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