Measurements of the size-dependence of solute concentrations in cloud droplets

Measurements of the particulate material and water vapor released when cloud droplets evaporate are used to determine the amount and concentration of material contained in the droplets. Droplets are sampled with a counterflow virtual impactor (CVI), which extracts them from the surrounding air and evaporates them in a flow of warm, dry, filtered air. The resulting concentrations of water vapor, particle number, and particulate light scattering coefficient are determined, allowing calculation of average droplet and particle radii, as well as the aqueous-phase concentration of non-volatile, soluble and insoluble material (“solute concentration”). By changing the internal flows of the CVI, the minimum radius of droplets sampled is varied within the interval 4—15 μm. Measurements obtained in central Sweden in the summer of 1986, near the base of stratocumulus clouds, show that solute concentrations increase with droplet size by roughly a factor of 3 over the radius range 5—9 μm. This observation is consistent with the hypothesis that condensational growth controls the droplet size distribution near cloud base.