AFM has visualized droplet formation on hygroscopic nanoparticles, but only reveals droplet surface topography with no information about the droplet interior, such as the aerosol particle dissolution dynamics. Optical microscopy can observe particle dissolution and recrystallization, but has a spatial resolution limit of ~500 nm. Electrodynamic balance measures mass growth factor of individual aerosol particles as a function of RH, but provides little information about the aerosol particle morphology or dissolution dynamics during deliquescence. Environmental transmission electron microscopy (ETEM) and environmental scanning electron microscopy (ESEM) can image individual hygroscopic nanoparticles with nanometer scale spatial resolution during deliquescence. In ETEM the use of differential pumping apertures around the sample region enables keeping the sample, typically a standard carbon coated TEM grid, at a higher pressure (~10 mbar) than the high vacuum in the microscope.