Airborne measurements of tropospheric ice-nucleating aerosol particles in the Arctic spring

Instrumented aircraft flights were made during field experiments in the Arctic Ocean, the NASA FIRE Arctic Cloud Experiment and SHEBA (Surface Heat Budget of the Arctic). Airborne measurements of ice nucleating aerosol particles (IN) used a continuous flow diffusion (CFD) chamber, covering -10 degrees to -30 degreesC and humidities from ice saturation to water supersaturation. During selected time periods, ice crystals that grew on ice nuclei in the chamber were sampled onto electron microscope (EM) grids for later examination of the nucleating particles. Samples of total aerosol (IN and non-IM) were also collected for comparative analyses. Concentrations of IN ranged from zero to rare very high values (hundreds per liter at -25 degreesC), making the frequency distribution of IN highly skewed: when accumulated as 10 s average concentrations (volume similar to0.17 L), 50% were zero. Additional evidence of few IN was seen in thin low-level stratus clouds at -10 degrees to -20 degreesC that persisted for several days with low concentrations of ice crystals (similar to0.1 L-1) and a few tenths g m(-3) liquid water. On occasion, small regions of high IN concentrations (hundreds per liter) were detected near the surface. The EM analyses indicated that ice nuclei a few tenths micrometer in size contained crustal materials (primarily Si) and had widely varying morphology. Many IN particles produced weak or no X-ray signatures, suggesting a dominant low-molecular-weight component not detected by the energy dispersive X-ray (EDX) system, and were probably carbonaceous. In contrast, for the total aerosol, S or S and Si were dominant components, and few particles had no X-ray signature.