State transformations and ice nucleation in amorphous (semi-)solid organic...

Baustian, K., M. Wise, E. Jensen, G. Schill, M. Freedman, and M. Tolbert (2013), State transformations and ice nucleation in amorphous (semi-)solid organic aerosol, Atmos. Chem. Phys., 13, 5615-5628, doi:10.5194/acp-13-5615-2013.

Amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual amorphous (semi-)solid particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic and ammonium sulfate were also investigated.

Results from water uptake experiments followed the shape of a humidity-induced glass transition (Tg(RH)) curve and were used to construct state diagrams for each organic and corresponding mixture. Experimentally derived Tg (RH) curves are in good agreement with theoretical predictions of Tg(RH) following the approach of Koop et al. (2011). A unique humidity-induced glass transition point on each state g′ diagram, T (RH), was used to quantify and compare results from this study to previous works. Values of Tg′(RH) determined for sucrose, glucose and citric acid glasses were 236, 230 and 220 K, respectively. Values of Tg′ (RH) for internally mixed organic/sulfate particles were always significantly lower; 210, 207 and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively.

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Atmospheric Composition Modeling and Analysis Program (ACMAP)