DIMETHYL SULFIDE AND ITS OXIDATION …...Dimethyl sulfide and its oxidation products in the atmosphere 1897 atmosphere. Filters were stored refrigerated in double poly- thene bags

Pergamon Atmospheric Environment Vol. 30, Nos 10/11, pp. 1895-1906, 1996 Copyright © 1996 Published by Elsevier Science Ltd

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1352-2310 (95) 00428-9

DIMETHYL SULFIDE AND ITS OXIDATION PRODUCTS IN THE ATMOSPHERE OF THE ATLANTIC AND

SOUTHERN OCEANS

B. DAVISON,* C. O ' D O W D , t C. N. H E W I T T , * M. H. S M I T H , t R. M. HARRISON,~ D. A. PEEL,§ E. WOLF,§ R. MULVANEY,§

M. S C H W l K O W S K I ¶ and U. B A L T E N S P E R G E R ¶ *Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, U.K.;

tDepartment of Pure and Applied Physics, UMIST, P.O. Box 88, Manchester M60 1QD, U.K.; :[:Institute of Public and Environmental Health, University of Birmingham, Birmingham B 15, U.K.; §British Antarctic Survey, Madingley Rd, Cambridge CB3 0ET, U.K.; and ¶Paul Scherrer Institut, CH-5232 Villigen PSI,

Switzerland

(First received 10 December 1994 and in final form 31 August 1995)

Abstract--Dimethyl sulfide, methane sulfonate, non-sea-salt sulfate and sulfur dioxide concentrations in air were obtained during a cruise between the U.K. and the Antarctic during the period October 1992-January 1993. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S)m -3 with an average of 18 ng(S)m -3. In the polar waters and regions south of the Falkland Islands concentrations from 3 to 714 ng(S)m -3 were observed with a mean concentration of 73 ng(S)m -3. Methane sullbnate concentrations were also enhanced in the vicinity of the Antarctic Peninsula and in the Weddell Sea. A simple model of DMS oxidation was used to estimate the ocean to atmosphere flux rate, and this was found to be within the range of previous estimates, with a mean value of 1011 ng (S) m- z h- 1.

Key word in,tex: Dimethyl sulfide, Antarctica, aerosol.

]iNTRODUCTION

Dimethyl sulfide is one of the breakdown products of dimethyl sulfoniopropionate (DMSP), a compound involved in regulating cellular osmotic pressure in marine algae (Dickson et al., 1980). Once released into sea water, the gas transfers across the water-air interface, as a result of its appreciable concentration gradient. Indeed, relatiw~ to its DMS concentration in air, ocean surface waters are typically supersaturated by two orders of magnitude (Andreae, 1986). This transfer process has been described by Liss and Slater (1974) in terms of the concentration gradient and a transfer velocity, which itself depends upon wind speed. This, and a variety of methods based upon ambient air concentration measurements, have al- lowed estimates of the global flux of DMS to be made. However, the strong temporal and spatial variations in DMS concentrations in both sea water and air render such global flux estimates uncertain. Current predictions are in the range 8-51 Tg(S)yr -1 with a best estimate of about 16 Tg(S)yr -~ (Rodhe and Langer, 1993). As the number of reliable DMS measurements increase, so will the accuracy of its flux estimates improve.

The spatial variability in the concentration of DMS in sea water is illustrated by measurements made in

the Atlantic Ocean by Barnard et al. (1982) when a range of 17-1700ng(S)( -1 with a mean of 90 ng(S)f-1 was observed. The seasonality in concentrations is most enhanced in temperate latitudes. For example Bates et al. (1987) determined average concentrations of 130 and 20 g(S)f-1 in summer and winter, respectively, in the waters of the north Pacific. However concentrations are generally higher in more productive coastal waters and in the vicinity of river estuaries where extreme patchiness is commonly observed (e.g. Turner et al., 1989; Leek et al., 1990). The biochemical processes underlying dimethyl sulfoniopropionate production, and hence the resulting DMS concentrations in sea water, are not clearly understood, although information on the variability in production rates from certain species of phytoplankton is available (Liss et al., 1994). The production and breakdown of DMS in the water column and the dependence of emissions on air and sea water temperature, solar flux and changing sea water ecology need to be understood better before it will be possible to predict sea water-air exchange rates in the future (Brimblecombe and Shooter, 1986; Leck et al., 1986; Bates et al., 1994; Gabric et al., 1994).

Concentrations of DMS in air are, of course, much lower than in sea water and show a greater variability. For example, measurements made at Cape Grim,

1895

1896 B. DAVISON et al.

Tasmania, gave concentrations in the range 30-380 ng(S)m -3 with a mean of 160 ng(S)m -3 (Andreae et al., 1985). Measurements made in Atlantic air at a coastal site in northwest Scotland over a two year period were in the range < 1-885 ng(S)m -3, with a summer mean of 110 ng(S)m -3 and a mean during the spring phytoplankton bloom period of 490 ng (S) m -3 (Davison and Hewitt, 1992). Winter values of < 1-10 ng (S) m - 3 were seen. Similar seasonal trends

have been observed elsewhere. A diurnal cycle in the concentration of DMS was observed by Andreae and Raemdonck (1983) in the Pacific Ocean, with mean daytime minima and nighttime maxima of 120 and 200 ng(S)m -3, respectively. Saltzman and Cooper (1988) measured a similar diurnal cycle in the trade winds of the Caribbean, with mean daytime minima and nighttime maxima of 56 and 83 ng(S)m -3, respectively. This cycle is attributed to the rapid oxidation and removal of DMS by reaction with the photochemically produced hydroxyl radical (OH). In clean air, OH concentrations follow a strong diurnal pattern, with essentially zero concentrations during the hours of darkness (Logan et al., 1981).

