DMS oxidation in the Antarctic marine boundary layer - Douglas Davis

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 103, NO. D1, PAGES 1657-1678, JANUARY

20, 1998

DMS oxidation in the Antarctic marine boundary layer: Comparison of model simulationsand field observationsof DMS, DMSO, DMSOz, HzSO4(g), MSA(g), and MSA(p) D. Davis,• G. Chen,• P. Kasibhatla, 2 A. Jefferson, 3 D. Tanner,3 F. Eisele,3 D. Lenschow, 3W. Neff,4 andH. Berresheim s Abstract. A sulfurfield study(SCATE) at PalmerStationAntarctica(January18 to February25) hasrevealedseveralmajor new findingsconcerning(dimethylsulfide) DMS oxidationchemistryandthe cyclingof sulfurwithin the Antarcticenvironment. Significantevidencewas foundsupportingthe notionthatthe OH/DMS additionreaction is a major sourceof dimethylsulfoxide(DMSO). Methanesulfonicacid (MSA(g)) levels were also foundto be consistentwith an OH/DMS additionmechanisminvolvingthe sequentialoxidationof the productsDMSO andmethanesulfinicacid (MSIA). Evidence supportingthe hypothesis thatthe OH/DMS additionreaction,as well as follow-on reactionsinvolvingOH/DMSO, are a major sourceof SO2was significant,but not conclusive.No evidencecouldbe foundsupportingthe notionthat reactiveintermediates (i.e., SO3)otherthanSO2were an importantsourceof H2SO4. Quiteclearly, oneof the major findingsof SCATE wasthe recognitionthat a largefractionof the Antarctic oxidativecyclefor DMS (nearPalmerStation)tookplaceabovethe boundarylayer (BL) in what we have labeledhere as the atmosphericbuffer layer (BuL). Althoughstill speculative in places,the overallpictureemergingfrom the SCATE field/modeling resultsis one involvingmajorcouplingbetweenchemistryanddynamicsin the Antarctic. At Palmerthe evidencepointsto frequentepisodesof rapid verticaltransportfrom a very shallowmarineBL into the overlyingBuL. Due to the combinationof a longphotochemical lifetime for DMS andthe frequencyof shallowconvectiveevents,a large fractionof oceanreleasedDMS is transportedinto the BuL while still in its unoxidizedstate.There, in the presenceof elevatedOH andlow aerosolscavenging, high levelsof oxidizedsulfur accumulate.Parcelsof this BuL air are then episodicallyentrainedback into the BL, therebyprovidinga controllinginfluenceon BL SO2,DMSO, andDMSO2.Additionally, becauseSO2andDMSO are majorprecursors to H2SO4 andMSA, BuL chemistry,in conjunctionwith verticaltransport,alsoact to controlBL levelsof the latter species. Althoughmanyuncertainties remainin our understanding of AntarcticDMS chemistry, the abovepicturealreadysuggests thatpreviouschemicalinterpretations of Antarctic field data may need to be altered.

1. Introduction

are estimatedto exceedthosefrom anthropogenic emissions by factorsof 2 or more [e.g., Berresheimet al., 1995; Bates Sulfur is recognizedas one of the critical elementsfor whichanunderstanding of itsbiogeochemical cycleispivotal et al., 1992; Pham et al., 1995]. By far the largestnatural source comes from the world's oceans in the form of to climatechangestudies[Charlsonet al., 1991; Wuebbles, dimethylsulfide (DMS). 1995, andreferencestherein].Althoughman-madeemissions In the Antarctic, sulfurhasalsobeenrecognizedas a key in the northernhemispherelong ago surpassed thosefrom element, being readily found in atmospheric aerosols naturalsources,in the southernhemispherenaturalsources

