JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 102, NO. D3, PAGES 3819-3830, FEBRUARY
20, 1997
Modeling sea-salt aerosols in the atmosphere
2. Atmospheric concentrations and fluxes
S. L. Gong,• L. A. Barrie,• J. M. Prospero,2 D. L. Savoie,2 G. P. Ayers?
J.-P. Blanchet,4 and L. Spacek4
Abstract. Atmosphericsea-saltaerosolconcentrationsare studiedusingboth long-term
observations
andmodelsimulations
of Na+ at sevenstationsaroundthe globe.Good
agreementis achievedbetweenobservationsand model predictionsin the northern
hemisphere.A strongerseasonalvariation occursin the high-latitudeNorth Atlantic than
in regionscloseto the equatorand in high-latitudesouthernhemisphere.Generally,
concentrationsare higher for both boreal and australwinters. With the model, the
productionflux and removalflux at the atmosphere-ocean
interfacewas calculatedand
usedto estimatethe global sea-saltbudget.The flux also showsseasonalvariation similar
to that of sea-saltconcentration.Depending on the geographiclocation,the model
predictsthat dry depositionaccountsfor 60-70% of the total sea-saltremovedfrom the
atmospherewhile in-cloudand below-cloudprecipitationscavengingaccountsfor about
1% and 28-39% of the remainder,respectively.The total amount of sea-saltaerosols
emitted from the world oceansto the atmosphereis estimatedto be in the vicinity of
1.17x 1016g yr-•. Approximately
99%of the sea-salt
aerosol
massgenerated
bywind
falls back to the seawith about 1-2% remaining in the atmosphereto be exportedfrom
the originalgrid square(300 x 300 km). Only a smallportionof that exported(-4%) is
associatedwith submicronparticlesthat are likely to undergolong-rangetransport.
1.
Using the mean monthlysurfacewind speedat a resolution
of 5ø x 5ø over the world oceansand an empiricalrelationship
betweenatmosphericsea-saltconcentrationand wind speedat
a referenceheightof 15 m, Ericksonet al. [1986]calculatedthe
horizontal distributionof atmosphericsea-saltover the world
Introduction
Since sea-saltaerosolsnot only scatter solar radiation but
alsoparticipatein cloudprocesses,
the seasonalvariation and
total loading may have an impact on both local and global
climate.
The concentration
and flux of sea-salt aerosols in the
high-latitudeNorth Atlantic is about twice as high as that in
Bermuda [Ericksonet al., 1986]. This is largely due to the
differencein surfacewind speed.However,the large difference
in sea-saltloadingin the atmosphereis likely to affectclimate
because of the interactions
between
sea-salt and clouds and
solar radiation.Togetherwith biogenicsulphuraerosols,seasalt aerosolsform the natural backgroundaerosolsurfacearea
and concentrationof cloudcondensation
nuclei (CCN) in marine areasuponwhichatmosphericsulphuraerosolsare superimposed.In addition, sea-saltaerosolparticlesare chemical
carriersof speciescontainingC1,Br, I, andS andthereforeplay
a role in the atmosphericcyclesof theseimportant elements.
The halogensBr and C1, once mobilized by heterogeneous
reactionsfrom sea-saltinorganicforms to reactive gaseous
forms(e.g.,Br2,C12)[e.g.,Mozurkewich,1995],canplay a role
in atmosphericozonedepletionand destructionof light hydrocarbons[Jobson
et al., 1994].Thus,in order to understandthe
impactof anthropogenicaerosolson climate,it is importantto
model
sea-salt aerosols.
1Atmospheric
Environment
Service,Downsview,
Ontario,Canada.
2Rosenstiel
Schoolof MarineandAtmospheric
Sciences,
University
of Miami, Miami, Florida.
3Division
of Atmospheric
Research,
Mordialloc,
Victoria,Australia.
4EarthSciences
Department,Universityof Quebecat Montreal,
oceans on a seasonal basis. The sea-salt aerosols in the north-
ern hemisphericmarine tropospheredisplayed a substantial
seasonalvariation (a factor of 2-3 betweenthe boreal winter
and summer),while lessvariationoccurredin the high-latitude
southernhemisphereand little variationoccurredin the equatorial areas.The globalsea-saltbudgetwasalsoestimated[Erickson
andDuce, 1988].Dry depositionwascalculatedasa functionof
wind speedby the methodof Slinnand Slinn [1981]and belowcloud wet depositionby a scavengingratio formulation.
In a previouspaper [Gonget al., this issue],an atmospheric
sea-saltaerosolmodelwasdevelopedto estimatesea-saltaerosol massand number concentrationsin terms of meteorological conditionsgeneratedby a one-dimensionalclimate model
(FIZ-C) [Themien,1993]. It was found that sea-saltaerosol
concentrationsare modulated strongly by the local surface
wind speed, and to a lesser extent by long-range transport.
Both experimentsand model predictions[Gong et al., this
issue]have shownthat the total sea-saltmassconcentration
can be expressedas an exponentialfunction of surfacewind
speedbut that the parametersof the function depend on the
wind and precipitationclimatologyof an area. In other words,
the assumptionof a singlerelationshipbetweensea-saltmass
concentrationand wind speedusedby others [Ericksonet al.,
1986;EricksonandDuce,1988]entailsconsiderable
uncertainties.
Canada.
Observational data for total aerosol Na + mass concentration
in the atmosphereare availableat a numberof globalbaseline
observatories
(Figure 1) includingAlert in the Arctic [Bartie,
1995],Heimaey,Mace Head, Bermuda,Oahu, Palmer Station,
and Cape Grim, enablingan assessment
of the globalsea-salt
Copyright1997 by the American GeophysicalUnion.
Paper number 96JD03401.
0148-0227/97/96JD-03401 $09.00
3819
3820
GONG ET AL.' ATMOSPHERIC SEA-SALT MODELING, 2
180' E
1
150•E ,/'
30øW
GONG ET AL.' ATMOSPHERIC
climatologyand budget. The aim of this study is to test the
ability of our model to predict a realisticsurfacelevel sea-salt
massdistributionon a globalbasisand estimatethe budget.