In continentally influenced air masses, DMS concentrations are generally lower than in maritime air, due to a lack of sources and an increase in oxidant concentrations. The presence of the nitrate radical (NO3) in polluted air during the night and its rapid reaction with DMS (Hewitt and Davison, 1988) may result in a reduction in amplitude of the diurnal cycle of DMS concentrations in such air masses.

Vertical profile measurements of DMS in the atmosphere suggest a steady decline in concentration with altitude within the boundary layer, followed by a rapid decrease in the free troposphere above. For example, Ferek et al. (1986) found a decrease in DMS concentrations under stable meteorological conditions in the vicinity of Barbados from 100ng (S) m - 3 at sea level to 60 ng (S) m - 3 at cloud top level at 1 km, with a rapid decline to a few ng(S)m -3 at 2 km, above the boundary layer. Under more turbu- lent convective conditions, the DMS concentrations in the free troposphere were an order of magnitude higher. Similar results have been observed elsewhere (e.g. Luria et al., 1986; Andreae et al., 1988). The short atmospheric lifetime of DMS of a few days (Hewitt and Davison, 1988) prevents its transfer to the free troposphere other than by rapid convective uplift.

The formation of aerosol phase methane sulfonate (MSA) and non-sea-salt sulfate (nssS) from DMS and their effects on the atmospheric number density of cloud condensation nuclei (CCN) has been the focus of much interest since Charlson et al. (1987) and others postulated that they may play a key role in regulating the Earth's radiation balance by their effects on cloudiness and on cloud albedo. Several at- tempts have been made to investigate this hypothesis, for example, by looking at links between DMS emissions and non-sea-salt sulfate concentrations (Ayers and Gras, 1991) and between DMS and particle con-

centrations (Hegg et al., 1991). In addition, satellite data of cloud cover, sea surface temperature and chlorophyll concentrations have been used to look (indirectly) at the direct link between DMS production and cloud albedo, without consideration of the intermediate physical and chemical steps (Falkowski et al., 1992; Boers et al., 1994). While evidence is available to support some, at least, of these links (e.g. Hobbs et al., 1993) other aspects of the DMS- climate hypothesis are far from proven. In fact, Schwartz (1988) has posed a major test of the hypothesis by suggesting that anthropogenic emissions of sulfur dioxide (SO2) in the Northern Hemisphere (NH) have not altered the cloud albedo of the NH relative to the Southern Hemisphere.

Here we report on the concentrations of DMS and its oxidation products, MSA, nssS and SO2 measured along a transect through the Atlantic Ocean from the United Kingdom to Halley Bay in Antarctica, to- gether with information on the physical and chemical properties of the ambient aerosol.

EXPERIMENTAL METHODS

Dimethyl sulfide

Full details of the sampling and analytical method used for DMS have been given elsewhere (Davison and Allen, 1994). The method allows DMS to be quantitatively sampled with a time resolution of 2-3 h, with a detection limit of 10 pptv and an analytical precision and accuracy of _+ 5% without the need for cryogenic liquids and without losses due to oxidation by ozone.

Methane sulfonate and other ionic species

Methane sulfonate (MSA), sulfate, chloride and a range of cations including sodium, magnesium, calcium and potas- sium in the ambient aerosol (0.1/~m < d > 10/~m) were obtained by high volume sampling through QMA quartz fibre filters at 0.7 m3min -~ with sampling periods of 24 h. All filters were prewashed by leaching for 3 days with Milli-Q water changed daily with a final rinse prior to drying in a filtered air clean room environment. Clean filters were stored in double polythene bags. Samples were extracted into Milli-Q water and analysed by ion chromatography using an AS4-A column for anions and a CS 12-A column for cations on a Dionex 4000i instrument (Davison and Hewitt, 1992). Blank filters, handled in the same manner as samples but without the passage of air, were collected periodically during the cruise to calculate detection limits of ~ 1 nmol m -3 for CI- and NO~- and 0.3 nmolm -3 for SO 2-. Experi- mental precision was 8% for MSA and NO; and < 5% for all other species. Sodium concentrations were also obtained by flame photometry. Na ÷ was used as a sea-salt tracer to allow calculation of non-sea-salt sulfate concentrations using a sea-salt Na+/SO 2- ratio of 0.251 (Walton Smith, 1974).

Sulfur dioxide

Sulfur dioxide samples were collected concurrently with those of DMS on prewashed carbonate impregnated What- man 54 filters (Ferek et al., 1991). The filters were treated with the carbonate/glycerol mixture 24 h before use and dried in a desiccator. A 0.45/~m PTFE membrane prefilter was used to remove particulate sulfate already present in the

Dimethyl sulfide and its oxidation products in the atmosphere 1897

atmosphere. Filters were stored refrigerated in double polythene bags and sealed bottles to minimise contamination after collection and were analysed immediately on return to the U.K. The prefilters underwent ultrasonic extraction in 20 ml Milti-Q water. The impregnated filters were extracted by shaking in Milli-Q water with addition of 0.1 ml hydrogen peroxide (30°,/0 w/v) solution to assure complete oxidation to SO 2- before analysis by ion chromatography. Blank filters undergoing the :~ame procedure as sampled filters were collected periodically throughout the trip to allow a detection limit for SO2 of 50 ng(S)m -3 to be calculated.

