1for

APO:CO2and5ppbppm‐1forCO:CO2(verysimilartoffCO2showninFig.5.8,onlyusingan

inflexiblebaseline).AlsoshownisffCO2calculatedusingemissionratiosthathavebeen

‘calibrated’bytheTAC14CO2data,andfortheAPOmethodonly,ffCO2calculatedusingthe

meanAPO:CO2ratiooftheatmosphericmeasurementsatUEAduringthesummer2014

period.ffCO2(CO)wascalculatedusingthemeanCO:CO2ratiooftheatmospheric

measurementsaswell,butthevaluesproducedwereextremelyhigh(upto350ppm)andnot

realistic;hence,thesedataarenotshowninFig.5.12.Thereasonwhythemeanmeasured

CO:CO2ratioistoolow,causingffCO2tobebiasedtoohigh,isduetolargenon‐fossilfuelrelated

219

CO2signalsfromtheterrestrialbiospherecoincidingwithfossilfuelrelatedCOsignals.In

contrast,themeanAPO:CO2ratioduringthisperiodisnotsoseverelyaffectedbytheactivityof

theterrestrialbiosphere.

Figure5.12.ffCO2fromAPOatUEA(toppanel)andCOatTAC(bottompanel)calculatedusingavarietyofemissionratios(seetextaboveandfigurelegends).TheffCO2fromtime‐varyingratiosisthesameastheffCO2showninFig.5.11(toppanel),onlywithouttheuncertaintyshading,toaidvisualcomparisonwiththeffCO2calculatedusingtheotheremissionratios.AlsoshownisffCO2fromTAC14CO2data(blacksymbols).

Fig.5.12demonstratesthattheffCO2(APO)values(toppanel)areallquitesimilarto

eachother,despiteusingdifferentfossilfuelemissionratiosources.Theonlyexceptionisthe

ffCO2(APO)calculatedfromtheemissionratiosthatwerecalibratedusingtheTAC14CO2data,

whichislowerthanthatcalculatedusingtheotherthreetypesofemissionratios.The14CO2

calibratedAPO:CO2emissionratiohadtobeadjustedtothehighestpossiblevalueforfossilfuel

emissions(0.9molmol‐1)inordertobeabletocalculateffCO2thatwaslowenoughtomatch

theffCO2fromthe14CO2data.Infact,insomecases,itwasnotpossibletomatchthe

ffCO2(14CO2)valuewithoutusinganAPO:CO2emissionratiothatishigherthanthemaximum

possiblefossilfuelemissionratiovalue,whichsuggeststhateventhoughtheffCO2(14CO2)was

correctedfornuclearinfluences,thevaluesarestillaffectedandarebiasedlow.Thisis

supportedbytheffCO2(CO)calculatedusingthe14CO2calibratedemissionratios,whereitwas

alsooftennecessarytouseextremelyhighemissionratios(upto100ppbppm‐1)inorderto

producealowenoughffCO2valuethatwouldmatchtheffCO2(14CO2)value.

220

UnlikeAPO,theffCO2(CO)showninFig.5.12ishighlydependentupontheemission

ratiosused,withthetime‐varyingratios,fixedratios,14CO2calibratedratios,andmean

measuredratios(notshown)producingverydifferentffCO2values.Asmentionedbefore,the

meanmeasuredratiosandtime‐varyingratiosfromtheEDGARdatabaseproduceffCO2from

COthatistoohighforarelativelyrurallocationsuchasTACorWAO,andthe14CO2calibrated

ratiosproduceffCO2valuesthatarebiasedlowbynuclearpowerplantemissions,giventhat

sometimesveryhighCO:CO2emissionratiosarerequiredtoreproducetheffCO2(14CO2)values.

ThefixedemissionratiosproducetheffCO2valuesthatmostcloselymatchthosecalculated

usingtheAPOmethod(fromdifferenttypesofemissionratios)atUEA,andarealsointhe

expectedrange,consideringthelocationofTAC.Thus,Fig.5.12suggeststhataswellastheAPO

methodbeingmoreprecisethantheCOmethodforquantifyingffCO2,itisalsoverylikelythat

theAPOmethodisalsomoreaccuratethantheCOmethod,giventhatthemagnitudeofffCO2

calculatedfromCOissovariable,dependingonthechoiceofemissionratiosused.

