Phase transition and ice nucleation behaviour of organic-containing aerosols

The atmospheric aerosol consists of a mixture of different inorganic and organic compounds. Depending on altitude and geographic region, the organic fraction may contribute to even more than 50% of the total aerosol mass. A significant fraction of the total organic mass is water soluble and contains oxygenated functional groups such as COOH, OH, and C=O. Given the broad diversity of organic species, it is not possible to formulate a uniform concept for describing the ice-nucleating potential of organic-containing aerosols. The ice nucleation ability will depend on whether an aerosol particle will tend to remain in a supercooled liquid state at low temperatures or will exhibit phase transitions to crystalline or amorphous solid phases (see Mikhailov et al., Atmos. Chem. Phys. 9, 9491-9522, 2009). In recent AIDA investigations, citric acid, various dicarboxylic acids, and secondary organic aerosol from ozonolysis of α-pinene were used as proxy substances for studying the ice nucleation behaviour of atmospheric organic material.

The experiments with citric acid were motivated by recent observations that aerosol particles containing certain organic molecules may form glasses (disorderd amorphous solids) at temperatures encountered in the upper troposphere (Zobrist et al., Atmos. Chem. Phys. 8, 5221-5244, 2008; B. J. Murray, Atmos. Chem. Phys., 8, 5423-5433, 2008). In such glassy or ultra-viscous aerosol particles, the diffusion of liquid water molecules to ice nuclei will be very slow, thereby explaining the experimentally observed inhibition of ice crystallization. Citric acid was chosen as a proxy because it contains OH and COOH functional groups typical of oxygenated organic compounds known to exist in atmospheric aerosols and because it features a well-characterized glass transition at temperatures found in the upper troposphere. In cooperation with Benjamin J. Murray from the University of Leeds, controlled expansion cooling experiments with airborne citric acid aerosol particles were performed in the AIDA chamber at temperatures above and below the glass transition. Above the glass transition temperature, the liquid citric acid solution droplets froze at supersaturation levels close to the values expected for homogeneous nucleation within aqueous solution droplets. Below the glass transition temperature, however, a fraction of the glassy solution droplets nucleated ice at much lower relative humidity than that required for homogeneous freezing of liquid particles.

Reference:
Heterogeneous nucleation of ice particles on glassy aerosols under cirrus conditions
Benjamin J. Murray, Theodore W. Wilson, Steven Dobbie, Zhiqiang Cui, Sardar M.R.K. Al-Jumur, Ottmar Möhler, Martin Schnaiter, Robert Wagner, Stefan Benz, Monika Niemand, Harald Saathoff, Volker Ebert, Steven Wagner, and Bernd Kärcher
Nature Geoscience, 3, 233-237, 2010 (doi:10.1038/ngeo817).

Dicarboxylic acids experiments

The hygroscopic behaviour (deliquescence and efflorescence phase transitions) and ice nucleating abilities of low molecular weight dicarboxylic acids have been investigated in various recent laboratory studies. These compounds have been identified as an important contribution to the water-soluble organic fraction of tropospheric aerosols, typically comprising 1 – 3 % of the total particulate organic carbon in urban and semi-urban areas and up to 10 % in remote continental and marine environments. Oxalic acid is usually found to be the most abundant species, followed by malonic and succinic acid.

The starting point for our AIDA investigations were the ice freezing experiments from Zobrist et al. (Atmos. Chem. Phys., 6, 3115-3129, 2006). The authors have specifically investigated the potential of several dicarboxylic acids as heterogeneous ice nuclei in the immersion mode, arguing that in most cases these substances would be part of a multi-component mixture in atmospheric aerosol particles. The freezing experiments were performed with emulsified binary dicarboxylic acid/water solutions of different solute concentrations and selected ternary solutions with e.g. sulphuric acid, ammonium sulphate, and sodium chloride as additional solute species. Our intention was to investigate whether it is possible to reproduce the results from cooling cycles with emulsified solutions by controlled expansion cooling experiments with airborne particles, conducted in the AIDA chamber. In a first manuscript, currently under review for ACP, we report on the high variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate particles in the deposition and condensation mode at temperatures between 244 and 228 K.

Reference:
High variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate
Robert Wagner, Ottmar Möhler, Harald Saathoff, Martin Schnaiter, and Thomas Leisner
Atmos. Chem. Phys. Discuss., 10, 11513-11575, 2010.

Secondary organic aerosol experiments