Measurement programme

Measurements were made on board the RRS Bransfield during the period 2 October 1992 to 14 January 1993. The Bransfield is a supply ~hip that services the British Antarctic Survey bases in Antarctica. The cruise consisted of five legs:

(i) from Grimsby in the United Kingdom to Montevideo (2-26/10/92: Julian days 276-300),

(ii) Montevideo to Port Stanley in the Falkland Islands (29/10-1/11/92: Julian days 303-306),

(iii) Port Stanley to Faraday on the Antarctic Peninsula at 65°15 ' S, 64°16 ' W and back to Port Stanley (5-14/11/92: Julian days 310-319),

(iv) a repeat passage to Faraday and back (18/11~/12/92: Julian days 323 339),

(v) Port Stanley to Halley Bay at 75°35 ' S, 26°46 ' W in the east Weddell Sea and back to Port Stanley (7/12/92-14/I/93: Julian days 342-380).

The period 21 30 December 1992 was spent at Halley Bay. The cruise track south of the Falkland Islands is shown in Fig. 1. For ease of presentation of the data, 1 January 1993 has been designated as Julian day 367.

Air sampling systems were housed in a cabinet on the monkey deck of the ship at a height of 20 m above the sea

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Fig. 1. Route ofthe RRS Bransfield ffomthe Falkland Islandsto Antartica,4November1992-10January 1993.

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surface in front of the ship's stack. Wind speed and direction were logged at this location at 3 rain intervals and this information used to identify periods of time when contamination from the ship's emissions was likely to have occurred. Data from these periods are excluded from the presentation and discussion here.

Air mass back trajectories

Isobaric three-dimensional air mass back trajectories were used to indicate any periods of long-range transport which may have brought continental contamination to the ship. Analysis of air mass back trajectories for the first part of the cruise (4-8 October 31046 ' N) were consistent with the observed local wind direction from the north which caused contamination of samples from the ship's chimney stack. From 8-13 October (5086 ' N) lighter winds were encountered. During a transect from the Cape Verde Islands to south of Recife (Brazil) trajectories indicate surface winds with a southerly direction and no continental component.

From 22 October until reaching port in Montevideo, surface winds were from the south running parallel to the coast. Westerly winds prevailed during the two cruises to Faraday on the Antarctic Peninsula. The trajectory for the 5 November shows air masses passing the tip of South America. During the cruise to Halley Bay katabatic winds flowing from the Antarctic continent dominated the surface air masses reaching the ship with circum-polar winds pre- vailing at higher altitudes.

RESULTS AND DISCUSSION

Dimethy l sulfide

Figure 2 shows the concentrations of D M S in air. A summary for the whole cruise and each leg is given in Tables 1-6. D M S concentrations with latitude are shown in Fig. 3. Very low D M S concentrations (3-46 ng(S)m -3) were observed through the Nor th and South Atlantic as far south as 58 °, with an average of 18ng(S)m -3. During the transect from the Cape Verde Islands to Brazil, atmospheric D M S concentrations dropped as the ship passed from tropical to equatorial waters and rose again as the southern tropics and the continental shelf of South America was reached.These data are consistent with low biological activity during the northern hemisphere winter and in the open ocean waters of the southern hemisphere and with those of Putaud et al. (1993) for measurements in the area of the Cape Verde Is- lands and of Andreae et al. (1994) for measurements taken in tropical regions off the coast of Brazil.

Further south, much higher concentrations were observed: during November and the beginning of December two cruises were undertaken from the

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,Julian Daye

Fig. 2. Concentrations of dimethyl sulfide in air with time during the cruise of the RRS Bransfield, 2 October 1992-10 January 1993.


Table 1. Concentrations of chemical species in air determined during the whole of the cruise of the RRS Bransfield from the U.K. to Antarctica, 3/10/92-10/1/93

Species Mean Maximum Minimum n

DMS (ng(S) m - 3) 54 714 0 140 CI- (#gm -3) 8.2 35 0.1 65 NO3 (#gm -3) 0.8 12 0.01 34 SO 2-(pg m - 3) 3.0 16 0.36 67 ns,;S (/tg m - 3) 0.9 4.0 0.06 50 N~L+(FP) (#gin -a) 6.8 29.8 0.17 64 N~ ÷ (IC) ( #g m - 3) 5.5 14.5 0.25 56 MSA (ngm -3) 77 362 1.72 62 M'SA: nssS 0.16 1.1 < 0.01 49 NH~(pg m - 3) 0.86 10.1 0.01 45 K~(/~gm -3) 0.33 5.6 0.01 52 Mg 2+ (/lgm -3) 0.82 4.9 0.003 63 Ca 2 ÷ (/~g m - 3) 0.33 1.6 0.01 60 SO2 ( ng m - 3) 804 1593 95.9 50

Table 2. Concentrations of chemical species in air determined during the northern part of the cruise of the RRS Bransfield from the U.K. to the Falkland Islands,

