Dust and Haze
Hazy Atmosphere
Urban Air Pollution
Smog at Crete
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Sahara Dust
Seasalt Aerosol



Seasalt Aerosol

Mechanism: Very small droplets ("sea spray") are released into the air as a consequence of a dispersion process ("mechanical generation") which is driven by breaking waves and spume of the oceans. A significant fraction of these droplets is transported upwards by turbulence within the marine boundary layer. During this process they dry and thus aerosol particles are formed which are summarized as "sea salt aerosol".

There are three different ways to generate them:
1. Release of so-called "spume drops"
2. Formation of "jet drops" in bursting bubbles
3. "Film droplets" deriving from surface films of bursting bubbles.

The "spume drops" are generated by the wind taking liquid from the wave combs which collapses into large droplets. Since these droplets have diameters larger than 10 µm, they fall back into the water with in seconds or minutes. If the air bubbles of the foaming sea surface burst, they emit an ascending, central jet from their base. This jet collapses into 1 to 10 small droplets, the so-called "jet drops". The central jet is generated by the surface tension of the bubbles when their surface energy is transferred into kinetic energy. After drying the jet drops are converted into sea salt particles in the micrometer range. Wind velocities larger than 3 m/s are sufficient to initiate this process. At 8 m/s around 1 % of the sea surface is covered with white foam ("white caps") which is responsible for the generation of sea salt particles. The smallest bubbles have a diameter of 100 µm because smaller ones would dissolve in the water. The largest bubbles generating "jet drops" can have a diameter of several millimetres. The larger the bubbles are the higher is the chance that they separate into smaller ones while they are ascending in the water. Jet drops reach sizes up to approximately 10 % of the bubble diameter. They can be thrown up to 15 cm into the air.

When the bubbles burst also their surface film collapses into "tiny fragments". By this means up to several hundreds of the so-called "film drops" in the micrometer to sub-micrometer range are generated. The haze which is caused by the different formation processes of the sea salt aerosol can be clearly recognized in Seaspray1.jpg to Seaspray4.jpg. (The pictures were taken at back light what means a forward scattering situation.) On this given day the wind velocity was larger than 50 km/h at the coast. In the case of Seaspray3.jpg the observer waited for relatively calm air resulting in an obviously small production of sea salt aerosol.

Chemical Composition:
Initially 96.8 mass percent of the "sea spray drops" is water and 3.2% mass percent is sea salt (predominantly NaCl). A fraction of the released droplets evaporate in the humid air above the ocean. Thereby they shrink by about 50 to 70 % of their original diameter depending on the ambient relative humidity. Because of the hygroscopicity of the salts in the droplets not the whole amount of water will be evaporated. Finally the droplets have a size which is a result of an equilibrium of the salt concentration in the droplet and the relative humidity.

Therefore sea salt particles rarely are true crystalline salt particles, but mostly droplets of concentrated solutions. The Chlorine to Sodium ratio is of 1.8:1 during sea spray emission. The relation decreases when Chlorine is driven off the particles by acidification of the droplets. For example, when sulphuric acid or nitric acid will be absorbed from the the gaseous phase (g) into the droplet (l), the Chlorine is forced to leave it as HCl(g) vapour. This process is called "sea spray acidification".

H2SO4 (g) + 2 NaCl (l) -> Na2SO4 (l) + 2 HCl (g)

Global Impact: The oceans are the second largest source of natural aerosol in the atmosphere after mineral dust stemming from the deserts. The number density of the sea salt particles has constant values between 10 and 1000 m above sea level. In dependence of the wind velocity at the sea surface concentrations between 1 and 100 particles per cm³ were observed. Above the marine boundary layer the number density quickly decreases. The production rates of aerosol particles from natural and anthropogenic emissions can only be roughly estimated.

Today it is assumed that the global production rate of natural sea salt aerosol is 1300 Tg/yr, whereas the mineral dust has a production rate of 1500 Tg/yr. Volcanoes contribute between 33 and 117 Tg/yr (Warneck 2000). The whole anthropogenic production rate was estimated as 400 Tg/yr (Warneck 2000). However, recent data suggest that the anthropogenic emissions contribute more than 50 % to the global aerosol emissions. In 1997 67 to 71 % of the aerosol production in the US derived from anthropogenic sources as roadworks, agriculture and traffic, industrial emissions, biomass burning, combustion of fossil fuels etc. (Jacobsen 2002). The total global aerosol production rate is estimated to be 3500 Tg/yr. Thereby direct emissions from natural sources have a greater contribution than the secondary emissions.




Seaspray1.jpg to Seaspray4.jpg: S. Borrmann, Beach at Loquillo, Puerto Rico, 18 December 2004, 1:49 p.m.