THE EFFECTS OF PARTICLE CHEMICAL CHARACTER AND ATMOSPHERIC PROCESSES ON PARTICLE HYGROSCOPIC PROPERTIES

Hygroscopic properties of atmospheric aerosol particles were studied b y constructing a simple model that relates the particle hygroscopic gr owth factor to the amount of soluble matter bound in the particulate p hase, to matter that is exchangeable with the gas phase, and to matter that reduces particle water absorption. The model was applied to expl ain 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 de monstrated that the ''less''-hygroscopic particles are likely to conta in a major insoluble part. ''More''-hygroscopic particles, being frequ ently almost totally water soluble, must have either (i) a significant fraction of soluble material absorbing water less efficiently than ty pical atmospheric inorganic salts, or (ii) substances that actively re duce the amount of water that particles are able to absorb. Of atmosph eric processes, coagulation and interphase chemical reactions graduall y turn hydrophobic particles into less-hygroscopic ones, whereas water -absorbing properties of less-hygroscopic particles are likely to be m odified mainly by the condensational gas-to-particle transport. The pr ocess responsible for turning less-hygroscopic particles into more-hyg roscopic ones in the atmosphere, and for maintaining the separation be tween the two hygroscopicity classes, is probably the processing of th e air through nonprecipitating clouds. Copyright (C) 1996 Elsevier Sci ence Ltd