Figure 3-5: Soot activation and heterogeneous freezing
in young aircraft exhaust plumes (K�rcher, 1998a). Observational evidence
suggests the existence of a sulfur-free pathway (starting with the dashed
arrow) for freezing of ice at threshold formation conditions. This pathway
is probably dominant for low and very low fuel sulfur levels. The sulfur-enhanced
path (solid arrows) is controlled by adsorption of oxidized sulfur molecules,
water vapor, and scavenging of H2SO4/H2O droplets. Activation into water droplets
occurs when the relative humidity in the plume exceeds 100%. A few ice particles
may nucleate below liquid water saturation (dashed arrow). Well below threshold
formation temperatures, homogeneous freezing of volatile droplets from the
nucleation mode (in which case ice crystals initially contain no soot inclusions)
is thought to dominate over soot-induced immersion freezing in the formation
of contrail ice particles. The hexagon is a schematic representation of ice
particle shapes that are close to spherical in young contrails but may vary
in aging contrails. Soot cores may reside inside the ice particles or be attached
at their surfaces.