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Airborne Measurements of Atmospheric Ice Nuclei in the Arctic |
Colorado State University
dcrogers@lamar.colostate.edu
Measurements of ice nucleating aerosol particles (IN) were made on the NCAR C-130 aircraft in support of field experiments in the Arctic Ocean during the May 1998 portion of NASA's FIRE Arctic Cloud Experiment. The IN measurements used the CSU continuous flow diffusion (CFD) chamber, which provides a real time measurement of the IN concentration at a particular temperature and humidity. IN measurements covered the range -10 to -30°C and ice saturation to +20% supersaturation with respect to water. During selected time periods, the ice crystals that grew on ice nuclei in the chamber were sampled onto electron microscope (EM) grids using inertial separation techniques, for later examination of the nucleating particles. Samples of total aerosol (IN and non-IN) were also collected for comparative analyses.
The observations showed that aerosol particle concentrations in the late winter/early spring Arctic atmosphere are remarkably low, with typical concentrations of CN (condensation nuclei) ~100/cm3 over extended regions, occasionally dropping to ~20/cm3. Such low values indicate air that has been cleaned by efficient natural scavenging mechanisms.
In general, the concentrations of IN were very low, although there were some occasional high values, making the frequency distribution highly skewed: when accumulated as 10 s average concentrations (~0.17 L), 50% were zero per liter. Additional evidence of the scarcity of IN was seen in thin low level stratus clouds at -15 to -20°C with low concentrations of ice crystals (~0.1/L) and a few tenths g/m3 liquid water.
Strong vertical stratifications were often seen in aerosol concentrations (CN and PCASP), coinciding with thermal stratifications (inversions and cloud top boundaries). Strong stratifications were not noticed for IN, except that small regions of high IN concentrations were occasionally seen at low altitudes (~100 m). The small area in which these were observed suggests that the sources may be local or that higher IN were in thin stable layers.
The EM analyses indicated that Arctic IN particles are generally a few tenths micrometer in size with widely varying morphology and composition. Elemental composition of IN particles was determined by energy dispersive x-ray microprobe techniques. Elements detected in ice nuclei include Si, S and some metals (Zn, Cr, Al and others). Many IN particles produced weak or no x-ray signatures, most likely due to the dominance of a low-molecular-weight component not detected by the EDX system; i.e., they were probably carbonaceous.
