Resolving the Role of Contact Ice Nucleation on the Earth�??s Climate System Using Laboratory and Field Studies

The most recent report of the Intergovernmental Panel on Climate Change (IPCC; Solomon et al., 2007) states that among the most uncertain processes in our understanding of climate change is the interplay between aerosol particles and cloud formation. More recently, this has been shown using direct observations of the Earth�??s energy balance by Murphy et al. (2009). Clouds can be composed of liquid water droplets, ice crystals, or mixtures of the two phases. The formation of water droplets is relatively well understood due to formation conditions near room temperature and clouds that can be located at or near ground level. Conversely, ice formation requires temperatures and water vapor contents considerably lower than what is typical found at ground level so that studies at high altitudes or latitudes are required. Because of this, ice formation is the less well understood process and currently limits our ability to determine future climate change. There are several mechanisms by which ice nucleation can take place (Pruppacher and Klett, 1997). The most studied are the deposition of water vapor to a particle surface (�??deposition freezing�??) and from within a droplet (�??immersion freezing�??). Less studies, and consequently less well understood, is ice nucleation upon the contact of two particles (�??contact freezing�??). It is noteworthy that due to current instrumental limitations and experimental complexity recent efforts at parameterizing ice nucleation have attempted to address deposition and immersion but not contact freezing (e.g. DeMott et al., 2010). Recent field studies at mountain-top locations and from research aircraft at high altitudes and latitudes have suggested which particles are most likely to form ice in the atmosphere via the deposition and immersion modes. What are now needed are in-depth studies of the ice nucleation processes, in particular contact nucleation, and these are best conducted in the laboratory using controlled conditions and high precision characterization techniques. We propose a two-part study of contact ice nucleation. We assume the particles that have been suggested as ice nuclei during recent field studies of immersion and deposition nucleation as a starting point for contact freezing: mineral dusts and metallic particles (e.g., DeMott et al., 2003a). Particles such as soot and biological material will also be studied. The first part of this study will be to determine the ice formation conditions of each particle type due to contact freezing. These experiments will utilize a Fourier transform infrared spectrometer and a single particle mass spectrometer which will be used to detect freezing onset conditions and determine if contact nucleation has taken place, respectively. The second part of this study will be to deploy this technique in a field study. The proposed location is a mountaintop site with access to free tropospheric aerosol. As a results we will be able to determine onset conditions and the composition and size of atmospherically relevant contact ice nuclei.