Hygroscopic growth measurements on aerosolized particles are ideally performed in the aerosol phase; however, a sample substrate was required to image the nanoscale dynamics of particle dissolution and droplet growth. Prior work has shown that the surface could have an impact on the hygroscopic growth. Chatre et al. showed that anisotropic droplets can grow on ammonium sulfate on a hydrophilic surface during imaging with environmental SEM. However, there was no indication in this study that a hydrophilic surface significantly impacted the deliquescence RH. Indeed, several prior studies measured the deliquescence point of salt particles on hydrophobic surfaces and achieved reasonable values. The contact angle of a saturated NaCl solution on the silicon nitride was measured to be 86 ± 0.17° (Figure ). Control experiments indicated that the SiN surface alone did not condense water. Supporting movie 3