Surface winds cause the production of aerosols from the sea as well as from the land. The effects of the wind over the ocean are mediated by breaking waves, bursting bubbles, and to a lesser extent the formation of large spume droplets torn from waves by strong winds. The seasalt injected into the atmosphere is another of the large sources for aerosols on a mass basis, of the order of 1000 to 10,000 Tg/yr (Blanchard, 1983). One reason for the large uncertainty in this estimate is that there is no strict definition of what constitutes a sea salt aerosol particle, i.e., very large particles have such short atmospheric residence times that one might question whether they are truly suspended in the atmosphere.
The production of sea salt particles is mainly due to oceanic whitecaps. Bubbles from the whitecaps burst at the sea surface producing film droplets and jet droplets, which have quite different properties and whose proportions vary as a function of bubble size (Blanchard, 1980). Model estimates by Erickson and Duce (1988) indicate the mass median radii (MMR) for sea salt over the oceans (50% of the sea salt mass occurs on particles smaller than the MMR and 50% on particles larger than the MMR) should range between 3.0 and 7.5 m, which is in good agreement with observations. Both the amount of salt produced and the sizes of the particles vary in response to wind speed, but a consideration of the relative amounts of ocean and land covering Earth's surface together with the atmospheric loadings of dust and sea salt shows the production of sea salt particles may be less efficient than dust.
For many substances, including sulfate, the composition of fresh, bulk, sea salt aerosols is similar to that of seawater, but as noted for mineral aerosol, one must recognize that the elemental composition of individual sea salt particles may be quite different from that of the bulk aerosol. Once the jet and film drops are ejected into the atmosphere, the water in them begins to evaporate, leading to a droplet of high ionic strength, which can undergo repeated cycles of dilution and concentration. Fractionary recrystallization within the evaporating drops can be followed by shattering of the particles, and this process can lead to variations in the content of elements such as Mg, S, K, and Ca among individual sea salt particles (Mouri et al., 1993).
Some substances, including some heavy metals, organic matter, radionuclides, and nutrient species are enriched in the sea salt aerosols as a result of the scavenging of surface-active material as bubbles pass through the water column and rupture the sea surface microlayer (e.g., Maclntyre 1974; Wallace and Duce, 1975). The enrich ments of trace elements in experimentally produced sea salt particles can reach several tens of thousands (Weisel et al., 1983), and this enrichment process can lead to a recycling of material between the surface ocean and the atmospheric marine boundary layer.
Sea-salt particles contain variable amounts of organic material, and large salt particles from over the remote oceans typically contain organic carbon with an isotopic composition similar to that of source materials in seawater (Buat-Menard et al., 1989). The carbon concentrations in sea salt particles (normalized to Na) are several 100-fold higher than that of seawater, presumably as a result of the same physicochemical processes causing the enrichments of inorganic materials (Wallace and Duce, 1975). In some areas of the oceans, terrestrial sources also can contribute significant amounts of carbon to large particles, but those continental sources, whether anthropogenic or natural, are more important for submicrometer marine aerosols.
Although the organic composition of marine aerosols is only partially characterized at best, numerous organic compounds have been detected in marine aerosols. For example, Peltzer and Gagosian (1989) investigated aliphatic hydrocarbons, wax esters, fatty alcohols, sterols, fatty acids, and long-chain unsaturated ketones in aerosols from several sites in the Pacific Ocean. These compounds were used as biomarkers in studies of the sources, transport, and transformation of organic material in the marine atmosphere. Kawamura and Usukura (1993) investigated dicar-boxylic acids in the western North Pacific, and they concluded the diacids were mainly from Asia, but some diacids were produced by photochemical reactions in situ.
Sea-salt particles also react with a variety of gaseous components of the marine atmosphere; most notably HN03, methanesulfonic acid, and H2S04 are sorbed by liquid sea salt particles and HC1 and HF are displaced (Ericksson, 1960; Okada et al., 1978). The modeling of acid displacement reactions is made difficult by the nonideal behavior of the high solute concentrations in the sea salt droplets (Brimblecombe and Clegg, 1988). However, analyses of individual particles from the North Atlantic suggest that CI loss from sea salt can be accompanied by the formation of NaN03 (Posfai et al., 1995). Reactions of sea salt with various N gases, such as N02, C1N03, and N205, have been observed, and those reactions could lead to the formation of NaN03 (Schroeder and Urone, 1974; Finlayson-Pitts et al., 1989; Keene et al., 1990). Such reactions also could generate reactive CI atoms; and analogous to the hydroxyl radical, the atomic chlorine would participate in photochemical reactions with various organic substances.
Other aqueous-phase reactions in sea salt aerosols involve the oxidation of S02 by 03 to non-sea-salt (NSS) sulfate (i.e., the sulfate in excess of what can be attributed to sea salt from unfractionated seawater) (Sievering et al., 1992, 1995; Chameides and Stelson, 1992). Recent analyses by Keene et al. (1998) indicate that the oxidation of S02 by ozone in sea salt aerosols is only a minor source for NSS sulfate, and these authors suggested that oxidation of S02 primarily occurs via another pathway, possibly involving HOX, where X is chlorine or bromine. As S02 originates from anthropogenic as well as natural sources, these reactions not only show how heterogeneous reactions can affect the composition of aerosols but also illustrate how intimately the chemistry of pollutants can be linked with the cycles of natural substances in the atmosphere.
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