The Hybrid Activity Model (HAM) for water uptake was developed to consider water uptake by both adsorption and hygroscopic growth. While this model and prior light scattering experiments have suggested water uptake follows this mechanism, the model and measurements could not capture the dynamics of water uptake or particle shape change and dissolution. Our work clearly demonstrates with single particle measurements that water absorption is an important water uptake mechanism for hygroscopic particles. It further demonstrates that empirical single particle measurements are required to understand water uptake and particle dissolution dynamics prior to and at the deliquescence RH. relevant to the atmosphere and well-known for its propensity to swell and form droplets at both sub and supersaturated RH conditions. This work enhances our understanding of deliquescence of hygroscopic aerosol particles using direct nanoscale visualization to unveil complex dissolution phenomena occurring prior to deliquescence, even for highly hygroscopic particles such as NaCl.