Direct observations of aerosols attached to falling snow crystals

Abstract

Aerosols attached to falling snow crystals were directly observed utilizing optical and electron microscopes in Sapporo in March 1973 and 1974. It was determined that the aerosols were picked up by snow crystals during their fall under the cloud base. Because the slope of size distribution of aerosols attached to snow crystals was nearly the same as Junge distribution in the range 0.1 to 5 μ, it was considered that falling snow crystals captured aerosols with a collection efficiency almost similar to neighbouring size ranges against the general theory of collection efficiency. The collection efficiency was also calculated by the use of surface density of aerosols attached to the surface of snow crystals. The results of calculations showed that the apparent collection efficiency was as high as near unity. With regard to the collection phenomena of aerosols by falling snow crystals, all possible mechanisms considered were checked to define the high collection efficiency, however it was hopeless to explain the high collection efficiency by the use of the mechanisms. This suggests that there are some unknown mechanisms remaining in the natural washout of aerosols by falling snow crystals.