A NEW MECHANISM FOR THE GENERATION OF HIGH SULFATE ENRICHED
AEROSOIS IN THE MARINE ATMOSPHERE
Roger J. Cheng
Atmospheric Sciences Research Center
State University of New York
Albany, New York 12222 U.S.A
The high concentration of sulfate in
marine aerosols has long been suggested to be the products of atmospheric
oxidation of gaseous dimethyl sulphide
(DMS) emitted by phytoplankton and the photo chemical reaction of S02
originated from contented and distance anthropogenic sources. But these
mechanisms cannot explain for observed high concentration of sulfate in remote
marine atmosphere
Recent macroscopically
investigation and field observation revealed that high enriched sulfate
aerosols were ejected from seawater droplets, generated by whitecaps at the
oceanic surface, during their phase transition.
Close examination of an individual
seawater droplet in an environment of relative humidity less than 75% and
temperature of 25 C have resulted in the following observations: Evaporation of
water increased the salt concentration- At first, a thin layer of brine formed
at the surface of the evaporating droplet; this was followed by clearly
sequential precipitations of numerous microcrystal of sulfates, and of a crust
of many relatively larger cubic NaCl crystals beneath
the surface correspondent with their individual solubilities
and concentrations in the droplet During this process, solution of chlorides (MgCl/2, KCI) mixed with microcrystals
of sulfates and/or NaCl were expelled from the surface
of the droplet. Brine film bursting at the interstices of cubic NaCl crystals was also observed and was followed by the
formation of a hollow sphere of sea-salt particle with many holes. A larger
number of aerosols were generated from rapid evapom6on followed by crystalline
of seawater droplets on a hot (35-40 C) rock surface. A secondary ejection of
sulfate aerosols was also detected during melting by the bursting of air
bubbles which formed when hollow particle was moved into a high-moisture
environment or dissolved into another water droplet.
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; aerosols were composed predominantly of sulfate
particles (CaSO/4, MgSO/4)
with a size range of 0.1 um to I0 um. High concentration of [S04=] and (CI-]
from the ejected aerosols were also measured by ion chromatographic analysis.
The shell structure of chlorides (MgCl/2, KCI)
present on the surfaces of the ejected aerosols was detected and confirmed via
polarizing microscopy.
To be presented at: 9th World Clean Air Congress
August 30 - September 4, 1992, Montreal , Quebec, Canada