Influence of the source formulation on modeling ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 106, NO. D21, PAGES 27,509-27,524, NOVEMBER

16, 2001

Influence of the source formulation on modeling the atmospheric global distribution of sea salt aerosol Walter Guelle, Michael Schulz, and Yves Balkanski Laboratoire des Sciencesdu Climat et de l'Environnement,Commissariath l'Energie AtomiqueCentre National de la RechercheScientifique,Gif-sur-Yvette, France

Frank

Dentener

European Commission,Joint ResearchCenter, Ispra, Italy

Abstract. Three different sea salt generationfunctionsare investigatedfor use in global three-dimensionalatmosphericmodels.Complementaryobservationaldata are used to validatean annualsimulationof the whole size range (film, jet, and spumedroplet derived particles).Aerosol concentrationsare correctedfor humiditygrowth and samplerinlet characteristics. Data from the North American depositionnetwork are correctedfor mineral dust to derive sea salt wet fluxes.We find that sea salt transportto inner continentalareasrequiressubstantialmassin the jet droplet range, which is best reproducedwith the sourceof Monahah et al. [1986]. The resultsfrom this source formulation also showsthe best agreementwith aerosolconcentrationseasonalityand sea salt size distributionsbelow 4/•m dry radius.Measuredwind speeddependenceof coarse particle occurrencesuggests that above4/•m the sourcefrom Smithand Harrison [1998] is most appropriate.Such sea salt simulationsare relevant for assessing heterogeneous chemistryand radiative effects.Sea salt aerosolprovideson an annual average,in marine regions,an aggregatesurfacearea equal to 1-10% of the area of the underlyingEarth's surface.Together with mineral dust, sulfate, and carbonaceousaerosol the total aerosol surfacearea globallyamountsto 13% of that of the Earth's surface.On the basisof atmosphericcolumnburdens,sea salt represents21% of the total global aerosolsurface area. Equal partitioningof the aerosolsurfacearea amongthe four componentssuggests that one has to considerall of them if the global aerosolimpact is to be fully determined. 1.

Introduction

Sea salt aerosolproducedby the action of wind at the ocean surfaceconstitutesthe most abundantaerosolcomponenttogether with mineral dust. Sea salt studieshave addressedits impacton troposphericchemistry,radiationbalance,or air/sea exchangeof matter and energy.Finlayson-Pitts[1983], Finlayson-Pittset al. [1989],andBehnkeet al. [1997] have shownthat the reactionof NO2 and N205 with NaC1 aerosolin the marine boundary(MBL) providesan effectiveinitiation pathwayfor atomicchlorine.Seasaltis alsoresponsiblefor a large fraction of the non-sea salt sulfateformation sinceit is an important sink for SO2 in the MBL [Sieveringet al., 1991, 1992; Chameidesand Stelson,1992;Gurciulloet al., 1999]. Furthermore,the alkalinityof seasalt ascloudcondensationnucleiaffectsaqueous chemistry[vanden Berget al., 2000]. Murphy et al. [1998] and Quinn and Coffman [1999] have

and thus participates into the aerosol indirect effect [Latham and Smith, 1990; O'Dowd et al., 1999a, 1999b]. Finally, giant sea salt particles have been invoked in the transfer

of heat

and moisture

between

the ocean

and the

atmosphere [Andreaset al., 1995]. Global

chemical

models

used to estimate

the chemical

and

radiativeimpactsof seasalthavehad simplifiedrepresentation of the size spectrum.The representationof the aerosol size and of its distribution is key for such estimatesderived from models.We studyin this paper three different sea salt generation functionsthat havebeen publishedand are widely used. We focuson their abilityto reproduceaccuratelythe whole sea salt size distribution.

Paper number 2001JD900249.

