title: Internally mixed sulfate and organic particles as potential ice nuclei in the tropical tropopause region.
abstract: Cirrus clouds are ubiquitous in the tropical tropopause region and play a major role in the Earth's climate. Any changes to cirrus abundance due to natural or anthropogenic influences must be considered to evaluate future climate change. The detailed impact of cirrus clouds on climate depends on ice particle number, size, morphology, and composition. These properties depend in turn on the nucleation mechanism of the ice particles. Although it is often assumed that ice nucleates via a homogeneous mechanism, recent work points to the possibility that heterogeneous ice nucleation is important in the tropical tropopause region. However, there are very few studies of depositional ice nucleation on the complex types of particles likely to be found in this region of the atmosphere. Here, we use a unique method to probe depositional ice nucleation on internally mixed ammonium sulfate/palmitic acid particles, namely optical microscopy coupled with Raman microscopy. The deliquescence and efflorescence phase transitions of the mixed particles were first studied to gain insight into whether the particles are likely to be liquid or solid in the tropical tropopause region. The ice nucleating ability of the particles was then measured under typical upper tropospheric conditions. It was found that coating the particles with insoluble palmitic acid had little effect on the deliquescence, efflorescence, or ice nucleating ability of ammonium sulfate. Additional experiments involving Raman mapping provide new insights into how the composition and morphology of mixed particles impact their ability to nucleate ice.
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Wise
authors.forename: Kelly J
authors.initials: KJ
authors.lastname: Baustian
authors.forename: Margaret A
authors.initials: MA
authors.lastname: Tolbert
mesh_terms: D000645:Ammonium Sulfate / Q000737:chemistry; D001272:Atmosphere; D057231:Climate Change; D003460:Crystallization; D005615:Freezing; D007053:Ice; D007259:Infrared Rays; D008853:Microscopy / Q000379:methods; D019308:Palmitic Acid / Q000737:chemistry; D010316:Particle Size; D052638:Particulate Matter; D013059:Spectrum Analysis, Raman / Q000379:methods; D013431:Sulfates / Q000737:chemistry*; D013696:Temperature
chemical_list: D007053:Ice; D052638:Particulate Matter; D013431:Sulfates; D019308:Palmitic Acid; D000645:Ammonium Sulfate
references.citation: Environ Sci Technol. 2002 Jun 1;36(11):2422-8
references.pmid: 12075799
references.citation: Science. 2006 Sep 22;313(5794):1770-3
references.pmid: 16946035
references.citation: Phys Chem Chem Phys. 2009 Jan 21;11(3):483-94
references.pmid: 19283265