Aerosol scavenging by falling snow

TitleAerosol scavenging by falling snow
Publication TypeConference Proceedings
Year of Conference1993
AuthorsCragin, J. H., and Hewitt A. D.
Conference Name61st Annual Western Snow Conference
Series TitleProceedings of the 61st Annual Western Snow Conference
Date PublishedJune 1993
PublisherWestern Snow Conference
Conference LocationQuebec City, Quebec
KeywordsCrystals, Scavenging, Snowflakes

Removal of atmospheric aerosols by falling snow flakes was studied during several winters in Hanover, New Hampshire, using micrometer-sized particles. Experiments were performed in open air and within static and dynamic aerosol chambers. The primary scavenging mechanism for particles in this size range was inertial impaction. Scavenging efficiencies were determined by collecting and chemically analyzing snow flakes that were allowed to fall naturally through the static chamber containing a cloud of known aerosol concentration. The averaging scavenging efficiency (defined as the ratio of the mass of aerosol collected by the snow flake to the total mass of aerosol in the swept volume) of several different types of snow flakes and ice crystals was 0.11 ± 0.08. Higher scavenging efficiencies were observed for three-dimensional spatial dendrites than for planar crystals, such as planar dendrites and hexagonal plates. Overall, snow was found to be four to five times more efficient that rain in scavenging 0.3-to 6µm-sized particles. Laser attenuation measurements within a dynamic aerosol chamber indicated that particle scavenging can cause relative transmission increases of as much as 15% for each minute of exposure to snowfall. Model calculations predicted aerosol cloud half-lives of 2 to 20 minutes for snow precipitation rates of 2.5 to 0.5 cm/hr.