The observationaldata and model predictionsare usedin a
complementaryway. Comparisonof modelled and observed
climatologicalmean seasonalvariation of sea-saltaerosolconcentrationis neededto test the parameterizationschemesused
in the modelfor variousatmosphericprocesses
influencingthe
atmosphericcycleof sea-saltaerosols.Sincethe size distribution of sea-saltaerosolconcentrationis resolvedby the model,
the effect of particle size on major sea-saltaerosol removal
processes
in the atmospheresuchas dry deposition,in-cloud
and below-cloudscavengingis investigated.Finally, the model
is used to estimatethe global sea-saltaerosol.
SEA-SALT MODELING,
2
3821
was extractedfrom the model resultsfor the first atmospheric
layer (0-167 m) and averagedmonthly for each size range.
Finally, the monthly concentrations,production, and removal
fluxeswere averagedto yield the monthly mean massconcentrations and fluxes over 5 years. These concentrations and
fluxesform the basisfor sea-salttrend and budget analysis.In
orderto compareour predictions
with Na+ concentration
observationsin the atmosphere,the sea-saltmassconcentration
is convertedinto Na+ massconcentration
by assumingthat
sodium ion content in sea-saltaerosolis 30.77% [Fergusson,
1982].
3.
Sea-Salt Climatology: Observed and
Predicted
3.1.
2.
Model Description
A detaileddescriptionof the sea-saltaerosolmodel usedin
this studyis givenby Gong et al. [this issue].A brief summary
of key featuresof the model is presentedhere.
The model includesthe following processes:(1) sea-salt
generationdue to surfacewind; (2) vertical transportby turbulenceand convection;(3) dry depositionand gravitational
settling;and (4) wet removal processes
which include both
in-cloud and below-cloudscavenging.A one-dimensionalclimate model(FIZ-C) [Therrien,1993]coveringa grid squareof
300 x 300 km was coupled with it to provide time-variant
meteorologicalconditionssuchas surfacewind, precipitation,
temperature,and relative humidity. Taking the above mentioned processesinto consideration,a generalizedprognostic
massbalanceequationfor any aerosoltype in a discretesize
range (i) can be written as
O!-
O!
dynamics
surface
clear air
dry
below-clouds
where;•,sis the sea-salt
concentration
expressed
asthe mass
mixingratio for i th size range of typej aerosols.In (1), the
aerosolconcentrationchangehasbeen dividedinto tendencies
for dynamics,surface,clear air, dry deposition,in-cloud and
below-cloudprocesses.
The dynamicsincludesresolvedmotion
as well as subgrid turbulent diffusion and convection.The
surfaceprocesses
includesurfaceemissionof both natural and
anthropogenicaerosolsand serveas boundaryconditionsfor
the model.Particlenucleation,coagulationand chemicaltransformation are included in clear-air process.It shouldbe emphasizedthat the resolutionof this model is approximately
equal to that of the GCMII from whichits inputsare derived
(300 x 300 km). Thus model predictionsrepresentspatial
averageconcentrationson this scale.
Sea-saltaerosolradius(dry) from 0.03 to 8 p•mwas divided
into eightdiscreteranges.For eachrange,(1) wassolvedfor 10
layers extendingfrom ocean surfaceto about 33 km in the
atmosphere.Except for Alert and Palmer Station which are
not situatedon the ocean,the modelwasintegratedfor each of
the pointsin Figure 1 for 5 yearswith a time step of 20 min.
Monthly averagedsea-saltmassconcentrationas well as the
standarddeviationwas computedfor comparisonwith the observations.The productionflux of sea-saltaerosolmassdue to
surfacewind and removal flux due to dry and wet depositions
Variations
of Sea-Salt
Aerosols
3.1.1. High-latitude North Atlantic. Two sites in the
high-latitude North Atlantic were chosenfor this area: Mace
Head on the west coast of Ireland (53.19øN, 9.54øW) and
Heimaey in Iceland (63.40øN, 20.30øW). The 24-hour mean
Na + concentrations
were measuredcontinuouslyfor 5 years
(August1988to July1993) at Mace Head and for 3 years(July
1991 to August 1994) at Heimaey. Observed and predicted
monthlyaveragedNa + are comparedin Figures2a and 2b,
respectively.At Mace Head, the sampleris sectoredtoward the
west and it should effectively sample winds associatedwith
westerly stormsmoving through the region. At Heimaey, the
sectoris from 90ø to 270ø, encompassingthe entire ocean area
south of Iceland.
The comparisonsare reasonable.Seasonalvariations occurredboth for the observationsand the model predictionsfor
the two points. Sea-saltconcentrationsare a minimum in the
summertime and peak from Decemberto February. However,
according to the observations, the seasonal variation is
stronger at Heimaey than at Mace Head. The minimum is
lower
m-cloud
Seasonal
for the model
than
for the
observations.
The
model
performance, defined as the absolute percentage difference
betweenobservationand model predictionat the samemonth,
P1 :
•l
-- Ol lot X 100%, rangesfrom 10% to 160%,
dependingon month and location.The annual averageperformance (AAP) is quantifiedas follows:
12
•=1
AAP: 12 :
12
-
O•
12
x 100%,
(2)
where M, is the average concentration for month i from a
prediction
overthemodeling
periodand•, froman observation over the measuringperiod. It is 54% and 26% for Mace
Head and Heimaey, respectively.
The observedpoint for February at Heimaey, Iceland, needs
some scrutinysince it is much higher than the neighboring
months.In the originaldata,Na + concentration
wasobserved
for three consecutiveFebruarysfrom 1992 to 1994. For February 1992,the monthlyaveragedNa + concentration
was28.0
p•gm-3 with a standarddeviationof 59.8.A concentration
as
high as 296.3 p•gm-3 was recordedon February13, 1992,
possiblybecauseof a winter storm. As a result of this high
episode,the averagedNa+ concentration
in Februarywasexceptionallyhigh. If this observationis excluded,the curve for
the observation in Figure 2b would be much closer to the
model predictions.
