Hygroscopic growth and measured and modeled critical super‐saturations of an atmospheric HULIS sample

[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 super-saturation for the activation of HULIS particles with dry diameters of 50, 75, 100, and 125 nm. Simple Kohler theory was used to simulate the measured hygroscopic growth factors. For this, a new technique was used, where the ionic density (ρion) was defined as a combination of the HULIS physical properties for which values could not be reliably determined. By adjusting ρion in the Kohler 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 ρion determined with our procedure can be reproduced from combinations of physically realistic values of the physical properties represented by ρion. 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 ρion were used in the Kohler model to derive critical super-saturations. For more dilute droplets, measured and modeled critical super-saturations 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.