Vertical variability of seawater DMS in the South Pacific Ocean and its implication for atmospheric and surface seawater DMS

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Vertical variability of seawater DMS in the South Pacific Ocean and its implication for atmospheric and surface seawater DMS
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  Vertical variability of seawater DMS in the South Pacific Ocean and its implicationfor atmospheric and surface seawater DMS Gangwoong Lee a , Jooyoung Park a , Yuwoon Jang a , Meehye Lee b, * , Kyung-Ryul Kim c , Jae-Ryoung Oh d ,Dongseon Kim d , Hi-Il Yi d , Tong-Yup Kim e a Department of Environmental Science, Hankuk University of Foreign Studies, Yongin, South Korea b Department of Earth and Environmental Sciences, Korea University, Seoul, South Korea c School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea d Korea Ocean Research & Development Institute, Ansan, South Korea e Korea Polar Research Institute, Songdo, South Korea a r t i c l e i n f o  Article history: Received 29 April 2009Received in revised form 20 October 2009Accepted 23 October 2009Available online 15 January 2010 Keywords: Dimethyl sulfideDissolved DMSPParticulate DMSPSouth PacificSea-to-air fluxMixed layer depth a b s t r a c t Shipboard measurements of atmospheric dimethylsulfide (DMS) and sea surface water DMS were per-formed aboard the R/V Onnuri across the South Pacific from Santiago, Chile to Fiji in February 2000.Hydrographic profiles of DMS, dissolved dimethylsulfoniopropionate (DMSP d ), and particulate DMSP p in the upper 200m were obtained at 16 stations along the track. Atmospheric and sea surface waterDMS concentrations ranged from 3 to 442pptv and from 0.1 to 19.9nM, respectively; the mean valuesof 61pptv and 2.1nM, respectively, were comparable to those fromprevious studies in the South Pacific.The South Pacific Gyre was distinguished by longitudinal-vertical distributions of DMS, DMSP d , andDMSP p , which was thought to be associated with the characteristic modification of biological activitiesthat occurs mainly due to significant change in water temperature. The averaged DMS maximumappeared at 40m depth, whereas DMSP p  and DMSP d  maxima coincided with that of dissolved oxygencontent at 60–80m. The sea-to-air fluxes of DMS were estimated to be 0.4–11.3 l mold  1 m  2 (mean=2.8 l mold  1 m  2 ). A fairly good correlation between atmospheric DMS and sea-to-air DMS fluxindicatedthatatmosphericDMSconcentrationwasmoresensitive tochangeinphysical parametersthanits photochemical removal process or surface seawater DMS concentrations.   2009 Published by Elsevier Ltd. 1. Introduction Dimethylsulfide (DMS), the most dominant sulfur speciesthroughout the ocean, is formed by enzymatic cleavage of dim-ethylsulfoniopropionate (DMSP), which is produced by a varietyof phytoplankton species. Most DMSP is consumed by bacteriaandonlyafractionisusedtoproduceDMS(Kiene,1996). Although DMSisremovedbybacterialconsumption(KieneandBates,1990), seasurfacelayers arealwayssupersaturatedwithit, whichimpliesa net fluxof DMS to the atmosphere (Huebert et al., 2004). As a re- sult,approximately1%of theDMSPproducedinseawateristrans-ported to the air in the form of DMS through sea-to-air flux (Bateset al., 1994; Simó and Pedrós-Alió, 1999). After being released into the atmosphere, DMS is readily oxi-dized to non-sea-salt sulfate (nss-SO 2  4  ) and methane sulfonate(MSA) in the atmospheric boundary layer. Atmospheric DMS ismainly oxidized by OH during the day and nitrate radical (NO 3 )at night. The oxidation of atmospheric DMS seems to contributelargely to the formation of aerosol containing nss-SO 2  4  in the mar-ine troposphere. According to the CLAW hypothesis (Charlsonet al., 1987), sulfate and MSA produced from oceanic DMS affectthe Earth’s radiation balance through the formation of cloud con-densation nuclei (CCN), thereby altering cloud properties. Theoverall effect of these couplings on climate is negative feedback,meaning that it tends to stabilize the climate. In recent studies,DMS was positively correlated with atmospheric CCN (Vallinaet al., 2006, 2007) and solar radiation (Toole and Siegel, 2004; Val- linaandSimó,2007)overmostoftheglobalocean,whichsupportsthe DMS–climate feedback loop for open-ocean environments.Researchers estimate that the gaseous DMS flux fromthe oceantotheatmosphereliesbetween23and35TgSyr  