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[1] An atmospheric HULIS (humic-like substance) sample dissolved in water was used to generate particles with different dry diameters. A HHTDMA (High Humidity Tandem Differential Mobility Analyzer) and LACIS (Leipzig Aerosol Cloud Interaction Simulator) were used to measure hygroscopic growth of the HULIS particles. LACIS also was used to measure the critical supersaturation for the activation of HULIS particles with dry diameters of 50, 75, 100, and 125 nm. Simple Köhler theory was used to simulate the measured hygroscopic growth factors. For this, a new technique was used, where the ionic density (r ion ) was defined as a combination of the HULIS physical properties for which values could not be reliably determined. By adjusting r ion in the Köhler equation, modeled hygroscopic growth could be brought into agreement with the measurements, even without the explicit knowledge of the different HULIS properties. It was demonstrated that the values of r ion determined with our procedure can be reproduced from combinations of physically realistic values of the physical properties represented by r ion . Adjustments of the ionic density were done for two different surface tensions, that of water as the upper limit, and the lowest value that had been measured for this HULIS sample (published previously) as the lower limit. The two adjusted values of r ion were used in the Köhler model to derive critical super-saturations. For more dilute droplets, measured and modeled critical supersaturations were in agreement for both values of the surface tension, whereas for the less dilute solutions, agreement only could be achieved when a lowering of the surface tension due to HULIS was taken into account.

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May and June 2006 in Tinfou, Morocco. A H‐TDMA system and a H‐DMA‐APS system were used to obtain hygroscopic properties of mineral dust particles at 85% RH. Dynamic shape factors of 1.11, 1.19 and 1.25 were determined for the volume equivalent diameters 720, 840 and 960 nm, respectively. During a dust event, the hydrophobic number fraction of 250 and 350 nm particles increased significantly from 30 and 65% to 53 and 75%, respectively, indicating that mineral dust particles can be as small as 200 nm in diameter. Log‐normal functions for mineral dust number size distributions were obtained from total particle number size distributions and fractions of hydrophobic particles. The geometric mean diameter for Saharan dust particles was 715 nm during the dust event and 570 nm for the Saharan background aerosol. Measurements of hygroscopic growth showed that the Saharan aerosol consists of an anthropogenic fraction (predominantly non natural sulphate and carbonaceous particles) and of mineral dust particles. Hygroscopic growth and hysteresis curve measurements of the ‘more’ hygroscopic particle fraction indicated ammonium sulphate as a main component of the anthropogenic aerosol. Particles larger than 720 nm in diameter were completely hydrophobic meaning that mineral dust particles are not hygroscopic.

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Abstract. This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435 ± 87 cm −3 and compares well with the observed median droplet number concentration of N d = 464 cm −3 . Furthermore, a 50 % activation diameter of D p50 ≈ 115 nm was derived, which was linked to a critical supersaturation S crit of 0.16 % via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σ S = 0.09 %. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S ) are calculated based on highly-resolved thermodynamic data showing a standard deviation of S ranging within 0.1 % ≤ σ S ≤ 0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S with σ S = 0.1 % at cloud base and a decreasing σ S with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.