Supplement of Atmos. Chem. Phys., 16, 3743–3760, 2016
http://www.atmos-chem-phys.net/16/3743/2016/
doi:10.5194/acp-16-3743-2016-supplement
© Author(s) 2016. CC Attribution 3.0 License.
Supplement of
Mercury oxidation from bromine chemistry in the
free troposphere over the southeastern US
Sean Coburn et al.
Correspondence to: Rainer Volkamer ([email protected])
The copyright of individual parts of the supplement might differ from the CC-BY 3.0 licence.
S1
InstrumentandMeasurementSite
TheinstrumentandmeasurementsiteareidenticaltothatdescribedinCoburnetal.(2011).Only
abriefoverviewwillbegivenhere.Forthedurationofthemeasurementsdiscussedinthisstudy,
aresearch-gradeMAX-DOASinstrumentwaslocatedataUnitedStatesEnvironmentalProtection
Agency(USEPA)facilityinGulfBreeze,FL(30.3N87.2W)andmeasuredfortimeperiodsbetween
May2009andFebruary2011.Thissiteis~10kmsoutheastofPensacola,FL(populationappr.
50,000) and ~1 km from the coast of the Gulf of Mexico, which enables the measurement of
urbanandmarineairmasses.Thespectrometerandcontrollingelectronicswereset-upinthe
warehouseoftheEPAfacility,whilethetelescopewasmountedonasupportstructureonthe
roofofthewarehouse(~10-12mabovesealevel)connectedviaanopticalfiber.Thetelescope
wasoriented~40°westoftruenorthinordertorealizeaclearviewinthelowestelevationangles
tothecoast.Duringoperationthefull180°elevationanglerangeofthetelescopewasutilizedto
enablethecharacterizationofair-massesovertheseawaterlagoontotheNorth,andoverthe
coastalregionoftheGulfofMexicototheSouth.Forthepurposesofthisstudy,thenorthviewing
direction will be considered to minimize changes in the radiative transfer calculations due to
azimutheffectsthroughouttheday.
TheinstrumentusedforthisstudyconsistsofaPrincetonInstrumentsActonSP2300iCzernyTurnergrating(500groove/mmwitha300nmblazeangle)spectrometerwithaPIXIS400BbackilluminatedCCDdetector(Coburnetal.,2011).Thissetupwasoptimizedtocoverthewavelength
range ~321-488 nm with an optical resolution of ~0.68 nm full-width at half the maximum
(FWHM).Thespectrometeriscoupledtoaweather-resistanttelescope(capableofrotatingthe
elevationangleby180°,50mmf/4optics)viaa10mlong1.7mmdiameterquartzfiber.During
normal field operation this instrument was routinely able to realize values of the root mean
square(RMS)oftheresidualremainingaftertheDOASfittingprocedureontheorder0.9-3x10-4.
Thissystemwasverystable,withlittleneedformaintenance,andwasoperatedremotelyfor
periodsbetweenMay2009andFebruary2011tomeasuremultipletracegases,including:BrO,
IO,nitrogendioxide(NO2),formaldehyde(HCHO),glyoxal(CHOCHO),andtheoxygenmolecule
collisioninducedabsorptionsignal(referredtoasO4).
S2
DOASretrievalsensitivitystudies
S2.1
Retrievalwindow:Severalsensitivitystudieswereperformedtodeterminethemost
suitableanalysissettingsfortheBrOretrieval.Thiswasaccomplishedthroughacomparisonof
bothO3andHCHOdSCDvaluesfromtheBrOfittingwindowwithdSCDspredictedusing
WACCMverticalprofiles.Also,theeffectofusingdifferentO4referencecross-sectionswas
testedwithrespecttotheO4dSCDintheBrOfittingwindow.