1 (KettleandAn-dreae, 2000; Simó and Dachs, 2002; Kloster et al., 2006). The oce- anic DMS flux compromises   30% of global sulfur sources (IPCC,2001) and its contributionto global nss-SO 2  4  is 27%, both of whichare similar in magnitude (Kloster et al., 2006). The mean annualcontribution of DMS to the climate-relevant nss-SO 2  4  column bur-den is the greatest (43%) in the relatively pristine Southern Hemi-sphere, where a lower oxidative capacity of the atmosphere, alarger sea-to-air transfer of DMS, and a larger surface area of  0045-6535/$ - see front matter    2009 Published by Elsevier Ltd.doi:10.1016/j.chemosphere.2009.10.054 *  Corresponding author. Tel.: +82 2 3290 3178; fax: +82 2 3290 3189. E-mail address:  meehye@korea.ac.kr (M. Lee).Chemosphere 78 (2010) 1063–1070 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere  emission lead to an elevated atmospheric DMS burden (Gondweet al., 2003). Therefore, the vast area of the South Pacific is thekey region in which to test the validity of the CLAW hypothesis.DMS flux from the ocean has been estimated through theparameterization of wind fields and the maps of DMS concentra-tions in the global ocean (Liss and Merlivat, 1986; Wanninkhof,1992; Kettle et al., 1999). Although Kettle et al. (1999) compiled aseawaterDMSandDMSPdatabaseofmorethan15000measure-ments over the global ocean, the temporal and spatial coverage of DMS is still poor. To obtain a global view of DMS distribution bytime, determining oceanic DMS concentration has to be ap-proached using various empirical parameterizations of field obser-vation datasets, such as chlorophyll  a  distribution (Anderson et al.,2001), climatological mixed layer depth (Simó and Dachs, 2002), and SeaWiFS ocean color measurements (Belviso et al., 2006).Kettle et al. (1999) found no significant correlations betweenDMSandotheroceanographicparametersandnosimplealgorithmto create temporal fields of sea surface DMS concentrations basedon these parameters. Thus, to reduce the great uncertainty inher-ent in estimates of DMS flux, more measurements with greatertemporal and spatial resolution are necessary. This is particularlytrue for the South Pacific, where measurements of DMS and DMSPare still very sparse. DMS in the South Pacific has been studiedmost extensively by Bates and his group (Bates and Quinn, 1997;Bates et al., 1998; Bates, 2004). Most of their research, however,has been concentrated on the equatorial Pacific, which is a regionthat exhibits relatively high DMS emissions throughout the year(Bates and Quinn, 1997). Unlike the equatorial Pacific, the central SouthPacific(20–50  S)shouldhavelargeseasonalandspatialvari-ations of DMS levels due to distinct seasonality and latitudinalvariations in sea surface temperature. The central South Pacifichas an area of 30  10 6 km 2 and covers about 8% of all oceansand seas worldwide, yet only four sets of latitudinal transit dataare available for this region in the Global Surface Seawater DMSDatabase(Bates,2004).Convincingevidencealsoexistsforthesea-sonality of DMSP and DMS concentrations and DMS flux in theSouthern Ocean (Simó and Dachs, 2002; Vallina et al., 2007). Kettle et al. (1999) reported that having DMS measurements isnot enough to explain the global DMS distribution, particularly inthe South Pacific and Indian Ocean. To evaluate the role of DMSin climate change at regional to global scales also requires mea-surements of atmospheric and sea water DMS concentrations,quantificationof its sea-to-air flux, and identification of the factorsthat control them. In this experiment, we concurrently measuredsea water DMS, dissolved DMSP, and particulate DMSP at variousdepths, mainly within the thermocline, and the atmospheric DMSalong the ship track from Chile to Fiji. We then characterized thebehaviors of atmospheric DMS and its sea-to-air flux with regardto various factors such as sea surface DMS and DMSP, momentumflux, and mixed layer depth MLD. The results of this study will beuseful to evaluate global ecosystem models for DMS productionand to accurately determine the global DMS budget. 