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

Abstract

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 (~2–2.5km height), mixed hygroscopic (anthropogenic, biomass burning and marine) particles arrive over the GAA, and contribute to the formation of convective non-precipitating PBL clouds (with droplets of ~16–20μm mean diameter) with vertical extent up to 500m. Under these conditions, high updraft velocities (1–2ms−1) and cloud condensation nuclei (CCN) concentrations (~2000cm−3 at 1% supersaturation), generated clouds with an estimated cloud droplet number of ~600cm−3. Under Saharan wind flow conditions (S-SW) a shallow PBL (<1–1.2km height) develops, leading to much higher CCN concentrations (~3500–5000cm−3 at 1% supersaturation) near the ground; updraft velocities, however, were significantly lower, with an estimated maximum cloud droplet number of ~200cm−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.