Asian Dust Particles Identified with Microscopes: Aging, Mixing and Subsequence

Abstract:

Asian dust particles in Japan and China were identified by their size, shape and composition through the use of electron microscopes. Knowledge of salt formation, sea salt mixing and bacterial transportation associated with the particles, and the subsequence of these processes and phenomena were investigated and discussed.

(1) Sulfate and nitrate were efficiently produced on dust particles in marine air but the production of the salts in the upwind continental air was very weak. Some studies reported substantial sulfate and nitrate in mineral particles in the urban air of Chinese cities. However, the reported mineral particles were more likely from anthropogenic activities rather than desert dust. The formation of sulfate on Ca-rich particles did not enhance the hygroscopicity of the particles. In contrast, nitrate produced on dust particles changed the particles from hydrophobic to hydrophilic to deliquesce at ~30% RH or less. (2) Evidence for HCl deposition on dust particles in marine air was obtained. In the absence of substantial sulfate and nitrate, chloride formation on dust particles could be a major process capable of converting the particles into aqueous droplets. (3) Dust particles frequently became mixture of mineral dust and sea salt in marine air, causing the change of particles in size, hygroscopicity and other properties. Estimates showed significant enhancement of dust particle gravitational settling by sea salt adherence, resulting in more efficient gravitational removal of the dust particles than without sea salt adherence. Thus the settling of dust particles to sea surface is fundamentally different from that in continental atmosphere. (4) A close relation between bacteria and long-range transported Asian dust was confirmed in dust plumes and their concentrations were proportional and comparable.