By using a polyester sailboat as sampling platform, a series of duplicate aerosol samples was collected by cascade impactors on a trip from Panama to Tahiti in 1979. Elemental analysis mainly by particle‐induced X ray emission (PIXE) indicated, in the samples collected between Panama and the Galapagos Islands, the presence of a substantial crustal component (∼0.4 μg/m 3 ), fine Cu (∼0.4 ng/m 3 ) and Zn (∼0.6 ng/m 3 ), and excess fine S and K (∼100 and ∼2.4 ng/m 3 , respectively) in addition to the major sea salt elements. The crustal component and fine Cu and Zn are suggested to result from natural continental sources (i.e., eolian dust transport from the American continents and perhaps geothermal emissions). Samples collected west of the Galapagos Islands in the southern trades showed significantly lower concentrations for the nonseawater components. The average Si and Fe levels were as low as 4.8 and 3.3 ng/m 3 , corresponding to a maximum of 0.066 μg/m 3 for an assumed mineral dust component, whereas heavy metal concentrations were all below the detection limits (typically ranging from 0.05 to 0.15 ng/m 3 for V, Cr, Mn, Ni, Cu, Zn, and Se). Excess fine S decreased to a mean of 46 ng/m 3 , a level similar to those reported for other remote marine and continental locations. This all indicates that the marine atmosphere west of the Galapagos was little influenced by natural continental source processes or by anthropogenic emissions. Under these truly marine conditions, several concentration ratios of the major seawater elements were significantly different from those in bulk seawater. Ca, Sr, and S in >1 μm diameter particles were enriched relative to K and Na, with the enrichment being substantially more pronounced (up to 50% or higher) for l–4‐μm diameter particles than for particles >4 μm. Comparison of these data with a similar data set from samples collected over the Atlantic indicates that the departures from seawater composition are significantly larger for the Pacific. Differences in sea‐to‐air fractionation processes, probably involving binding of divalent cations to organic matter in the oceanic surface microlayer, are suggested as being responsible for these observations.
We have performed over 900 measurements of atmospheric dimethyl sulfide (DMS) in five different marine locations: the equatorial Pacific; Cape Grim, Tasmania; the Bahamas; the North Atlantic; and the Sargasso Sea. At all locations, DMS concentrations were usually in the range of 100–400 ng S m −3 , with similar average concentrations of approximately 150 ng S m −3 (107 parts per thousand by volume). Highest concentrations occurred during, but were not limited to, periods of sustained high winds and overcast skies, presumably owing to faster exchange from surface seawater and less photochemical activity in the atmosphere. Lowest values occurred during airflow from continental regions, which provides higher levels of oxidants and free radicals to react with DMS. Averaged over time, the concentrations in clean marine air reached a maximum at night and a minimum in the afternoon, when concentrations were about one third lower than during the nighttime maximum. The observed concentrations of DMS in the marine atmosphere and their diurnal variability agree well with model simulations involving OH and NO 3 oxidation of DMS and are consistent with a global sea‐to‐air DMS flux of about 40±20 Tg S yr −1 . DMS may represent a major sink for NO 3 in the marine troposphere.
Dimethyl sulfide (DMS) has been identified as the major volatile sulfur compound in 628 samples of surface seawater representing most of the major oceanic ecozones. In at least three respects, its vertical distribution, its local patchiness, and its distribution in oceanic ecozones, the concentration of DMS in the sea exhibits a pattern similar to that of primary production. The global weighted-average concentration of DMS in surface seawater is 102 nanograms of sulfur (DMS) per liter, corresponding to a global sea-to-air flux of 39 × 10 12 grams of sulfur per year. When the biogenic sulfur contributions from the land surface are added, the biogenic sulfur gas flux is approximately equal to the anthropogenic flux of sulfur dioxide. The DMS concentration in air over the equatorial Pacific varies diurnally between 120 and 200 nanograms of sulfur (DMS) per cubic meter, in agreement with the predictions of photochemical models. The estimated source flux of DMS from the oceans to the marine atmosphere is in agreement with independently obtained estimates of the removal fluxes of DMS and its oxidation products from the atmosphere.
Aerosols from the marine boundary layer were collected during a cruise from the Peru/Ecuador shelf through the equatorial and tropical Pacific to the Hawaiian Islands. A variety of samplers of different design and independent analytical techniques were used to validate the results obtained. On large particles, little or no chloride deficiency relative to seawater composition was observed, while on submicrometer particles, as much as 40% of chloride had been lost. A silicate component with a composition similar to the crustal average was associated with sea‐salt particles as an internal mixture. Along the Peru coast, substantial concentrations of Cu, Zn, As, and Pb were found which must be related to large emissions from the metallurgical industry in that region. High levels of selenium in the remote equatorial region suggest a biological source of volatile selenium in this area of high productivity. The concentrations of excess sulfate were found to be consistent with the flux of dimethylsulfide determined on the same cruise. The presence of soot carbon, particulate organic carbon, and fine excess potassium showed evidence of long‐range transport of combustion‐derived aerosols to this remote region. Enhanced soot and crustal elements in the Intertropical Convergence Zone suggest downward mixing of materials transported in the upper troposphere in this region of active vertical exchange.
