Effect of Secondary Organic Coating on the Ice Nucleation Ability of Solid Ammonium Sulphate Aerosol

Cirrus clouds are cold clouds that form in the upper troposphere. Climatology of cirrus cloud coverage varies from 15-20% in South America to 50% over equatorial regions of Africa (Hahn and Warren, 2007). Cirrus clouds are made of ice crystals that can form both via homogeneous and heterogeneous ice nucleation mechanisms. It is important to determine when and which of these two modes prevails in order to describe cirrus microphysical properties. These, in fact, are needed to properly evaluate the impact of cirrus clouds on climate. The competition between homogeneous and heterogeneous ice nucleation strongly depends on the thermodynamic conditions and on the abundance and types of ice nucleating particles.

Sulphate and organics are among the most abundant aerosol components measured in cirrus residuals (Froyd et al., 2009 and 2010). Ammonium sulphate in its solid form can act as an ice nucleating particle, promoting ice formation via the heterogeneous nucleation mode (Abbatt et al., 2006). However, when crystalline ammonium sulphate is suspended in the atmosphere for hours or days it can undergo different ageing mechanisms that modify its chemical-physical properties. Condensation of secondary organic material on the aerosol surface is one of these aging mechanisms. A couple of studies have already shown the effect of organic coatings on the ice nucleation ability of mineral dust at cirrus conditions (e.g., Möhler et al., 2008). In this contribution, we report the effect of a thin coating layer of secondary organic material derived from the oxidation of α-pinene and toluene on crystalline ammonium sulphate.

The coating was performed at different temperatures (-45°C, -50°C and -60°C) and the ice nucleation ability of the aerosol particles was measured at different temperatures as well (-50°C, -54°C and-60°C). The experiments were performed at the AIDA cloud chamber. The ice nucleation ability of the aerosol was investigated by means of a cylindrical continuous flow diffusion chamber (the Ice Nucleation instrument of the Karlsruhe Institute of Technology, INKA) and via expansion cooling experiments in the AIDA chamber. The amounts of ammonium sulphate and organic material condensed on the seed aerosol particles was monitored with an Aerosol Mass Spectrometer (HR-ToF-AMS). Our experiments show the progressive deactivation of the solid ammonium sulphate particles to form ice heterogeneously with the increase of condensed organic material. The ice onset is progressively shifted towards higher values of supersaturation with respect to ice, until the homogeneous ice nucleation threshold is reached. These results highlight the importance of understanding how the different aging mechanisms can modify the aerosol particles’ properties and therefore their impact on clouds and climate.