The smoke number is dominated by the largest soot particles collected onto a filter. Sampling soot particles smaller than about 300 nm on such filters becomes inefficient. Because soot emissions depend strongly on engine types, power settings, and flight levels, additional information is generally needed to relate smoke number to emissions under flight conditions. However, details of the size distribution and physicochemical properties of soot under flight conditions are generally not known and cannot be inferred from smoke number data. These values continually decrease as the plume dilutes.
Much less information is available concerning the hydration properties of exhaust soot.
In the initial stages of formation, graphite-like soot particles are hydrophobic. Soot particles fresh from jet engines very likely become hydrophilic by oxidation processes or deposition of water-soluble species present in the exhaust. Irregular surface features and chemically active sites can also increase chemical reactivity and amplify heterogeneous nucleation processes. Heterogeneous nucleation of H 2 SO 4 hydrates on soot was found to be unlikely under plume conditions. 3.2.3.2 Metal Particles Aircraft jet engines also directly emit metal particles.