WACCMmodeloutputprofilesforO3andHCHOwereusedtoforwardcalculatedSCDsfor
comparisontomeasureddSCDsofO3andHCHOretrievedusingtheBrOfittingwindow.Five
differentBrOanalysissettingwindowsweretested:1)fittingwindow345-359nmwitha2nd
orderpolynomial(encompassestwoBrOabsorptionpeaks–2-bandanalysis);2)fittingwindow
346-359nmwitha2ndorderpolynomial(2-bandanalysis);3)fittingwindow340-359nmwitha
3rdorderpolynomial(3-bandanalysis);4)fittingwindow340-359nmwitha5thorder
polynomial(3-bandanalysis);and5)fittingwindow338-359nmwitha5thorderpolynomial(4bandanalysis).Itwasdeterminedthatanalysissetting5)(4-bandanalysiswith5thorder
polynomial)includingaconstrainedintensityoffset(Sect.S2.2)bestrepresentedbothO3and
HCHO.ThesearetheanalysissettingsthatwerethenusedtoassesstheeffectsofdifferentO4
crosssectionsintheBrOfittingwindow:1)Hermans(2002);2)Greenblattetal.(1990);and3)
ThalmanandVolkamer(2013).TheO4dSCDsretrievedfromtheBrOfittingwindowwere
comparedwiththeO4dSCDsretrievedfromanO4optimizedfittingwindowintheUV(353-387
nm);andwhilenoneofthecrosssectionsareabletofullyreproducetheO4dSCDsfromtheO4
optimizedwindow,theThalmanandVolkamer(2013)cross-sectionwasdeemedtobean
improvementoverHermansandGreenblatt/BurkholderinrepresentingO4intheBrOfitting
window.
S2.2
Intensityoffset:AnadditionalparameterthatcanbeutilizedintheDOASretrievalisan
intensityoffset,whichwouldbeusedtohelpaccountforanyinstrumentstraylight.The
instrumentemployedforthisstudywasdesignedtoactivelyminimizespectrometerstraylight
throughtheuseofcut-offfilters(BG3andBG38),aswellasthemethodofbackground
correction.ThebackgroundcorrectionissimilartothatdescribedinWagneretal.(2004)and
utilizesdarkregionsontheCCDdetectortocorrectfordarkcurrentandoffsetnoiseaswellas
straylight.Itwasdeterminedthatstraylightintheinstrumentwasonlyafewpercent(before
correction)inthewavelengthrange330-360nm.Fittinganintensityoffsetshouldonlyaccount
foruncorrectedstraylightandisexpectedtobeontheorderofmagnitudeoftheerrorinthe
backgroundcorrection.ThefittingofthisparametertypicallyhelpsreducetheRMSofthe
fittingroutine,thusimprovinginstrumentsensitivity.However,preliminarystudiesfounda
significanteffectontheretrievedBrOdSCDsdependingonwhetherornotthisparameterwas
includedinthefittingroutine,andthatthiseffectwasmostpronouncedinthenarrowerfitting
windows.Inthemostextremecase(analysiswindow346-359nm),retrievedBrOdSCDs
changedfrom~1x1014moleccm-2withoutfittingtheintensityoffsettovalueslessthanzero
whenanunconstrainedintensityoffsetwasincluded.Inallfittingwindowstested,utilizingan
unconstrainedintensityoffsetresultedinthehighestfitfactorfortheoffsetandlowestvalues
fortheBrOdSCDs,andinsomecasesleadtosignificantlynegative(non-physical)values.In
thesecases,itwasfoundthatperiodsoftimeexistedwhenthefitfactorfortheintensityoffset
wasmuchgreaterthanwhatwasdeterminedtobeareasonablevalueforthisinstrument.For
thisreason,theintensityoffsetwaskeptintheretrieval(tohelpwithRMS),butlimitedtoa
rangedeterminedbytheupperlimitofthisestimatedcorrection(±3x10-3).
BasedonthefindingsoftheoffsetsensitivitytestsandthedSCDcomparisontestspresentedin
thissection,thedSCDsusedfortheBrOinversionarefromthe338-359nmfittingwindow
utilizinga5thorderpolynomial,theconstrainedintensityoffset,andtheThalmanandVolkamer
(2013)O4cross-section.
