SULFATE AEROSOL GENERATION
in the MARINE ATMOSPHERE

Roger J. CHENG ASRC UALBANY

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 (CaSO/4,MgSO/4) 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 (CaSO/4,MgSO/4) and (C) Chloride droplets (MgCI/2, 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 (MgCl/2, 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.