3/10/92-1/11/92


DMS (ng (S) m - 3) 18 47 2.8 48 CI- (/tgm -3) 8.9 19.6 1.2 22 NO~" (/~gm -a) 1.2 12.0 0.03 20 SO42- (#gm -3) 4.6 14.7 1.33 22 nssS (/~g m - 3) 1.11 4.0 0.06 17 Na + (FP) (/~g m - 3) 9.0 27.8 2.34 22 Na+(IC) (#gm -3) 7.5 14.5 1.93 21 MSA (ng m - 3) 14.3 39.7 1.72 18 MSA: nssS 0.026 0.29 0.002 18 NI-I~- (#gm -3) 2.8 10.1 0.09 10 K ÷ (#gm -3) 0.3 0.7 0.11 20 M~, 2 + ( #g m - 3) 0.9 1.7 0.22 20 Ca z+ (#gm -3) 0.44 1.6 0.08 20 SO2 (ngm -3) 913 1562 95.9 19

Table 3. Concentrations of chemical species in air determined during all legs of the cruise of the RRS Bransfield south of the Falkland Islands, 5/11/92-10/1/93


DMS (ng(S) m -3) 73 714 < d.l. 92 Cl- (pgm -3) 7.9 35 0.1 43 NO~ (#gm -3) 0.2 0.74 0.01 13 SO 2- (pgm -3) 2.2 15.8 0.36 45 nss ~ (/tg m - 3) 0.8 3.42 0.08 33 Na÷(FP) (#gin -3) 5.6 9.44 0.17 40 Na ÷ (IC) (/~g m - a) 4.5 11.7 0.25 37 MSA (ngm -a) 103 362 1.94 44 MSA: nssS 0.2 1.05 0.01 35 NI~I~ (#gm -3) 0.3 0.97 0.01 35 K ÷ (/tgm -3) 0.34 5.55 0.01 42 Mg z+ (/lgm -3) 0.79 4.86 < 0.01 43 Ca :~+ (pgm -3) 0.27 1.6 0.01 40 SO z ( ng m - 3) 738 1593 432 30

Fa lk land Is lands to Fa r aday Base on the Antarct ic Peninsula. Dur ing the first cruise in early N o v e m b e r a mean D M S concen t ra t ion of 3 6 n g ( S ) m -3 was measured, with a mean of 44 n g ( S ) m -3 being observed dur ing the second cruise in late N o v e m b e r and

early December. These values are lower than the mean of 1 4 0 n g ( S ) m -3 observed dur ing M a r c h by Berresheim (1987) in the Bransfield and Ger lache Straits and suggest tha t the Antarct ic summer p h y t o p l a n k t o n b loom was not fully underway.

1900 B. DAVISON e t al.

Table 4. Concentrations of chemical species in air determined during the first leg of the cruise of the RRS Bransfield south of the Falkland Islands, 5/11/92-14/11/92


DMS (ng(S)m 3) 36 106 < d.l. 20 C1- (,ugm -3) 11.1 35 2.1 10 NO3 (#gm -3) 0.3 0.5 0.16 2 SO 2- (pgm -3) 2.5 7.8 0.38 10 nssS ( #g m- 3) 0.9 2.5 0.21 5 Na+(FP) (#gm -3) 7.2 23.6 0.69 8 Na ÷ (IC) (/~g m - 3) 4.2 7.6 1.18 8 MSA ( ng m- 3) 27. I 99 1.94 9 MSA: nssS 0.07 0.12 0.01 l 5 NH2 (pgm -3) 0.17 0.22 0.06 6 K + (/tgm -3) 0.25 1.08 0.3 10 Mg z+ (/tgm -3) 0.87 3.39 0.13 10 Ca z + ( pg m - 3) 0.31 0.96 0.04 10 SO2 ( ng m- 3) 808 1593 464 7

Table 5. Concentrations of chemical species in air determined during the second visit of the RRS Bransfield to the Antarctic Peninsula, 19/11/92-2/12/92


DMS (ng (S) m - 3 ) 44 204 9.37 23 C1- (,ugm 3) 8.7 16.3 1.75 11 NO3 (#gin -3) 0.2 0.74 0.01 8 SO 2- (#gin -3) 1.8 3.13 0.41 12 nssS (#gm -3) 0.47 1.05 0.14 9 Na+(FP) (/~gm -3) 5.9 10.6 0.24 12 Na ÷ (IC) ( t~g m- a) 5.9 10.9 1.59 11 MSA (ngm -3) 97 200 2.54 12 MSA: nssS 0.27 1.05 0.018 9 NH,~ ( #g m- 3) 0.29 0.94 0.09 11 K ~ (#gm -3) 0.66 5.55 0.07 12 Mg 2÷ (/lgm 3) 0.98 4.86 0.18 12 Ca 2 + ( #g m- 3) 0.26 0.53 0.1 11 SOz (ng m- 3) 731 1375 456 6

Table 6. Concentrations of chemical species in air determined during the visit of the RRS Bransfield to Halley Bay in the east Weddell Sea, 8/12/92-10/1/93


DMS (ng (S) m - 3) 102 714 3.9 51 C1- ( pg m - 3) 6.02 28.1 0.1 22 NO~- (/zgm -3) 0.18 0.23 0.12 4 SO~- (#gm -3) 2.28 15.8 0.36 23 nssS (/~g m - 3) 0.87 3.42 0.08 19 Na+(FP) (#gm -3) 4.7 29.9 0.17 20 Na ÷ (IC) (/zg m - 3) 3.8 11.7 0.25 18 MSA (ngm -3) 136 362 2.48 23 MSA: nssS 0.22 0.56 0.01 21 NH~ ( #g m- 3) 0.36 0.97 0.01 18 K ÷ (#gm -3) 0.2 1.17 0.01 20 Mg 2÷ (pgm -3) 0.64 3.3 < 0.01 21 Ca 2 ÷ ( pg m - 3) 0.25 1.6 0.01 19 SO2 (ng m- 3) 711 1258 432 27

Observa t ions of phy top l ank ton activity in the region indicate tha t the summer b loom generally starts a round mid/ la te December (Krebs, 1983; Bunt, 1968).

