The Johnstown steel mill was observed to produce aerosols containing ice nuclei active at threshold temperatures between -6¿ and -8¿C (warmer than those observed for background air) and in concentrations up to a factor of 5 greater than background air in the temperature range of -10¿C to -12¿C, as tested with a combination membrane filter-drop freezing technique. These ice nuclei were not observed in the plume at distances greater than 15 km downwind of the source.