[Fischer et al., 1969; Cadle et al., 1968; Maenhaut and Zol!er, 1977; Cunninghamand Zoller, 1981; Shaw, 1980, •Georgia Institute ofTechnology, Atlanta. 1988]. During summermonthsit is the dominantelement, 2DukeUniversity, Durham,NorthCarolina. accountingfor 80-90% of the total massdepositedto the 3National Center forAtmospheric Research, Boulder, Colorado. 4National Oceanic andAtmospheric Administration, Boulder, Colorado. Antarcticplateau.Even for wintertimeconditions,it fallsjust 5Deutscher Wetterdienst/MOHp, Hohenpeissenberg, Germany. behindNa andC1, reflectingtheimpactof intenseintrusions of marine air (i.e., salt storms) at a time when most other Copyright 1998bytheAmerican Geophysical Union. sources of sulfur are weak or nonexistent [Shaw, 1980]. Early speculationabout the sourceof Antarctic sulfur was Papernumber97JD03452. 0148-0227/98/97JD-03452509.00 centeredupontransportfrom the stratosphere andAntarctic 1657

1658

DAVIS

ET AL.:

DMS OXIDATION

IN THE ANTARCTIC

volcanicactivity. More detailedstudies,however,led to the conclusionthat local volcanicsourcescouldexplainno more them10% of the yearly Antarcticsulfurbudget[e.g., Radke 1981, 1982a, b; Delmas and Servant, 1982; Rose et al.,

1984, 1985; Chuang et al., 1986]. Similarly, efforts to evaluatethe stratosphereas a sourcerevealedit alsowas of rather minor importance,the possibleexceptionbeingtimes of major volcanicinjectionsinto the stratosphere [Komhyr and Grass, 1967; Delmas and Servant, 1982; Legrand and Delmas, 1984; Oltmansand Komhyr, 1976; Hogan et al., 1982].

The most important sourceof Antarctic sulfur came to light as a resultof the pioneeringwork by Lovelockandcoworkersin the early 1970s[Lovelocket al., 1972]. This was followedby an extensiveseriesof field studiesin the earlier 1980s [Andreae and Raemdonck, 1983; Andreae and Barnard, 1984; Andreae et al. , 1985; Andreae, 1986].

Collectively, this work identified DMS releasesfrom the world's oceansas the singlelargestnaturalglobalsourceof sulfur.

Numerous

follow-on

studies have now corroborated

MARINE

BOUNDARY

LAYER

currentunderstanding of AntarcticDMS oxidationchemistry. This effort was uniquein providingfor the first time measurements of severalnew sulfur species,for example, MSA(g), DMSO(g), DMSO2(g), and H2SO4(g),most of which were measured with a time resolution of < 15 min.

In addition,this studymeasuredfor the first time in the Antarcticenvironmentthe criticaloxidizingspecies,OH. In thispaperwe examinein greaterdetailthe productionand lossprocesses controllingthe levels of DMSO, DMSO2, H2SO4(g),andMSA(g)basedondirectobservations of these speciesas well as that of OH.

2. Sulfur Chemistry Field observations andlaboratorystudieshavecontributed significantlyto our current understandingof atmospheric DMS chemistry. Extensivereviewsof thisearlierwork have been published by Yin et al. [1990], Turnipseed and Ravishankara [1993], and Berresheim et al. [1995]. The oxidation

mechanisms

summarized

in these reviews

have

the early findings of Andreae, including several in the Southern Ocean/Antarctica [Berresheim, 1987; Gibson, 1990, Fogelqvist, 1991; Sullivanet al., 1993; Turner et al.,

