Imaging\nDissolution Dynamics of Individual NaCl Nanoparticles\nduring Deliquescence with <i>In Situ</i> Transmission Electron\nMicroscopy

Water\nvapor condensation on hygroscopic aerosol particles plays\nan important role in cloud formation, climate change, secondary aerosol\nformation, and aerosol aging. Conventional understanding considers\ndeliquescence of nanosized hygroscopic aerosol particles a nearly\ninstantaneous solid to liquid phase transition. However, the nanoscale\ndynamics of water condensation and aerosol particle dissolution prior\nto and during deliquescence remain obscure due to a lack of high spatial\nand temporal resolution single particle measurements. Here we use\nreal time <i>in situ</i> transmission electron microscopy\n(TEM) imaging of individual sodium chloride (NaCl) nanoparticles to\ndemonstrate that water adsorption and aerosol particle dissolution\nprior to and during deliquescence is a multistep dynamic process.\nWater condensation and aerosol particle dissolution was investigated\nfor lab generated NaCl aerosols and found to occur in three distinct\nstages as a function of increasing relative humidity (RH). First,\na < 100 nm water layer adsorbed on the NaCl cubes and caused sharp\ncorners to dissolve and truncate. The water layer grew to several\nhundred nanometers with increasing RH and was rapidly saturated with\nsolute, as evidenced by halting of particle dissolution. Adjacent\ncube corners displayed second-scale curvature fluctuations with no\nnet particle dissolution or water layer thickness change. We propose\nthat droplet solute concentration fluctuations drove NaCl transport\nfrom regions of high local curvature to regions of low curvature.\nFinally, we observed coexistence of a liquid water droplet and aerosol\nparticle immediately prior to deliquescence. Particles dissolved discretely\nalong single crystallographic directions, separated by few second\nlag times with no dissolution. This work demonstrates that deliquescence\nof simple pure salt particles with sizes in the range of 100 nm to\nseveral microns is not an instantaneous phase transition and instead\ninvolves a range of complex dissolution and water condensation dynamics.