Heterogeneous ice nucleation on phase-separated organic-sulfate particles: Effect of liquid vs. glassy coatings
Atmospheric ice nucleation on aerosol particles relevant to cirrus clouds remains one of the least understood processes in the atmosphere. Upper tropospheric aerosols as well as sub-visible cirrus residues are known to be enhanced in both sulfates and 5 organics. The hygroscopic phase transitions of organic-sulfate particles can have an impact on both the cirrus cloud formation mechanism and resulting cloud microphysi- cal properties. In addition to deliquescence and efflorescence, organic-sulfate particles are known to undergo another phase transition known as liquid-liquid phase separa- tion. The ice nucleation properties of particles that have undergone liquid-liquid phase 10 separation are unknown. Here, Raman microscopy coupled with an environmental cell was used to study the low temperature deliquescence, efflorescence, and liquid-liquid phase separation be- havior of 2 :1 mixtures of organic polyols (1,2,6-hexanetriol, and 1 : 1 1,2,6-hexanetriol +2,2,6,6-tetrakis(hydroxymethyl)cycohexanol) and ammonium sulfate from 240–265K. 15 Further, the ice nucleation efficiency of these organic-sulfate systems after liquid-liquid phase separation and efflorescence was investigated from 210–235 K. Raman map- ping and volume-geometry analysis indicates that these particles contain solid ammo- nium sulfate cores fully engulfed in organic shells. For the ice nucleation experiments, we find that if the organic coatings are liquid, water vapor diffuses through the shell 20 and ice nucleates on the ammonium sulfate core. In this case, the coatings minimally affect the ice nucleation efficiency of ammonium sulfate. In contrast, if the coatings be- come semi-solid or glassy, ice instead nucleates on the organic shell. Consistent with recent findings that glasses can be efficient ice nuclei, the phase separated particles are nearly as efficient at ice nucleation as pure crystalline ammonium sulfate.