2. Methods As a part of the Southern Pacific Ocean Dynamic Studies, mea-surementsof atmospheric DMSandsurfacewater DMSweremadeonboardtheresearchvessel(R/V)Onnuri,whichleftPuntaArenas,Chile on February 5, 2000 and arrived at Fiji on March4, 2000. Thestudyarealiesbetween20  Sand50  SandrunsfromtheequatorialPacific to the Southern Ocean, in which scarce sets of DMS andDMSP measurements are available. Fig. 1 shows the ship trackandthestationsatwhichhydrocastsamplesweretaken.OnFebru-ary 20 and 21, hydrocast sampling was cancelled in order to keepthe ship on schedule.For vertical profiles of DMS and DMSP, sea water samples werecollectedusing11Niskinbottlesfrom3mbelowthe surfacedownto 200m at 16 stations. During each hydrocast, conductivity, tem-perature, and depth were continuously determined with a CTDalong with dissolved oxygen content. Water samples were takenfrom3mbelowsurfacedownto200mateachstation:sevensam-plesbetween3mand100m,andfoursamplesbetween100mand200m at a 25m interval. The hydrocasts were conducted at dawnso that bacterial and phytoplankton populations would be mini-mally affected upon exposure to ultraviolet radiation (Kiene andLinn, 2000). Upon retrieval of the bottles, sea water was gentlydrawnfromeachNiskinbottleintoa130mLDObottlethroughty-gon tubing; the new bottle was overflowed with sea water 2–3timessothatnoairwouldbetrappedinside.Ateachhydrocaststa-tion, surface water was collected using a bucket for the surfaceDMS measurement. Along the cruise track, surface seawater alsowas sampled 4–6 times a day for DMS analysis using the continu-ous seawater pumping system on the ship.To measuresea water DMS, analiquot of 30mL was withdrawnfrom the DO bottle with a 50mL syringe. The water-filled syringethen was connected to an air-tight filter holder equipped with a47mm Whatman GF/F glass fiber filter. By applying gentle pres-sure to syringe, the water sample was filtered into a 100mL gasstripper bottle. Next, the filtrate was purged with high purity he-lium at 100mLmin  1 for 20min and the purged air was passedthrough a Nafion dryer (Perma-Pure, Inc., USA) to remove watervapor. Finally, DMS was captured in a carbosieve 300 adsorptiontrap(Supelco,USA). ThedissolvedDMSP(DMSP d )inthepurgedfil-trate and the particulate DMSP (DMSP p ) on the filter were con-verted to DMS using a strong alkaline solution for 1–2h and thenmeasured as DMS (Kim and Andreae, 1987). A common practiceis to leave the particulate DMSP samples in the alkaline solutionat least 12h. As a result, our DMSP p  values were likely underesti-mated due to incomplete hydrolysis of DMSP p . DMS and DMSP insea water were measured right after the water samples were col-lected from the upper depths. Some samples from lower depths,however, could not be analyzed immediately and had to wait amaximum of 4h; during this time they remained in the dark at Fig. 1.  Ship track and stations (closed circles) for vertical seawater sampling.Numbers above and below each station indicate the station number and date inFebruary 2000, respectively.1064  G. Lee et al./Chemosphere 78 (2010) 1063–1070  room temperature. Traps containing seawater DMS were stored ina freezer at   70  C until gas chromatography (GC) analysis couldbe conducted.Air DMS was collected in adsorption traps (carbosieve 300)every 3–6h (4–6 times a day) at the upper bridge deck of the ship.AirwaspassedthroughaNafiondryerfilledwithsilicagel andaKItrap at  100mLmin  1 (1atm, 25  C) ahead of an adsorption tubeto remove water vapor and oxidants, respectively. AtmosphericDMS was analyzed immediately after being collected.The DMS in a trap was desorbed by rapid heating to 300  Cusing a thermal desorption unit (Supelco, USA) for 3min and in- jected directly into a HP 6890 GC equipped with a 30-in. packedsuper Q column (Alltech, USA) and a sulfur chemiluminescencedetector (Siever, USA). The flow rate of carrier gas (He) was set at30mLmin  1 . The oven temperature was programmed to initiateat 60  C for 3min, then the temperature was raised to 200  C at arate of 15  Cmin  1 , held there for 5min. Detection limits of atmo-spheric and seawater DMS were 1.5pptv and 0.01nM, respec-tively. DMS concentrations were calibrated using a permeationdevice (13ngmin  1 at 30  C, VICI, USA). The analytical precisionsof atmospheric and seawater DMS measurements were 12% and13%, respectively, which accounted for uncertainties associatedwith variance of daily DMS standards, air flow, and water volumeover the course of the experiment.Meteorological data, such as wind speed, wind direction, airtemperature, and pressure, were obtained from the shipboardautomated weather system(Vaisala, Finland), which was designedfor accurate measurement of true wind direction and