A modeling study of the formation of cloud condensation nuclei in the jet regime of aircraft plumes

The formation of cloud condensation nuclei in the jet regime of a B‐747 airliner at cruise has been investigated by modeling studies. Both the formation of H2O/H2SO4 clusters by homogeneous nucleation and the deposition of water vapor on soot particles activated by the adsorption of gaseous H2SO4, sulfuric acid hydrates, and H2O/H2SO4 clusters were taken into account. H2SO4 has been assumed to be formed only by OH oxidation of SO2 in the plume. Whereas at ambient temperatures between 219 and 224 K the heterogeneous condensation leads to ice particles with average diameters between 3.0 and 1.1 μm for soot emission indices of EI(soot) = (0.05 − 0.5) g/kg, respectively, no heterogeneous condensation occurs at higher temperatures. Homogeneously nucleated H2O/H2SO4 clusters, on the other hand, have diameters of less than 7 nm and do not contribute to visible contrail formation. Assuming different sulfur emission indices (0.1 g/kg≤EI(SO2)≤10 g/kg), we conclude that the contrail onset is essentially independent of this quantity and that a fractional H2SO4 surface coverage corresponding to a 0.1 monolayer (ML = 0.1) must be sufficient to activate the soot particles for H2O uptake, at least for EI(SO2)≥0.5 g/kg fuel. Calculations based on higher threshold values (i.e., 0.1