Salt aerosols, enrichments

Sea salt aerosol initially consists mainly of seawater (see Table 1). Organic carbon is present in sea salt particles as well, typically enriched in smaller sea salt aerosols compared to bulk seawater carbon (e.g., Blanchard, 1964 Hoffman and Duce, 1977 Blanchard and Woodcock, 1980 Middlebrook et al., 1998 Turekian et al., 2003). This organic carbon originates from three... 

Volpe et al. (1998) analyzed the isotopic composition of chlorine in marine aerosol and found enrichment of Cl, i.e., the heavier chlorine isotope, in chlorine depleted aerosol, as expected, thereby providing further evidence for acid displacement in mid-size sea salt aerosols (mean aerodynamic diameter up to 2 p.m). In larger sea salt aerosol (mean aerodynamic diameter between... 

As shown by Knipping et al. (2000) and Knipping and Dabdub (2002) for chlorine, surface segregation has the potential of increasing the release of halogens from sea salt aerosol. This should be pursued further, especially for Br and I , where the surface enrichment effects are predicted to be highest. These effects also likely play a role in polar ODEs. 

From inspection of the more recent sea salt aerosol data, it appears that only soluble salt or total aerosols compensated for dust (39,40) were analyzed. These show definite trends in ion enrichments when normalized to Na and compared with sea water ratios. Even with the strict requirements of what the authors consider to be a pure marine sea salt aerosol, the majority of aerosols collected by various workers show elemental ratios different from those of sea water. 

Sea salt contains (by weight) 55.1% Cl, 0.199 Br, and 0.00002% I. Depletion of the Cl and Br content of marine aerosol relative to bulk seawater, as measured by Cl/Na and Br/Na ratios, indicates that there is some net flux of these two halogens to the gas phase. Interestingly, the ratio I/Na in marine aerosol is typically much greater than that in seawater, often by a factor of 1000. The large enrichment for iodine in sea salt aerosols relative to seawater has been attributed, in part, to the enhanced level of organic I compounds in the surface organic layer on the ocean that become incorporated in the aerosol formation mechanism. 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

The possibility that the depletion of chloride in the marine aerosol is due to fractionation during the formation of sea-salt particles by bursting bubbles can be discounted. Laboratory studies of Chesselet et al (1972b) and Wilkness and Bressan (1972) showed no deviation of the Cl /Na+ mass ratio from seawater in the bubble-produced sea-salt particles. It may be mentioned in passing that bromide in marine aerosols shows a deficit similar to chloride, whereas iodide is present in excess. The latter observation is attributed to both chemical enrichment at the sea s surface and scavenging of iodine from the gas phase. A portion of iodine is released from the ocean as methyl iodide, which in the atmosphere is subject to photodecomposition and thereby provides a source of scavengable iodine. The process has been reviewed by Duce and Hoffman (1976). In continental aerosols, chloride and bromide are partly remnants of sea salt, but there exists also a contribution from the gas phase. 

Hoffman et al. (1974) found the same procedure applicable to data obtained from measurements on board of ships in the central Atlantic Ocean. Table 7-15 includes mean (X)/(Na) ratios from their work. Shown in parentheses are the values derived from the slopes of regression lines. They are distinctly lower than the averaged data. Hoffman et al. (1974) measured also the abundance of iron in the aerosols. Since the samples were taken in a region partly affected by fallout from the Saharan dust plume, iron serves as a convenient indicator for the contribution of material from continental sources. Not surprisingly, the enrichment of the elements Mg, Ca, K, and Sr was well correlated with the iron content. The (X)/(Na) ratios approached those of sea water only when the Fe concentrations were very low. These results demonstrate that materials from both marine and crustal sources are present over the open ocean. In addition, they provide some verification for the existence of a tropospheric background aerosol having the continents as a source, and they confirm the absence of a significant fractionation of alkali and alkaline earth elements in the production of sea salt. 

---