Abstract
Abstract. 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 organics. The hygroscopic phase transitions of organic-sulfate particles can have an impact on both the cirrus cloud formation mechanism and resulting cloud microphysical properties. In addition to deliquescence and efflorescence, organic-sulfate particles are known to undergo another phase transition known as liquid–liquid phase separation. The ice nucleation properties of particles that have undergone liquid–liquid phase 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 behavior of 2 : 1 mixtures of organic polyols (1,2,6-hexanetriol and 1 : 1 1,2,6-hexanetriol + 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol) and ammonium sulfate from 240–265 K. Further, the ice nucleation efficiency of these organic-sulfate systems after liquid–liquid phase separation and efflorescence was investigated from 210–235 K. Raman mapping and volume-geometry analysis indicate that these particles contain solid ammonium 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 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 become 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.
Highlights
Methods and Cirrus clouds are ubiquitouDs ianttaheSupypsetretrmopsosphere and are efflorescence, organic-sulfate particles are known to undergo known to have a large effect on the Earth’s climate
Raman microscopy coupled with an environmenpresent in the tropical tropGopeaousseclaieyenr t(iTfiTcL) 50–80 % of tshpehetriimc eim(pJeonrtsaeMnnceoetbdeacel.a,lu2sDe01teh0ev).yeTclahonepsdeemhcyleodurnadttse are of atmoair ascending tal cell was used to study the low temperature del- through the TTL and, effectively regulate stratoiquescence, efflorescence, and liquid–liquid phase separation behavior of 2 : 1 mixtures of organic polyols (1,2,6-hexanetriol and 1 : 1 1,2,6-hexanetriol + 2,2,6,6tetrakis(hydroxymethyl)cyclohexanol) and ammonium sulfate from 240–265 K
Ments, we find that if the organic coatings are liquid, wa- ground aerosols as well as sub-visible cirrus residues are enter vapor diffuses through the shell and ice nucleates on the hanced in sulfates and organics and have little aerosol typammonium sulfate core
Summary
Methods and Cirrus clouds are ubiquitouDs ianttaheSupypsetretrmopsosphere and are efflorescence, organic-sulfate particles are known to undergo known to have a large effect on the Earth’s climate. Wise et al (2010) have shown that coating ammonium sulfate with palmitic acid minimally affected the ice nucleation efficacy of ammonium sulfate over a similar temperatures range to Koehler et al (2010) Raman analysis of these particles, revealed that this behavior could be attributed to uneven coating by the organic material. Laboratory work in our group has shown glassy sugars are excellent heterogeneous ice nucleators (Baustian et al, 2012b) These combined works suggest that (semi-)solid amorphous organic coatings may enhance, not deactivate, the ice nucleation ability of phase-separated particles. We studied the low temperature deliquescence, liquid–liquid phase separation, and efflorescence behavior of mixed organic-sulfate particles using 1,2,6-hexanetriol or an equal mass ratio of 1,2,6hexanetriol/2,2,6,6-tetrakis(hydroxymethyl) cycohexanol as the organic species. We studied the heterogeneous ice nucleation behavior of the pure organics and mixed organic-sulfate particles to help elucidate the role of phaseseparated particles in ice cloud formation
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