These three generation functions of sea salt particles are Monahah et al. [1986] (hereafter referred to as Monahan); Smith and Harrison [1998] (referred to as Smithliar); Andreas [1998] (referred to as Andreas). Each one provided the input for a yearly simulation in the global transport model. Results from the three simulationsare compared to measurementsover different parts of the sea salt size spectrum. On the basisof the model/measurementscomparison, we discusswhich source function is most adequate to represent sea salt mass distribution and deposition on large scales. The physical characteristicsof the global sea salt aerosol distribution (mass burden, surface area, number, etc.) are also compared to sulfate, carbonaceousand min-

0148-0227/01/2001JD 900249509.00

eral aerosol

shown that over wide oceanic areas, sea salt is the most effi-

cient aerosolcomponentto scattersolarradiation.Haywoodet al. [1999] invokedseasalt to explainthe bias in solar reflection betweena generalcirculationmodel (GCM) and ERBE satellite. Submicrometer sea salt competes with sulfate to influencethe number of cloud condensationnuclei (CCN) Copyright2001 by the American GeophysicalUnion.

27,509

distributions.

27,510

2.

GUELLE ET AL.: SOURCE FORMULATION

Previous Global Modeling Studies of Sea Salt

To our knowledge,sea salt atmosphericcyclehas been includedinto onlyfour globalmodels.Genthon[1992]and Tegen et al. [1997]useda sourceformulationwhichfixesthe surface level concentration[Ericksonet al., 1986]asa functionof wind speed,insteadof estimatinga fluxat the air-seainterface.This formulationusesa singleset of coefficients(slopeand intercept), deducedfrom observations of Lovett[1978],to deduce the surfacelayer sea salt massconcentrationfrom the 10-m wind speed.The intercomparison of similarrelationshipsby variousauthors [Fitzgerald,1991; Gong et al., 1997a] showed significantdifferencesbetweenderivedsetsof coefficients. The different measurementtechniquespermit derivationof relationshipsoverlimitedpartsof the sizespectrum.Any extension to other parts might be hazardous. The most recent global simulationof sea salt aerosoluses the Canadiangeneralcirculationmodel (GCM) [Gonget al., 1997a, 1997b]which Ericksonet al. [1999] used to derive a reactivechlorineemissioninventory.The sourceflux follows Monahanusinghisformulationfor a dry radiusof particlesup to 8/am. The resultsof the simulationare comparedagainst monthlyand yearly averagedsea salt concentrations observed in surfaceair at five coastalstations.Seasonalvariabilitycontributes an important part to the overall variability of the atmosphericsea salt loading.

FOR MODELING

SEA SALT AEROSOL

Bubbleburstingis believedto producethe film and jet droplets,and tearingof wavecrestsby wind formsthe largerspume

drops.Particles withrdry•< 0.5 /amarecommonly referredto as film drop range,particleswith dry radii between0.5 and 4

/amasjet droprange,andparticles withrdry• 4 /amasspume droprange;rdrycorresponds to theradiusof theparticleat 0% RH. Hereinafter we will often refer to small particlesto in-

cludefilm dropandjet droprange(/'dry--7 m s-•. Thus,asidefromthe lowest2 m, the concentrationmeasuredat 15 m asl is representativefor the marine boundarylayer extendingfrom 2 to 15 m for sustainedwind speeds.For lower wind speeds,when sea salt production and turbulent exchangeare dampened, large particlesdisappearfrom the measurementsat 11 m asl [De Leeuw, 1986b]. We

method

SEA SALT AEROSOL

wind and then do not suffer from inlet problemsas discussed for the filter samplersabove. De Leeuw [1987] found no consistentdecreasein particle concentrations

Sea Salt,

Na Ca

FOR MODELING

are not aware

of measurements

of the sea salt size

distributionat heightsabove 15 m. Profile measurementsof aerosolmassperformednear the Hawaiiancoastby Blanchard et al. [1984]andDaniels[1989]showa weakto strongnegative gradientfrom 10 to 70 m asl.The samegradientis exhibitedin measurements

at elevations

between

10 and 30 m above the

open ocean[Extortet al., 1985].

5. Results From the Three Source

Simulations Functions

With

Each of the three seasalt sourceformulationswas ran globally in the transportmodel drivenby meteorologicalfieldsfor 1987.Table 1 presentsyearlyand globalaveragedvaluesof sea salt mass,surfacearea, and numberfor three sizerangesand for each of the model integration.