3822
GONG ET AL.: ATMOSPHERIC
(a)
20
SEA-SALT MODELING,
MadeHead,Ireland
2
(b)Heimaey,
Iceland
•8
16
•6
-
•
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•
bservation
14
12
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Jan
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Feb Mar Apr May Jun
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Jul
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Aug Sep
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0 •
Jan
Oct Nov Dec
Feb
Mar
Apr
May
Jun
•
[
,
•
,
Aug Sep
Oct
Nov
Dec
(d) Hawaii
(c) Bermuda
10
Jul
Month
Month
i
,
,
i
7
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8
•,
3
Observation
0
I
,
Jan
I
,
Feb
I
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Mar
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Apr
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May
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Jun
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Jul
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•
Aug
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Observation
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Sep
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Oct
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Nov
Jan
Dec
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Feb
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Mar
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Apr May Jun
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Jul
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Aug Sop Oct
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Nov Dec
Month
Month
(e) Cape Grim
6
i
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5
•4
2
.
1
Jan
Feb Mar
Apr May Jun
Jul
i
Aug Sep
Oct
i
Nov Dec
Month
Figure2. Comparison
of monthlymeansea-salt
[Na+] concentrations
betweenobservation
andsimulation.
The errorbar indicatesonestandarddeviation.(a) MaceHead,Ireland,(b) Heimaey,iceland,(c) Bermuda,
(d) Oahu, Hawaii, and (e) Cape Grim, Australia.
it isworthwhile
tOpointoutthatdueio thecoarse
resolution the paper,thesesporadiceventsshouldbe averagedout over
usedin the FIZ-C (300 x 300 km), somesporadicandextreme
events,suchasan isolatedstormor unusuallycalmconditions,
time.
The agreementof seasonalvariation betweenmodel and
maynot be resolved
in the model.On the otherhand,such observationis not as good at Mace Head as at Heimaey. Two
sporadicevents,shouldtheyoccur,will invariablycontributeto
sourcesof uncertaintiesexist:modelwind speedand represen-
the actualobservations.In a shortterm, theseeventsmay have
tativeness of measurement
stations. The first one is that the
variationof wind speedis too largefor the gridsquare
an effecton the comparison
betweenobservations
and model seasonal
predictions.
Sincesea-saltclimatology
is the majorconcernin includingMace Head.Becauseof the resolution(---300x 300
GONG ET AL.: ATMOSPHERIC
km), the model-derivedwind may representthe averagewind
speedin that area but not for Mace Head which is located on
the coast.The associated,seconduncertaintyrevolvesaround
the fact that the wind speedmeasuredat a coastalstationsuch
as Mace Head may have a weaker variation than the open
oceanwindsin that region.This influencecanbe seenfrom the
predictionsof other sitessuchas Bermudaand Oahu aswell.
3.1.2. Bermudaand Oahu. Bermuda(32.27øN,64.87øW)
and Oahu (21.33øN,157.70øW)are two islandslocatedin the
Atlantic and Pacific,respectively.Figures 2c and 2d showthe
seasonal
variationsof Na+ concentrations
for the two points.
The seasonalvariabilityand the concentrationis lessthan that
for high-latitudepoints suchas Mace Head and Heimaey. At
SEA-SALT MODELING,
2
3823
6
,-.
lOO
•
._
•
5o '•'
4
Precipitation
both locations, observations show a small decrease of the sea-
salt concentrationin the summertime, while predictionsshow
no significantdecreasefor Bermuda and a maximum at Oahu.
As opposedto the observations,a peak in summerfor Oahu
waspredictedby the model (Figure 2d). In view of the climatology of wind speed and precipitation at Oahu, the model
result is likely justified. In Figure 3, the monthly observed
meanwind speedand precipitationdata [Van Loon, 1984] are
plottedfor Oahu (21.20øN,157.55øW)at a 5-m elevation.The
higherwind speedand lower precipitationin the summertime
shouldproducea maximumsea-saltconcentrationaspredicted
by the model rather than a minimum concentration as observed.It shouldbe pointed out that the samplerat Oahu is
sectoredtoward the north. This may bias againstthe summer
windswhich are associatedwith southeastlytrades at Oahu.
Nevertheless,in terms of annual averagevaluesthe agreement betweenmodel predictionsand observationsfor the two
sites is reasonable.
AAP
is 33%
and 71%
for Bermuda
and
Oahu, respectively.
3.1.3. Cape Grim in the southernhemisphere. The Na +
trend at Cape Grim (40ø41'N,144ø41'E)on the northwesttip
of Tasmania from our 5-year simulation and in situ measurementsis shownin Figure 2e. The experimentaldata (Baseline
AtmosphericProgram(Australia),Bureauof Meteorologyand
CSIRO Division of AtmosphericResearch,Annual Reports,
1976-1991) were taken from December 1988 to May 1993 by
a samplerwith PM•o hivolheadto removeparticleswith radius
larger than 5 /•m. A 7-day samplinginterval was used during
this period. In Figure 2e, the observationsand predictionsof
sea-saltaerosol concentrationare shownfor r -< 5 /•m.
A generalagreementwasachievedbetweenthe observations
and modelpredictionsin termsof the annuallyaveragedNa+
massconcentration.The AAP for Cape Grim is 22%. Little
seasonalvariation was observedin the experimentat Cape
Grim. The observedannualaverageNa + concentrationis 3.3
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug Sep
Oct
Nov
Dec
Month
Figure 3. Observedmonthly mean wind speedand precipitation data at Oahu, Hawaii.
terrain.Figure 4a showsthe monthlyaveragedNa+ air concentrationobservedat Alert over 13 years[Bartie,1995]. Minimum concentrationsare observedin summer time (JuneAugust). The sea-salt aerosol peaks over a broader winter
period beginningin early October and endingin April. Being
far away from the open ocean,the Na + in Alert is mainly
contributedfrom longrangetransport.BartieandBartie [1990]
attributedthe winter peak to a combinationof longer aerosol
residencetimes and strongerseaspraysourcesin the northern
oceansat that time of year than in the summer.This is clearly
shownin Figures2a and 2b where the sea-saltproductionis at
a maximumin the high-latitudeNorth Atlantic ocean during
boreal winter
and a minimum
in the summer.
