Wetting and hydration of insoluble soot particles in the upper troposphere.

Wettability and hydration are determined for aircraft combustor and laboratory-made soots which are used as surrogates for the insoluble part of aircraft-generated black carbon particles in the upper troposphere (UT). The measured water/ice contact angles on the soot surfaces are in the range 60-80 degrees. Factors influencing the soot wetting show a tremendous dependence on the surface chemical composition and microstructure. Wetting characteristics of soots are directly related to its hydrophilicity. The inverse Kelvin effect is considered as a mechanism of ice nucleation which is facilitated by the soot agglomerated structure with interparticle cavities in which condensation takes place on the insoluble surface with a high water contact angle. Estimations of the critical supersaturations needed for the ice condensation growth of particles are provided to determine which of the wetting characteristics are required for cirrus cloud formation in ice saturated regions of the UT.