In thesizerange0.5-4 •m for rdry , seasaltmassdiffersby a factor of 2 between the three formulations, whereas the

surfaceand numberconcentrations varyby a factor of 3 and 1 order of magnitude,respectively.The largestsurfacearea and number concentrationare predictedby the Monahah formulation sincefar more of the smallerparticleswithin this size to observed sea salt concentrationsover the continents,but we range are produced(see Figures1 and 2a). The ratio mass/ expectthis sourceto be of minor importanceduringrainfalls. surfaceareaindicatesthe averagesizedistributionwithin a size Depositionfluxesare alsomeasuredaspart of the European range. The smaller ratio of mass/aerosolsurface area for Monitoringand EvaluationProgramme(EMEP) overwestern Monahah in the range 0.5-4 •m reflectsrelativelymore small Europe. The proximityof these stationsto the coastrenders particlesproducedin that formulationthan with the onesfrom any attempt of comparisonwith our model results a much Smithliar and Andreas.

more difficult

4.2.

task.

Large Sea Salt Particles (Spume Drop Mode)

Large particles(->4 •m) are the mostdifficultto measure. The verticalresolutionusedin globalchemicalmodelslimitsin the way in whichwe can accountfor the largeverticalgradients in the numberof theseparticles(the first modellayer consists of 70 m). Giventhisrestriction,we will devoteonlya smallpart of the model/measurement comparisonto theseparticle sizes. Simulationsof theselarge particlesin the marine boundary layerwere made by De Leeuwand Davidson[1989].De Leeuw [1986a] has gathered existingfield and laboratorymeasurementsof giant particlessampledat heightsof (.--X..

20

'x...x.'

I

i

I

i

'l

i

i

I

i

i

i

i

Isl. (28N,177W)

?-. .,,'

"..

20

"x'"•"•

o

J

40

I

30-• x...x,

30

2 i

i

40 '"55/xm for all wind speeds.A better agreementwith observations at all wind speeds is provided by the formulation Monahan

+ Smithliar.

The measurements by O'Dowd et al. [1997b]showa distinct maximum in aerosolmassaround 60/xm which is reproduced in neither simulation(seeFigures9b, 9d, 9f, and 9h). This very

27,518

GUELLE

ET AL.: SOURCE FORMULATION

FOR MODELING

SEA SALT AEROSOL

ticorenaandBergametti, 1995],andthe occurrence of absorbing aerosolsover desert areas [Hermanet al., 1997]. The TM3 simulationfor BC and POM wasdoneby C. Liousse(personal communication, 2000) usingLiousseet al.'s [1996]sourcefor-

10

mulation.

Since both the dust and the BC+POM

simulation

assumea lognormalsize distribution,with constantspreadobut varyingmediandiameter,computationof the aerosolsurface area is based on the simulated fields of mass and number

concentration.The sulfatefieldswere producedusinga detailed troposphericchemistryschemerepresentingthe main speciesinvolvedin the sulfurcycleand emissioninventoriesfor natural and anthropogenicsulfurcomponents[Jeuken,2000; Jeukenet al., 2001]. Aerosolloadsof the differentaerosolcomponents are sum-

0.1

J _,• 0.01

- -• - Sierra impactor

ß

' '''"l

'

0.1

' ' '''"l

'"0'" Model '

1

' ' '''"1

'

10

r70(gm) Figure 8. Comparisonbetweenthe sea salt masssize distribution measuredduringthe PSI-91 campaignwith two different cascadeimpactorsand the one simulatedwith the Mona-

marized in Table 4. Aerosol surfacearea, mass,or number are

dominatedby different aerosolcomponentssinceeach component has a size distributionwith different characteristics. The aerosol surface area is dominated by dust and sulfate followed by the contribution of sea salt and the combined

organicaerosolfraction (BC+POM). The aerosolmassof mineraldustandseasaltare comparable,exceeding by several folds any other component.In contrast,number concentrations are dominatedby BC+POM and sulfatewith a substantial contribution