S3
AerosolRetrieval
Aerosol profiles are determined through an iterative comparison of measured O4 dSCDs
(analyzed in the wavelength window 437-486 nm) with O4 dSCDs calculated from the RTM
McArtim3(Deutschmannetal.,2011)outputsbasedonspecificaerosolextinctionprofiles.This
processisperformedoneachsetofMAX-DOASviewingangles(forthispointforwardreferred
toasascan)ofthecasestudyday(totalof56scans)inordertodetermineindividualaerosol
profiles.Theinitialaerosolprofileusedforeachscandecreasedexponentiallywithaltitudefrom
avalueof0.01km-1at483nm(scaleheightof0.6km).Thiswavelengthischosenforitsproximity
totheO4peakabsorptionstructureat477nmwhileavoidingthefeatureitself,aswellasavoiding
absorptionstructuresfromothertracegases(i.e.NO2).ThisO4fittingwindowwaschosendueto
the better general agreement achieved between measured and forward calculated O4 dSCDs
(calculatedafteraerosolextinctionprofilesweredetermined)ascomparedwiththeO4retrieval
intheUVrange.ThisissimilartofindingsfromVolkameretal.,(2015),whereitisdetermined
thatusingthe477nmO4bandforderivingaerosolprofilesismuchmorerobustthanusingthe
UV bands due to the increased Rayleigh scattering at shorter wavelengths, which can mask
aerosolextinction.
TheO4verticalprofilesusedforallcalculationsandasinputtotheRTMarebasedontemperature
andpressureprofilesavailablefromNOAA’sESRLRadiosondeDatabaseforlocationsclosetothe
measurementsite,whichareingoodagreementwiththecorrespondingprofilesfromWACCM.
IneachstepoftheiterationthemeasuredO4dSCDsarecomparedtotheforwardcalculated
dSCDsateachelevationangleofthescanbeinganalyzed,andthedifferencesbetweenthese
valuesareusedasinputforoptimizingthemodificationoftheaerosolprofileforthesubsequent
iteration.Forthisstudy,theconvergencelimitissetatapercentdifferenceof5%betweenthe
lowest two elevation angle dSCDs, or if the process reaches 5 iterations without finding
convergencethelastaerosolprofileisused.Thelimitof5iterationsischosenasacompromise
betweenachievingoptimalagreementbetweentheO4dSCDsanddatacomputationtime.For
this case study, the 5% criterion is reached for every sequence. The resulting time series of
aerosolextinctionprofilesareshowninFig.S10(Supplement).
Theaerosolextinctionprofilesat483nmwerescaledtoderiveextinctionat350nmusingthe
relationshipfoundinEq.(S1).
𝜀"#$ =𝜀%&" ·(
"#$ )*.,#
)
%&"
(S1)
whereε350andε483representaerosolextinctioncoefficientsat350and483nm,respectively.An
Angstromexponentof-1.25waschosenasamoderatevaluethatwouldberepresentativeofan
averageatmosphere(Duboviketal.,2002).Theretrievedaerosolprofilesat350nmprovided
inputtotheRTMinordertocalculatetheappropriateweightingfunctionsforBrO.
S4
BoxModelDescription
ThemodelingportionofthisstudyisdesignedtoassesstheconcentrationofBrradicalsavailable
toparticipateinthemercuryoxidationreactionbasedonBrOverticalprofilesprovidedfromthe
MAX-DOASmeasurementsandGEOS-Chem.Othertracegasandatmosphericparameterinputs
to the diurnal steady-state box model (Dix et al., 2013, Wang et al., 2015) are median daily
profilesderivedfromtheMAX-DOASmeasurements,WACCM,andGEOS-Chem.Asinglemedian
profile representing the entire day (daily median, for each model input) is used rather than
individualprofiles.Fromtheseprofiles,theboxmodelthencalculatesthepartitioningbetween
bromine species throughout the troposphere (including: reactions with other trace gases;
photolysis; and some aqueous phase partitioning and chemistry) in order to derive vertical
profilesoftheBrradical,whicharesubsequentlyusedforcalculatingmercuryoxidationrates.A
summaryofallthereactionsinvolvingmercurycompoundsconsideredintheboxmodelalong
withassociatedratecoefficientscanbefoundinTable2.Themodelconceptuallyfollowsthe
framework of Crawford et al., (1999), where model inputs are initiated and allowed to reach
steady-stateoverseveraldays.Intheboxmodel,theBrOandIO(toassesstheimpactofiodine
radical species in the mercury oxidation reactions) profiles are taken from the MAX-DOAS
measurements;however,theGEOS-ChemBrOprofileisalsousedinordertoassesstheimpact
of the differences between these profiles. Other box model inputs are taken as the output
profilesfromtheexternalmodelsutilizedinthisstudyandtheseinclude:temperature,pressure,
andHCHOfromWACCM;andO3andNO2fromGEOS-Chem.AsasensitivitytestWACCMO3and
NO2werealsousedtoassesstheimpactonthemercuryoxidationschemetowardsdifferences
in the vertical distributions of these molecules. The resulting vertical profiles of the rate of
mercuryoxidationusingthesetwoprofilesasboxmodelinputsisfoundinFig.S11(Supplement),
whichfollowsthesameformatasFig.7.AerosolsurfaceareameasurementsfromtheTORERO
dataset(Volkameretal.,2015;Wangetal.,2015)areassumedrepresentativeofconditionsin
the marine atmosphere, and therefore used as model inputs for lack of independent
measurements. The vertical distribution of Hg0 from GEOS-Chem was used in all scenarios.
