Abstract
Abstract. Homogeneous nucleation of ice within aqueous solution droplets and their subsequent crystallisation is thought to play a significant role in upper tropospheric ice cloud formation. It is normally assumed that homogeneous nucleation will take place at a threshold supersaturation, irrespective of the identity of the solute, and that rapid growth of ice particles will follow immediately after nucleation. However, it is shown here through laboratory experiments that droplets may not readily freeze in the very cold tropical tropopause layer (TTL, typical temperatures of 186–200 K). In these experiments ice crystal growth in citric acid solution droplets did not occur when ice nucleated below 197±6 K. Citric acid, 2-hydroxypropane-1,2,3-tricarboxyllic acid, is a molecule with similar functionality to oxygenated organic compounds which are ubiquitous in atmospheric aerosol. It is therefore thought to be a sensible proxy for atmospheric organic material. Evidence is presented that suggests citric acid solution droplets become ultra-viscous and form glassy solids under atmospherically relevant conditions. Diffusion of liquid water molecules to ice nuclei is expected to be very slow in ultra-viscous solution droplets and nucleation is negligible in glassy droplets; this most likely provides an explanation for the experimentally observed inhibition of ice crystallisation. The implications of ultra-viscous and glassy solution droplets for ice cloud formation and supersaturations in the TTL are discussed.
Highlights
Ice clouds that form in the Earth’s upper troposphere (UT) are known to play a significant role in the planet’s radiation budget and climate (Liou, 1986), but mankind’s impact on these clouds is uncertain (Denman et al, 2007)
An important mechanism of UT ice cloud formation is through the homogeneous freezing of aqueous solution droplets
It is well known in the food industry and the field of biological tissue preservation that the addition of oxygenated organic compounds, such as sugars, to aqueous solution can lead to the formation of glassy solids (Angell, 2002)
Summary
Ice clouds that form in the Earth’s upper troposphere (UT) are known to play a significant role in the planet’s radiation budget and climate (Liou, 1986), but mankind’s impact on these clouds is uncertain (Denman et al, 2007). An important property of oxygenated organic compounds is their ability to form extensive hydrogen bonding; molecules of relatively large mass may be soluble in water It is well known in the food industry and the field of biological tissue preservation that the addition of oxygenated organic compounds, such as sugars, to aqueous solution can lead to the formation of glassy solids (Angell, 2002). The properties include the glass transition temperature (Maltini et al, 1997) The objective of this laboratory investigation was to investigate the impact of an oxygenated organic compound relevant to the Earth’s atmosphere on the nucleation and growth of ice crystals in aqueous droplets. The implications of highly viscous or glassy solution droplets for UT cloud formation are discussed
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