The Na + concentrations at Alert are •60
times lower on
averagethan at sitescloseto open ocean(compareFigures2a,
2b, and 2c). As pointed out by Gonget al. [thisissue],mostof
the large particles(e.g., r - 4-8 •m) generatedover open
oceanhave a short atmosphericresidencetime (•1/2 hour)
and are unlikelyto survivelong-rangetransportto Alert over
Arctic ocean ice [Battle et al., 1994]. Aerosols at Alert are
associated
with smallersea-saltparticles(massmedian diameter •1 •m) which representonly a small portion of sea-salt
aerosolmassgeneratedat the source.There is strongcorrela-
tion (Figure4a) betweenNa+ air concentration
at Alert and
the production rate of sea-saltat Heimaey, Iceland. This is
consistentwith transportof sodiumaerosolsfrom high-latitude
North
Atlantic
ocean to Alert.
Although,observedNa+ is approximately
5 timeshigherat
/•g m-3 whilethemodelpredicts
an annualaverage
of 3.0/•g Palmer Station in the Antarctic than at Alert in the Arctic
m-3 for sea-salt
particles
of lessthan5/•m in radius.However, (Figure4), it is still lowerthan predictedfrom the open ocean
a seasonalvariation of sea-saltaerosolswas predictedby the
model.The maximumconcentration
wasobtainedduringthe
point about 600 km north of the Palmer Station (Figure 4b).
monthsof July to October.This maximumis mainly causedby
the high wind speedfrom the climate model. As was pointed
out by Ericksone! al. [1986] the seasonalvariation of Na +
concentrationin the midlatitudesouthernhemisphereis much
lessthan that in the northern hemisphere.
distance than to Alert.
3.1.4.
arctic.
Alert
in the Arctic
and Palmer
Station
in the Ant-
Unlike the other stations discussedabove, these two
sitesare remote from oceanwater. Consequently,the concentrations observedare not directly comparableto the onedimensionalmodel predictionssincethe model doesnot simulate removal during transportto the sites over ice-covered
This is due to removal
3.2.
as discussed above but over a shorter
Sea-Salt Flux and Budget at the Sea-Atmosphere
Interface
Comparedwith observations,
the modelpredictednot only a
reasonable
seasonal variation
of sea-salt mass concentration
for somelocationsbut alsoa ratherconsistent
yearlyaveraged
concentrationthroughoutthe 5-yearsimulationasindicatedby
the values of AAP for various stations.This agreementdemonstratesthat the parameterizationschemesused for sea-salt
generationand removalprocessesand the climatologygener-
3824
GONG ET AL.: ATMOSPHERIC
SEA-SALT MODELING,
2
(a) Alert, Canada
0.,5
•
,
•
.
•
,
-4
•
-'
0.4
Observation
concentration
+
Model predictionof Productionflux
at Heimaey, Iceland
Apr
May
E
ß• 0.3
o
'.•
c
o
0.2
0.1
-0
0.0
Jan
Feb
Mar
Jun
Jul
Aug Sep
Oct
'
'
Nov
Dec
Month
(b) Palmer Station
'
I
'
I
'
I
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'
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'
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'
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10
E
8
6
Vlodel Prediction
o
o
4
2 -
Observation
of
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug Sep
Oct
Nov Dec
Month
Figure 4. (a) Monthly mean sea-salt[Na+] concentrations
at Alert, the Arctic from observations.
The
productionflux of sea-saltaerosolsat Heimaey,Iceland,is alsoshown.The error bar indicatesone standard
deviation.Note that a differentscaleare usedfor the flux.(b) Comparison
of monthlymeansea-salt[Na+]
concentrations
betweenobservations
at Palmer Station,Antarctic,and a model prediction600 km north of
Palmer
Station.
ated by the FIZ-C climate model are reasonable.In this section, a fluxandbudgetanalysisof sea-saltaerosolsispresented.
The productionflux for different size rangesas a functionof
surfacewind speedwas calculateddynamicallyin our model
usingthe empiricalparameterizationof Monahanet al. [1986].
Dry deposition,gravitationalsettling,andwet removalcontrib-
3.2.1.
Seasonal
variation
of sea-salt
flux.
The model
re-
sultsshowa strongseasonalvariation of sea-saltconcentration
at Mace Head, Heimaey,and a smallweakerone at Bermuda,
Oahu, and Cape Grim. These variationsare associatedwith
the seasonalvariationsin surfaceproductionand atmospheric
removalof sea-saltaerosols.In Figure 5, the seasonaltrend for
uted to the removal flux of sea-salt aerosols. The fluxes in units
the monthlysurfaceproductionflux and the ratio of removal/
of kgm-2 s-• werecalculated
at eachtimestepandaveraged productionflux are presentedfor Mace Head, Heimaey, Bermonthly.
muda, Oahu, and Cape Grim, respectively.For Mace Head
GONG ET AL.' ATMOSPHERIC
SEA-SALT MODELING,
2
3825
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3826
GONG ET AL.' ATMOSPHERIC
SEA-SALT MODELING,
2
0.0
,
-0.4
ß
•,,,1 I •''-.•
.•.
-0.8
ß
x
Total
Total Regr
mm Ory Deposition
Dry Oep. Regr
ß
o
E
In-Cloud
In-CloudRegr
•,
....
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Below-Cloud
Below-CloudRegr
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Balanced
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Line
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0.0
I I I I
0.4
I I I I
0.8
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1.6
Production
Flux[kgm-2s-1]x10-9
Figure 6. Correlationbetweenremovaland productionflux at Bermudafor differentremovalprocesses.
Each marker in the plot representsa monthlyaveragedvalue.
and Heimaey,the minimumflux duringJune to September
corresponds
to the minimummassconcentration
of sea-salt
aerosolsin the sameperiod obtainedfrom both observation
andmodelprediction.Production
fluxat Bermudais similarto
that at Mace Head and Heimaeywith lessseasonalvariation
andsmallermagnitude.Thisimpliesthat the cyclingof seasalt
in the atmosphere
in the high-latitudeNorth Atlanticis much
more intensive than at Bermuda.
