Ice Nuclei from Automobile Exhaust and Iodine Vapor

Abstract

When exposed to a trace of iodine vapor, the submicroscopic particles of lead exhausted by automobiles produce nuclei for the formation of ice crystals. Concentrations of particles exceeding 10⁶ per liter can be directly sampled from the exhaust pipe of an idling motor. Concentrations of from 10⁴ to 10⁵ per liter have been found in rural air downwind of auto roads; the concentration at one rural site has increased by an order of magnitude in 13 years. The phenomenon may provide a method of modifying clouds, and of determining (and monitoring) the percentage of automobile exhaust in a polluted atmosphere. It may be an important factor in inadvertent modification by man of the climate.