shows that droplets only grew on the salt particles when the RH was approaching the deliquescence point and no droplets formed on the bare SiN. Our experimental deliquescence point was always near 75%, which was close to the literature value. The previous studies and our data strongly suggest the SiN surface did not significantly impact the water uptake dynamics or deliquescence point. Initially, images showed dry NaCl nanoparticles with irregular cuboid shape in contact with 45% RH room air (Figure , See Supporting Movie 4 for full in situ movie). Increasing RH to 66% caused the sharp corners of the NaCl cubes to dissolve and formed nearly isotropic cubes with truncated corners. A ~100 nm water layer was observed around the NaCl droplet as a lighter 'halo' around the dark contrast particle. As we increased RH to near the deliquescence RH the water layer grew into a circular droplet (303.1 s) and the NaCl particle dissolved (307.8 s). The slow increase in RH over time and high contrast between the NaCl and water droplet enabled tracking their projected areas with time and RH (Figure ). Deliquescence occurred at a measured RH of about 74%, marked by complete dissolution of the NaCl particle and a rapid increase in the growth rate of the water droplet. We compared the growth factor of an NaCl particle as a function of RH to prior measurements by Gupta et al. (Figure , Supporting Movie 3). The volume of the particle and droplet cannot be determined from the TEM images, which provide 2D projections of the objects. Instead we utilized the "2D Area Ratio" as implemented by Gupta et al., which is the ratio of the droplet projected area to the initial dry NaCl projected area as determined from optical microscopy images. Interestingly, recasting the 2D area ratio as a function of the RH revealed hygroscopic growth of the NaCl prior to the deliquescence point starting at about RH = 65%, whereas the literature measurements showed essentially no growth until the deliquescence point.