Aviation and the Global Atmosphere

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Figure 3-5: Soot activation and heterogeneous freezing in young aircraft exhaust plumes (Kļæ½rcher, 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.

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