Water activity and surface tension of aqueous ammonium sulfate and D-glucose aerosol nanoparticles

Abstract. Water activity (a_w) and interfacial energy or surface tension (σ) are key thermodynamic parameters to describe the hygroscopic growth of atmospheric aerosol particles and their ability to serve as cloud condensation nuclei (CCN) influencing the hydrological cycle and climate. Due to size effects and complex mixing states, however, these parameters are not well constrained for nanoparticles composed of organic and inorganic compounds in aqueous solution.

In this study, we determined a_w and σ by differential Köhler analysis (DKA) of hygroscopic growth measurement data for aerosol particles smaller than 100 nm composed of aqueous ammonium sulfate (AS), D-glucose (Gl), and their mixtures. High-precision measurements of hygroscopic growth were performed at relative humidities (RH) ranging from 2.0 % to 99.6 % with a high humidity tandem differential mobility analyzer (HHTDMA) in three complementary modes of operation: hydration, dehydration, and restructuring. The restructuring mode (hydration followed by dehydration) enabled the transformation of initially irregular particles into compact globules and the determination of mass equivalent diameters. The HHTDMA-derived growth factors complemented by the DKA, allows for determination of water activity and surface tension from dilute to highly supersaturated aqueous solutions that are not accessible with other methods. Thus, for mixed AS/Gl nanoparticles with mass ratio of 4:1 and 1:1, the upper limit of solute mass fraction (X_s) was 0.92 and 0.98, respectively.

For pure AS and Gl, the DKA-derived a_w is in a good agreement with electrodynamic balance and bulk measurement data. For AS particles, our a_w data also agree well with the Extended Aerosol Inorganic Model (E-AIM III) over the entire concentration range. In contrast, the UNIFAC model as a part of AIOMFAC was found to overestimate a_w in aqueous Gl particles, which can be attributed to unaccounted intermolecular interactions.

For mixed AS and Gl nanoparticles, we observed a non-monotonic concentration dependence of the surface tension that does not follow the predictions by modelling approaches constructed for mixed inorganic/organic systems. Thus, for AS/Gl particles with a 1:1 mass ratio exhibited a strong decrease of σ with increasing solute mass fraction, a minimum value of 56.5 mN m^-1 at X_s ≈ 0.5, and a reverse trend of increasing σ at higher concentrations. We suggest that D-glucose molecules surrounded by ammonium sulfate ions tend to associate, forming non-polar aggregates, which lower the surface tension at the air-droplet interface.

We analyzed the uncertainty in the DKA-derived water activity and surface tension, related to the instrumental errors as well as to the morphology of the nanoparticles and their phase state. Our studies have shown that under optimal modes of operation of HHTDMA for moderate aqueous concentrations, the uncertainty in a_w and σ does not exceed 0.2–0.4 % and 3–4 %, respectively, but it increases by an order of magnitude in the case of highly concentrated nanodroplet solution.