Additionally, photolysis rates for a variety of species, calculated using the Tropospheric
UltravioletandVisible(TUV)Radiationmodel,areincluded.TheTUVmodelwasinitiatedfora
Rayleighatmosphere(aerosolextinction=0),withO3andNO2columnsof380and0.3Dobson
Units(DU),respectively,whicharederivedfromtheaverageverticalprofilesfromWACCM.Since
themedianBrOprofilederivedbytheMAX-DOASmeasurementscloselyresemblestheprofiles
retrievedaroundsolarnoon(seeFig.5),theTUVcalculationsfromthistimeareused.Forthe
modelrunscomparingtheBrOprofilesfromthemeasurementsandGEOS-Chem,onlytheBrO
profileischanged;allotherinputsremainconstant.
Forthedeterminationofthedominantoxidativepathways,thereactionratesforoxidationof
Hg0againstBrandO3arecalculatedasafunctionofaltitudeforthedifferentreactantvertical
profiles, which is important due to atmospheric temperature gradients. This also allows the
assessmentoftherelativecontributionsofthesereactionstotheoverallrateofoxidationasa
functionofaltitude.OxidationbyO3isincludedintheboxmodelduetotheamountofevidence
from laboratory studies indicating that this reaction might play a role in the atmosphere;
althoughpotentiallynotcompletelyinthegasphase.
FollowingWangetal.(2015),theboxmodelisinitiatedundertwodifferentmodestoinvestigate
thesensitivityofoxidationrates,andlikelyproductdistributionstothemechanisticassumptions
aboutmercuryoxidation.ThetwomodesdifferinthescavengingreactionsoftheHgBradduct.
A“traditional”scenarioonlyincludesBrandOHradicalsasscavengers(Holmesetal.,2009),and
a“revised”modeincludesspeciessuggestedbyDibbleetal.,2012(BrO,NO2,andHO2)aswell
asadditionalhalogenspecies(IandIO).Themodelalsotrackstheconcentrationsofallspecies
as a function of altitude, which gives indications for the product distributions of the various
reactions.
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FigureCaptions
Figure S1: BrO box-AMFs for two elevation angles (25° and 90°, solid and dashed lines
respectively)atdifferentSZAs.Forthesehigherpointingelevationangles,thebox-AMFspeakat
altitudesof2-15km(freetroposphere)forSZAslowerthan70°,whileathigherSZAthebox-AMFs
indicatethatthesensitivitytowardsthefreetroposphereisdecreasing.
FigureS2:Overviewofbox-AMFSforSZAslessthan70°for4elevationangles:a)3.8°;b)10°;c)
25°;andd)90°.
FigureS3:OverviewoftheWACCMBrOverticalprofilesasafunctionofaltitudeandtimeofday
(panela),andthencollapsedintotothecorrespondingpartial(greentrace)andtotal(bluetrace)
VCDsinpanelb.