from sea salt. The aerosol size distribution

of

seasaltis muchbroaderthan of dust(seeFigure 11). The partitioningof the aerosolsurfacearea amongthe four aerosolcomponents suggests that one needsto considerevery singleone to estimatethe globalaerosolimpacton heterogelarge mode contributessignificantlyto the total seasalt mass. neouschemistryand radiativeeffects.The importanceof the Sucha mode for the largeparticlesis not seenin the measure- sea salt is remarkablein remote marine areas.Sea salt reprementsof TaylorandWu[1992]at windspeeds > 15m s-1 (see sents34% of the total aerosolsurfaceareaon a yearlyaverage. Plate 1 illustratesthe spatialdistributionof the seasaltsurface Figures8d and 8h).

han formulation for 1987.

at the same location and the same month but

area when normalized

7. Sea Salt Surface Area Versus Total Aerosol Surface Area

One reasonfor performing the size-resolvedsea salt simulations

is the relevance

of the aerosol

surface area in hetero-

to the total aerosol surface area. Sea salt

is not pervasiverelative to other aerosolspeciesover continentsanddownwindfrom importantcontinentaloutflows,east of North America and East Asia, west of Sahara and Central

Africa. It becomes much more prominent over southern oceans,where it constitutesthe dominantaerosolcomponent.

geneouschemistryand for the computationof aerosolradiative effects. The relevance

of the sea salt aerosol surface area

8.

Conclusions

can be illustratedby comparingit to the Earth's surfacearea and to the aerosolsurfacearea of the other major aerosol We haveevaluatedthree differentseasaltgenerationfunccomponents(sulfate,mineraldust,blackcarbon,and particu- tions (Monahah,Andreas,and SmithHat) in order to derive late organicmatter). We will discuss the casewherethe aerosol the more realisticsea salt distributionfor globalatmospheric is treated as an external mixture. models.Our studywasnot restrictedto a specificsizerangebut The sea salt aerosol surface area is computedfrom the rather addressedthe whole sea salt size spectrum.This was median diameterof each sizeclassand the averagedensityas motivatedby the dependenceof the radiativeeffectsand the given in Table 4, which also tabulatesthe model parameters chemistryof seasalt upon the aerosolsize. To evaluate the different sourcefunctions, we selectedcomusedfor the other aerosolcomponents.Figure 10 showsthe globalseasalt aerosolsurfacearea as columnburdenwith the plementary observations,so that the model/measurements Monahan source formulation. Sea salt aerosol surface in macouldbe comparedfor the wholesizespectrum.Consideration rine regionsrepresents1-10% of the area of the underlying of humidityeffectson particle size and aerosolsamplerinlet Earth's surface. characteristics is required for a proper comparisonbetween For the comparisonwith the other aerosolcomponents we measurements and model results. For instance, we estimated combinerecent simulationsfor dust, for black carbon(BC) that the aerosolinletsusedto measurethe seasonalcyclehave and particulateorganicmatter (POM) and sulfatewhichwere a cutoffthatcorresponds to anrd• of 2/•m. Hadwenottaken performedwith the sametransportmodel (TM3) and meteo- the cutoff into account,the mean discrepancy betweensimurologicalinput fields. The mineral dust simulationswere de- lated and annual observed concentrations would be 77% with scribedby Schulzet al. [1998]and Guelleet al. [1998a]usinga Monahan's formulation, + 224% with SmithHat, and + 1237% dust sourcedescribedby Claquin [1999] on the basisof the with Andreasas comparedto 34%, -62%, and + 144%, remineralogyof desert areas [Claquinet al., 1999]. The dust spectively. sourceis constrainedfrom detailedinvestigations of the spatial We showedthat the evaluationof the sourcesrequirescomvariabilityof the thresholdvelocityin the Saharanregion[Mar- paring resultsagainstdifferent observationaldata sets. For

GUELLEET AL.:SOURCEFORMULATION FORMODELINGSEASALTAEROSOL

Honahan+Andreas

Honahan + Smith Har

0 obs.: 5ms'1

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0.01

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0.01

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100

rdry(pm)

Figure 9. Simulated seasaltmass size distributions offthewest coast ofIreland. Thesolid lines correspond totheaverage distribution from4 months forthewindspeed indicated. Minimum andmaximum values of simulated distributions duringthesemonths forman errorenvelop, whichis shaded. (left) The Monahan +Smithliar simulations and(right) theMonahan +Andreas formulations. Observations arefromDe

Leeuw [1987] (crosses), fromTaylor andWu[1992] (pluses), fromSmith etal.[1993] (circles), andfrom O'Dowdet al. [1997b](squares).