The seasonal variation
of
productionfluxin Oahudiffersslightlyto thatin Bermudaand
the North Atlantic. A higherflux betweenApril and August
wasobtained.Althoughthe trend doesnot agreewell with the
observation,
it is supportedby the climatedatafor that region
(Figure3). An explanation
hasbeengivenin subsection
3.1.2.
for the discrepancy.
CapeGrim is the onlysimulation
pointlocatedin the southern hemisphere.Taking into accountaustralwinter being 6
monthsout of phasein the southernhemisphere,
CapeGrim
has a similar variation to the northern hemisphericsites.
Higher productionflux wasassociated
with winter monthsof
May to September.
The analysisin thisstudyfor the seasonal
variationof atmo-
sphericsea-saltconcentration
and flux generallyagreeswith
that by Ericksonet al. [1986].They foundthat seasonal
variation in the northern hemisphereis higher than that in the
high-latitudesouthernhemisphere.
The smallestseasonal
variation is found around the equatorialregion.The variability
predictedby our modelat Cape Grim in the southernhemispherewasnot evidentin the observations.
The ratio of removal to production flux at the ocean/
atmosphere
interfaceis alsoshownin Figure5 for eachsite.
This ratio indicatesthe degreeof atmosphericremovalof seasaltaerosolsin the grid square.As canbe seenfrom Figure5,
the mean ratio is between 0.98 and 0.99 for all sites and does
not vary substantially
with seasonexceptfor August at Heimaeywhere the ratio is 0.95.
3.2.2. Sea-saltbudget. The budgetof sea-saltaerosolsis
investigated
in two scales:localgrid scalesandglobalaverage.
Local budget: For eachpoint, the removalflux was separatedinto dry,in-cloudandbelow-cloud
flux.In Figure6, the
surfaceproductionflux at Bermudais comparedto the removalflux of total mass(r - 0.03-8 •m). The removalflux
of total masscorrelatesverywell with the surfaceproduction
flux (slope- -0.99). A verysmallfractionof the production
flux is not balanced,probablydue to vertical movementof
smallerparticlesby turbulentdiffusionthat contributeonly a
little to total mass.Dry deposition
contributes
morethan60%
to removalof total sea-saltmass.Of wet scavenging,
in-cloud
and below-cloudscavenging
processes
accountsfor 1% and
30%, respectively.
In the literature,there existsa very largerangein estimates
of relative contributionsof dry and wet removalprocessesto
total sea-saltremoval.Ericksonand Duce [1988] estimatedan
annualrate of 1.5 x 1016g for globaldry depositionand
1.8-6.0 x 1015g forglobalwetdeposition.
Thiscorresponds
to
70% dry and 30% wet removal.Blanchard's[1963]computation yieldeda globaldry depositionof atmospheric
seasaltof
3 x 10•5 g yr-• andwetdeposition
of 6 x 10• g yr-•. This
corresponds
33% dry and67% wet removal.There are several
reasonsfor this difference.First, the dry depositionrate either
by measurement
or by modelcalculationdiffersa lot between
GONG ET AL.: ATMOSPHERIC
Table
1.
Various
Annual
Sea-Salt
Removal
Flux Fraction
than wet removal.
at
Stations
Station
Dry Deposition
In-cloud
SEA-SALT MODELING,
Below-cloud Slope*
2
3827
Since no clouds are allowed
in the lowest
model layer (e.g., 0-166 m) in FIZ-C physicsto prevent developmentof excessive
low cloudiness[McFarlaneet al., 1992],
the in-cloudremovalfraction is quite small,that is, 1-2%. It is
Bermuda
60%
1.0%
39%
-0.99
attributed
Cape Grim
Heimaey
68%
67%
1.0%
1.0%
31%
32%
-0.99
-0.98
Mace Head
Oahu
68%
71%
1.0%
1.0%
31%
28%
-0.98
-0.97
lowestlayer and above.
Note that in all casesremovaldid not matchproduction,that
is, -1-2% of massgenerated at the surface remained in a
columnabovethe grid square(300 x 300 km) of the FIZ-C.
This unbalancedmassenablessea-saltaerosolsto be exported
beyondthe grid squareand to engagein long-rangetransport.
*The slopeis for the regressionline of downfluxupflux.A value of
- 1.000 meanscompleteremoval.
to the in-cloud
This is consistent
thesetwo studies.Spatialand temporalvariationof dry deposition rate is not understoodwell. The uncertaintyin dry depositionrate will inevitablyaffectthe fractionalloss.In addition,
the concentrationusedbyEricksonet al. [1986]to calculatethe
flux was based on the global wind distribution and a single
equationrelatingwind speedand sea-saltmassconcentration.
The equationchosenmaynot necessarily
applygloballyasboth
measurements
and modelpredictionshaveindicatedthat there
existsa differentrelationshipbetweensea-saltmassconcentration and wind speedfor different locations[Gong et al., this
issue].Furthermore, a singlevalue of 500 for the scavenging
ratio in the wet depositioncalculation[Ericksonet al., 1986]
would result in a large uncertainty.
