Jun 20, 1995 - the face velocity used: --1.2/â¢m [John et al., 1983]). The fine fraction ..... O'Dowd and Smith also measured smaller sea salt particles and found ...
JOURNAL OF GEOPHYSICALRESEARCH,VOL. 100,NO. D6, PAGES 11,335-11,356, JUNE 20, 1995
Biogenicsulfur emissionsand aerosolsover the tropical South Atlantic
3. Atmospheric dimethylsulfide, aerosolsand cloud condensation
nuclei
MeinratO. Andreae,WolfgangElbert, andStephenJ. de Mora Biogeochemistry Department,Max PlanckInstitutefor Chemistry,Mainz, Germany
Abstract. We measureddimethylsulfidein air (DMSa) and the numberconcentration, sizedistribution,and chemicalcompositionof atmosphericaerosols,includingthe concentration of cloud condensation nuclei (CCN), during February-March1991 over the tropicalSouthAtlantic along 19øS (F/S Meteor, cruise 15/3). Aerosol number/size distributionswere determinedwith a laser-opticalparticle counter,condensation nuclei (CN) concentrationswith a TSI 3020, and cloud condensationnuclei (CCN) with a Hudson-typesupersaturation chamber.Aerosolsampleswere collectedon two-stage stackedfilters and analyzedby ion chromatography for solubleion concentrations. Black carbonin aerosolswas measuredby visible light absorptionand usedto identify and eliminateperiodswith anthropogenic pollutionfrom the data set. Meteorologicalanalysis showsthat most of the air massessampledhad spentextendedperiodsover remotemarine areasin the tropical and subtropicalregion. DMS a was closelycorrelatedwith the sea-toair DMS flux calculatedfrom DMS concentrations in seawaterand meteorologicaldata. Sea salt made the largestcontributionto aerosolmassand volume but provided only a small fraction
of the aerosol number concentration.
The submicron
aerosol had a mean
compositionclose to ammoniumbisulfate, with the addition of somemethanesulfonate. Aerosol(CN and CCN) numberand non-sea-saltsulfateconcentrations were significantly correlatedwith DMS concentrationand flux. This suggeststhat DMS oxidationfollowed by aerosolnucleationand growth in the marine boundarylayer is an important,if not dominating,sourceof CN and possiblyCCN. The degreeof correlationbetweenDMS and particle concentrations in the marine boundarylayer may be stronglyinfluencedby the different time scalesof the processesregulatingtheseconcentrations.Our results provide strongsupportfor severalaspectsof the CLAW hypothesis,which proposesthe existenceof a feedbackloop linking DMS emissionfrom marineplanktonto sulfate aerosoland global climate. more shortwavesolar radiationbeing reflectedback into
Introduction
The CLAW hypothesis(namedafter the initials of its authors, Charlson, Lovelock,
Andreae, and Warren
spaceand thusin a lower globaltemperature. Finally, changesin global temperaturewould influencethe
speciation andabundance of marine plankton, andthereby
[Charlsonet al., 1987]), postulatesthat biogenicdimethyl theproduction andemission of DMS,andbythatclosethe sulfide (DMS) productionby marine phytoplankton influ- feedbackloop. encesglobalclimatethrougha multistepfeedbackmechaFollowing the publicationof the CLAW hypothesis,the nism. It arguesthat biogenicDMS diffusesfrom the sea sulfur cycle in the marine atmospherehas been the subject surfaceinto the atmosphere and is thenoxidized,at leastin of intensivestudy and debatein the literature. Since this part, to sulfateaerosol.This aerosolthenactsas the main hypothesiscannotbe testedby conductinga global experisource of cloud condensation nuclei (CCN) over the ment, attemptsat validationhave been confinedto investioceans.The numberof available CCN regulatesthe number gations of the various stepsinvolved in the CLAW feedof cloud dropletsand thus indirectlythe reflectivity (albackmechanism.It has been shownconvincinglythat DMS bedo) of marine clouds.A higher cloud albedoresultsin is producedby marine plankton and that it is the main volatile sulfur compound released by the marine biota
1Now at Dtpartement d'Octanographie, Centre [Andreae, 1990, and references therein; Bates et al., 1992; Octanographique deRimouski, Rimouski, Qutbec,Canada. Liss et al., 1993]. The relationshipbetween DMS concenCopyright1995by AmericanGeophysical Union Papernumber94JD02828. 0148-0227/95/94JD-02828505.00
trations and variables describing marine phytoplankton populationsis very complex, and attemptsto correlate seawaterDMS with parameterssuchas chlorophyllconcentrations and primary productivity have met with mixed success.This issue is discussedin detail in a companion 11,335
11,336
ANDREAE ET AL.: DMS AND AEROSOLS OVER THE SOUTH ATLANTIC
