Seasalt aerosols

The major ions have two main escape routes from the ocean (1) incorporation into sediments or pore water and (2) ejection into the atmosphere as seasalt spray. This spray is caused by bursting bubbles that produce small particles, called aerosols, that range in diameter from 0.1 to 1000 pm. The annual production rate of seasalt aerosols is large, on the order of 5 x lO kg/y, but virtually all of it is quickly returned when the spray fells back onto the sea surfece. A small fraction (about 1%) is deposited on the coastal portions of land masses and carried back into the ocean by river runoff. As shown in Table 21.6, seasalts represent a significant fraction of dissolved solids in river runoff, especially for sodium and chloride. Due to the short timescale of this process, seasalt aerosol losses and inputs are considered by geochemists to be a short circuit in the crustal-ocean-atmosphere fectory. The solutes transported by this process are collectively referred to as the cyclic salts. ... 

Chameides, W. L., and A. W. Stelson, Aqueous-Phase Chemical Processes in Deliquescent Seasalt Aerosols, Ber. Bunsenges. Chem., 96, 461-470 (1992a). 

Heterogeneous reactions on the surface of seasalt aerosols have been suggested as a potential source of atomic chlorine in the marine boundary layer [10,105]. The tropospheric relevance of the reaction of nitrogen oxide species N203 with Nad and NaBr was demonstrated in a smog chamber experiment on dry and deliquiescent NaCl aerosol and on salt solutions [74,78,106,107] ... 

In continental air sulfate tends to be associated with finer particles, and as ammonia is more likely to be present in the air this can neutralize the sulfuric acid with the formation of ammonium sulfate- or bisulfate-containing particles over land. Sulfuric acid can displace chloride from seasalt aerosols and represent a source of hydrogen chloride ... 

Sea salt contains (by weight) 55.7% Cl, 0.19% 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 seasalt 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. 

Reactions of the nitrogen oxides N02 or N205 with NaX contained in seasalt aerosol can lead to the formation of XNO or XN02, respectively, for example... 

Hydrogen halides can be liberated from seasalt aerosol by the action of strong acids, such as H2S04 and HN03 ... 

Chlorine in Nature. Volcanic eruptions are the sources of great masses of hydrogen chloride, they also contain free chlorine gas. Breaking waves and winds over the oceans produce large amounts of seasalt aerosols in the atmosphere, corresponding to ten billion tons of chloride per year. These aerosols are partly deposited on the continents. 

Examples of inferred enhancements of atmospheric primary aerosol concentration in the glacial atmosphere relative to the modem are factors of 4 to 7 for insoluble particles from continents, and 3 for seasalts (Alley et al, 1995), over Greenland. 

To determine the non-seasalt (nSS) contributions of trace elements we have normalized mean concentrations of ions to Cl and subtracted the contributions from seasalt (Nozaki, 1997). We assume that all of the Cl" in the sample was from marine aerosol or marine-derived salt and nSS major ions and trace elements can be calculated. For example the percentage of nSS-S042 is calculated as ... 

The first three components suggest regional sources of acidic anthropogenic aerosol, the marker elements of a copper smelter, and seasalt, respectively. The fourth component or the ammonium In component three do not provide a ready Interpretation of a known emission or meteorological source of variability. The negative correlation of nitrate with component two Is consistent with separate Influences of the copper smelter and automobile emissions. 

Vogt, R P.J, Crntzen, and R. Sander (1996) A new mechanism for halogen release from seasalt-salt aerosol in the remote marine boundary layer. Nature, 383, 327-330. 

Tables on the "reaction probalility or "uptake coefficient" have been summarized for various heterogeneous reactions in a recent review article [87], and by the IUPAC [88] and NASA-JPL [86] evaluation teams. For the purpose of this article, a rough comparison is made of the uptake rates for the reactions (1) to (5) on the different type surfaces. Three major type of surfaces have been considered a) NAT, or Type I PSC, b) Water ice, or Type II PSC and c) sulfuric acid aerosol, which is normally a liquid surface generally composed of 60-80 wt % H,S04 and 40-20 wt % H,0 also considered is the solid form SAT (sulfuric acid tetrahydrate) with a composition of 57.5 wt % H,S04. The importance of chlorine activation on sulfuric acid solutions has been demonstrated in a recent article [89]. Halogen activation on seasalt material will shortly be reviewed as part of the tropospheric processes.

The two EUSAAR/ACTRIS stations in the Britain and Irish stations show not only a significant inter-station variability, but also many similarities. The stations have high seasonal variation and large variance in intra-seasonal concentrations. The station data show a prominent spring-summer maximum in all sizes from 30 to 70 nm. The variability was probably due to occurrences of both clean Atlantic and polluted local air masses, and the maximum value at Mace Head during summer months can be attributed to enchanted marine biota activity, which increases the sub-micron particle mass concentration of non-seasalt sulfate and organic aerosol [21],... 

Atmospheric Aerosol Non-Seasalt Sulfate Non-Remote Marine -10 to +13 San Francisco Bay -10 to +12 N.W. Atlantic + 7 to +13 HBEF, non-urban US +1 to +4 Miami, Florida +1 to +2 Mauna Loa Observatory +4 to +6 Ludwig (22) Gravenhorst (22) Saltzman et al. (24) Saltzman (pers. commun.) Zoller Kelly (pers. commun.)... 

Atmospheric Aerosol Sulfate. Isotope measurements of non-seasalt sulfate in marine aerosols (24.52.631 require that sulfate from sea spray be either physically or mathematically removed from the sample medium. Mathematically, mass balance relationships are used to correct the value for the presence of seasalt sulfate in the sample. Physical means employ impactors or cyclone separators to segregate particles based on size so that value for non-seasalt sulfate can be directly measured. 

The sulfate that derives from seasalt is called seasalt sulfate. It is important to distinguish it from sulfates that are found in the marine aerosol from the oxidation of gaseous sulfur compounds, which are known as non-seasalt sulfate. 

Dimethylsulfoxide can readily be removed onto particles and there it can undergo an efficient oxidation through to methanesulfonate. This adds a significant pathway to the gas-phase production of methanesulfonic acid, which is present largely in the submicron aerosol fraction. Peak summer concentrations are 0.6 0.3 nmol m and at times this can amount to almost a quarter of the non-seasalt sulfate in the remote marine atmosphere (Jourdain and Legrand, 2001). 

Jourdain B. and Legrand M. (2001) Seasonal variations of atmospheric dimethylsulfide, dimethylsulfoxide, sulfur dioxide, methanesulfonate, and non-seasalt sulfate aerosols at Dumont d Urville (coastal Antarctica) (December 1998 to July 1999). J. Geophys. Res. Atmos. 106(D13), 14391-14408. 

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