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). There were also measurements of total particles(condensation nucleiBCN). 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. While one of the instruments obtained ambient CCN spectra, the other was often measuring the ambient sample that was processed. Two processing techniques, volatility and size vs. supersaturation(S) (Hudson and Da 1996), were employed to indirectly determine the composition and size of CCN, which is difficult to determine because of the small size of the particles (<0.1 micrometers).
Particle volatility can produce information about the internal state of the aerosol mixture if CN is also measured along with CCN. The results of volatility measurements show that most CCN are most likely to be (NH4)2SO4 rather than NaCl. Generally CN in cleaner air are more volatile than in dirtier air (Hudson and Da 1996). Compared to CCN, CN often have higher volatility temperatures, especially in more polluted air. This suggests an insoluble less-volatile component to many anthropogenic CCN. Thus anthropogenic CCN tend to be internally mixed soluble and insoluble material but natural CCN are only soluble material.
Particles from different sources displayed different size-Sc relationships. Most of the natural particles are pure substances such as sulfate, whereas anthropogenic particles often are the products of combustion, which produces carbon material that may be more insoluble.
The high concentrations above cloud along with polluted-air type size-Sc relationship and less-volatile particles imply that long range transport from Europe and Asia still persisted throughout May.
References
Hudson, J.G., 1989: An instantaneous CCN spectrometer. J. Atmos. & Ocean. Techn., 6, 1055-1065.
Hudson, J.G. and X. Da, 1996: Volatility and size of cloud condensation nuclei. J. Geophys. Res., 101, 4435-4442.