paper [T. W. Andreaeet al., 1994]. Becauseof experimental limitations, the air-sea transfer of DMS can at this time not be measureddirectly; it is estimatedusing various parameterizationsthat predict fluxes based on sea-to-air concentration gradientsand meteorologicalparameters[Liss andMerlivat, 1986; Erickson, 1993]. That theseparameterizations are basically correct is suggestedby some studies which show closecorrelationbetweenthe flux predictedby
In recent years, empirical evidence for a relationship between DMS on the one hand and sulfate aerosol mass or number concentration on the other has been found in
studiescomparingseasonalcovariationsbetweenDMS and non-sea-salt (nss) sulfate aerosol [Andreae et al., 1991; Ayers et al., 1991; Ayers and Gras, 1991; Nguyenet al., 1992]. Other investigators have observed correlations betweenatmosphericDMS (DMSa) and condensation nuclei the transfer models and the concentration of DMS in the (CN) or CCN concentrations[Hegg et al., 1991a; Putaud atmosphericboundarylayer [T. W. Andreae et al., 1994; et al., 1993]. However, suchcorrelationsare by no means Putaud et al., 1993; Suhre et al., 1995; Thompsonet al., universal, and several investigatorsfound no or only 1993]. sporadiccorrelationbetweenDMS and nss sulfate, CN or The chemical and physical pathwayswhich lead from CCN [Bates et al., 1992; Berresheirnet al., 1993; Huebert atmospheric DMS to sulfate aerosol particles are very et al., 1993; Schiiferet al., 1993]. complex and are still poorly understoodin spite of a conThusin spiteof the considerableeffort that has goneinto siderableresearcheffort in recentyears. Reactionwith OH studyingthe DMS-sulfate-CCN systemin recentyears, the appears to be the dominant first step in DMS oxidation number of studiesshowingclear relationshipsbetweenthe [Hyneset al. , 1986; Tyndall and Ravishankara,1991; Iqn chemicaland physicalparametersthat make up the links of et al., 1990]; only in polluted atmospheres,where appre- theCLAW feedbackloop remainssmall. We think that this ciable levels of NO3 build up at night, can the DMS +NO3 is to a large extent due to the measurementstrategies reaction lead to significantDMS removal [ChaOSeldand chosen,particularlyto the choiceof studyregionin many Crutzen, 1990; Jensenet al., 1991]. The proposedoxida- of the investigationscited above. The complexmeteorologition pathways lead to the production of SO2, which is cal conditionsprevailingin the temperatezone, combined in the temperate further oxidized to H2SOn and to methanesulfonicacid with the patchinessof marine ecosystems oceans,introducesa great degreeof short-termvariability (MSA). Laboratory studiesshow that the yields of MSA and SO2 are temperaturedependent[Hyneset al., 1986], and make day-to-daycorrelationsof the variablesinvolved with colder temperaturesfavoring MSA production.SO2 in the CLAW cycle difficult or impossibleto detect. A can be oxidized to H2SOnby reactionwith OH in the gas similarproblemappliesto cruiseswhich cover large latituphaseor to SOn= by reactionwith O3 or H202 in the liquid dinal rangesat relatively high speeds.The presentstudy phase of aerosols and cloud droplets [Charneldesand was conceivedto make optimal use of a zonal ship track along 19øS, which coveredthe tropical SouthAtlantic at a Stelson, 1992; Langner and Rodhe, 1991; Sieveringet al., 1992; Suhre et al., 1995]. The liquid phase mechanism ßslow averagespeed(--7.5 km h-•). As the shipremained appearsto be the dominantsink of SO2 over most of the withinthe tradewindbelt for almost6 weeks,atmospheric oceans.Bandy et al. [1992] have argued that apparently measurementscould be made under very consistentcondipoor correlations between DMS and SO2 in the marine tions over an extendedperiod. We attemptedto measure boundarylayer (MBL) are due to low efficiencyof conver- simultaneouslyas many of the variables of the CLAW desion of DMS to SO2; they proposedthat H2SOncan be cycle as possiblein order to obtain a comprehensive formed from DMS without SO2 as an intermediary.How- scriptionof the system. In a companionpaper, we have analyzed the relationever, problems with the accuracy of SO2 determination [Gregory et al., 1993a] and the existenceof a complex, ships between chlorophyll and DMS in seawater, the multi-pathoxidationmechanismfor SO2 may also account parameterizationof the air-sea transfer of DMS, and the for the lack of observed correlation. correlationbetweenthe sea-to-airflux and the atmospheric Sulfate and MSA from the oxidationof DMS may enter concentration of DMS [T. W. Andreae et al., 1994]. The