Electron Microscopic Study of Ice-Fog and Ice-Crystal Nuclei in Alaska

[摘要] At air temperatures around -40°C in the Fairbanks, Alaska, area, dense ice fog is formed by freezing of water droplets condensed from water vapor in flue gases derived from the chimneys of power and heating plants, and the exhausts of automobiles. Ice fog crystals, ice crystals and supercooled fog droplets which formed at temperatures around -40°C, -25°C and -10°C respectively were collected in this area on electron microscope grids covered with collodion film. After sublimation or evaporation, the residues of these specimens were examined with an electron microscope. A solid particle of 0.1μ to 3μ t diameter was observed in the center of the residue of each ice-fog crystal, ice crystal and supercooled droplet. By comparing them under the electron microscope with combustion products from coal, fuel oil and gasoline, the main nucleus substances were identified as organic and inorganic particles, and hygroscopic particles formed by combustion of coal, fuel oil and gasoline. As there is no difference in composition between ice fog nuclei and supercooled fog nuclei in the Fairbanks area, it is concluded that the substances work first as condensation nuclei, then as freezing nuclei when cooled to about -25°C or below. All stages in the process from water droplet formation to ice fog formation with falling temperatures were observed directly during the collection of these specimens.Ice fog crystals formed at around -40°C were mainly spherical ice crystals of 2μ to 15μ diameter, the remainder being hexagonal and columnar crystals of 10μ to 30μ diameter. Ice sintering caused by collision of ice-fog crystals was found even at a temperature of -40°C.

[学科分类] 大气科学