Ice cloud processing of ultra-viscous/glassy aerosol particles leads to enhanced ice nucleation ability

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.

Contact freezing: a review of experimental studies

“…Natural aerosol particles such as bioaerosols (e.g., bacteria, pollen and fungi), volcanic ash and soil particles (e.g., mineral dust and clays) have been found to be good IN. Amorphous organic aerosols, such as citric acid, levoglucosan and raffinose (Murray et al, 2010;Wagner et al, 2012;Wilson et al, 2012), secondary organic aerosols (Wang et al, 2012;Ladino et al, 2013); and crystalline particles, such as ammonium sulfate (Abbatt et al, 2006) or hydrated sodium chloride (Wise et al, 2012) may also serve as IN. Artificial particles such as silver iodide (AgI) have been used in the laboratory and in cloud seeding studies (Wieringa and Holleman, 2006) because they were found to be efficient IN.…”

Pre-activation of aerosol particles by ice preserved in pores

“…Wagner et al (2014) called this process depositional ice growth. A detailed description of these mechanisms can be found in Wagner et al (2012Wagner et al ( , 2014. Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds has also been discussed by Adler et al (2013).…”

“…Pre-activation disappeared when the chamber temperature was raised above the glass transition temperature of the substance under investigation, but remained even if the aerosol was kept for 2.5 h below ice saturation with a minimum RH i value of 70 %. Wagner et al (2012) hypothesized that vitrification in the presence of ice crystals may leave behind a structured surface with defects or pores filled with ice. This hypothesis was confirmed by Adler et al (2013), who showed that freeze drying leads to highly porous particles.…”

“…Because aerosol particles have low terminal velocities, they need to show persistent pre-activation at the humidity conditions of the air masses they pass through. To withstand dry air conditions between cloud layers, ice in swelling pores or ice completely shielded in pockets of effloresced or glassy particles might be required (Wagner et al, 2012(Wagner et al, , 2014.…”

“…If the crustal particles detected by Prenni et al (2009) included clay minerals, these would be suited for pre-activation with ice crystal growth as soon as RH is above ice saturation. If ice is contained in pockets of carbonaceous particles, relative humidity needs to increase above the deliquescence RH or up to water saturation to release the ice and to initiate ice crystal growth (Wagner et al, 2012(Wagner et al, , 2014Adler et al, 2013).…”