the aerosolphaseeither by condensation of H2SOnand theoretical aspectsof DMS oxidation, SO2, sulfate, and MSA formedin thegasphaseor asa resultof liquidphase particle productionare discussedin the paper by Suhreet oxidationof SO2.In thelattercase,or if H2SOn andMSA al. [1994]. The presentpaper describesthe resultsobtained on the atmosphericconcentrationsof DMS, the ionic are depositedontoexistingparticles,the oxidationof DMS leads to an increase in aerosol mass but not to an increase constituentsof the aerosol, especiallynss sulfate, and the in the number concentration of aerosolparticles.The number concentrationof CN, CCN, and size-segregated will be interpretedin the CLAW hypothesisis based,however, on the effect of the aerosolparticles.The observations numberconcentration of sulfateCCN on clouddroplet contextof air mass history, sulfur speciesoxidation,and numberand thusrequiresthe production of new particles particleproductionmechanisms.
(or the growthof CN into the CCN size range)for the existence of a feedback betweenbiogenicsulfurproduction and climate. Somemodelsof DMS and SO2oxidationin Methods the MBL do predictthe formationof newparticlesby this Sampling
process[Hegg et al., 1992; Kreidenweiset al., 1991; Lin et
al., 1992; Raes and van Dingenen, 1992; Russellet al., 1994], whereasother modelsrequireDMS to be transported into the free troposphereand to be oxidizedthere for new particle productionto occur [Raeset al., 1993; Raes, 1995]. Some experimentalevidencealso supports particleproductionin the MBL [Hoppeland Frick, 1990; Covert et al., 1992].
Sampleair for the determination of gaseousconstituents (DMSa, 03) was broughtinto the laboratorythroughan inlet securedto the mast at approximately33 m abovesea level. The inlet was protectedfrom moistureby a plastic funnel; a Teflon cyclone at the inlet preventedsea salt aerosolfrom enteringthe sampleline. Approximately10 m of 9.5 mm OD Teflon (fluorinatedethylenepropyleneor
ANDREAE ET AL.: DMS AND AEROSOLS OVER THE SOUTH ATLANTIC
FEP) tubingconnected the inlet to the DMS and03 analyzers. Sampleswere collectedover 30-rain periods and analyzedby the systemwhile the next samplewas being collected,resultingin continuous coverageand a sampling frequencyof two measurements per hour. The filter holdersfor aerosolsamplingwere mountedon themastnextto the gas-sampling inlet. Aerosolsamplesfor the determination
of soluble ionic constituents were col-
lected using two-stage, stacked filter holders (47-mm diameter;NucleporeCorporation,Pleasanton,Ca., USA). The coarsefraction (•l.2-/•m diameter)of the aerosolwas collectedon the first filter stage,consistingof a Nuclepore filter (nominalpore size of 5.0/•m; 50% cutoffdiameterat the face velocityused: --1.2/•m [Johnet al., 1983]). The fine fraction, which had passedthrough the Nuclepore filter, was collected on a Zefluor membranefilter (Gelman,
11,337
CN concentrations were determined with a TSI model
3020 CN counter(TSI Incorporated, St. Paul, Minnesota). This device countsindividualparticlesat densitiesup to 1000 cm'3, and uses integral light scatteringat higher concentrations. Aerosolsize spectrawere obtainedwith a
Rokyomodel5120 laser-optical particlecounter(Pacific Scientific Instruments,Silver Spring, Maryland). The instrumentcountsparticlesin the followingsize classes' 0.2-0.3 /•m, 0.3-0.5 /•m, 0.5-1.0/•m, 1.0-2.0/•m,2.0-5.0
/•m, and > 5.0 /•m diameter.It was calibrated by the manufacturer usinglatex beadaerosols.Accordingto the manufacturer's specification, the particlesize accuracyis betterthan 10% in the rangebelow2/•m, andthe sample volumeaccuracy is betterthan10%. Experience showsthat the actualsizingaccuracy obtainable in practiceusuallyis worsethan10%, especially whenaerosols of a composition
nominalpore size of 1.0 /•m). The flow ratesand sample different from the calibration aerosol are measured. To
intothe operating volumeswere measuredwith an integratingmass-flowmeter bringthe sampleaerosolconcentrations (TeledyneHastings-Raydist, Hampton,Va., USA). Filters were collected over 48- or 24-hours intervals. Sampling was automaticallyinterruptedby a Weathertronicssampler controller if the relative wind direction was > 90 ø off the
range of the instrument,ambientair was dilutedwith filteredair to yieldan effectivesampleair flowrateof 390 cm3 min-1.Sampleanddilutionair streams wereat ambient humidity.Measurements were takenat 20-minintervals
bow. This was rarely necessary,however,sincethe ship's coursewas into the prevailingeasterlytrade windsduring
usinga samplingtime of 14.5 min.