FigureS4:ExampleresultsfortheinversionofIOfromtheMAX-DOASmeasurements.Panela)
containsthethreea-prioriprofilesusedintheinversion(dashed,coloredlines),thea-posteriori
resultsfromaMAX-DOASscanat~45°SZA(morning)correspondingtothea-prioriprofiles(solid
lines,colorscorrespondwiththea-priori),andthemedianIOprofile(redtrace,wheretheerror
barsreflectthe25thand75thpercentiles).Panelb)showstheaveragingkernelsfortheinversion
using a-priori “Prf1”. Panel c) shows the diurnal variation of the IO VCD for the BL (0-1 km),
troposphere(0-15km),andtotal(0-25km).
FigureS5:OverviewofthesensitivityofSCDRefandthederivedVCDsonthechoiceofreference
spectra/scananda-prioriprofileassumption.Panel(a)containstheSCDRefdeterminedfromboth
forwardcalculationsofthea-prioriprofiles(greytraces)andtheiterativeapproach(bluetraces)
for47differentzenithspectra.Thea-prioricasescorrespondingtothemedianBrOprofilebased
on WACCM (WACCM*1.4) are denoted with thicker and darker lines. The error bars on the
forwardcalculatedmedianBrOprofilecase(darkgrey)reflectthe±1x1013moleccm-2criteriafor
selectingsuitablereferences.Panel(b)containsthecorrespondingVCDsderivedforoneMAXDOAS scan (near solar noon) for each of the references and a-priori combinations. The blue
shadedverticalboxesdenotereferencesthatmeetthecriteriaforbeingusedintheinversion;
andthehorizontalgreybox(panelb)coverstherangeof2.3±0.9x1013moleccm-2,whichfully
capturesallthereferencescontainedwithintheshadedblueregion.
FigureS6:TimeseriesoftheDoFsandinversionRMSforthreedifferentanalysisprocedures:1)
changingreferenceanalysis(referenceisselectedfromeachmeasurementscanthroughoutthe
day,bluetrace);2)fixedreferencewithoutaccountingforanySCDRef(greentrace);and3)fixed
referenceanalysisaccountingforSCDRef(redtrace).IncludedintheplotisSZA(blacktrace)for
reference.
FigureS7:ShowsacomparisonofthemeasuredBrOdSCDsandthedSCDscalculatedfromtheaposterioriprofiles(inversionusingtheWACCMprofile)forboththeentirecasestudyday(panel
a)andforasmallsubsetofscansbeforesolarnoon(panelb).PanelccontainstheRMSofthe
differencebetweenthemeasuredandcalculateddSCDsforeachscan.
FigureS8:Overviewofverticalprofilesofparametersusedasinputtotheboxmodelutilizedin
this study which were: 1) from WACCM (temperature, pressure, HCHO); 2) from GEOS-Chem
(BrO,O3,NO2,andHg0);3)fromtheTOREROfieldexperiment(totalsurfacearea);and4)derived
duringthisstudy(BrOandIO).
FigureS9:ObservedconcentrationsofselectedtracegasesduringApril2010atthePensacola
MDNsite.Notescalefactorsforsomespeciesgiveninthelegend.
FigureS10:ComparisonofmeasuredO4dSCDsanddSCDcalculatedbasedonthederived
aerosolprofilesfortheentirecasestudyday(panela)andasubsetofscans(panelb).Panelc
containsthediurnalvariationinthederivedaerosolprofiles,andanexampleprofileisfoundin
paneld.
FigureS11:Boxmodelresultsoftherateofmercuryoxidationasafunctionofaltitudefortwo
species:1)ozone(red);and3)bromineradicals(solidblue:MAX-DOAS;dashedpink:GEOSChem)(panela);thecorrespondinglifetimesarefoundinpanelb.Theblackdashedlineat4km
showswheremeasurementsensitivitystartstodropbecauseofthedecreasingamountofBrO
(themeasuredparameter)inthelowerlayersoftheatmosphere.Thisfigureiscomplementary
toFig.7,butusestheWACCM,ratherthanGEOS-Chem,O3andNO2verticalprofilesasinput.
FigureS1
FigureS2
FigureS3
FigureS4
FigureS5
FigureS6
FigureS7
FigureS8
FigureS9
FigureS10
FigureS11