By providing simultaneous data on cloud droplet number concentration (Nd), cloud liquid water content, and the physical properties of cloud droplet residue particles (CDRP), the counterflow virtual impactor (CVI) offers a new approach to studying how aerosol perturbations may modulate Nd. We report here results from a preliminary study in which a CVI optimized for this task was deployed on a coastal mountain in Washington State for 2 weeks in the spring of 1990 as part of the Pacific Sulfur/Stratus Investigation. Additional instrumentation measured total particle number and basic meteorological parameters. Two main features of this data set emerge from a comparison of selected cloud episodes. First, the majority of CDRP are smaller than 0.1-μm diameter, implying that the population of aerosol particles controlling Nd may be distinct from the population controlling aerosol mass or light scattering. Second, while large variations in aerosol loading (number and volume) were observed, the variation of Nd was dominated by a combination of mesoscale and turbulent factors such that an unambiguous aerosol effect on Nd could not be detected. These results highlight some of the difficulties in quantifying the indirect, or cloud-mediated, climatic effect of aerosol perturbations stemming from either natural or anthoropogenic emissions. ¿ American Geophysical Union 1994