most of the cruise.
continuously operatingthermal-gradient-diffusion cloud chamber similarto the systemdescribed by Hudson[1989]. The systemwas constructed by J. G. Hudson(Desert
The intakes for the continuousaerosol analyzers (CN, CCN, laser-optical particlecounter,and aethalometer) were locatedon a beam extendinginto the airflow just abovethe flying bridge (--30 m abovethe sea surface),wherethe ship's atmosphericlaboratoryis located. This made it possible to keep the tubing lengths between inlet and instrumentsto 3 m or less. Electrically conductivetubing
For the determination of CCN concentrations,we used a
ResearchInstitute,Reno, Nevada).We operatedbothcold
and warm plates isothermally,so that the instrumentacted as a CCN counterat a specificsupersaturation value rather than as a CCN spectrometer as described by Hudson [1989]. The droplets emerging from the cloud chamber with an inner diameter of 9 mm was used for the connecwere countedby a Royko optical counter and recorded in tions. 128 voltage bins. Most CCN counts were obtainedat a Meteorologicalinformationwas acquiredby the ships supersaturationof 0.3 %; supersaturationspectra were on-boardsystem,which is operatedby the GermanMeteo- recordedperiodicallyby varying the temperaturedifference rological Service.The data recordedincludeair tempera- between the chamber wall plates. The counter was caliture, dew point, relative humidity, pressure,wind speed bratedusingNaC1 aerosolsof known particle size produced and direction, seawatertemperatureand salinity, global with an aerosol generator and an electrostaticclassifier. radiation,UV radiation,visibility, and two daily radiosonde Count rates were calibrated by comparisonbetween the soundings. outputof the CCN counterand a TSI 3020 when both were samplinga standardNaC1 aerosolfrom the aerosolgeneraAnalysis tor/classifier. Data were collectedusing 10-min integration The techniqueusedfor the determination of DMS in air periods,andpooledinto 1-houraveragesfor further statistical analysis. is describedin detail in the companionpaper by T. W. Black carbonwas measuredwith an aethalometer(Magee Andreae et al. [1994]. Oxidant interferencesare scrubbed from the sampleair by a cottonscrubber,and DMS is Scientific, Berkeley, California). This instrumentmonitors preconcentrated on gold wool. After thermaldesorption, continuouslythe light absorptionby aerosoldepositedon a sulfurcompounds are separated by gaschromatography and quartz filter throughwhich sampleair is pumpedat a flow detectedby a flame photometric detector.The systemis rate of --20 L min-1;the black carbonaerosolconcentration automated, andoperatescontinuously with a time resolution is determinedfrom the increaseof light absorptionover a of 30 min. Our analyticalsystemhas beenvalidatedin a specifiedtime interval. We recordedthe data at 10-min double-blindintercalibration experiment;its detectionlimit intervals, but becauseof the extremely low black carbon is about5 ppt, precisionis 5 /•m) were plotted arbitrarily at 7.5 /•m, the width of this channelwas assumedto be 5/•m (5-10/•m). The solid symbols represent periods with an average relative humidity > 75 %; the open symbols represent periodswith lower humidity.
aerosols duringthe "clean"periodsof our cruisemusthave beenwell below50 cm-3,probablyevenbelow15 cm-3.CN concentrations below100cm-3wereobserved occasionally (e.g., 17, 23 and26 February;Figure2), andit is possible thatcontinental aerosols mayhavecontributed significantly (10-50%) to thesevery low CN concentrations. During most of the "clean"periods,however, the contributionof
1oo
pollutionevent of 14-15 February, where a considerable fraction(30%) is also presentin channel2. Together, channels1 and 2 accounton averagefor 82% of the total Royko count.
