There are crucial uncertainties about the behavior of organic material in clouds. The large organic component of the atmospheric aerosol may significantly contribute to both natural and anthropogenic cloud condensation nuclei (CCN). Therefore, laboratory measurements of the critical supersaturations as a function of dry particle size are presented for several common organic compounds. These include sodium lauryl (dodecyl) sulfate, oxalic, adipic, pinonic, hexadecanedioic, glutaric, stearic, succinic, phthalic, and benzoic acids. These are compared with theoretical and measured size versus critical supersaturation relationships of common inorganic compounds (e.g., sodium chloride, ammonium sulfate, potassium iodide, calcium sulfate). Unlike most inorganic compounds some organics display variations in solubility as a function of particle size; some showing relatively greater solubility at smaller and some at larger sizes; this is then in effect a variation of the van't Hoff factor with particle size. Relatively greater solubility for smaller sizes may be attributable to surface tension reduction (Facchini et al. 1999), which would be greater with less water dilution, as is the case for smaller particles, which have higher critical supersaturations that are less diluted at the critical sizes. Observations of relatively greater solubility for larger particles may be caused by greater dissolution of material at the higher dilutions that accompany larger particles, which have lower critical supersaturations; this is partial solubility (Laaksonen et al. 1998). Measurements are also presented of internal mixtures of various organic and inorganic substances.

These laboratory measurements were done with two CCN spectrometers (Hudson 1989) operating simultaneously. These two instruments usually displayed similar results in spite of the fact that they are configured differently (i.e., different flow rates and supersaturation profiles). The degree of agreement between these cloud chambers then tests and defines the limits of possible alterations of conventional Kohler theory (e.g., Kulmala et al. 1993), i.e., that some organics enhance or inhibit nucleation and droplet growth. Delayed nucleation and growth of some cloud droplets would have important implications for the indirect aerosol effect and for investigations of the indirect aerosol effect. The latter pertains to the relevance of CCN measurements to atmospheric clouds. This could mean that other factors than the critical supersaturation may be relevant to the interaction of some particles with clouds.

The supersaturation range of the Desert Research Institute (DRI) CCN spectrometers is also shown to have been extended to lower values--0.005%, which corresponds to dry particle sizes of nearly 1 µm diameter. Ambient measurements over the full range of supersaturations (1.5 to 0.005%) in various conditions will be presented to show the range of variability of giant nuclei and how they relate to CCN concentrations. Comparisons of the two DRI CCN spectrometers over this full range of possible cloud supersaturations for a variety of ambient conditions are also presented. These comparisons also test the applicability of Kohler theory to the atmosphere.