Water-soluble organic compounds have recently received much attention because of their ability to absorb water and alter the hygroscopic properties of inorganic aerosols. The effects of glycerol, succinic acid, malonic acid, citric acid, and glutaric acid on the water cycles (water activities during particle evaporation and growth), crystallization relative humidities (CRH), and deliquescence relative humidities (DRH) of sodium chloride (NaCl) and ammonium sulfate (AS) were studied using an electrodynamic balance (EDB). The growth factors of these inorganic and organic mixtures were lower than those of the pure inorganic species. The presence of all these organics in the mixed particle reduce the water absorption of NaCl but enhance that of AS relative to that of the pure inorganic salts. Glycerol and succinic acid did not affect the deliquescence properties of NaCl and AS, although succinic acid increased the CRH of NaCl and AS. Malonic acid and citric acid, behaving as nondeliquescent species in single particle studies, caused NaCl and AS particles to absorb a significant amount of water before deliquescence. Glutaric acid caused NaCl and AS to deliquesce gradually, spanning a wide range of relative humidity. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. However, the detailed crystallization and deliquescence behaviors of the organic/inorganic mixtures cannot be easily predicted from the hygroscopic properties of the individual components.

Amino acids and organic species derived from biomass burning can potentially affect the hygroscopicity and cloud condensation activities of aerosols. The hygroscopicity of seven amino acids (glycine, alanine, serine, glutamine, threonine, arginine, and asparagine) and three organic species most commonly detected in biomass burning aerosols (levoglucosan, mannosan, and galactosan) were measured using an electrodynamic balance. Crystallization was observed in the glycine, alanine, serine, glutamine, and threonine particles upon evaporation of water, while no phase transition was observed in the arginine and asparagine particles even at 5% relative humidity (RH). Water activity data from these aqueous amino acid particles, except arginine and asparagine, was used to revise the interaction parameters in UNIQUAC functional group activity coefficients to give predictions to within 15% of the measurements. Levoglucosan, mannosan, and galactosan particles did not crystallize nor did they deliquesce. They existed as highly concentrated liquid droplets at low RH, suggesting that biomass burning aerosols retain water at low RH. In addition, these particles follow a very similar pattern in hygroscopic growth. A generalized growth law (Gf = (1 - RH/100)-0.095) is proposed for levoglucosan, mannosan, and galactosan particles.

Water-soluble organic compounds have recently received much attention because of their ability to absorb water and affect the radiation balance and the climate. Partly because of their relatively high volatility, thermodynamic data on water-soluble organic compounds are scarce. Recently, we have developed a method based on the scanning electrodynamic balance (SEDB) that enables the measurement of water activity data of evaporating droplets within an hour, which can potentially be used to measure volatile species. This paper demonstrates the use of the SEDB to study the hygroscopic growth of selected atmospheric species, including semivolatile organic species with vapor pressure up to 1 × 10 -4 mmHg. We also measured the water activities, the crystallization relative humidity, and the deliquescence relative humidity (DRH) of aqueous solutions of maleic acid and glutaric acid. The DRHs of maleic acid and glutaric acid are in general agreement with the literature, except that glutaric acid shows a small delay in the completion of deliquescence due to masstransfer limitation. The water activities of equal molar mixtures of maleic acid and malic acid and of malonic acid and glutaric acid were also measured. The Zdanovskii-Stokes-Robinson (ZSR) predictions agree well with the measurements of the mixtures. The UNIFAC (UNIQUAC functional group activity coefficients) predictions, using the modified functional group interaction parameters of COOH, OH, and H 2 O derived from our earlier measurements of the water activities of aqueous droplets of a list of dicarboxylic and multifunctional acids, are also in agreement with the mixture data.