Correlationanalysisof the Roykodata showsa high degreeof mutualcorrelationbetweenthe countsin channels
in the range> 0.5/xm, consisIn the Meteor 15/3 data set, we have two independent 3 to 6, i.e., all sizeclasses but comolementarymeasurementsthat we can use to tentwiththisrangebeingdominatedby the seasaltaerosol. of particlesin channels estimatethe contributionof sea salt particlesto the aerosol The averagenumberconcentration in population observed:the chemical compositionof the 3-6 is 4.6 cm-3, and the measurednumberconcentrations on wind speed aerosol,and the number/sizedistributions from the Royko this size rangeshowa clear dependence of seasaltparticles[Woodopticalparticlecounter.The Royko numberspectrawere (Figure 8) as is characteristic groupedinto time periodswith relativelyconstantshapeof cock, 1953; Gras and Ayers, 1983; Hoppel et al., 1990; the size distribution; the number/size distributionsfor these
periodsare shownin Figure 7. The dataare plottedin the conventionalrepresentation of dN/dlog(D) versusdiameter D; for the largestsize classan upper size limit of 10 /xm was assumed.The sharpfalloff at diameterslarger than 5 /xmmay be due at leastin part to lossesin the intakeline. Because of the low nurnber concentrationof these large particles,even a large error in the measurementof this size class would be irrelevant for our discussion of CCN and CN number concentrations but could lead to an underestimarion of the mass and volume of the sea salt aerosol. The
shapeof the distributionsshowsrelativelylimitedvariation, with moisterperiods(solid symbolsin Figure 7) showinga tendencyfor a shift in the size distributionfrom channel1 (0.2-0.3 /•m) to channel 2 (0.3-0.5 /•m). The number concentrationof aerosolsin the size range detectedby the Roykocounter(> 0.2-/•m diameter)is clearly dominatedby the smallestsize fraction(76-86% of counts),exceptfor the
O'Dowd and Smith, 1993]. The regression of log(N) on windspeedyieldsa slopeof 0.109-t-0.004andan intercept of-0.34-t-0.04 at u=O (r2=0.42; the standarderror is indicated withtheslopeandintercept estimates). Thisslope is very closeto that obtainedby O'Dowdand Smith[1993] over the North Atlantic, but the interceptcorresponds to onlyabout50% of theparticleconcentration foundby these authors at a givenwind speed.This appearsto be the result of ourcuttingoff the seasaltsizerangeat 0.5/xm, whereas O'DowdandSmithalsomeasured smallerseasaltparticles and found that about half of the sea salt particleshad diameters below0.4/xm. Whentheirdatafor thesizerange >0.4 /•m are comparedwith ours, the resultsagreeto within 50%. This suggests that a considerable fractionof the totalnumberof seasaltparticlesis presentin channels 1 and2 but thattheycannotbe statistically discriminated in these size classes because of the dominance of the nss
sulfate particlesthere. The true number concentrationof
ANDREAE ET AL.' DMS AND AEROSOLS OVER THE SOUTH ATLANTIC lOO
11,343
eachchannel.Most of the aerosolvolumeis presentin the size range2- to 5-/•m diameter,and the potentialcontribution of the 0.3- to 0.5-/•m size classto the sea salt volume is no more than some 2 %. Therefore this fraction is of little
importance as potentialreactionvolumefor the oxidationof SO2in the liquid phaseof aerosolsand can be ignoredin themodelcalculations presentedin the companion paperby