A higher mean concen t ra t ion of 102 ng ( S ) m - 3 was observed in the Weddell Sea dur ing the passage to Halley Bay in late December and early January. Ex-

t reme D M S concent ra t ions of 296 (on 18/12/92), 543 (on 20/12/92), 382 (on 21/12/92) and 713 ng (S)m -3 (on 2/1/93) were observed in b roken sea ice and a long the f ront of the polar ice shelf in the area 67-74°S. This spatial patchiness is characterist ic of algae growth on sea ice (Bunt, 1968) and its release f rom the

Dimethyl sulfide and its oxidation products in the atmosphere

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Fig. 3. Concentrations of dimethyl sulfide in air with latitude during the cruise of RRS Bransfield, 2 October 1992-10 January 1993.

ice edge in pulses related to localised melting (Fryxell and Kendrick, 1988).

These data are broadly consistent with previous measurements obtained in these regions by Staubes and Georgii (1993) who observed lower than expected atmospheric DMS concentrations, attributed to extensive sea ice cover during their passage through the Weddell Sea. The), are also consistent with observa- tions of spring, summer and autumn phytoplankton blooms in the vidnity of the Antarctic Peninsula (Krebs, 1983; Leakey, 1991)

Simplistically, a dependence between DMS concentrations in air and both wind speed and the height of the atmospheric boundary layer might be expected, with high wind speeds enhancing the water-air flux rate (Liss and Slatcr, 1974) and an increasing boundary layer height leading to a dilution of DMS in the atmosphere. However, a very poor correlation ( r 2 = 0.14, n > 100) was found between wind speed and DMS concentrations. This is not surprising since the measurements were made from a ship moving through areas of varying DMS production rates.

In order to estimate the sea to air exchange rate of DMS by the use of a piston or transfer velocity (Liss

and Slater, 1974) it is necessary to know the DMS concentration in sea water. Such measurements were not made on this cruise. However it is also possible to arrive at a first-order estimate of the flux rate from the height of the boundary layer, h, the air concentration of the gas, Ca, and the atmospheric lifetime of the gas, t:

F = hCa/t .

The maritime boundary layer height was calculated from balloon sonde data. Meteorological balloons were launched twice daily (midnight and noon GMT) from the ship while at sea. The DMS lifetime may be calculated from the known rate constants for the DMS-OH and DMS-NO3 reactions (Hewitt and Davis•n, 1988) and estimates of the OH concentration with latitude (Logan et al., 1981). The resultant fluxes are shown in Fig. 4. A mean flux of 1011 ng(S) m - E h - 1 was estimated (range 25-5336 ng(S) m - 2 h -1, n = 140). When four extreme data points are excluded, a mean value of 788 ng (S) m - 2 h - 1 is calculated. The lowest values were found in equatorial waters, with increased fluxes being observed in Tropi- cal regions and the highest fluxes being observed south of 50°S. The presence of significant amounts of


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Fig. 4. Calculated flux of dimethyl sulfide from the ocean to the atmosphere with latitude during the cruise of the RRS Bransfield, 2 October 1992-10 January 1993.

DMS in the atmosphere in regions of complete sea ice cover, for example, in areas of the Weddell Sea, suggests that transport of DMS from areas of open water must be occurring. This suggests that the steady-state approximation used to calculate fluxes from atmospheric DMS concentrations is not valid in some regions. However, only seven data points in Fig. 4 come from areas of significant ice cover and so do not significantly alter the estimated average flux for the entire cruise. From measurements in the Gerlache Straits area of the Antarctic Peninsula, Berresheim (1987) calculated a flux of 2000 ng (S) m - 2 h - 1. This higher value reflects the greater phytoplankton activity in the region during March. The measurements collected during this cruise in the same region of the Antarctic Peninsula were taken during November, before the phytoplankton bloom had started and so the flux calculated is generally lower. Staubes and Georgii (1993) calculated a flux from DMS measurements over the ice covered Weddell Sea of 234 ng(S)m-2 h-1. They attributed this low value to

a reduced gas exchange due to the extensive sea ice c o v e r .

Methane sulfonate

Very low concentrations of methane sulfonate were observed from the U.K. through the Atlantic to 58°S (mean of 14 ngm-3), with much higher concentrations measured in the vicinity of the Antarctic Penin- sula and in the Weddell Sea (mean 103 ng m-3). The data are summarised in Table 1. The concentrations of MSA observed during the cruise with time and latitude are shown in Figs 5 and 6, respectively.

