The results of measurements of hygroscopic properties and chemical analysis of atmospheric aerosol samples collected from June 10 to 20 and December 15 to 25, 2011, at the ZOTTO background sta tions (60.8° N, 89.35° E) in Central Siberia are presented. The sorption properties of aerosols are studied with the help of a differential analyzer of absorbed water mass in the relative humidity range of 5 to 99%. It has been found that the hygroscopic growth factor of aerosol particles collected during the winter campaign is on average 45% higher than that of the aerosol collected in the summer campaign, which leads to a 40% decrease in the critical supersaturation threshold of cloud activation of particles. The measurement data are analyzed and parameterized using a new approach that takes into account the concentration effects in the particle–water vapor system at low humidities. Based on the chemical analysis, the content of watersoluble substancesin the winter sample is 2.5 times higher than in the summer sample.