•ElO
Suhre et al. [1994]. The chemicalanalysisof the aerosols
collectedduringMeteor 15/3 confirmstheseresults.Only 4 % of the sodium content of the aerosol is found on the
secondfilter stage,i.e., in the submicronfraction. Figure10 showsa time seriesplot of the aerosolvolume (daily means).Over the time of the cruise,the sea salt aerosol volume varied over an order of magnitude(3-30
/xm3 cm-3),mostlyin responseto wind speedvariations. Thisrangeagreeswith previousobservations, e.g., the data 0 5 10 from the Bermudaregion presentedby Kim et al. [1990] Windspeed,rnsec-1 and the North Atlantic data of Hoppel et al. [1990]. The volume of the aerosolfraction >0.5 /xmis well correlated Figure 8. The number concentrationof sea salt particles with the sodium content of the supermicronaerosol (i.e. particles> 0.5 ttm) plottedagainstwind speed. (r2=0.70; Figure 11a). The correlationslopeis 2.3+_0.4 o.1
/xm3 /xg-3,corresponding to a saltcontent of about44+_7% (massper volume)in the aerosolparticles.The correlation seasaltparticlesmaythushavebeenabouttwicethevalue is furtherimprovedto r2=0.89 whenthe effectof varying derived from channels 3-6. Similar observations were made humidity is removedby convertingthe moist aerosol by GrasandAyers[1983],whoalsofoundthatthenumber volumesto dry equivalentsusingthe equationsof Hanel
[1976] (Figure 1lb). The correlation slope is then to a densityof 2.0 g cm-3 of 0.1 ttm.Thisis of relevancefor the discussion of the sea 0.495+_0.045,corresponding salt contribution to CCN concentrations,since these small whichis closeto the densityof NaC1(2.17 g cm-3). Figure 10 alsodemonstrates that the submicron aerosol sea salt particlescan providea significantproportionof modemakesonly a smallcontribution to the total aerosol CCN under circumstances where other CCN sources are weak or missing,e.g., at higherlatitudesduringthe cold volume.The averagevolumein the size fraction < 0.5 ttm diameteris 1.08+_0.47/xm3 cm-3, comparedto a mean season,when DMS productionis low. The contributionof submicronsea salt particlesto the coarse volume of 12.0+_6.4 /xm3 cm-3. This submicron aerosolvolumedistribution,however,is quitesmall. Figure volume is somewhat smaller than the clean air data from 3 9 showsthe averagenormalizedvolumesize distribution theNorthAtlanticgivenby Hoppelet al. [1990](2.27/xm cm -3) and Kim et al. [1990] (1.64/xm 3 cm-3), consistent with for the "clean"periods.The averageparticlevolumefor sulfateaerosolsover theRoykochannels wascomputed basedon theassumptiona greaterinfluenceof anthropogenic of a Junge-type exponential number/size distribution within the North Atlantic.
distribution of sea salt aerosol extended down to diameters
0.8
0.6
0.4
0.2
0.2-0.3
0.3-0.5
0.5-1
1-2
2-5
>5
Diameter, pm Figure 9. Normalizedmeanvolume/sizedistributionof the atmospheric aerosolbasedon measurements with the laser-opticalparticle counterduringMeteor 15/3.
11,344
ANDREAE ET AL.' DMS AND AEROSOLS OVER THE SOUTH ATLANTIC
3O
lO
2O
10
14-Feb 18-Feb 22-Feb 26-Feb 02-Mar 06-Mar lO-Mar 14-Mar 18-Mar
1991 Figure 10. Daily averageaerosolvolumein the coarse(lower, denselyshadedportionof the bars)and fine fractionsof the aerosolduringMeteor 15/3. The wind speedis shownas a superimposed continuous line.
