Aerosol particles seawater

This simple two component model for the Fe isotope composition of seawater does not consider the effects of the Fe isotope composition of dissolved Fe from rivers or from rain. Although the dissolved Fe fluxes are small (Fig. 19) the dissolved fluxes may have an important control on the overall Fe isotope composition of the oceans if they represent an Fe source that is preferentially added to the hydrogenous Fe budget that is ultimately sequestered into Fe-Mn nodules. In particular riverine components may be very important in the Pacific Ocean where a significant amount of Fe to the oceans can be delivered from rivers that drain oceanic islands (Sholkovitz et al. 1999). An additional uncertainty lies in how Fe from particulate matter is utilized in seawater. For example, does the solubilization of Fe from aerosol particles result in a significant Fe isotope fractionation, and does Fe speciation lead to Fe isotope fractionation ... 

Figure 20. Calculated Fe isotope composition of seawater from different ocean basins based on a simple two-component mixing between Fe from aerosol particles and Fe from mid-oceanic-ridge (MOR) hydrothermal solutions. Atmospheric Fe fluxes (Jatm) for different ocean basins from Duce and Tindale (1991) MOR hydrothermal Fe flux ( mor) to different ocean basins were proportioned relative to ridge-axis length. Modified from Beard et al. (2003a).

Arctic at polar sunrise. The mechanism likely involves regeneration of photochemically active bromine via heterogeneous reactions on aerosol particles, the snow-pack, and/or frozen seawater. The source of the bromine is likely sea salt, but the nature of the reactions initiating this ozone loss remains to be identified. For a review, see the volume edited by Niki and Becker (1993) and an issue of Tellus (Barrie and Platt, 1997). 

In short, although photochemistry in bulk seawater has been extensively studied, very little is known about the micro layer. Even less is known of the photochemical reactions that may be occurring in aerosol particles. Since marine aerosol particles may undergo wet deposition as rain, fog droplets, or cloud waters, photochemical studies of these atmospheric waters over the ocean and in coastal areas can shed some light on potential aerosol processes. Since hydrogen peroxide is formed from the photolysis of DOM and nitrates in aqueous solutions, its presence may serve as a potential marker for the occurrence of these photoreactions within aerosol particles or atmospheric water droplets. Peroxides also play significant roles in oxidation reactions and OH production in atmospheric waters. 

Taketani, F., Kanaya, Y., Akimoto, H. Heterogeneous loss of HO2 by KCl, synthetic sea salt, and natural seawater aerosol particles. Atmos. Environ. 43, 1660-1665 (2009)... 

The concentration of metals in atmospheric aerosols and rainwater (Table 7.1) is therefore a function of their sources. This includes both the occurrence of the metals in combustion processes and their volatility, as well as their occurrence in crustal dust and seawater. As a result of this, the size distribution of different metals is very different and depends on the balance of these sources. For a particular metal this distinction is similar in most global locations (Table 7.2), although some variability does occur as wind speed and distance from source exert an influence on the particle size distribution spectrum (Slinn, 1983). Once in the atmosphere particles can change size and composition to some extent by condensation of water vapour, by coagulation with other particles, by chemical reaction, or by activation (when supersaturated) to become cloud or fog droplets (Andreae et al., 1986 Arimoto et al., 1997 Seinfeld and Pandis, 1998). 

The seas are a source of aerosol (i.e. small particles), which transfer to the atmosphere. These will subsequently deposit, possibly after chemical modification, either back in the sea (the major part) or on land (the minor part). Marine aerosol comprises largely unfractionated seawater, but may also contain some abnormally enriched components. One example of abnormal enrichment occurs on the eastern coast of the Irish Sea. Liquid effluents from the Sellafield nuclear fuel reprocessing plant in west Cumbria are discharged into the Irish Sea by pipeline. At one time, permitted discharges were appreciable and as a result radioisotopes such as Cs and several isotopes of plutonium have accumulated in the waters and sediments of the Irish Sea. A small fraction of these radioisotopes were carried back inland in marine aerosol and deposited predominantly in the coastal zone. While the abundance of Cs in marine aerosol was refiective only of its abundance in seawater (an enrichment factor - see Chapter 4 - of close to unity), plutonium was abnormally enriched due to selective incorporation of small suspended sediment particles in the aerosol. This has manifested itself in enrichment of plutonium in soils on the west Cumbrian coast,shown as contours of 239+240p deposition (pCi cm ) to soil in Figure 3. 

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... 

Analogous to the process releasing Ra to seawater, decay of Th in sediments releases dissolved Ra which is then mixed into the ocean interior. Radium-226 decays through a series of short-lived nuclides to Pb (half-life 22.3 yr) which, like thorium and protactinium, is insoluble and readily sorbs to particles. Radioactive decay of gaseous Rn in the atmosphere also produces Pb, which is then deposited on the sea surface with aerosols and in precipitation. Although Pb and, to a lesser extent, Pa have found many applications as tracers of particle transport, by far the greatest use has been made of thorium isotopes, which form the focus of this review. 

Radke, 1981 Radke et al, 1976). Although these small particles contribute little to the total aerosol number density (see Fig. 7-1), their origin is still undetermined. The problem is that bubbles with radii much smaller than 50p.m dissolve too rapidly in seawater. Blanchard and Woodcock (1980) suggest that the absorption of organic matter onto the bubble surface stabilizes some of the bubbles in the 10 p.m size range and prevents their dissolution. The possibility that these bubbles are responsible for the production of sea-salt particles in the submicrometer size range must be considered. 

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. 

In coastal seawater such as the Western Mediterranean basin, soil-derived particles originated from arid areas (in this case the Sahara). The atmospheric flux of anthropogenic trace metals, however, was dominated by aerosols from industrialized regions of Western Europe. Volcanic activity (Mount Etna) contributes selenium. The atmospheric input of Cr, Hg, Pb, and Zn into the Western Mediterranean basin is of the same order of magnitude as the riverine and coastal inputs of these components (Arnold et al. 1983). For the southern bight of the North Sea, estimates even indicate a predominance of the atmospheric input of... 

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