speed. How-ever, the data for the first 2weeks of the experiment were lost be-cause they were overwritten daily under the automatic savingmode. After February 16, meteorological data were manuallysaved. Modeled wind speeds and momentum fluxes were derivedfrom the FNL archive produced by the National Centers for Envi-ronmental Prediction (NCEP). The FNL model data used in thisstudy were archived in the Air Resources Laboratory of the Na-tional Oceanic and Atmospheric Administration (NOAA, http://www.arl.noaa.gov/ready). While the FNL wind speed and directionandthoseobservedontheshipweresimilarintheirpatternofvar-iation, their one-to-one correlation was poor. 3. Results and discussion  3.1. Distribution of atmospheric and surface seawater DMS  Fig. 2 shows the variations of atmospheric and seawater DMS,momentum fluxes, and wind speeds over the course of the study.The atmospheric DMS concentrations ranged from 3 to 442pptvwith a mean of 61pptv. For the first week of the experiment, theatmospheric DMS concentrations remained below 50pptv. Mostof the high atmospheric DMS concentrations (>100pptv) occurredbetween February 14 and 24 and between stations 4 (37  59 0 S,101  58 0 W) and 13 (28  51 0 S, 153  33 0 W), which is the subtropicalregion of the South Pacific. Although atmospheric DMS over theocean srcinates from the ocean, we did not find a significant cor-relation between atmospheric and surface seawater DMS concen-trations in this study. Although sea surface and atmospheric DMSconcentrations were raised over the same period, their variationsclearly differed from each other. Watanabe et al. (1995) observeda strong positive correlation between the spatial distributions of surface seawater and air DMS over the temperate North Pacific.However,otherpreviousstudiesconductedinvariousmarineenvi-ronments failed to find similar correlations due to complexities insea-air transfer mechanisms of DMS (Berresheim et al., 1991;Church et al., 1991) and removal processes of atmospheric DMS(Kieber et al., 1996). In this study, meteorological parameters affecting sea-to-airflux played a more pronounced role in controlling atmosphericDMSthandidsurfacewaterDMSconcentrations.Theperiodofele-vated atmospheric DMS concentrations, especially between Febru-ary 17 and 24, was characterized by higher wind speed (>5ms  1 ).Although wind data were lost and the overall correlation was notavailable for the first 10d of the experiment, atmospheric DMSconcentrations were closely related to the momentum fluxes.However, the poor correlations between observed wind and FNL data hinder further discussion about the relationship betweenatmospheric DMS and the calculated momentum flux. Nonethe-less, it is evident that the intimate coupling of sea-to-air flux withwind speed and momentumflux played a significant role in deter-mining atmospheric DMS concentrations over the study region.The surface seawater DMS concentrations varied considerablyfrom 0.1 to 19.9nM (mean=2.1nM) and were elevated concur-rently with atmospheric DMS at stations 4–13 in the subtropicalregion.Ourresultsindicatethatinsummer,seawaterDMSconcen-trations of the subtropical region in the South Pacific clearly dif-fered from those of tropical and temperate regions. These spatialdifferences in surface seawater DMS will be examined in detail ina later section.Our measurements of atmospheric and surface seawater DMSare in reasonable agreement with DMS concentrations from previ-ous studies conducted in the Pacific Ocean. For atmospheric DMS,ourmeanwasclosetothatobtainedbyNguyenetal.(1983)duringthe spring, but their measurements (40–139pptv) were less scat-teredthanours(3–442pptv);ourvaluesalsofallwithintherangesobserved at a coastal site (36  16 0 S) in New Zealand (Wylie and deMora, 1996) and in the North Pacific (Watanabe et al., 1995). Over the South Pacific Subtropical Gyre, the levels of atmo-spheric and seawater DMS are highest during February and March(  3–4nM), based on the global database of measurements (Kettle 0100200300400500     p    p     t    v 0481216     n     M 6-Feb11-Feb16-Feb21-Feb26-Feb2-Mar 04812     m     /    s    e    c 00.40.81.2      N     /    m      2  Atmospheric DMSMomentum FluxWind SpeedSea surface DMS (a)(b)(c)(d) Fig. 2.  