The two Desert Research Institute (DRI) cloud condensation nuclei (CCN) spectrometers (Hudson 1989) were mounted on the NCAR C-130 aircraft during both phases of SHEBA (May and July, 1998). At least one of these instruments obtained ambient spectra throughout all of the flights. Sometimes both instruments simultaneously obtained spectra but often the second instrument measured processed sample. There were also measurements of total particles (condensation nuclei--CN). Most of the measurements were made near the SHEBA ice camp but there were also measurements between Fairbanks and SHEBA. The spectral range extended from 1 to 0.01% supersaturation (S) in at least 20 increments of supersaturation.
There was considerable variability in the total CCN (1% S) and CN concentrations--three orders of magnitude. Vertical gradients in CN and CCN concentrations were similar to those found in other maritime regions (Hudson and Frisbie 1991). However, the difference in concentrations between below and above cloud was actually more pronounced in SHEBA than in the other projects. The concentrations below cloud were as much as an order of magnitude lower than above cloud. During periods without boundary layer clouds the concentrations were almost uniform with altitude. Concentrations as low as 20 cm-3 below cloud were lower than most previous maritime measurements and concentrations as high as 800 cm-3 above cloud were higher. This implies strong cloud scavenging and great vertical stability.
The spectral shape of the CCN sometimes implied cloud scavenging for the below cloud aerosol. The spectra below cloud sometimes had lower critical supersaturation values implying that they are the result of coagulation and collection of cloud droplets.
There were several measurements of extensive areas of extremely low CCN concentrations--less than 5 cm-3. Such low CCN concentrations have been measured by Hindman et al. (1994) and Hudson and Xie (1998). These were usually in conjunction with very high CN concentrations. Such areas are indicative of small particle production (Clarke 1993), which is set up by the removal of larger particles (CCN) probably due to cloud scavenging.
The high concentrations above cloud imply that the Arctic haze, which is a result of long range transport from Europe and Asia still persisted throughout May. The results of the July study should determine if the lower concentrations that have previously been observed are due to less long range transport or to greater cloud scavenging because of more cloudiness in the summer.
References
Clarke, A.D., Atmospheric nuclei in the Pacific midtroposphere: Their nature, concentration, and evolution, J. Geophys. Res., 98, 20,633-20,647, 1993.
Hindman, E.E., W.M. Porch, W.M., J.G. Hudson and P.A. Durkee, Ship-produced clouds lines of 13 July 1991, Atmos. Environ., 28, 3393-3403, 1994.
Hudson, J.G., An instantaneous CCN spectrometer, J. Atmos. & Ocean. Tech., 6, 1055-1065, 1989.
Hudson, J.G. and P.R. Frisbie, Cloud condensation nuclei near marine stratus, J. of Geophys. Res., 96, D11, 20,795-20,808, 1991.
Hudson, J.G. and Y. Xie, 1998: Cloud condensation nuclei measurements in the high troposphere and in jet aircraft exhaust. Geophys. Res. Let., 25, 1395-1398.