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Efflorescence as a Contact Sport:  Phase changes in Atmospheric Particulate upon Collisions

Because homogeneous salt efflorescence typically requires low relative humidity (RH), atmospheric salt particles are often assumed to be aqueous throughout much of their atmospheric lifetime.  Here we use a long working distance optical trap to examine heterogeneous efflorescence that occurs when a supersaturated salt droplet comes into contact with a solid particle.  We compare our findings for pure salt droplets with those obtained for salt/organic droplets where the organic may be found either in a core shell structure or as an amorphous/glassy solid.   We find that in many cases, single collisions with a range of nuclei promote efflorescence at relatively high RH.  Further, we find that  the same nucleus is much more effective in initiating nucleation in contact mode than when immersed within the droplet.  These laboratory experiments challenge our theoretical understanding of nucleation in microscopic salt and mixed organic salt particles.   However, the observations of effective contact nucleation implies that salt particles could be solid for more of their atmospheric lifetime than currently assumed.