The detailed regression of individual DMS and MSA concentrations gave a very poor correlation (r 2 = 0.3, n = 100). This poor correlation might be expected given the inhomogeneous spatial nature of DMS production from phytoplankton and of its flux into the atmosphere, the variable lifetime of DMS in the atmosphere (which depends on the oxidant concentration), the transport of DMS and its oxidation products from source regions by advection to the


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Fig. 5. Concentrations of methane sulfonic acid in air with time during the cruise of the RRS Bransfield, 2 October 1992-10 January 1993.

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Fig. 6. Concentrations of methane sulfonic acid in air with latitude during the cruise of the RRS Bransfield, 2 October 1992-10 January 1993.

moving measurement station, the different atmospheric lifetimes of the gas-phase primary reactant and its aerosol-phase product and the different sampling averaging times used for DMS (3-4 h) and MSA (24 h). However, some broad agreement was seen between DMS and/vISA concentrations, with periods of enhanced DMS and MSA concentrations coinciding,

consistent with MSA production from the oxidation of DMS.

Non-sea-salt sulfate and sulfur dioxide

For the first part of the cruise through the North Atlantic a mean nssS concentration of 1.1 #gin -a (range 0.06-4.03 #g m - 3) was observed. These


concentrations are rather higher than the observa- tions of Harrison et al. in the vicinity of the Azores (personal communication) and of Luria et a l. (1989) in the vicinity of Bermuda. Strong northerly winds were encountered during some of this period and although precautions were taken to try and exclude local pollu- tion events from the ship the possibility of such events being overlooked cannot be ignored. Long-range transport of anthropogenic sulfur dioxide-derived sulfate from continental areas, emissions of which are enhanced during the winter, must be considered a possibility (the measurements of Harrison and Luria having been made during the summer months). The mean nssS concentrations measured south of the Falkland Islands, 0.78#gm -3 (range < 0.001-3.4 #g m -3) are similar to those reviewed by Fitzgerald (1991) for four studies in remote areas of the Pacific and Indian Oceans. Using model calculations for the mid-North Atlantic, Luria et al. (1989) estimate that DMS oxidation could give rise to about 1/~gm -3 sulfate in air, which suggests that the observed nssS in the Southern Ocean might all be due to DMS oxidation.

In areas north of 58°S, the ratio of MSA to nssS was very low (0.03). Air mass back trajectories for this period show surface winds were from the south running parallel to the coast. These air masses may have contained some continental contamination or contri- butions from oil and gas rigs positioned along the coast of South America. Further south the ratio was enhanced with a mean of 0.2. A maxima for this ratio of 0.7 was seen on the west side of the Peninsula, which is in close agreement to the value of 0.6 seen by Berresheim (1987) in the same area. Extremely wide ranges for this ratio have been previously observed (e.g. < 0.01-0.9 in the NE Pacific, Quinn et al., 1993), but this bulk aerosol-derived parameter may in fact obscure more useful size-dependent information which was not obtainable with the methods used here. The enhanced MSA/nssS ratio observed in the colder Antarctic air adds credence to a temperature-dependent oxidation pathway observed during laboratory kinetics and mechanism experiments (Hynes et al., 1986). The addition of OH to the DMS sulfur atom favoured at lower temperatures leads to an enhanced MSA yield (Yin et al., 1990a, b; Davison and Hewitt, 1994).

Sulfur dioxide concentrations observed for the period ranging from 0.1 to 1.6 ~gm -3 with a mean of 0.8 pgm -3. These were rather higher than those of Putaud et al. (1993) measured in the tropical northeastern Atlantic Ocean (mean 0.1 ~gm -3) and of Harrison, measured near the Azores (0.1-0.5 ~g m-3). Luria et al. (1989) found SO2 concentrations near Bermuda to be less than the detection limit of the method used (0.3klgm -3) but predicted higher concentrations of 0.8/~gm -3 from DMS oxidation. Considerable uncertainty still surrounds the concentrations and sources of SO2 in the maritime atmosphere. Whether or not the relatively high con-

centrations measured in this study are due to uniden- tified analytical problems (which cannot be ruled out), long-range transport of SO2 from continental areas or from DMS oxidation is not yet clear.

Other aerosol ions

Na + concentrations ranged from 0.25 to 15/~g m - 3 with a mean of 5.5 ~gm -3 (analyses carried out by both flame photometry and ion chromatography gave similar mean value). This is about twice the mean value reported by Putaud et al. (1993), from samples collected in calmer tropical North Atlantic waters, and is consistent with the higher wind speeds, and hence higher concentrations of sea spray, experienced during the cruise.

Some enhancement in Mg 2+, K + and particularly Ca 2 + concentrations in the aerosol, relative to their sea water concentrations, was observed for samples collected in the Atlantic from the U.K. to the Falk- land Islands. This suggests that long-range transport of continental crustal material might be important, even in the mid-Atlantic regions.

A high proportion of samples collected south of the Falkland Islands were deficient in non-sea salt Mg 2 +, K + and Ca 2+. This however may be related to the method for calculating non-sea-salt, being the differ- ence of two large numbers. The higher wind speeds found in this region of the Antarctic would cause a larger sea-salt contribution on the sampled filter, so increasing the error in the calculated non-sea-salt concentration. This aspect of the data requires further analysis.