Our predictionagreesbest with the predictionof Erickson
and Duce [1988].Accordingto our simulation,similarrelative
removalby dry and wet depositionswas obtainedfor all stations. Table 1 summarizesthe productionflux, separatedremoval flux, and the correlation between production and removal flux for different stations.Dry depositioncontributed
more to the major removalprocesses
of total sea-saltaerosols
a)
ß
r = 0.03 - 0.06
ß
r = 0.06 - 0.13 p.m
removal
of sea-salt from the second
with observations
of sea-salt aerosol
b)
0.0000
0.0000
-0.0002
-0.0009
-0.0004
-0.0018
-0.0006
-0.0027
ß
r = O.13 - 0.25 p.m
ß
r = 0.25 - 0.50 p.m
-0.0036
-0.0008
0.0000
0.0002
0.0004
0.0006
0.0008
0.0000
0.00
0.0009
0.0018
0.0027
0.0036
Production
Flux[kgm-2s-1]xl 0-9
Production
Flux[kgm-2s-1]x10-9
c)
ß
r = 0.5 - 1 p.m
ß
r=2 -4 txm
ß
r=l
ß
r=4-8[tm
- 2 I.Lm
0.0
-0.07
-0.14
-0.4
-0.21
-0.6
-0.28
-0.8
0.00
0.07
0.14
0.21
0.28
Production
Flux[kgm-2S-1]xl0-9
at sites
remote from open oceansuchasAlert and Palmer Station.To
investigatethe size dependenceof this net export, we considered size-segregatedproduction and removal fluxes in the
model. Figure 7 showsthe correlationbetweenproductionand
removal flux for eight size rangesat Bermuda. For all particle
size ranges,the production flux is nearly balancedby the removal flux. In view of the wide sizespectrumconsideredin the
model(r = 0.03-8/•m), there existsmore than one dominant
removal mechanismfor large and small aerosolparticles.This
is clearlyshownin Figure 8 where the size-segregated
removal
fractionsof three processesfor eight size bins are plotted at
Bermuda for the month of January.The fractionsin the graph
reflect the relative importance of three removal processesof
sea-saltaerosolsat different sizes.For large particles (r 0.5-8 /•m), dry depositionaccountsfor more than 50% of the
total removalprocessand is the dominantremovalmechanism.
Becauseof their large dry depositionvelocity,these particles
will fall back to the seasurfacein a very short time. Gonget al.
[this issue]calculatean atmosphericresidencetime of several
hours for these particles. The remaining removal fraction
0.0
0.2
0.4
0.6
0.8
Production
Flux[kgm-2s-1]xl 0'9
Figure 7. Size-segregated
correlationbetweenremovalflux and productionflux at Bermuda.Each marker
in the plot representsa monthly averagedvalue.
3828
GONG ET AL.: ATMOSPHERIC
SEA-SALT MODELING,
2
0.8
Dry
Deposition.•
0.6
0.4
0.2
0.0
0.03-0.06 0.06-0.13 0.13-0.25 0.25-0.50 0.50-1
6
7
8
1- 2
2- 4
i,
Size Bin Number
4- 8
Radius[lLtm]
Figure 8, Size-segregated
fractionallossfor three separatedremovalprocesses.
(-40%) of particlesin these size rangesis attributedto the
below-cloudremoval processes.For smaller particlesin bins
1-4 (r = 0.03-0.5 /•m), the dry depositionvelocityis much
lower and the wet removal processesbecomecomparableto
the dry deposition.For sea-saltaerosolsin thesesizebins,the
combined below- and in-cloud scavengingaccountsfor more
than 60% of total sea-saltmassremovedfrom the atmosphere.
In termsof massfluxeswhichcyclethroughthe atmosphere/
ocean interface, large particles (r = 0.5-8 /•m] dominate
(Figures7c and 7d) both generationand removalprocesses.
The flux for theseparticlesis muchlarger than that in Figures
7a and 7b. To gain further insightinto the cyclingprocess,the
unremoved
number
flux and mass fraction
ported far from their sourcegrid. On the other hand, submicron sea-salt aerosolshave a long atmosphericresidence
time and are likely to be transportedby turbulent motion of
the air into the higheratmosphereand later engagedin longrange transport by advection.This is consistentwith the ob-
servationof Na+ aerosols
at Alert [Bartieet al., 1994]which
showsaerosolconcentrationabout60 timeslowerthan in open
oceanicregionsand a particlemassmediandiameterof Na+
below 1 /•m [-0.49 /•m]. The analysisseemsto suggestthat
although the large particles contribute most of total unremoved sea-saltmass,only a small fraction [4%] of it [due to
sub-micronparticles]will participatein long range transport
and be exportedfrom oceansto surroundingice or land sur-
of sea-salt aerosols
faces. In contrast to the massfraction, however, the sub-micron
in eachsizebin was calculatedat Bermuda(Figure 9). About
96% of the total unremovedmassis attributed to particles
larger than 1/•m in diameter(r = 0.5-8/•m) with remaining
fraction attributable to the submicronparticles. Because of
their shortresidencetime (<1 day) in the atmosphere,mostof
the particlesin bins 5-8 (r = 0.5-8 /•m) will not be trans-
10 6
,
,
particles dominate the number flux of unremoved sea-salt
aerosols(Figure 9).
Global budget: The productionand removalfluxesof seasalt aerosolat the sea/atmosphere
interfaceenableus to estimate the global sea-saltbudget.In addition to the five simu-
,
,
lOO
10 5
Mass Fr
[• 104
>
>
o
o
[:: 103
10
E
•)X 102
o
•
o
101
E
Z
10o
10 -1
i
i
i
i
i
i
i
I
1
2
3
4
5
6
7
8
0.03-0.06 0.06-0.13 0.13-0.25 0.25-0.50 0.50-1
1- 2
2- 4
4- 8
1
Size Bin Number
Radius[lLtm]
Figure 9. Size-segregated
unremovedmassfraction and number flux at Bermuda.
GONG ET AL.: ATMOSPHERIC
Table 2.
SEA-SALT MODELING,
2
3829
Global Sea-Salt Annual Flux Estimate
SouthernHemisphere
Northern Hemisphere
Best
MaceHead
Dry
In-cloud
Below-cloud
Total Out
Total In
Net Input
Heimaey
Bermuda
Oahu
CapeGrim
1165
1292
1922
Estimate*
8.9
6.4
3.6
5.2
6.0
13.6
11.3
6.1
7.9
0.10
4.0
13.1
13.3
0.10
3.0
9.5
9.6
0.06
2.1
7.3
7.5
0.06
2.1
7.3
7.5
0.07
2.7
8.8
8.9
0.14
5.9
19.6
19.9
0.11
4.1
15.6
15.8
0.07
2.6
8.9
9.0
0.08
3.4
11.5
11.7
0.14
0.10
0.15
0.14
0.17
0.30
0.20
0.16
0.17
Estimates
arein g yr-• x 10•-s.
