The role of mineral dust in cloud droplet formation processes: Insights from the Central Asian dust belt subregion

Abstract. In this article, the contribution of aged desert dust, contaminated with hygroscopic material, to droplet formation in liquidwater-containing tropospheric clouds in Central Asia is investigated. The study is based on long-term lidar observations of aerosol and cloud properties at Dushanbe (38.6° N, 68.9° E; 864 m height), Tajikistan. The continuous measurements, performed between June 2019 and July 2023, cover three summer and three winter seasons in terms of cloud condensation nucleus concentration (CCNC) of the dust and non-dust (anthropogenic) CCN reservoirs and cloud droplet number concentration (CDNC) and effective radius of the water droplets in the base (lowest 100 m) of liquid-dominated cloud layers. The CDNC contains information about the actually activated CCN fraction of the total CCN reservoir. The data analysis reveals that aged, hygroscopic dust particles are able to sensitively influence or even control CDNC levels during the dust season (April to October) and that the impact is relatively weak during the main winter months (December to February) when the dust particle concentration is partly very low and aerosol pollution from anthropogenic sources dominates. Case studies are presented to check the consistency in the retrieval products and to show the complexity of aerosol-cloud interaction in a region, probably representative for large parts of Asia, with aerosol mixtures of fresh and aged dust, anthropogenic haze, rural background aerosol, and even wildfire smoke after long-range transport. Water uptake by dust particles, observed at high relative humidity close to cloud base, is a clear sign for the presence of dust particles containing hygroscopic material.