significantareasof agreement,but they alsohave areasof disagreement. In Figure 1 we presentan abbreviated version of a DMS oxidationmechanismwhich attemptsto summa1995; Giacomo and Smith, 1995]. These studieshave shown rize where many of the current uncertaintieslie. Areas that when large populations(blooms)of the phytoplankton where significantsupportingevidenceexists,either in the Phaeocystis Antarcticaare presentthereare correspondingly form of laboratorydataand/orfield observations,includethe elevatedvalues for many DMS related parameters.These identification of the reactive intermediates DMSO and MSIA include the following: levels of marine and atmospheric as well as the final oxidationproductsSO2, MSA(g), and DMS and DMS photochemicaloxidationproductssuchas DMSO2. Quitenoteworthy in theproposed mechanism isthat methanesulfonate(MSA(p)) and non-sea-saltsulfate(NSS) virtuallyall of theabovecited"final"oxidationproductsare [e.g., Berresheim, 1987; Berresheimet al., 1990; Gillett et shownas having been formed by more than one reaction al., 1993; Turner et al., 1995]. Thus, it has been concluded pathway. The fact that some of thesehave not been conthat the highly reproducibleseasonalcycling of Antarctic firmed, or in othercasesquantified,hasbeenindicatedin sulfuraerosolis a reflectionof the seasonality in biological Figure 1 with the symbol"?". Of particularinterestto this activity [Prospero et al. , 1991; Savoie et al. , 1993, work will be thosepathwaysinvolvingthe species DMSO, Wagenbachet al. , 1988, Wagenbach,1996;Bodhaineet al., DMSO2, MSA(g), SO2, andSO3. 1986; Cunninghamand Zoller, 1981; Tuncelet al., 1989]. Species notshownin thisscheme butfrequently shownin The atmosphere-ocean sulfur link hasalsobeenutilized othercitedmechanisms includemethylsulfenic acid(MSEA, by Antarcticglaciologists in evaluations of long-termtrends CH3SOH) and several methylsulfurnitrate compounds in climate. These ice core studies have focused on both the absolute levels of MSA and NSS as well as the ratio of these

(MSN). The absenceof MSEA in our mechanismreflects

planetarytemperatureandconcentrations levelsof MSA and

(pptv)).

kineticresultsrecentlyreportedby Zhaoet al. [1996],which two species[SaigneandLegrand,1987;LegrandandFeniet- showedthatthedirectproduction of thisspecies is at the5 % Saigne,1991;Legrandet al., 1992].In mostcasesa simple or lesslevel. Sulfurproductsof the MSN type havealso proportionalsource-receptor relationshiphasbeenassumed beenneglected because of theverylowNOxvaluesobserved between oceanic DMS emissions and the levels of NSS and in Antarctica.As will be discussed in subsequent sections of MSA depositedto the ice. Analysesof ice coresfrom the thetext, observed NO levelsduringSCATEweretypically Antarctichaveshowna stronginverserelationship between at thedetectionlimit (e.g., 3-4 partsper thousand by volume

NSSdatingbackto morethan160,000yearsbeforepresent. As noted in the abovetext, laboratorychambertype Thesestudiesnow spanup to two entireglacial-interglacial studieshave historicallyprovidedstrongevidencefor the cycles(E.S. Saltzman,personalcommunication, 1997)with DMS oxidationproductsSO2 and MSA [e.g., Niki et al. the resultingpredictedtemperaturerecordsfoundto be in 1983; Grosjean, 1984; Yin et al., 1986, 1990]. The relative relativelygoodagreementwith thosepredictedfrom marine yields of theseproducts,however,appearto be quite sediment cores.With thesuccess alreadygainedin theuseof dependenton the exact conditionsand type of chamber sulfur compounds as an ice core proxy species,one can employed[Berresheimet al., 1995]. As a result, it has not expectsuchstudiesto be still further expanded. Thus it beenpossibleto sort out quantitatively the productyields becomesimportant,if we are to gaincompleteconfidence in from the OH initiatingadditionand abstractionchannels. the conclusions drawn from theseinvestigations, that the More recently, chamber studieshave also revealedthe simplifyingchemicalanddynamicalassumptions continue to presenceof still other DMS oxidationproducts,e.g., be critically examined. DMSO, DMSO2, andMSIA [Sorensen et al., 1996;Barnes, As outlinedby BerresheimandEisele[thisissue],project private communication,1997]. Quite significanthere have SCATE was designedto help fill someof the gapsin our beenstudiesby theseinvestigators usingthe reactiveinter-

DAVIS ET AL.: DMS OXIDATION

IN THE ANTARCTIC

MARINE

CH4,Other prods? ß

CH3 [• SO2 IIMSA(p)l rxsbelow*) SO2 I SO• z (see

_•

•'•

BOUNDARY

LAYER

1659

HO (see rxsbelow __2

or SO

4CH,AerosolScav. riCH •xx 3 mu•ti-•lH2SO4(g)l

'] heterd/rx /•7 • D• DP,W•9•

/

4 O.,Ln4steps

•

Part. Grth.

