Abstract
Abstract. The ice nucleation potential of airborne glassy aqueous aerosol particles has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 247 and 216 K. Four different solutes were used as proxies for oxygenated organic matter found in the atmosphere: raffinose, 4-hydroxy-3-methoxy-DL-mandelic acid (HMMA), levoglucosan, and a multi-component mixture of raffinose with five dicarboxylic acids and ammonium sulphate. Similar to previous experiments with citric acid aerosols, all particles were found to nucleate ice heterogeneously before reaching the homogeneous freezing threshold provided that the freezing cycles were started well below the respective glass transition temperatures of the compounds; this is discussed in detail in a separate article. In this contribution, we identify a further mechanism by which glassy aerosols can promote ice nucleation below the homogeneous freezing limit. If the glassy aerosol particles are probed in freezing cycles started only a few degrees below their respective glass transition temperatures, they enter the liquid regime of the state diagram upon increasing relative humidity (moisture-induced glass-to-liquid transition) before being able to act as heterogeneous ice nuclei. Ice formation then only occurs by homogeneous freezing at elevated supersaturation levels. When ice forms the remaining solution freeze concentrates and re-vitrifies. If these ice cloud processed glassy aerosol particles are then probed in a second freezing cycle at the same temperature, they catalyse ice formation at a supersaturation threshold between 5 and 30% with respect to ice. By analogy with the enhanced ice nucleation ability of insoluble ice nuclei like mineral dusts after they nucleate ice once, we refer to this phenomenon as pre-activation. We propose a number of possible explanations for why glassy aerosol particles that have re-vitrified in contact with the ice crystals during the preceding homogeneous freezing cycle exhibit pre-activation: they may retain small ice embryos in pores, have footprints on their surface which match the ice lattice, or simply have a much greater surface area or different surface microstructure compared to the unprocessed glassy aerosol particles. Pre-activation must be considered for the correct interpretation of experimental results on the heterogeneous ice nucleation ability of glassy aerosol particles and may provide a mechanism of producing a population of extremely efficient ice nuclei in the upper troposphere.
Highlights
IntroductionSeveral recent studies have addressed the hygroscopic behaviour, reactivity, and ice nucleation ability of glassy aerosol particles (Bodsworth et al, 2010; Koop et al, 2011; Li et al, 2011; Mikhailov et al, 2009; Murray, 2008b; Murray et al, 2010, 2012; Shiraiwa et al, 2011; Tong et al, 2011; Zobrist et al, 2008, 2011)
The ice nucleation threshold in run 2C with Raffinose (Sice = 1.12) is slightly higher than in run 1D with hydroxy-3-methoxy-DL-mandelic acid (HMMA) (Sice = 1.05, conducted only 32 min after the homogeneous freezing run), the results presented clearly indicate that the pre-activation behaviour of ice cloud processed glassy aerosol is conserved even if the www.atmos-chem-phys.net/12/8589/2012/
– Glassy aqueous aerosol particles reveal an enhanced ice nucleation ability after having been processed in a homogeneous freezing cycle that was conducted in the regime below Tg and above the upper threshold temperature where heterogeneous ice nucleation in the deposition mode on the vitrified particles could be observed
Summary
Several recent studies have addressed the hygroscopic behaviour, reactivity, and ice nucleation ability of glassy aerosol particles (Bodsworth et al, 2010; Koop et al, 2011; Li et al, 2011; Mikhailov et al, 2009; Murray, 2008b; Murray et al, 2010, 2012; Shiraiwa et al, 2011; Tong et al, 2011; Zobrist et al, 2008, 2011). Tong et al (2011) have shown that there is a time delay of several 10 to 100 seconds before an initially glassy particle of 1–8 μm in radius returns to equilibrium conditions when the relative humidity is increased from below RHg to above RHg, with RHg denoting the glass transition relative humidity. Such gradual deliquescence transition when going from amorphous to liquid particles has been observed by Mikhailov et al (2009) for levoglucosan aerosol particles. Murray et al (2012) show how the shards of shattered glassy droplets gradually merge to form a single droplet on increasing RH using optical and Raman microscopy
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