The properties of atmospheric aerosol under a cumulus base were studied using three-wavelength lidar. A growth of hygroscopic aerosol particles in a convective updraft and the activation of cloud condensation nuclei (CCN) were observed. A simple and robust model of droplet formation in a rising parcel was used as a closing assumption to the original approach of cloud droplet number concentration (CDNC) retrieval. The potential to retrieve the vertical profile of the effective radius of droplets near the cloud base and the height of activation and condensation of nuclei is demonstrated.