*Thebestestimate
wasobtained
byFB= 0.428/'North
4-0.572/'South,
where
PNorth
andPSouth
aretheaveraged
fluxfornorthern
andsouthern
hemisphere,
respectively.
Thefractions
of worldoceanat northernandsouthern
hemisphere
are0.428and0.572.
lationpoints(station2, 3, 4, 5, and6 of Figure1) usedabove, land surface,while Petrenchuk[1980] increasedthe value to
of this
threemorepointsin the southernhemisphere
(1165,1292,and 3.70 x 10TMg yr-•. Two factorsaffectthe accuracy
1922in Figure1) were addedto estimatethe globaldistribu- estimate.First, river run-off of chlorideis contributednot only
tion.Usinganapparent
areaoftheworldocean
of3.61x l0ts by sea-saltdepositionon the landsurfacebut alsoby industrial
cm2 [Baumgartner
andReichel,1975],the annualproduction activitiesalongrivers,dry soil salts,igneousrocksand more.
andremovalratesof globalatmospheric
sea-saltaerosols
were The extent of contributionsother than sea-saltis very difficult
calculatedby integratingthe productionflux and removalflux to estimate.Second,a sea-saltbudgetestimatebasedon chloof each station over 12 months. The estimated values are listed
ride river run-off suffers the weakness of local effects. The
sampling
locations
andtimeall haveaninfluenceon the global
averagevalue.A bettermethodfor the net exportis to calcuatmosphere
wasestimated
to be 1.15x 1016g of whichdry late the dry and wet depositionof sea-saltaerosolson global
in Table
2.
The model-predicted
annualsea-saltremovalrate from the
deposition,
in-andbelow-cloud
account
for7.9x l0isg,8.0x
land surfaces.Sincethe net exportinvolvesboth the produc-
1013g, and3.4 x 10•s g, respectively.
Thesevalueswerecal- tion and transportof sea-saltaerosols,a three-dimensional
culatedfrom the weightedaveragefluxesover northern and globalclimatemodelis ultimatelyrequiredto couplewith this
southernhemispheres
by considering
the fractionof the world aerosolmodel for a more accurateglobalsea-saltbudgetestioceanin both hemispheres.
The relativecontributionsof dry, mate.
in- and below-cloud to total annual removal are 69%, 0.7%,
The generalagreementbetweenmodelingand observation
of sea-saltaerosols
aspresentedabovewith a oneand30%, respectively.
The annualproductionrate of sea-salt climatology
fromtheoceanto the atmosphere
is 1.17x 10•6 g. The dif-
dimensional climate model indicates that a much better value
ferencebetweenproductionand removalratesyield a global for global sea-saltbudget can be achievedif the model is
globalGCM. It is anticiatmospheric
netinputof sea-salt
of 1.7x 10TM
gyr-• fromthe coupledwith a three-dimensional
world oceans. This is the amount of sea-salt aerosols unrepatedthatglobalsea-saltproduction,
deposition,
andtransport
movedin the atmosphere
overthe oceansand likelyexported will be calculateddynamicallyin the modeland globalsea-salt
to the continents. Because of one-dimensional nature of the
budgetisthusobtaineddynamically.
The initialeffortat sucha
run is currentlybeingundertakenat
climatemodel,the net input may reflect someuncertainties globalthree-dimensional
since the horizontal advection is not considered in the model.
AtmosphericEnvironmentServiceof Canada.The Northern
Despitedifferenttechniques
usedto arriveat a globalsea- AerosolClimate Model (NARCM) coveringmostof anthrosalt flux, the value estimatedfrom our model is comparable pogenicaerosolsourceregionsin the northernhemisphere
of 1ø x 1øisbeingdeveloped
with estimatesfrom previousstudies.The removalrate esti- (>35øN)andhavinga resolution
matedby Erickson
andDuce[1988]is 1.8 x 1016g yr-•.
with this aerosol model and used to study the production,
of seasalt,sulphur,
andsoildustaerosols.
Assuminga globaldry depositionflux of atmospheric
sea-salt removal,andtransport
aerosols
of 5.5 x 10-6 /•g cm-2 s-• andthat the tradewind
regionswere representative
of the globalwind field,Eriksson
4.
[1959]estimated
theglobalsea-salt
production
to exceed1.0 x
10•s g yr-•. By a similarbut improvedapproach,
Blanchard
Conclusions
Both observations
and modelpredictionsdemonstratethat
sea-saltaerosolconcentrations
haveseasonal
vari[1963]gavetheannual
removal
rateof 9 x 10•sg yr-1. In an atmospheric
attemptto calculatethe budgetof seasalt and sulfurin the ationswhosemagnitudedependson the geographiclocation.
atmosphere,
Petrenchuk
[1980]estimatedthe intensityof sea- Sea-salt concentrationsin the high-latitude North Atlantic
saltaerosolsources
of 1.3 x 10•s g yr-•. Usingthe whitecap (Mace Head and Heimaey) show a substantialseasonal
atlas,Spillaneet al. [1986]estimatedthe annualsea-saltmass change.The seasonaldependenceis decreasedin lowerlatituderegions(BermudaandOahu).Highersea-saltconcenfluxto be 3.50x 10•s g.
Anotherimportantparameterrelevantto the globalsea-salt trationsare generallyfoundin borealwintersexceptfor Oahu
ispredictedin summertimeby the
budgetis the net exportof sea-saltto the land surfaces.Our wherehigherconcentration
estimate
fromthe modelis 1.7 x 10TMg yr-1. Riverrunoffof model. The sea-saltconcentrationat Alert in the high Arctic
chloridewas used to approximatethe quantityof chloride resemblesthe seasonalvariation in the high-latitudeNorth
deposited
on landfromthe ocean[Edksson,
1960;Petrenchuk, Atlantic but is •60 times lower. Observations at Palmer Sta1980].By this method,Eriksson[1960]estimateda value of tion in the Antarcticand Cape Grim revealedlittle seasonal
1.01x 10TMg yr-• for an annualnet exportof sea-salt
to the variations.
