Aerosol-cloud interactions in Saharan mineral dust over the Eastern Mediterranean

Abstract. Mineral dust is a major component of atmospheric aerosol mass, yet its role in cloud processes remains poorly understood. While laboratory studies have explored its ability to act as cloud condensation nuclei (CCN) and ice nucleating particles (INPs), in situ atmospheric measurements are scarce.

This study presents unique in situ and remote sensing observations of dust-embedded clouds from the A-LIFE campaign in the Eastern Mediterranean. In situ observations on 20 April 2017 including shadow images and size distributions of aerosol and cloud particles combined with Doppler wind lidar (DWL) measurements were used to characterize dust and cloud microphysical properties. A novel size distribution retrieval for the second-generation Cloud, Aerosol, and Precipitation Spectrometer (CAPS) was developed – accounting for instrumental uncertainties, particle non-sphericity, and refractive index via a Monte Carlo method – and a machine learning-based calibration of the CAPS hotwire sensor for LWC measurements was introduced.

Using the newly developed analysis tools, this study presents a quantitative assessment of two different aerosol-cloud interaction processes in dust-embedded clouds: Measurements at the cloud-top indicated heterogeneous ice nucleation, in accordance with laboratory experiments and INP parameterizations. Furthermore, small-scale CCN activation of mineral dust particles into liquid droplets (~10 µm) was observed in the middle part of the cloud, with activation diameters as small as 0.13–0.23 µm, consistent with laboratory findings. DWL observations revealed vertical lifting as the driver of increased water supersaturation, enabling CCN activation. These findings provide new insights into the microphysical processes of dust-embedded clouds and their interactions with the atmosphere.