Water absorption by organics: Survey of laboratory evidence and evaluation of UNIFAC for estimating water activity

The composition of organic material in atmospheric particles and the influence of these organics on aggregate particle properties have remained less well characterized than that of the inorganic ionic fraction. While laboratory and atmospheric studies strive to quantify the formation rates and concentrations of water-soluble and other organic compounds in atmospheric particles, concerted efforts are being devoted by many scientists to develop models for simulating the formation and gas-particle distribution of condensible organics in the atmosphere. Within this research milieu, as a first step toward developing a capability to simulate the thermodynamics of aqueous, organic-containing submicron droplets under atmospheric conditions, in this paper we (i) synthesize published laboratory data to evaluate the water absorption behavior of multifunctional oxygenated organic compounds and (ii) test the reliability of the UNIFAC group contribution method (1) for estimating water activities of aqueous organic solutions. The laboratory data show that multifunctional oxygenated compounds can absorb water over the entire range of relative humidities. For a wide variety of compounds (e.g., glycols, dicarboxylic acids, keto acids) and a wide range of solute concentrations (0 to >90% by wt), we find that, in most cases, water activities can be estimated to within approximately 15%.