Heterogeneous nucleation of ice particles on glassy aerosols modifies TTL cirrus

Experiments at the AIDA chamber, Karlsruhe Institute of Technology, have shown that glassy aqueous citric acid aerosol can nucleate ice at temperatures relevant to the tropical tropopause layer (TTL)(1). Modelling suggests this new route to the formation of TTL cirrus can provide an explanation for the very low ice particle number density observed in cirrus clouds in this region and may lead to high in-cloud supersaturations(1). Nucleation of ice on glassy aerosol is consistent with the absence of traditional ice nuclei in sampled TTL cirrus residue(2). In addition, we will present new data from experiments performed in July 2010 at the AIDA chamber using glassy aerosols composed of other atmospherically relevant compounds (levoglucosan, raffinose) and an internal mixture of five dicarboxylic acids and ammonium sulphate (raffinose/M5AS)(3). All four systems tested nucleate ice when in a glassy state. This indicates that heterogeneous ice nucleation is a general property of glassy aerosols and that natural aerosols which are composed of similar molecules will also nucleate ice if glassy. Glassy aqueous levoglucosan and raffinose/M5AS aerosol nucleated ice at temperatures similar to those found for glassy aqueous citric acid aerosol (<202 K). Whereas raffinose, which forms a glass at much higher temperatures, nucleated ice heterogeneously at up to ~220 K. This activity at higher temperatures suggests that ice nucleation by glassy aerosol may also play a role in the formation of warmer ice clouds. (1)B. J. Murray et al., Heterogeneous nucleation of ice particles on glassy aerosols under cirrus conditions, Nature Geosci, 2010, 3, 233-237. (2)K. D. Froyd et al., Aerosols that form subvisible cirrus at the tropical tropopause, Atmos. Chem. Phys., 2010, 10, 209-218. (3)B. Zobrist et al., Do atmospheric aerosols form glasses?, Atmos. Chem. Phys., 2008, 8, 5221-5244.