[DMS[•• ••[DMSO [•• • •MSIA • • •[MSA(g)[ •M• O2N• * , ,•u2• CH3O2• DwDP,W• •

-

•erosm •cav.•

• DWDP,W/R • •

nounaary Layer •

k

9

HO 2

•

DM•

Addition

• •

•rt. •rth.

N 9

-:so3(g) uc.. CH3

Part. Grth.

DP,Wm

BoundaryLayer DWDP,W/R DWDP,W/R

Abstraction { Sulfur Peroxides ?

7t

i-step

processes

Sulfur Peroxides

Part. Grth.

lmSA(g)[

9

?¾,4

IMSA(p)]

Dry DP,W/R

•-'• •/? IDMS[ orZCH*-•-' "'- 30•O2 • • SO3 multi_step• I$2504(g)• Ultrafin½--•-N• HO 2

Iol

D•

- •SO2 multi-steps DP,

he•ero. rx

Part.Grth.

ß

Dry DP,W/R

Figure 1. AbbreviatedDMS oxidationchemistryschemefor marineboundarylayer conditions. [3valuesdenotebranchingratiosfor elementary reactions,¾valuesrepresent overallconversion efficiencyfactorsfor multistepprocesses.Speciesenclosed in boxesare thosemeasuredduring projectSCATE. Abbreviatedspeciesincludethe following: DMS, dimethylsulfide;DMSO, dimethylsulfoxide;DMSO2, dimethylsulfone;MSIA, methanesulfinicacid; H2SO3, sulfurous acid;MSA, methansulfonic acid. Otherabbreviations include:"?", areasof majoruncertainty, i.e., no direct evidencefrom laboratoryor field studies,Dry DP: dry deposition,herProrx: hererogenous reaction,Nuc.: Nucleation,W/R: Wash out/Rainout.

mediate, DMSO. These investigationshave provided additional insight concerningproductsformed from the

mentwouldthensuggestthatthe valueof the MSA(p)/NSS ratio shouldincreaseat higherlatitudesdue to lower mean

OH/DMSO reactionwhichincludeSO2, DMSO2, MSA, and

temperatures. Under these conditions the addition channel

MSIA.

becomes the majorchannel.Addingto the strength of this One of the objectivesof this paper will be to examine argument is thegeneralbeliefthatMSA is predominantly severalof the hypothesizedpathwaysshownin Figure 1 to formedthrough theaddition channel eventhough nolaboradetermineif SCATE observations can qualitatively,and in toryor fieldstudyhasconvincingly demonstrated thispoint. some casesquantitatively,confirm their importance. Of In fact,fieldresults supporting thissimple kineticinterpretaspecialinterestwill be thosephysicalandchemicalprocesses tion of DMS productyieldshavebeenreportedin the that influence the DMS product ratio MSA(p)/NSS. This literature,[e.g., Berresheim, 1987;Berresheim etal., 1990; ratiopotentiallycontainsadditionalinformationfor interpret- Pszennyet al., 1989;andProsperoet al., 1985]. ing ice core data. It also has been the subject of much Still other observations, however,suggestthat this discussionin literature becauseof its potential value as a simplistickineticview may havesignificant limitations.In sulfur sourceapportionmentcriterium. At tropicallatitudesthisratiohasroutinelybeenmeasured at valuesnear 0.07 [e.g., Prosperoet al., 1985; Saltzmanet al., 1983; Savoieet al., 1989]. This hasled to the suggestion

several cases, values for this ratio have been found to be

NSS for this environment.Assumingthat the final rate controllingexpressionfor DMS productformationalways involvesthe OH initiatingreaction,the samekinetic argu-

Gillett et al. [ 1993] alsoobservedthatthe valueof this ratio tendedto maximizeduringthe Australsummer,not winter.

