Continuous Particle Sampler

Abstract

A continuous particle sampler has been developed which captures atmospheric particles in a Formvar solvent liquid film. The Formvar solution completely encapsulates the particle; then as the solvent evaporates the film hardens quickly, preserving a replica of the particle. The method yields exact replicas of ice crystal shapes. Liquid droplets are somewhat flattened by surface tension during replication and so a calibration factor is necessary to ascertain the original droplet size from the replica. This calibration factor has been found experimentally by using spores distributed in the droplets to show the volume of the droplet creating each replica. The sampler provides continuous information on particle concentration, particle sizes, and whether the particles are ice or water. The Formvar solution is ordinarily applied to transparent 16-mm movie leader film, and the replicas are viewed by projecting the film with a stop-motion microscope projector. Several versions of the instrument have been built and used on cloud physics research aircraft. Special adaptations have been made for ground use: to sample fogs, to measure crystals in a freezing nuclei counter, and to collect snowflakes. Various design compromises are required in the operational instruments to overcome, over a broad range of meteorological conditions, problem such as those associated with film coating, droplet encapsulation, droplet migration and spurious crystal growth. These factors and others associated with particle collection, data reduction and interpretation are discussed. Examples of the field use of the sampler are given.