Effect of soot on immersion freezing of water and possible atmospheric implications

More than 3000 water droplets with soot particles immersed were monitored individually to identify the effect of heterogeneous freezing and establish the link between soot properties and freezing efficiency. A set of soot samples from many combustion sources, including an aircraft engine combustor were examined. The list of key soot properties affecting freezing were determined. It appears that soot effectiveness in causing the droplets to freeze mainly depends on the soot behaviour in water defined by the mass density of soot agglomerates, size and wetting. When large mass particles were homogeneously distributed through droplets and were stable to sedimentation effects median freezing temperatures of such droplets were significantly warmer than pure water droplets. The long-term presence of large soot agglomerates in water leads to an increase in the freezing efficiency over time because of the slow kinetics of wetting. Small sizes of soot agglomerates stimulate aggregative instability in droplets with large soot mass which decreases the total soot surface area and the freezing temperature. The highest freezing efficiency is found for homogeneously distributed soot agglomerates of high porosity. A gradual increase of the freezing temperature in correspondence with number density of active oxygen-containing sites is observed for soots having surfaces of low or intermediate polarity. However, highly soluble surface compounds, such as sulfates, may be easily dissolved in water leading to a decrease in the potential freezing efficiency. This was the case found for aircraft engine combustor generated soot. Therefore relatively hydrophobic soot with a significant number of oxygen-containing functional groups but not covered totally by hydrophilic sites and soluble compounds is proposed to act as the most efficient immersion freezing nuclei in the atmospheric cloud droplets.