From AMS Glossary
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(Also called slow ion, heavy ion.) An ion of relatively large mass and low mobility that is produced by the attachment of a small ion to an Aitken nucleus.
Large ions were discovered by P. Langevin and are sometimes referred to as "Langevin ions." Large ions have ion mobilities of the order of 10-8 m s-1 per volt m-1, or some 10 000 times lower than those of small ions. As a result these atmospheric ions contribute practically nothing to the conductivity of the air, except in rare cases where small ions are nearly absent. Typically, they bear only a single electronic charge, as is true of small ions. Large ions move so slowly that they are not destroyed by being neutralized by still other large ions of paired signs, for such collisions are too infrequent. Instead, they are neutralized by union with a small ion of opposite sign. Their mean lifetimes are of the order of 15–20 minutes over the oceans, but may approach 1 h in very polluted air. The ion density of large ions varies widely depending upon the degree of atmospheric pollution. Representative low-altitude values might be 109 m-3 in clean country air, 1010 m-3 in an industrial area, and 108 m-3 over the oceans.
Wait, G. R., and Parkinson, W. D. 1951. Compendium of Meteorology. 120–121.