Aerosol particles affect the climate system by directly absorbing and scattering solar radiation or by acting as cloud condensation nuclei (CCN) and modulating cloud radiative properties. Cloud particle activation is at the heart of these aerosol-cloud interactions, but it is important to quantify the degree to which aerosol (size distribution and composition) or dynamical aspects (vertical velocity) contribute to cloud droplet number concentration, as they determine in the end the cloud sensitivity to aerosol variations.

In this study, we use a comprehensive dataset of number-size distributions and meteorological data observed at 11 sites throughout the E. Mediterranean (Greece) during the summers of 2020 and 2021 and use them as input into a state-of-the-art cloud activation parameterization to determine the potential activated cloud droplet number and maximum supersaturation. Remote sensing retrievals of droplet number complement the analysis and are used to evaluate the droplet number calculations carried out with the parameterization. We then examine the droplet formation characteristics of each region (urban, rural, remote, and mountain), determine when clouds are velocity- and aerosol-limited, link them to airmass origin, and discuss the implications for cloud formation in the region.