5.4Summaryandfuturework

InthischapterIhavepresentedanewmethodforquantifyingffCO2usingAPOdata,

whichIhavecomparedtoffCO2calculatedfromCOand14CO2data.Overall,IfoundtheAPO

methodtobesignificantlymoreprecisethantheCOmethod,whichislargelyowingtothe

reduceduncertaintyintheAPO:CO2fossilfuelemissionratioscomparedtotheuncertaintyin

theCO:CO2fossilfuelemissionratios.ThelargestsourceofuncertaintyintheAPOmethodis

currentlythebaselinefittingprocedure.Futuretechnicalimprovementsinmakinghigh‐

precisionO2measurementswillhelptoreducetheAPObaselineuncertainty.Iwouldalso

expectthatshort‐termdeviationsfromtheAPObaselinewillbecomeeasiertodetermineina

moreurbansetting,wherethemagnitudeofthesignalsarelarger,andthatthiswillalsohelpto

221

reducetherelativeuncertaintyintheAPObaseline.Incontrasttothetwocontinuousmethods,

Ifoundthatrelyingonthe14CO2dataaloneledtosignificantunderestimationofffCO2in

Norfolk,partlyduetonuclearpowerplantinfluencesthathavenotbeenadequatelycorrected

for,andalsopartlyduetocleanairsampling(AngelinaWenger,personalcommunication,

2016).

WhencomparingtheCOandAPOmethods,IfoundthattheAPOmethodwasvery

likelytobemoreaccuratethantheCOmethod.Thisconclusionislargelybasedonasensitivity

analysisoffourdifferentemissionratiosources.FortheAPOmethod,Ifoundthattherangein

ffCO2valuesassociatedwiththefouremissionratiosourceswasmuchsmallerthantheffCO2

rangefortheCOmethod.IwasalsoabletodeterminethatsomeofthelargestffCO2peaksfrom

theCOmethodcouldnotpossiblybereal,sincetheywerelargerthanthemeasuredCO2

enhancementabovethebackgroundCO2molefraction.Ideally,Iwouldhavedeterminedthe

accuracyoftheAPOandCOmethodsbycomparingtoffCO2from14CO2atalocationthatisnot

affectedbygas‐coolednuclearpowerplantinfluences,becauseffCO2from14CO2isgenerally

expectedtobethemostaccuratewayofdeterminingffCO2.AsfarasIamaware,however,

thereisnoexistingdatasetofconcurrent,high‐precisionAPO,COand14CO2dataatalocation

thatalsoexperiencespollutedair,andisnotaffectedbynuclearpowerplantinfluences.

Despitethislimitation,theresultsIhavepresentedhereindicatethatitisverylikelythatthe

APOmethodismoreaccuratethantheCOmethod.Mostencouragingly,Ihavefoundthatat

WAOinparticular(wherethemeasurementsareco‐located),theffCO2variabilitybetweenthe

twomethodsisoftenverysimilar,andperiodswheretheffCO2magnitudeisalsoinagreement

affordsmeextremelyhighconfidenceintheffCO2accuracy,giventhattheCOandAPO

methodsarereliantontwocompletelyindependenttracers.

TheUKgovernmentstatesthatUKannualfossilfuelCO2emissionsfor2013areknown

towithin±2%uncertainty,basedonbottom‐upinventorymethodsanda95%confidence

level.Whilethisuncertaintysoundsverysmall,itisapproximatelyequivalenttotheUKmean

annualCO2footprintsofover950,000people.Inaddition,theuncertaintiesassociatedwiththe

UKinventoryarenotquantifiedforhigherspatialresolutionthannational,orforhigher

temporalresolutionthanannual(StephenForden,DECC;personalcommunication,2016).