completely outof phasewiththeearliertemperature predictions. For example,in independent Antarcticstudies by Prospero etal. [1991]atMawsonandWagenbach, [1996]at that the OH/DMS abstractionchannel, which dominates the G.v. Neumayer,abruptchanges havebeenreportedin the initiating kinetics for high-temperatureconditions [e.g., valuesfor thisratioduringearlysummermonthsbutwithno Hyneset al., 1993], is the majorsourceof SO2 andhence concomitant change in ambient temperature. At CapeGrim,

Finally,Berresheim et al. [1990],in anairborne studyover

1660

DAVIS ET AL.: DMS OXIDATION

IN THE ANTARCTIC

the SouthernOcean, reportedthat this ratio decreasedwith altitudeas the temperaturesteadilydeclined.Overall, these results would seem to suggestthat the yields of DMS oxidation products cannot be routinely predicted as a functionof temperatureusingsimplistickineticarguments. Among the complicatingfactors in understandingthe MSA(p)/NSS ratio is the potential role played by atmosphericverticalmixing. It may well be, for example,thatin somecasesthe final mix of oxidationproductsfrom DMS reflectsoxidationthathasoccurredat morethanonetemperature,reflectingrapidverticalmixingwithinthetroposphere. Equally importantis the questionof the temperaturedependencies of the numerous kinetic processesfollowing the initiatingOH/DMS additionand abstractionreactions.For someproductselementaryreactionsseveralstepsremoved from the initiatingreactioncouldeitherdefineor contribute to the rate determiningexpressionfor a givenproduct.Just the possibilitythat SO2 might be formed from reactions involvingboth the additionand abstractionchannelscreates a very complex picture for interpretingthe MSA(p)/NSS ratio. There is alsothe questionof heterogeneous reactions. If importantin the formationof MSA(p), astheyare known to be in the conversion of SO2to NSS, stilllargeruncertainties mightexistin the interpretationof MSA(p)/NSS values.

MARINE BOUNDARY

LAYER

As shown in figures 2a and 2b, one of the most unexpectedobservations recordedduringthe SCATE campaign are the numerousabruptenhancements seenin the mixing ratiosof DMSO andDMSO2. From the individualdayplots many of these perturbationscan be seen as significantly

correlated.For the overalldatasettheR2 valuewas0.43;