Variations of (a) atmospheric DMS, (b) surface water DMS, (c) momentumflux, and (d) wind speeds during the course of the experiment. G. Lee et al./Chemosphere 78 (2010) 1063–1070  1065  to DMS ( Jones et al., 1998), the bacterial conversion of DMSP d would be greatest over the transition region. While the levels of DMS and DMSP p  within the water column rapidly decreased mov-ing north from 30  S (west of 150  W), DMSP d  increased noticeablyin this subtropical oligotrophic region; this indicates that the rateof bacterial conversion of DMSP d  to DMS or the direct release of DMS from phytoplankton likely decreased. Evidently, the changesin biological activity, including phytoplankton composition andbacterial activities, played a crucial role in the distribution of DMSovertheresearcharea.Furtherinvestigationisneededtoclar-ify the mechanisms responsible for this finding and their relativeimportance to DMS and DMSP distributions.In Fig. 4a, vertical profiles at all 16 stations were averaged forDMS, DMSP d , and DMSP p . The maximum depth appeared at 40mfor DMS and 60–80m for DMSP p  and DMSP d , which is within thedepth range of the Chl a maximum reported for the South Pacific(Maritorenaetal.,2002;Grobetal.,2007).Asshownintypicalpro- files of density, temperature, salinity, and oxygen contents(Fig. 4b), the surface mixed layer was well developed throughoutthe study region, even though the depth of the mixed layer variedslightly from station to station (mainly due to meteorological andoceanographicconditions). Thedepthformaximumdissolvedoxy-gen coincided with the depth at which the maximum concentra-tions of DMSP p  and DMSP d  were found. Considering thatdissolved oxygen content is an indication of biological productionwithin the euphotic zone, the conversion of DMSP d  to DMS wouldbe higher within the mixed layer. Indeed, the variations in DMSand DMSP d  were out of phase between 20m and 60m (Fig. 4a).  3.3. DMS flux into the air  We estimated the sea-to-air fluxes of DMS using the followinggas exchange model for the 12 stations for which wind speedand water temperature data were available: Flux DMS  ¼  k l ð C  l    C   g  = H  Þ ffi  k l    C  l where k l  isthegastransfervelocity; C  l  and C   g   aretheconcentrationsof gas in seawater and the atmosphere, respectively; and  H   is thesolubility of the gas in seawater. Because seawater is supersatu-rated relative to atmospheric DMS, the flux of DMS can be ex-pressed as the product of surface water concentrations and thetransfer velocity of DMS in the interface of sea and air. Variousmethods have been employed to parameterize  k l  using wind speedand physio-chemical properties of surface water (Liss and Merlivat,1986; Wanninkhof, 1992; Nightingale et al., 2000). Here, we used theparameterizationschemesof LissandMerlivat(1986)(hereinaf-ter LM86) and Wanninkhof (1992) (hereinafter W92) that havebeen most widely used in calculations of gas transfer velocity. Intheir schemes, the gas transfer velocity is regarded as being depen-dent on wind speed at 10m height ( u 10 ) and the Schmidt number(Sc DMS ), which is expressed as a function of temperature (Saltzmanet al., 1993): Sc DMS  ¼  2674 : 0  147 : 12 t   þ 3 : 726 t  2  0 : 038 t  3 where  t   is water temperature (  C). In LM86, the gas transfer velocity is defined differently accord-ing to wind speed: K  l ; DMS  ¼  0 : 17 u 10 ð 660 = Sc DMS Þ 2 = 3 ½ cm h  1  u 10  6 3 : 6  ð smooth regime Þ K  l ; DMS  ¼ ð 2 : 85 u 10   9 : 65 Þð 660 = Sc DMS Þ 1 = 2 ½ cm h  1  3 : 6  <  u 10  6 13  ð rough regime Þ K  l ; DMS  ¼ ð 5 : 9 u 10   49 : 3 Þð 660 = Sc DMS Þ 1 = 2 ½ cm h  1  u 10 P 13  ð wave-breaking regime Þ Incomparison,thegastransfervelocitygivenbyW92issimplerthan that of LM86: K  l ; DMS  ¼  0 : 31 u 210 ð 660 = Sc DMS Þ 1 ½ cm h  1  Using these equations, we calculated the DMS fluxes for eachstation and the results are given in Table 1. The sea-to-air fluxesof DMS by LM86 ranged from 0.4 to 11.3, with a mean of  DMSDissolved DMSPParticulate DMSP 012345 Concentration (nM) 20016012080400    D  e  p   t   h   (  m   ) 812162024 Temperature o C 20016012080400      D    e    p     t     h 24242525262627 Sigma- θ 55666 Oxygen mg/l 34.034.2 34.434.634.835.0 Salinity %o TemperatureSalinitySigma- θ Oxygen (a) (b) Fig. 4.  Vertical profiles of (a) DMS and dissolved and particulate DMSP concentra-tions (averaged for all stations) and(b) density (sigma h ), temperature, salinity, andoxygen contents at station 8. G. Lee et al./Chemosphere 78 (2010) 1063–1070  1067
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