CONCLUSIONS

The DMS concentrations obtained during the cruise of the RRS Bransfield in this study were as anticipated, with maximum concentrations occurring in the vicinity of the Antarctic Peninsula and in the Weddell Sea. A mean ocean to atmosphere flux rate of DMS of 1011 ng(S)m-2h -1 was calculated using a simple model of DMS oxidation rate and the marine boundary layer height derived from meteorological balloon launches from the ship. This gave values within the range of previous estimates (e.g. Erickson et al.,

1990). Enhanced methane sulfonate concentrations were also observed in the vicinity of the Antarctic Peninsula and in the Weddelt Sea, with a mean of 103 ng(S)m -3. Although broad agreement was seen between DMS and MSA concentrations, with maxima in both data sets observed in the same general areas, good correlation was not observed between individual data pairs. This may be due to the different lifetimes of these species in the atmosphere. The ratio of MSA to nssS was very low in areas north of 58°S but increased further south. A limitation of the data set was that the chemical composition of the aerosol was obtained from bulk filter samples; this may obscure more informative size-dependent information.


Acknowledgements--We thank Captain Stuart Lawrence and the officers and crew of the RRS Bransfield for their help during the cruise and the Natural Environment Research Council for funding. "We also thank Joyce Harris at NOAA for help with air mass back trajectories.

REFERENCES

Andreae M. O. (1986) The ocean as a source of biogenic gases. Oceanus 28, 27-35.

Andreae M. O. and Raemdonck H. (1983) Dimethyl sulfide in the surface ocean and the marine atmosphere: a global view. Science 211,744 747.

Andreae M. O., Ferek R. J., Bermond F., Byrd K. P., Engstrom R. T., Hardin S., Houmere P.D., LeMarrec F. and Raemdonck H. (1985) Dimethyl sulfide in the marine atmosphere. J. geophys. Res. 90, 12,891 12,900.

Andreae M. O., Berresheim H., Andreae T. W., Kritz M. A., Bates T. S. and Merrill J. T. (1988) Vertical distribution of dimethyl sulfide, sulfur dioxide, aerosol ions and radon over the northeast Pacific Ocean. Atmos. Chem. 6, 149-173.

Andreae T. W., Andreae M. O. and Schebeske G. (1994) Dimethylsulfide in seawater and in the atmospheric boundary layer. J. geophys. Res. 99, 22,819-22,829.

Ayers G. P. and Gras J. L. (1991) Seasonal relationships between cloud condensation nuclei and aerosol methane sulphonate in marine air. Nature 353, 834-835.

Barnard W. R., Andrcae M. O., Watkins W. E., Bingemer H. and Georgii H. W (1982) The flux of dimethyl sulphide from the oceans to the atmosphere. J. geophys. Res. 87, 8787 8793.

Bates T. S., Cline J. D., Gammon H. and KeUy-Hansen S. R. (1987) Regional and seasonal variations in the flux of oceanic dimethyl sulphide to the atmosphere. J. geophys. Res. 92, 2930-293~;.

Bates T. S., Kiene R. P., Wolfe G. V., Matrai P. A., Chavez F. P., Buck K. R., Blomquist B. W. and Cuhel R. L. (1994) The cycling of sulfur in surface seawater of NE Pacific. J. geophys. Res. 99, 7835-7843.

Berresheim H. (1987) Biogenic sulphur emissions from the subantarctic and Antarctic oceans. J. geophys. Res. 92, 13,245 13,262.

Brimblecombe P. and Shooter D. (1986) Photo-oxidation of dimethyl sulphide in aqueous solution. Mar. Chem. 19, 343--353.

Boers R., Ayers G. P. and Gras J. L. (1994) Coherence between seasonal variation in satellite-derived cloud op- tical depth and bc,undary layer CCN concentrations at a mid-latitude Southern Hemisphere station. Tellus 46B, 123-131.

Bunt J. S. (1968) Microalgae of the Antarctic pack ice zone. In Proc. Syrup. on Antarctic Oceanography, pp. 198-218. Scientific Committee for Antarctic Research (SCAR), Scott Polar Research Institute, Cambridge, England.

Charlson R. J., Lovelock J. E., Andreae M. O. and Warren S. G. (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655-661.

Davison B. and Hewitt C. N. (1992) Natural sulphur species from the North Atlantic and their contribution to the United Kingdom sulphur budget. J. geophys. Res. 97, 2475-2488.

Davison B. M. and Allen A. G. (1994) A method for sampling dimethylsulfide in polluted and remote marine atmo- spheres. Atmospheric Environment 28, 1721-1729.

Davison B. M. and Hewitt C. N. (1994) Elucidation of the tropospheric reactions of biogenic sulfur species from a field measurement campaign in NW Scotland. Chemo- sphere 28, 543 557.

Dickson D. M., Wyn Jones R. G. and Davenport J. (1980) Steady state osmotic adaption in ulva lactuca. Planta 150, 158-165.

Erickson D. J., Ghan S. J. and Penner J. E. (1990) Global ocean to atmosphere dimethyl sulfide flux. J. geophys. Res. 95, 7543-7552.

Falowski P., Kim Y., Kolber Z., Wilson C., Wirick C. and Cess R. (1992) Natural versus anthropogenic factors af- fecting low-level cloud albedo over the North Atlantic. Science 256, 1311-1313.

Ferek R. J., Chatfield R. B. and Andreae M. O. (1986) Vertical distribution of dimethyl sulphide in marine atmosphere. Nature 320, 514-516.

Ferek R. J., Hegg D. A., Herring J. A. and Hobbs P. V. (1991) An improved filter pack technique for airborne measurements of low concentrations of sulfur dioxide. J. geophys. Res. 96, 22,373-22,378.

