Hygroscopic properties of atmospheric aerosol particles were studied by constructing a simple model that relates the particle hygroscopic growth factor to the amount of soluble matter bound in the particulate phase, to matter that is exchangeable with the gas phase, and to matter that reduces particle water absorption. The model was applied to explain the general results obtained from many TDMA measurements, such as the existence of two distinctive particle types, the ＇＇less＇＇-and the ＇＇more＇＇-hygroscopic particles, in the atmospheric aerosol. It was demonstrated that the ＇＇less＇＇-hygroscopic particles are likely to contain a major insoluble part. ＇＇More＇＇-hygroscopic particles, being frequently almost totally water soluble, must have either (i) a significant fraction of soluble material absorbing water less efficiently than typical atmospheric inorganic salts, or (ii) substances that actively reduce the amount of water that particles are able to absorb. Of atmospheric processes, coagulation and interphase chemical reactions gradually turn hydrophobic particles into less-hygroscopic ones, whereas water-absorbing properties of less-hygroscopic particles are likely to be modified mainly by the condensational gas-to-particle transport. The process responsible for turning less-hygroscopic particles into more-hygroscopic ones in the atmosphere, and for maintaining the separation between the two hygroscopicity classes, is probably the processing of the air through nonprecipitating clouds.