Severalstudieshaveshownthatemissionsuncertaintiesincreasewithincreasingspatialand

temporalresolution,andcanreach100%ormorefor1°latitude/longituderesolutions(also

fora95%confidencelevel)(Andresetal.,2012;Andresetal.,2016).Thus,evenifnational

scaleuncertaintiesinfossilfuelemissionsarerelativelysmallandareassumedtobewell‐

known,largedifferencescanbefoundatsmallerscales,asdemonstratedby(Ackermanand

222

Sundquist,2008),whofounddifferencesofupto25%inindividualUSApowerplantCO2

emissionscompiledbydifferentgovernmentagencies.

InordertobeabletosuccessfullyreduceanthropogenicCO2emissions,weneedto

haveaccesstohighresolution(spatialandtemporal)informationthatenablesustodetermine

whichbehaviourscauseincreasesanddecreasesinanthropogenicCO2emissions.For

example,howdoUKfossilfuelCO2emissionschangeifthereisanunexpectedlycoldwinter,or

ifelectriccarsbecomedominantoverpetrolcars,orifhouse‐holdelectricityandgassmart

metersaremadecompulsoryinUKhouseholds?Inthischapter,IhavecomparedffCO2from

APOandCOmeasurementswithmodelledffCO2frominventoryestimates.Thecomparison

indicatesthatboththeCOFFEE(derivedfromEDGAR)andtheUKNAEIinventoriesmaybe

over‐estimatingCO2emissionsinNorfolk.InthecaseoftheUKNAEI,someofthisdisparity

maybeexplainedbythefactthatIhavecompared2014ffCO2fromtheatmosphericdatato

modelledffCO2basedon2013values,becausethe2014valuesarenotcurrentlyavailable;

however,thereductionintheNorfolkNAEICO2emissionsbetweensummer2013and

summer2014wouldneedtoberelativelylargeinordertobringtheinventoryffCO2estimates

in‐linewiththeffCO2fromtheatmosphericmeasurements.Itshouldalsobenotedthatthe

modelledffCO2fromtheinventoriesthatIhavepresentedinthischapterarecalculatedusinga

singleatmospherictransportmodel.FurthersensitivityanalysisonthemodelledffCO2

emissionsshouldbecarriedoutusingotheratmospherictransportmodels,suchasSTILT

(StochasticTime‐InvertedLagrangianTransportmodel)(Linetal.,2003)andTM3

(HeimannandKörner,2003),toensurethatthemodelledffCO2isnotbiasedbymychoiceof

atmospherictransportmodel.

Tomyknowledge,therearecurrentlynocontinuoushigh‐precisionatmosphericO2

measurementsmadeinurbansettingsforthepurposeofffCO2quantification,andyet~70%of

allanthropogenicCO2emissionsarefromcities(IEA,2012a).Ithereforeproposeanew

directionforhigh‐precisionO2measurements,byadvocatingthatatmosphericO2isacurrently

under‐exploitedtoolforffCO2quantificationinurbanenvironments,andhasthepotentialto

provideprecise,accurate,hightemporalandspatialresolutionffCO2quantification,whichcan

alsobeusedinregionsthatareseverelyaffectedbygas‐coolednuclearpowerplantemissions,

suchaswesternEurope,Japan,easternUSAandCanada.Itshouldbenotedthatinordertouse

atmosphericO2measurementstosuccessfullyquantifyffCO2,veryprecisemeasurementsare

required(ontheorderof~5permegover1‐2minutes)andahighlevelofdataqualitycontrol

isrequired.Nevertheless,asdemonstratedinthischapter,itiscurrentlypossibletoachieve

suchmeasurementprecisionanddataqualitycontrolrequirementsinordertosuccessfully

quantifyffCO2evenatruralandcoastallocations,whereffCO2emissionsarerelativelylow.I

223

thereforeproposethatacombinationofatmosphericO2measurementsandinversemodelling

couldenablerobusttop‐downquantificationofCO2emissionsatbothnational,butalso

perhapsurbanscales,andatsub‐annualtemporalresolutions,dependingonthedensityofthe

atmosphericO2measurementnetwork,andlimitationsofatmospherictransportmodelsand

inversemodellingmethodologies.

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