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