Seasalt particles

Finally, there is a major flux of sulfur through the atmosphere in both seasalt particles ( 140 Tg S/yr) and terrestrial dust ( 20 Tg S/ yr). In each case, the form of sulfur is sulfate, originating mostly as the mineral gypsum in the... 

Thus N20, may react with Nad (and NaBr) on seasalt particles in polluted marine areas as well as in the arctic regions. The observed uptake coefficients, with 3xl0"J, imply that the reaction is sufficient fast and may lead to the formation of CINO, overnight C1N02 will photolyze at dawn to give d-atoms which will initiate the oxidation of organic compounds in a manner analoguous to OH radicals. Also the uptake of HNO, on salt powders (NaCl, NaBr) was measured and the release of Hd and HBr observed [108,109]. The latter compounds may release d or Br in the troposphere upon reaction with OH radicals. 

While the data in Fig. 7-13 cover only particles with radii larger than 1 pirn, there can be no doubt that the size spectrum extends toward smaller values. Seasalt particles as small as 0.05 p,m have been detected by electron microscopy (Meszaros and Vissy, 1974) and flame scintillation photometry... 

An overall mechanism for the DMS-OH reaction is shown in Figure 6.19. Many of the details of this mechanism are still uncertain. The principal stable products of the oxidation are DMSO, DMS02, MSA, S02, and H2S04. The ratio of MSA to SO4- is indicative of the path that is followed subsequent to formation of CH3S in the abstraction branch. This ratio is measured in the marine atmosphere as the ratio of MSA to nonseasalt sulfate (nss-S04). (Nonseasalt sulfate, the amount of sulfate present in particles in excess of that expected from seasalt particles, is a direct measure of the sulfuric acid formed.) Measurements as a function of latitude indicate that this ratio is quite temperature-dependent, with the ratio varying from about 0.1 near the equator to close to 0.4 in Antarctic waters. Thus the colder the temperature, the more favored the path to MSA formation as opposed to that to S02 and eventually to sulfuric acid. This behavior is consistent with a competition between a radical decomposition step, with a fairly large activation energy, namely... 

Formation of BrNO or BrN02 by reaction of NO2 or N2O5, respectively, with NaBr contained in seasalt particles. BrNO and BrN02 arc rapidly photolyzed to yield Br atoms. 

Values for NaC104 and NH4CIO4 were smaller than 1.1. These results indicate that the ozone reaction can be 2.6 times faster in a seasalt particle with an ionic strength of 1 M than in a solution with zero ionic strength. 

The composition of seasalt reflects the composition of seawater enriched in organic material (marine-derived sterols, fatty alcohols, and fatty acids) that exists in the surface layer of the oceans (Schneider and Gagosian 1985). Seawater contains 3.5% by weight seasalt, and when first emitted, the seasalt composition is the same as that of seawater (Table 8.8). Reactions on seasalt particles modify its chemical composition for example, sodium chloride reacts with sulfuric acid vapor to produce sodium sulfate and hydrochloric acid vapor... 

As a result of this reaction more nitrate is transferred to the aerosol phase and is associated with the large seasalt particles. At the same time, hydrochloric acid is liberated and the aerosol particles appear to be chloride -deficient. This deficiency may also be a result of... 

C Fresh seasalt particles in the marine atmosphere are alkaline and can serve as sites for the heterogeneous oxidation of S02 by ozone. Assuming that the diameter of these particles varies from 0.5 to 20 pm, their pH varies from 5 to 8, their liquid water content is 100 pgm-3, and typical mixing ratios are 50 ppt for S02 and 30 ppb for ozone ... 

The low amount of liquid water associated with particles (volume fraction 10-10, compared to clouds, for which the volume fraction is on the order of 10-7) precludes significant aqueous-phase conversion of S02 in such droplets. These particles can contribute to sulfate formation only for very high relative humidities (90% or higher) and in areas close to emissions of NH3 or alkaline dust. Seasalt particles can also serve as the sites of limited sulfate production (Sievering et al. 1992), as they are buffered by the alkalinity of seawater. The rate of such a reaction as a result of the high pH of fresh seasalt particles is quite rapid, 60 pM min-1, corresponding to 8% h 1 for the remote oceans (S02 = 0.05 ppb). Despite this initial high rate of the reaction, the extent of such production may be quite limited. For a seasalt concentration of 100 nmol m 3, the alkalinity of seasalt... 

Fig. 1-2 Chemical data from the Vostok ice core. The graph of 5D can be taken as a proxy for temperature changes, as described in Chapter 18. CO2 and CH4 are greenhouse gases and vary in the same direction as temperature. Non-seasalt sulfate and methane sulfonic acid are both sulfur species existing in the particle phase, and are positively correlated with each other, but negatively with T. Major variations for all of these variables seem to correlate either positively or negatively with each other, indicating a coupled system. <5D, non-seasalt sulfate, and methane sulfonic acid data kindly provided by Dr Eric Saltzman. CO2 data are from Bamola et al. (1987) and Jouzel et al. (1993). CH4 data are from Chappellaz et al. (1990) and Jouzel et al. (1993). (ppmv = parts per million by volume ppbv = parts per billion by volime)...

A small flux is shown between the land and atmosphere. This represents the transport of dust particles to the atmosphere (F28) and the deposition of these particles back on land either as dry deposition or associated with atmospheric precipitation (F82). Similarly, fluxes that represent the transport of seasalt from the surface ocean to the atmosphere (Fss) and the deposition of soluble (F85) and insoluble (F81) atmospheric forms are also shown. As already discussed for the river fluxes, the insoluble particulate flux is represented as a direct transport of P to the sediment reservoir. 

Figure 16-1 is a master-variable diagram corresponding approximately to the previous clean marine case, illustrating that HC03 derived from CO2 is only important at pH > 7, and that at equilibrium H", NH4, and SO4- are the dominant species. Figure 16-2 extends this approach to the small population of droplets without any SO - in them that are nucleated on particles of seasalt that is present. In this case, pH = 6.7 and the dominant cation is seawater "alkalinity" or Ak (alkalinity in seasalt is the sum of cation concentration due to dissolved... 

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. 

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. ... 

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],... 

The presence of non-seasalt sulfate particles in the remote marine atmosphere has important environmental consequences. As a result of their size and hydrophylicity, sulfate particles make good cloud condensation nuclei... 

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. 

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). 

Sulfate is the ultimate product of the oxidation of SO2. We might expect small concentrations of other species, such as dithionates that are intermediates in the oxidation process, although these have not been detected. Concentrations of sulfate particles in the remote atmosphere are typically at a few nanomoles per cubic meter. In the marine atmosphere the sulfate is found both in coarse particles where it derives from seasalt and in fine particles around a micron in diameter as... 

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