Phase State, Surface Tension, Water Activity, and Accommodation Coefficient of Water-Organic Clusters Near the Critical Size for Atmospheric New Particle Formation

This studyreports parametric relations quantitatively showingthat water contributes significantly to aerosol formation with implicationsfor human health. Interactions between water and organic molecules in sub-4nm clustersplay a significant role in the formation and growth of secondary organicaerosol (SOA) particles. However, a complete understanding of therelevant water microphysics has not yet been achieved due to challengesin the experimental characterization of soft nuclei. Here, we usemolecular dynamics simulations to study the phase-mixing states, surfacetension, water activity, and water accommodation coefficient of organic-waterclusters representative of freshly nucleated SOA particles. Our resultsreveal large deviations from the behavior expected based on continuumtheories. In particular, the phase-mixing state has a strong dependenceon cluster size; surface tension displays a minimum at a specificorganic-water mass ratio (m (org)/m (w) & SIM; 4.5 in this study) correspondingto a minimum inhibition of droplet nucleation associated with theKelvin effect; and the water accommodation coefficient increases bya factor of 2 with nanocluster hygroscopic growth, in agreement withrecent experimental studies. Overall, our results yield parametricrelations for water microphysical properties in sub-4 nm clustersand provide insight into the role of water in the initial stages ofSOA nucleation and growth.