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The generation of maritime cloud
condensation nuclei (CCN) through the ejection of jet and film
droplets from bursting whitecap-produced bubbles on the ocean
surface has been well documented. The processes involved in
the transformation (evaporation and crystallization) of these
liquid droplets into their solid form under varying
conditions, however, has not previously attracted much
attention from atmospheric researchers.
A set of laboratory
investigations and field observations of the characteristics,
both physical and chemical, of seawater droplets during phase
change in a controlled environment have revealed the following
startling and very significant phenomena:
(1) The ejection of sulfate
aerosols (CaSO4,MgSO4) with size range of 0.1mm to 10 mm. The
concentration and chemical composition of the ejected
aerosols, identified by the techniques of scanning electron
microscopy and energy dispersive x-ray spectroscopy, were
dependent on the rate of droplet evaporation. Sea-salt
aerosols could be classified into three categories: (A) NaCI
crystals, (B) Crystals of sulfates (CaSO4,MgSO4) and (C)
Chloride droplets (MgCI2, KCI).
(2) The formation of hollow
spherical sea-salt particles (>5 mm). A secondary ejection
of aerosols was detected during melting by the busting of air
bubbles, which were formed when the hollow particle was moved
into a high-moisture environment or dissolved into a water
droplet.
(3) A thin film of chlorides
(MgCl2, KCI) observed on the surface of sea salt particles
present a highly hygroscopic surface to initiate the
condensation of water vapor in an environment with RH As low
as 40%. Sodium Chloride (NaCI-75%RH) plays only a minor role
for the formation of cloud droplets in the marine atmosphere.
Characterization of the ejected
sulfate aerosols in comparison with field observation,
chemical processes inside the evaporating seawater droplets
and the mechanisms for the generation of secondary aerosols in
the marine atmosphere are presented with illustrations.
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