From hygroscopic aerosols to cloud droplets: The HygrA-CD campaign in the Athens basin -- An overview

The international experimental campaign Hygroscopic Aerosols to Cloud Droplets (HygrA-CD), organized in the Greater Athens Area (GAA), Greece from 15 May to 22 June 2014, aimed to study the physico-chemical properties of aerosols and their impact on the formation of clouds in the convective Planetary Boundary Layer (PBL). We found that under continental (W-NW-N) and Etesian (NE) synoptic wind flow and with a deep moist PBL (~ 2-2.5 km height), mixed hygroscopic (anthropogenic, biomass burning and marine) particles arrive over the GAA, and contribute to the formation of convective non-precipitating PBL clouds (of ~ 16-20 μm mean diameter) with vertical extent up to 500 m. Under these conditions, high updraft velocities (1-2 m s− 1) and cloud condensation nuclei (CCN) concentrations (~ 2000 cm− 3 at 1% supersaturation), generated clouds with an estimated cloud droplet number of ~ 600 cm− 3. Under Saharan wind flow conditions (S-SW) a shallow PBL (< 1-1.2 km height) develops, leading to much higher CCN concentrations (~ 3500-5000 cm− 3 at 1% supersaturation) near the ground; updraft velocities, however, were significantly lower, with an estimated maximum cloud droplet number of ~ 200 cm− 3 and without observed significant PBL cloud formation. The largest contribution to cloud droplet number variance is attributed to the updraft velocity variability, followed by variances in aerosol number concentration. © 2016 Elsevier B.V.