Fitzgerald J. W. (1991) Marine aerosols, a review. Atmo- spheric Environment 25A, 533-545.

Fryxell G. A. and Kendrick G. A. (1988) Austral spring microalgae across the Weddell Sea ice edge: spatial relationships found along a northward transect during AMERIEZ 83. Deep-sea Res. 35, 1-20.

Gabric A., Murray N., Stone L. and Kohl M. (1993) Model- ling the production of DMS during a phytoplankton bloom. J. geophys. Res. 98, 22,805-22,816.

Hegg D., Radke L. and Hobbs P. (1991) Measurements of Aitken nuclei and cloud condensation nuclei in the marine atmosphere and their relationship to the DMS-cloud- climate hypothesis. J. geophys. Res. 96, 18,727-18,733.

Hewitt C. N. and Davison B. M. (1988) The lifetimes of organosulphur compounds in the troposphere. App. Or- ganometallic Chem. 2, 407-415.

Hobbs P. V., Hegg D. A. and Ferek R. J. (1993) Recent field measurements of sulfur gases: particles and clouds in clean marine air and their significance with respect to the DMS- cloud-climate hypothesis. In Dimethyl Sulfide, Oceans At- mosphere and Climate (edited by Restelli G. and Angeletti G.). Kluwer, Dordrecht.

Hynes A. J. Wine P. H. and Semmes D. H. (1986) Kinetics and mechanism of OH reactions with organic sulphides. J. phys. Chem. 90, 4148-4156.

Krebs W. N. (1983) Ecology of neritic marine diatoms, Ar- thur Harbor, Antarctica. Micropaleontology 29, 267-297.

Leakey R. (1991) Signy Phytoplankton. British Antarctic Survey, Cambridge.

Leck C., Larsson U., B~gander L.E., Johansson S. and Hajdu S. (1990) DMS in the Baltic Sea: annula variability in relation to biological activity. J. geophys. Res. 95, 3353-3363.

Liss P. S. and Slater P. G. (1974) Flux of gases across the air-sea interface. Nature 247, 181-184.

Liss P. S., Malin G., Turner S. M. and Holligan P. M. (1994) Dimethyl sulphide and phaeocystis: a review. J. Marine Sys. 5, 41-53.

Logan J. A., Prather M. J., Wofsy S. C. and McElroy M. B. (1981) Tropospheric chemistry: a global perspective. J. geophys. Res. 86, 7210-7254.

Luria M., Van Valin C. C., Wellman D. L. and Pueschel R. F. (1986) Contribution of the Gulf area natural sulphur to the North American sulphur budget. Envir. Sci. Technol. 20, 91-95.

Luria M., Van Valin C. C., Galloway J. N., Keenes W. C., Wellmann D. L., Sievering H. and Boatman J. F. (1989) The relationship between dimethyl sulphide and particulate sulphate in the mid Atlantic Ocean atmosphere. Atmospheric Environment 23, 139-147.

Putaud J.-P., Belviso S., Nguyen B. C. and Mihalopoulos N. (1993) Dimethylsulfide, aerosols and condensation nuclei over the tropical Northeastern Atlantic Ocean. J. geophys. Res. 98, 14,863-14,871.

Quinn P. K., Covert D. S., Bates T. S., Kapustin V. N., Ramsey-Bell D. C. and Mclnnes L. M. (1993) Dimethyl-


sulfide/cloud condensation nuclei/climate system: relevant size-resolved measurements of the chemical and physical properties of atmospheric aerosol particles. J. yeophys. Res. 98, 10,411-10,427.

Rodhe H. and Langner J. (1993) Atmospheric concentrations of DMS and its oxidation products estimated in a global 3D model. In Dimethylsulfide, Oceans, Atmosphere and Climate (edited by Restelli G. and Angeletti G.). Kluwer, Dordrecht.

Saltzman E. S. and Cooper D. J. (1988) Shipboard measurements of atmospheric dimethyl sulfide and hydrogen sulfide in the Caribbean and Gulf of Mexico. J. atmos. Chem. 7, 191-209.

Schwartz S. E. (1988) Are global cloud albedo and climate controlled by marine phytoplankton? Nature 336, 441-445.

Staubes R. and Georgii H. W. (1993) Biogenic sulfur compounds in sea-water and the atmosphere of the Antarctic region. Tellus 45B, 127-137.

Turner S. M., Malin G. and Liss P. S. (1989) Dimethyl sulphide and (dimethylsulphonio)propionate in European coastal and shelf waters. Biogenic Sulphur in the Environ- ment. ACS Symposium Series 393.

Walton Smith F. G. (1974) Handbook of Marine Sciences, 620 pp. CRC Press, Cleveland.

Yin F., Grosjean D. and Seinfeld J. H. (1990a) Photooxida- tion of dimethyl sulphide and dimethyl disulphide. 1: Mechanism development. J. atmos. Chem. 11, 309-364.

Yin F., Grosjean D., Flagan R. C. and Seinfeld J. H. (1990b) Photooxidation of dimethyl sulphide and dimethyl disulphide. 11: mechanism evaluation. J. atmos. Chem. 11, 365 399.

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DIMETHYL SULFIDE AND ITS OXIDATION …...Dimethyl sulfide and its oxidation products in the atmosphere 1897 atmosphere. Filters were stored refrigerated in double poly- thene bags