Chemical Compositionof the Aerosol The
aerosol
collected
luted" periods (Figure 12b). There is, however, also
on the stacked filter
units was
considerable
variation
of nss sulfate concentration
within
analyzedfor soluble, ionic constituents.The coarseaerosol the "clean" periods. This variation is attributableto variafraction is dominatedby sea salt ions, which make up an tiom in the supplyof biogenicsulfurfrom the ocean,as we average of 92% of the soluble, ionic speciesdetected. will show below. Nss sulfateis presentin both the coarse Chloride and sodiumare presentat a ratio of 1.28_+0.26, andthe submicronfractions;on average,the coarsefraction whichis not significantlydifferent from the sea salt ratio of containsabout33+9% of the total nsssulfate.This agrees 1.16. Potassium,calcium, and magnesiumare also present well with our previousobservationsfrom severalcleanand in sea salt proportionswithin the limits of analyticalaccu- anthropogenicallyinfluenced marine regions [Andreae et racy. The only non-sea-saltions detectablein the coarse al. , 1988; Berresheim et at., 1990, 1991; Raemdonck et at., 1986] and confirmsthe significanceof the conversion fraction are nitrate and some sulfate in excess of the of SO2 to sulfatein marine aerosolsand in clouddroplets amount attributableto sea salt. Nitrate (Figure 12a) was detectable only in the coarse size fraction and presumably nucleatedon sea salt particles [Suhre et at., 1995]. Given originated from the depositionof HNO3 on sea salt parti- the shorter lifetime of the sea salt aerosol and its associated cles. This size distributionof nitrate is typical of marine nss sulfate burden as comparedto the sulfatein the f'me aerosols, and has been observedpreviously by ourselves fraction, the dry depositionof nss sulfatepresenton sea and by other authors [e.g., Andreae et al., 1988; salt particlesmay be an importantsink of biogenicsulfur from the MBL [Sieveringet at., 1992]. Berresheim et al., 1990; Church et al., 1991; Savoie and
Prospero, 1982]. The aerosol nitrate concentration was strongly influencedby long-rangetransportfrom the continents. During the cleanestperiodsin the middleof the cruise,nitrate levels were comparableto valuesfoundover the remotePacific Ocean[Prosperoand Savoie,1989]. The line at 40 ppt in Figure 12a represents the annualaverage
Methanesulfonate
was also found both in the submicron
(6.1 _+4.0ppt) and coarse(6.3+4.4 ppt) aerosolfractions, consistent with the size distribution of aerosol MSA
we and
other authorshave reported previously [e.g., Andreae et at., 1988; Berresheim et at., 1990; Huebert et at., 1993]. Thus almost half of the MSA was presenton the sea salt
aerosol and had not been involvedin new particleproducProsperoand Savoie's"cleanest"station.The shadedareas tion. Depositionof MSA to submicronparticles could, in Figure 12a are those periods where elevatedblack however, have contributedto the growth of suchparticles
at Funafuti Island in the middle of the South Pacific,
carbon
levels
indicated
some influence
of
continental
sources.
Nss sulfate in the submicron and coarse aerosol fractions
is alsoinfluencedby long-rangetransportduringthe "pol-
into
the
size
class
active
as CCN.
The
mean
ratio
MSA/(MSA+nss SO4=) was 10.4+5.6% in the coarse aerosol and 4.9+2.1%
in the submicron aerosol. For the
total aerosol,this ratio was 6.6+2.8%, in goodagreement
ANDREAE
•
E
ET AL.: DMS AND AEROSOLS
OVER THE SOUTH ATLANTIC
11,345
3o
o
•
Volwet = -(1.0+_3.5) + (2.3+_0.4).Salt
E
a
ß
r2 = 0.70
20
o
co ø
5
lO
V•)l•lry (7-+•0+._)0 .4
.•
b
• 4 r2= 0.88 3
.
,o
•
•
3
o
= 0
5
10
Sea saltaerosol,gg m3 Figure ll. Coarseaerosolvolumeplottedagainstseasalt mass(determinedby chemicalanalysisof filter pack samples);(a) at ambienthumidity,(b) normalizedto dry conditions.
with previousresultsfrom clean tropical and subtropical duringotherwisealreadyvery cleanconditions(e.g., on 26 regions [Saltzman et al., 1986 ; Savoie and Prospero, February), the highestvalueswere presentin pollutedair 1989; Berresheim et al., 1991]. These results show that at massesfrom Africa, particularlyon 18 and 19 March. The leastfor this region, MSA depositionon coarseparticlesis mean CN concentration during clean conditions was only a minor sink for DMS-derived sulfur and does not 220+_128 cm-3, within the rangetypicalof cleantropical have a significantimpacton the potentialfor CCN produc- and subtropicalair massesover the oceans[e.g., Hoppel tion from DMS. and Frick, 1990; Hoppel et al., 1990; Schiller et al., Ammonium was detectableonly in the submicronfrac- 1993]. As was pointedout already, anthropogenic aerosols tion. The mean molar ratio of ammonium to nss sulfate is seemed to have relatively little impact on CN concentra1.09+_0.27,with the highestvalues(up to 1.6) occurringat tions even under mildly pollutedconditions(exceptfor the the beginningand end of the cruise and the lowest ratios spikes on 15, 18 and 19 March). For the analysisof the (down to 0.66) in the middle part of the cruise. Again, relationshipsbetween DMS and presumably biogenic these results agree well with previousmeasurementsfrom aerosols in the MBL, the periods showing detectable cleantropicaland subtropicalareas[e.g., Berresheimet al., anthropogenicinfluence (i.e., the grey-shadedperiods in 1991; Quinn et al. , 1990]. Figure 2) have been eliminatedfrom the data set. In view of the highly complex sequenceof photochemiDimethylsulfide, CondensationNuclei, and CCN cal reactionsand phasetransferprocessesrequiredfor the The time series of CN concentrationsduring Meteor transformationof DMS into an aerosol particle, it may 15/3 is shownin Figure 2, with the time periodsaffected appear almost futile to look for simple relationshipsbeby anthropogenic pollutionindicatedby a grey background. tween atmosphericDMS, CN, and CCN concentrations.In CN concentrations rangedfrom 0.2/xm detectedby the laser-optical particlecounterplottedagainst CCN concentrations (0.3 % supersaturation) measuredwith a cloudchamber.
