1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, 80309, USA
2Department of Atmospheric and Oceanic Science, University of Colorado, Boulder, CO, 80309, USA
3NASA Ames Research Center, Moffett Field, CA, USA
4Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309, USA
5Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, USA
*now at: School of Earth and Environment, University of Leeds, Leeds, UK
**now at: College of Theology, Arts and Sciences, Concordia University, Portland, OR, 97211, USA
Abstract. Glassy or 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 aqueous organic glass 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 species and ammonium sulfate were also investigated.
Results from water uptake experiments were used to construct glass transition curves and state diagrams for each organic and corresponding mixture. A unique glass transition point on each state diagram, Tg', was used to quantify and compare results from this study to previous works. Values of Tg' determined for aqueous sucrose, glucose and citric acid glasses were 236 K, 230 K and 220 K, respectively. Values of Tg' for internally mixed organic/sulfate particles were always significantly lower; 210 K, 207 K and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively.
All investigated organic species were observed to serve as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at Sice=1.1–1.4 for temperatures between 235 K and 200 K. Particles consisting of 1:1 organic-sulfate mixtures remained liquid over a greater range of conditions but were in some cases also observed to depositionally nucleate ice at temperatures below 202 K (Sice=1.25–1.38).
Glass transition curves constructed from experimental data were incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol will be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation (Sice>1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Combined with the low saturation ratios measured for ice nucleation, this work suggests heterogeneous ice formation on glassy substances may play an important role in cirrus cloud formation.