Ice Nuclei from Automobile Exhaust and Iodine Vapor

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Science  23 Dec 1966:
Vol. 154, Issue 3756, pp. 1555-1557
DOI: 10.1126/science.154.3756.1555-a


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.