Some sources produce aerosols both by mechanical processes and by gas-to-particle conversion, the oceans being a good example of this. In addition to the mechanically generated sea salt aerosol, the oceans emit gaseous compounds that can be oxidized and eventually produce aerosol particles. The most important chemicals in this case contain sulfur, especially dimethylsulfide (DMS), methanesulfonic acid (MSA), and sulfate. Interest in the marine sulfur cycle increased enormously after a hypothesis was proposed connecting oceanic phytoplankton to aerosols to clouds and hence to climate (Charlson et al., 1987). In this hypothesis, DMS produced by marine phytoplankton evades from the ocean and is oxidized to sulfate aerosol (and MSA), which can act as cloud condensation nuclei (CCN). The number of CCN in the atmosphere affects the reflectivity of clouds (the cloud albedo), and in this way oceanic emissions of reduced sulfur gases are linked to climate. Sulfate aerosols, whether marine derived or originating from continental emissions, also can reflect solar radiation back to space, and in so doing influence weather and climate directly.
There is at present no universally accepted chemical mechanism for the formation of NSS sulfate via DMS oxidation. One of the controversies that arose with respect to the oxidation of DMS was whether sulfur dioxide (S02) was an important intermediate in the pathway leading to sulfate aerosol (Bandy et al., 1992; Lin and Chameides, 1993). Recent studies indicate that the pathway from DMS to NSS sulfate does indeed include S02 as an intermediate, but our knowledge of the other compounds and reactions in the pathway is far from complete (Keene et al., 1998). Whether from DMS or from pollution sources, S02 can be further oxidized both in the gas and liquid phase to H2S04, but these processes are sensitive to gasphase nitric acid and ammonia concentrations (Clegg and Toumi, 1997), again demonstrating links between the chemistry of aerosols and gaseous species.
Over vast areas of Earth's oceans, from about 30°N to 30°S, the ratio of biogenic NSS sulfate to MSA tends to be constant, so much so that a MSA/NSS sulfate mass ratio of ~ 18 to 20 has been used as a diagnostic for marine biogenic sulfate (Savoie and Prospero, 1989; Savoie et al., 1994; Arimoto et al., 1996). The relative amounts of MSA and NSS sulfate do change with latitude however; above — 30° (N or S) more of the biogenic sulfur occurs as MSA (Berresheim, 1987; Pszenny et al., 1989; Koga et al., 1991; Bates et al., 1992). One of the central issues of the marine sulfur cycle currently being investigated is the extent to which this latitudinal dependence in MSA/NSS sulfate ratios is driven by the temperature dependencies of various reactions in the DMS oxidation pathways versus the influences of other photoche-mically active compounds.
In the marine atmosphere gaseous H2S04 either can deposit on existing surfaces, again potentially involving sea salt, or it can form new sulfate particles via homogeneous nucleation. A fundamental issue concerning the marine sulfur cycle has to do with where new aerosol sulfate particles form. The cycling and fate of particles formed in the marine boundary layer (MBL) would be far different from those formed in the free troposphere, and therefore this issue has important implications for the direct and indirect effects of the aerosols on solar radiation. While new particles form in the marine boundary layer under certain conditions (Covert et al., 1992), most of the new particle production evidently occurs in the proximity of clouds (Hegg et al., 1990; Perry and Hobbs, 1994). Recent studies conducted for ACE-1 (aerosol characterization experiment) showed that few new particles formed in the MBL over the Southern Ocean south of Australia (Clarke et al., 1997). Instead layers of new particles, DMS, MSA, and H2S04 were observed in the free troposphere in the outflow regions of clouds at altitudes of several kilometers. Andreae and Crutzen (1997) suggest the DMS-aerosol-climate connection may still pertain because the subsidence of aerosol-laden air from the free troposphere into the MBL can supply particles that are initially too small to act as CCN but through heterogeneous or cloud processes can grow and become CCN.
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