A MODELING STUDY OF THE FORMATION OF CLOUD CONDENSATION NUCLEI IN THEJET REGIME OF AIRCRAFT PLUMES

The formation of cloud condensation nuclei in the jet regime of a B-74 7 airliner at cruise has been investigated by modeling studies. Both t he formation of H2O/H2SO4 clusters by homogeneous nucleation and the d eposition 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 ox idation of SO2 in the plume. Whereas at ambient temperatures between 2 19 and 224 K the heterogeneous condensation leads to ice particles wit h average diameters between 3.0 and 1.1 mu m for soot emission indices of EI(soot) = (0.05 - 0.5) g/kg, respectively, no heterogeneous conde nsation occurs at higher temperatures. Homogeneously nucleated H2O/H2S O4 clusters, on the other hand, have diameters of less than 7 nm and d o not contribute to visible contrail formation. Assuming different sul fur emission indices (0.1 g/kg less than or equal to EI(SO2) less than or equal to 10 g/kg), we conclude that the contrail onset is essentia lly independent of this quantity and that a fractional H2SO4 surface c overage 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) g reater than or equal to 0.5 g/kg fuel. Calculations based on higher th reshold values (i.e., 0.1 < ML less than or equal to 0.3) lead to resu lts which are in disagreement with the onset of contrail formation as deduced by visual observations. Moreover, the present modeling study p rovides an estimate of the effect of mutual coupling of homogeneous an d heterogeneous condensation pathways.