The ice nucleation potential of airborne glassy aqueous aerosol particleshas been investigated by controlled expansion cooling cycles in the AIDAaerosol and cloud chamber of the Karlsruhe Institute of Technology attemperatures between 247 and 216 K. Four different solutes were used asproxies for oxygenated organic matter found in the atmosphere: raffinose,4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and amulti-component mixture of raffinose with five dicarboxylic acids andammonium sulphate. Similar to previous experiments with citric acidaerosols, all particles were found to nucleate ice heterogeneously beforereaching the homogeneous freezing threshold provided that the freezingcycles were started well below the respective glass transition temperaturesof the compounds; this is discussed in detail in a separate article. In thiscontribution, we identify a further mechanism by which glassy aerosols canpromote ice nucleation below the homogeneous freezing limit. If the glassyaerosol particles are probed in freezing cycles started only a few degreesbelow their respective glass transition temperatures, they enter the liquidregime of the state diagram upon increasing relative humidity(moisture-induced glass-to-liquid transition) before being able to act asheterogeneous ice nuclei. Ice formation then only occurs by homogeneousfreezing at elevated supersaturation levels. When ice forms the remainingsolution freeze concentrates and re-vitrifies. If these ice cloud processedglassy aerosol particles are then probed in a second freezing cycle at thesame temperature, they catalyse ice formation at a supersaturation thresholdbetween 5 and 30% with respect to ice. By analogy with the enhanced icenucleation ability of insoluble ice nuclei like mineral dusts after theynucleate ice once, we refer to this phenomenon as pre-activation. We proposea number of possible explanations for why glassy aerosol particles that havere-vitrified in contact with the ice crystals during the precedinghomogeneous freezing cycle exhibit pre-activation: they may retain small iceembryos in pores, have footprints on their surface which match the icelattice, or simply have a much greater surface area or different surfacemicrostructure compared to the unprocessed glassy aerosol particles.Pre-activation must be considered for the correct interpretation ofexperimental results on the heterogeneous ice nucleation ability of glassyaerosol particles and may provide a mechanism of producing a population ofextremely efficient ice nuclei in the upper troposphere.
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