Using the comparative kinetic electrodynamic balance approach, hygroscopic growth curves for a water activity from ~0.5 up to ~0.99 are reported in Fig , along with equilibrium solution compositions (expressed as a mass fraction of solute) in Fig , and compared with sodium chloride (NaCl) solution droplets. For droplets larger than 100 nm diameter, the water activity in a solution droplet can be assumed to equal the environmental RH, neglecting the influence of droplet surface curvature. The diameter growth factor, GFd, is defined as the ratio of the wet droplet diameter at the specified RH to the dry particle diameter in the limit of no moisture content (0% RH). Using -Köhler theory, we can define the RH dependence of the equilibrium droplet diameter by: