Ferromagnetic particles and ice nuclei

Airborne ferromagnetic particles (20–0.1 −m in diameters) have been collected by different methods. Their ability to form ice crystals has been investigated by means of direct and indirect observations. No big (d>5 μm) particles (“black spherules”), placed on filters, have been found to initiate ice phase at −12 and −15°C. Millipore filters, located behind a magnetic and a geometrically identical reference precipitator, have shown no characteristic differences in their ice nuclei concentrations when “developed” in a small diffusion chamber at −18°C. The results show that also the smaller ferromagnetic particles cannot probably be considered as effective ice nuclei.