In this study, we employ a microelectromechanical system (MEMS) based in situ TEM sample cell to visualize water condensation and dissolution dynamics of cuboid NaCl nanoparticles with single nanometer spatial resolution prior to and during deliquescence. Atmospheric aerosols are generally a mix of inorganic and organic compounds and NaCl is representative of highly hygroscopic aerosol particle well-known to readily deliquesce under relevant atmospheric humidity conditions. Real time movies of water condensation and aerosol particle dissolution with increasing RH unveiled complex dynamics prior to deliquescence. The use of scanning TEM (STEM) imaging provided high contrast between the NaCl particle and surrounding water droplet that permitted detailed quantitative analysis of droplet growth and aerosol particle dissolution dynamics. Aerosol particle dissolution and water condensation occurred in three qualitatively distinct stages with increasing RH, including adsorption of a ~100 nm water layer, temporal fluctuations in particle shape, and intermittent dissolution dynamics not previously observed by other single particle methods. This work unveils complex dynamics leading up to deliquescence and challenges the common conception that deliquescence of nanosized aerosol particles is a single-stage instantaneous phase transition. We expect that the in situ TEM methodology established here will become a powerful approach to investigate more complex multicomponent, multiphase aerosols and their interaction with water.