however, this value increasedto 0.6 when only those observationswere comparedthat deviatedfrom the mean valueby 2 sigma.The highestvaluereachedfor DMSO was 25 pptv, i.e. January20, with most spike eventsranging from 2 to 16 pptv. Baselinevaluesrangeddownto a low of 0.2 pptv as seenon January19. Spike valuesfor DMSO2 reached 12 pptv (i.e., January23) and, as with DMSO, baselinevalues on January 19 approached0.2 pptv. The medianand meanvaluesfor DMSO were 1.5 and 2.5 pptv, respectively;whereas,thosefor DMSO2 were estimatedat 1.3 and 1.7 pptv [Berresheim et al., thisissue]. The unusualnatureof the DMSO and DMSO2 observationsraisedseveralsignificantquestions,themostimportant concerningthe origin of the spike eventsthemselves. For example, from figure 2c it is quite apparentthat pollution episodesinvolvingdieselexhaustsare notthe sourceof these events.Similarly, from figure2d it is apparentthatwhatever thephenomenon responsible, it is unrelatedto solaractivity. We note also that no relationshipwas found betweenthe 3. Observational Data averagedaily value for DMS and the daily averagevalues As discussed in the SCATE "Overview" paper for DMSO andDMSO2. If indeedDMSO andDMSO2 were formed due to local OH/DMS photochemistry,one would [Berresheimand Eisele,thisissue],DMSO, DMSO2, OH, expect thatwith the shortlifetimeestimatesfor thesespecies H2SO4,andMSA weremeasured usingtheSI/CI/MS (selecta modest correlationwouldprevail. ion chemical-ionization massspectrometry)technique.The Given the lack of a causalrelationshipbetweenchemical first two specieswere measuredwhile the instrumentwas operatedin the positiveion mode; whereasthe last three factors(e.g., localpollution,DMS levels,andsolaractivity) were detectedas negativeions. Thus, as configuredduring and DMSO and DMSO2 spike events, correlationswith SCATE, time sharing of the SI/CI/MS instrumentwas several meteorological parameters were examined. Of unavoidable.The measurementtime period for DMSO and particularinterestwere thoseparametersrelatedto shiftsin DMSO2 wasJanuary18-24, andthat for OH, H2SO4, and the wind field and/or in the intensityof verticaltransport. MSA was February 16-25. Overall, thismodeof operating The resultsfrom this examinationindicatedthat no significreatedan elementof uncertaintyin someof theinterpretive cant correlationexistedbetweenwind speedversusDMSO levels; however, a modest bias was found when the data analysis;but as discussedin section4.1, we believethat a significantcarryoverof the findingsfrom onemeasurement were examined versus wind direction. The wind sector showingthe highestDMSO valueswas 230 to 270ø. This period to the other has still beenpossible. sectorhasbeenlabeledby BerresheimandEisele [thisissue] Details concerningthe techniquefor measuringDMSO as the openoceansector(II). The fact that, on average,the and DMSO2 havebeenpreviouslydescribed by Berresheim et al. [1993a, b]; whereasthosefor H2SO4 andMSA have highestlevels might come from this sectormakessensein that it also showedthe highestlevelsof DMS. Even so, one beendiscussed in earlierpapersby Eiseleand Tanner[1991, muststill reconcilethe fact thatlargespikeeventswere also 1993] and Tanner and Eisele [1995]. As related to the SCATE observations,a more thoroughdiscussion of sample observed under dark conditions. Consideringthe short inlets, calibrations, etc., can be found in Berresheim et al. lifetime of DMSO and DMSO2 (i.e., see discussion in section4.2) the levelsof bothspecieswouldbe expectedto [this issue]. be near their minima if they were largely controlledby 3.1. DMSO and DMSOz horizontaladvectionin conjunctionwith photochemistry. To explorethe possiblerole of verticaltransport,DMSO Figures2a and 2b show DMSO and DMSO2 measureandDMSO2 mixingratioswereplottedasa functionof dew mentsfrom 3 daysspanningthe time periodof January19 to January23. The daysselectedillustrateboththe wide range point (DP) and equivalentpotentialtemperature(0e) as of valuesas well as the variabilityencountered duringthe 7shownin Platesla and lb. For purposesof this discussion dayDMSO/DMSO2measurement period.Figures2c and2d we will use the term "buffer layer" (BuL) to describethe showlocal pollutioneventreadingsfor thesesamedaysand region from just above the boundarylayer (BL) to free the UV solarirradiance.The pollutionreadingswere based troposphereas has been recently defined and discussedby on NO measurements,reflectingthe emissionsfrom a local Russellet al. [1998]. These investigatorsusedthis term to dieselgeneratorwhenthe wind was out of the 270 - 330ø describemixing betweentwo atmosphericlayers, both of sector. which are turbulent,where air parcelscouldbe transported

DAVIS ET AL.' DMS OXIDATION

IN THE ANTARCTIC

01/19/94

MARINE

01/22/94

BOUNDARY

LAYER

1661

01/23/94 ..

20

120

18 16

•' 14 '" 12

fA DMSO X10

100 80 • 60

...

0 10 u•

.....

•

8

r•

6

40 •.

4

20 •

2

120

15

B

DMSO2 x10 100

• lo

•>

80

•

60

c•

40

•

20 •

õ 0.8

n,,

0.2

200

E

,

øj >

150

o100

-

50 0 00:00

, , i 03:00

i

06:00

09:00

00. 00

i

i

06:00

12:00

Local

18:00

00:00

06:00

12'00

18:00

00:00

Time

Figure 2. Time line plotsfor January19, 1994, January22, 1994, and January23, 1994' (a) DMSO, (b) DMSO2, (c) relative pollution events,and (d) UV flux. Average DMS level is indicatedby dottedline on panels(a) and (b). AverageDMSO levelsfor January19, 22, and23 were 0.4, 1.6, and4.7 pptv, respectively.AverageDMSO2 levelsfor thistime periodwere 0.3, 1.3, and 3.1 pptv, respectively.