27,520

GUELLE ET AL.: SOURCE FORMULATION

FOR MODELING

SEA SALT AEROSOL

Table 4. Annual Global Average of Daily Values From TM3 Simulationsof This Studya Surface AreaLoad, Surface Area MassLoad, NumberLoad, mmd,d m2 m-2 Fraction,% mgm-2 10lø m-2 /xm

Seasalt(Monahan+SmithHar)

0.027

21

38.4

Dust Sulfate Black carbon

0.036 0.032 0.009

28 25 7

35.2 6.46 0.32

Particulateorganicmatter

0.025

Total

0.130

Seasalt(Monahan)

0.026

19

5.22

2.13

100

82.5

20

Sigma

ParticleDensity, kg m-3

6.49b

3.12b

2170

2.07 9.64 52.1

2.20 c 0.13 c 0.14 c

2.00 c 1.79 c 1.70 c

2650 1700 1500

59.9

0.34c

2.00c

1500

3.44b

2.08b

2170

128.9

24.8

5.2

aGlobalareal averageof columnintegrateddata (N = 72*48).

bSizedistribution parameters arecomputed frombinneddata. CSizedistributionparameterscorrespondto a lognormaldistribution.

dmmd,massmediandiameter.

example,in the caseof the sourceformulationfrom Andreas, range(rdry upto 80 txm).However, aspreviously pointed outby the observedsea-saltwet depositionfluxesin the inner conti- O•Dowdet al. [1997b],measurements of the seasaltsizedistribunental North America is rather well capturedwhereaswet tion that extendto largeparticlesare verysparse. deposition and monthly surface level concentrationsat the The seasaltsimulations are relevantfor realisticmodelingof coastal stations are shown to be overestimated. heterogeneous chemistryand radiativeeffects.Sea salt aerosol Sea salt particleswith dry radiusbelow 4/xm are well rep- provides,on an annualaverage,21% of the aerosolsurfacearea. resentedby the Monahan sourceformulation.We were able to Nearly equalpartitioningof the aerosolsurfacearea amongthe reproduceobservationsof surfacelevel concentrations,conti- four components confirmsthe necessity to considerall of themto nental wet depositionfluxes,and for size distributionsbelow determinethe globalimpactof aerosolon climateand troposphericchemistry. rdry• 1-2 txm. Future work will use the reanalyzedmeteorologicalfields However,to representgiantseasaltparticles,a combination of Monahan and Smithliar is requiredto havea completesea from ECMWF over either the !5 or 40 year period. This will saltgenerationfunction.The latterformulationshowsagreement allowus to bettercapturethe wind-generated fluxesandtransA with the few measuredsizedistributions in the very coarsesize port fieldsthat were presentat the time of the observations.

Yearlyaveraged columnseasaltsurface(m2 aerosol/ m2 earth) -180

-150

90

I

-120

-90

-60

-30

0

30

60

90

I

120

150

I

I

180

I 90 60

60

0.03•

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.03

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--. -60

-90 -180

- 150

- 120

-90

-60

-30

0

30

60

90

120

150

180

longitude

Figure 10. Annual averageof sea salt aerosolsurfacearea per Earth surfacearea usingthe sourceformulation

Monahan.

GUELLE ETAL.:SOURCE FORMULATION FORMODELING SEASALTAEROSOL

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I.iJ

O

O

O

o

27,521

27,522

GUELLE ET AL.: SOURCE FORMULATION

FOR MODELING

SEA SALT AEROSOL

1.20x 10 -3

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