Ice forming nuclei in the high Arctic

Concentrations of ice forming nuclei detected by the membrane filter technique were measured at temperatures of -12.5, -15 and -17.5 degrees C during a voyage on the icebreaker Oden to the North Pole between 1 August and 6 October 1991. Geometric mean concentrations ranged from 13 m(-3) at -15 degrees C during the first 17 days of the expedition to 2.9 m(-3) on the last 17 days of the voyage in good agreement with surface measurements made earlier north of latitude 70 degrees N. On average, concentrations increased by a factor of 4.5 for a 5 degrees C fall in temperature, less than is common in continental regions. Although air trajectory analysis showed that land sources occasionally influenced concentrations strongly, the time since the air had been over the open ocean was clearly the most important determining factor other than the seasonal decline. This implies an oceanic origin of the nuclei, the relationship being consistent with a halflife of 48.5 h (e-folding time of 70 h), about 60% longer than that of condensation nuclei. An apparent decrease in ice nucleus concentrations with temperature was mainly due to the seasonal change in concentrations but partly to the air trajectories associated with low temperatures. Elapsed time since the air was above the planetary boundary layer or over land also influenced concentrations, suggesting that the upper troposphere was deficient in ice nuclei while land was a weak source. Changes in the mixed depth of the atmosphere appeared to affect ice nucleus concentrations in the same way as condensation nucleus concentrations although poor time resolution limited this to two good examples. North of latitude 80 degrees N, a few ice crystals were present near the surface for a considerable proportion of the total time. Single stellar crystals constituted about 80% of the total, implying growth near -15 degrees C in predominantly supercooled clouds. Their concentrations were usually but not always consistent with surface ice nucleus concentrations.