3830
GONG ET AL.: ATMOSPHERIC
Depending on the geographiclocation, model estimatesindicate that dry depositionaccountsfor 60-70% of the total
sea-saltremovedfrom the atmospherewhile in-cloudand below-cloudprecipitationscavenging
is responsiblefor --•1% and
--•30% of the total removal.Size-segregated
removalflux analysis showsthat only 1-2% of the total sea-saltmassemitted
from the ocean surfacein all size ranges(r = 0.03-8 /•m)
remainsin the columnabovethegridsquareasa net inputto
the atmosphere.Of the net input sea-saltaerosols,particlesof
radiusr - 0.5-8 /zm contributemostof the mass(96%) with
the remaining 4% coming from submicronparticles in the
radius range, r = 0.03-0.5
The total amount of sea-salt aerosols emitted from the world
oceans
to theatmosphere
isestimated
tobe 1.17x 1016gyr-1,
while the annual removal
of sea-salt aerosols is calculated
to be
SEA-SALT MODELING,
2
Eriksson,E., The yearlycirculationof chlorideand sulfurin nature:
Meteorological,geochemicaland padologicalimplications,1, Tellus,
11(4), 375-403, 1959.
Eriksson,E., The yearly circulationof chloride and sulfur in nature:
Meteorological,geochemicaland padologicalimplications,II, Tellus,12(1), 63-109, 1960.
Erickson,D. J., andR. A. Duce, On globalflux of atmospheric
seasalt,
J. Geophys.
Res.,93(Cll), 14,079-14,088,1988.
Erickson, D. J., J. T. Merrill, and R. A. Duce, Seasonal estimates of
globalatmosphericsea-saltdistributions,
J. Geophys.
Res.,91(D1),
1067-1072, 1986.
Fergusson,J. E., Inorganic Chemistryand the Earth, Pergamon,New
York, 1982.
Gong,
S.L.,L.A.Barrie,
and
Ji-P.
Blanchet,
Modeling
sea-salt
aerosolsin the atmosphere,1, Model development,
J. Geophys.
Res.,this
issue.
Jaenicke,R., Troposphericaerosols,in Aerosol-Cloud-Climate
Interactions,edited by P. Hobbs, pp. 1-31, Academic,San Diego, Calif.,
1993.
1.15x 10•6 g. Thisestimate
is quiteconsistent
withprevious Jobson,B. T., H. Niki, Y. Yokouchi,J. Bottenheim,F. Hopper, and
estimates found in the literature.
R. Leaitch,Measurementsof C2-C6 hydrocarbons
duringthe Polar
In view of the current state of atmosphericsea-saltbudget
Sunrise1992Experiment:Evidencefor C1atom and Br atomchemistry,J. Geophys.
Res.,99(D12), 25,355-25,368,1994.
estimation,especiallythe longrangetransportrate of sea-salt
aerosolsfrom world oceansto ice or land surfaces,runningthis McFarlane, N. A., G. J. Boer, J.-P. Blanchet, and M. Lazare, The
aerosol model with a three-dimensional
climate model is nec-
essaryto provide a more accurateaccountof the sea-saltaerosol cyclein the atmosphere.
Canadian climate centre second-generationgeneral circulation
modelant its equilibriumclimate,J. Clim.,5, 1013-1044,1992.
Monahan,E. C., D. E. Spiel, and K. L. Davidson,A model of marine
aerosolgenerationvia whitecapsand wave disruption,in Oceanic
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editedby E. C. Monahanand G. Mac Niocaill,pp. 167174, D. Reidel, Norwell, Mass., 1986.
Acknowledgments. Funding for the University of Miami aerosol
measurementswas providedby Heimaey, Iceland--NOAA cooperative agreement NA90RAH00075; Oahu, Hawaii--NASA contracts
NAG8-621, NAG8-841, and NAGl-1229; National Science Foundation as part of the Sea-Air Exchangeprogram;Mace Head and Bermuda-National ScienceFoundation grants ATM-8703411, ATM9013125, ATM9414846; and Palmer Station--U.S. Department of
Energy contracts DEAC1791EV90116 and DEAC1794EV901. The
authorsacknowledgesupportfrom the Cape Grim BaselineAir Pollution programmanagedjointly by the AustralianBureau of Meteorology and CSIRO, the CGBAPS staff for many years of untiring
logisticalsupport,and the Australian GovernmentAnalytical Laboratories (Kingston)for the aerosolanalysis.Finally, the authorswould
like to expresstheir sincerethanksto G. Issacand R. Leaitch for their
valuablecommentsduringan internal reviewat AtmosphericEnvironment Service, Environment Canada.
Mozurkewich,M., Mechanismsfor the releaseof halogensfrom sea
salt particlesby free radicalreactions,J. Geophys.Res.,100(D7),
14,199-14,207, 1995.
Petrenchuk,O. P., On the budgetof seasaltsand sulfur in the atmosphere,J. Geophys.
Res.,85(C12), 7439-7444, 1980.
Slinn,S. A., and W. G. N. Slinn,Modelingof atmosphericparticulate
depositionto natural waters, in AtmosphericPollutantsin Natural
Waters,edited by S. J. Eisenreich,pp. 23-53, Ann Arbor Sci.,Ann
Arbor, Mich., 1981.
Spillane,M. C., E. C. Monahan, P. A. Bowyer,D. M. Doyle, and P. J.
Stabeno,Whitecapsand globalfluxes,in OceanicWhitecaps,edited
by E. C. Monahan and G. Mac Niocaill, pp. 209-218, D. Reidel,
Norwell, Mass., 1986.
Therrien, D., Le modalede circulationg•n•rale atmosph•riqueCanadien en versioncolomne:FIZ-C, M.S. thesis,123 pp., Univ. of
Quebec at Montreal, 1993.
Van Loon, H., Climatesof the Oceans,WorldSurv.Climatol.,vol. 15,
Elsevier, New York, 1984.
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