CLOUD DROPLET NUMBER STUDIES WITH A COUNTERFLOW VIRTUAL IMPACTOR

By providing simultaneous data on cloud droplet number concentration ( N(d)), cloud liquid water content, and the physical properties of clou d droplet residue particles (CDRP), the counterflow virtual impactor ( CVI) offers a new approach to studying how aerosol perturbations may m odulate N(d). We report here results from a preliminary study in which a CVI optimized for this task was deployed on a coastal mountain in W ashington State for 2 weeks in the spring of 1990 as part of the Pacif ic Sulfur/Stratus Investigation. Additional instrumentation measured t otal particle number and basic meteorological parameters. Two main fea tures of this data set emerge from a comparison of selected cloud epis odes. First, the majority of CDRP are smaller than 0. 1-mum diameter, implying that the population of aerosol particles controlling N(d) may be distinct from the population controlling aerosol mass or light sca ttering. Second, while large variations in aerosol loading (number and volume) were observed, the variation of N(d) was dominated by a combi nation of mesoscale and turbulent factors such that an unambiguous aer osol effect on N(d) could not be detected. These results highlight som e of the difficulties in quantifying the indirect, or cloud-mediated, climatic effect of aerosol perturbations stemming from either natural or anthropogenic emissions.