in either direction, dependingon the turbulencelevels in each layer and the stability jump between them. Thus, discrete events such as local increases in the wind shear

acrossthe layer or a decreasein the stabilityjump due to horizontal advectionhave the potential for generating episodesof enhancedentrainmenteitherfrom the BL into the BuL or vice versa. (Entrainmentratesfor turbulentlayers

havebeenfoundfrom laboratoryexperiments to be directly proportional to the cube of the characteristicturbulence velocityscalefor the layer andinverselyproportionalto the jump in stabilityacrossthe layer interface[Turner, 1968].)

As a generalrule, bothDP and0eare foundto decrease in the transition from the BL to the BuL. In the case of the

Palmerstudy,bothplotsinvolvingDMSO andDMSO2show a strongbias, with the highestmixing ratio valuesbeing more prominentat the lowest dew point and equivalent potentialtemperature.This suggests thatair parcelsassociatedwith thehighestmixingratiovaluesoriginatedfrom the BuL. Table 1 providestheseresultsin a more quantitative form by subdividing the DMSO and DMSO2 dataintotwo basicverticaltransportcategories.For DP values > 0 ø we haveassumedthatthe air parcelwas only minimallyinflu-

1662

DAVIS ET AL.' DMS OXIDATION

IN THE ANTARCTIC

MARINE BOUNDARY LAYER

ß

20

18 16

12 10 8

4 2

-$

-4

-3

-2

-1

0

I

2

3

4

DewPoint(øC) 20

................

20

B

18

18

16

,*

12

16 .

•o

:

12

-.

•o

8i- .--.. .:. $L_

ø

[ '.,'•6

ø

".-.*:

_ ß

ø ß

/

41-,. ß'".•.i;:,$.•:

•_.. .. ,,,$. :,, •', . 2 l- e*". ' , ;q•. ß .-._'..'ø . ß w'. ß I

282

.:- •,

284

' _ .,f', ß 286

.,0 ] ß.,

44 ,

288

,- .. 290

%

.= _..

292

, ß 294

Equivalent Potential Temperature (K) Plate1. (a) Scatterplotof observed DMSO andDMSO2versusdewpoint,(b) plotof DMSO and DMSO2 versusequivalentpotentialtemperature.

] -12 /

296

DAVISET AL.' DMSOXIDATION IN THEANTARCTIC MARINEBOUNDARY LAYER

1663

Table1. DMSOandDMSO2ValuesFilteredby DewPoint. Dew Point

< -2 øC

# data

> 0 øC

# data

AverageDMSO (pptv)

3.41

156

0.89

143

3.83

Average DMSO2(pptv)

2.55

156

0.59

143

4.32

encedby verticaltransport,whereasfor DP values< -2ø, reflecting thehighest valuesof DMSO observed, we argue thatthecorresponding mixingratioswerestrongly influenced by transport. The resultsshowthattherearenearlyequal amounts of datain eachextremecategory(e.g., 156versus 143)andthatthevalueof theratio(i.e., BuLversus uninfluencedBL air) is approximately four,i.e., favoringtheBuL. We believetheseresultsstronglysupportthe notionthat

Ratio

3.2. H2SO4(g ) and MSA(g)

Figures 3aand3baretheMSA(g)andH2504(g ) measurements fromthreedaysspanning thetimeperiodof February 20 to 22. Asin thecaseforDMSOandDMSO2,thedays selected arethosewebelievearemostrepresentative of the rangeof valuesobserved for thesespeciesas well as the

variability encountered overthesampling period ofFebruary verticaltransport is a majorfactorcontrolling BL mixing 16 to 25. Figures3c and3d providelocalpollution event ratiosforbothDMSOandDMSO2.(Further details concern- readings andtheUV solarirradiance for thesesamedays. ingthispointarepresented in sections 4.2 and4.5.) Significant trendsin thesedatainclude:(1) themeanvalues

...

2/20/94

•

2/21/94

6 ^

-•.

2/22/94 lOO •>