11,348
ANDREAE
ET AL.: DMS AND AEROSOLS
OVER THE SOUTH ATLANTIC
3OO
o oo•,o•,,,/
o 2OO
E
8 8•• IO0
o•o ø O0
0.68 : : : ::::::I
: ........
', ........
200
• I•
400
• •
600
CN, cm-3 80
60
z
4o
20
0
0.2
0.4
0.6
0.8
1,0
CCN / CN ratio
Figure 14.
(a) CCN concentrations (0.3% supersaturation) plottedagainst CN concentrations.(b)
Histogram of CCN/CN ratios.
covary strikinglywell, especiallyin view of the theoretical difficultiesof explainingCN formationfrom DMS oxidationat DMSalevelsas low as 50 ppt. There is no consistent phase relationshipbetweenDMSa and CN; the apparent phaseshiftduringsometime periodsmay just be coincidental. When the unsmootheddata are plottedagainsteach other (Figure 16) a good correlationis also seen. Correla-
tion analysisgivesa r2 of 0.48, whichimprovesto 0.61
hypothesisthat DMS oxidationis the dominantsourceof submicronaerosolparticlesover the remoteoceans. There are few previousdata with which this resultcan be compared. Putaud et al. [1993] found a correlation between daily mean DMS and CN concentrations over the NorthAtlantic,but this correlationis entirelydependent on a number of points from continentallyinfluenced and polluted air masses.The slope found in Putaudet at.'s
when CN is regressedagainst24-hour nmningaveragesof study(5.5 cm'3ppt4) is twiceas largeas foundin our data. A similar correlationis obtainedwhenCN is regressed DMS a to eliminatethe effect of its diet cycle. This shows that over 60% of the variance in CN can be attributed to against the sea-to-air flux of DMS instead of the DMS (r2=0.60; Figure17a). Here it is not necesD MS. The regressiongives a statisticallyinsignificant concentration intercept(-3___7 cm-3)and a slopeof 2.44___0.06 particles sary to introduceany smoothingof the independentvaricm'3 (pptDMS)-•. This suggests thatin the studyregiona able, sincethe DMS flux showsno diet cycle. The datain major fraction of CN can be accountedfor by aerosol Figure17a seemto suggestsomeflatteningof the CN/FDus production from DMS and that under the meteorological slopeat highervalues,an effectwhichis removedby logand photochemicalconditions prevailing in the study transformationof the independentvariable (Figure 17b). region, about250 particlescm-3can be generated from a Whilethereis visuallya betterfit usingthe log-transformed of typical DMS concentrationof -100 ppt. Since suchcon- relationship(which would imply that the concentration centrations of both DMS and CN are typicalfor remoteand CN is dependenton the log of the DMS flux), the correlasubtropicalregions, our results are consistentwith the tion coefficientremainsthe same(r2=0.61). Our dataare
ANDREAE
ET AL.- DMS AND
AEROSOLS
OVER THE SOUTH
600 t
ATLANTIC
11,349
[250 ß
200
l::: 400 o
o.
150 o.
100 C3
I 200
50
0
...........................
15-Feb
0
22- Feb
01 -Mar
08-Mar
15-Mar
1991
Figure 15. Condensation nuclei(CN) andatmospheric DMS concentrations for the periodsfree from continentalinfluence.To removethe effectof the diel cycleof DMS, the dataare plottedas 24-hour runningmeans.
thusequallyconsistent with a linearrelationship between significant(a
