THE INITIAL COMPOSITION OF JET CONDENSATION TRAILS

Physicochemical processes that generate and transform aerosols in jet aircraft plumes are discussed on the basis of theoretical models and r ecent observations of young contrails in the upper troposphere. The in itial evolution of optical depth and ice water content under threshold contrail formation conditions is studied. Constrained by the measurem ents, a lower bound is deduced for the number density of ice crystals initially present in contrails. This bound serves as a visibility crit erion for young contrails. An analysis of the primary contrail particl es (aqueous solution droplets nucleated in situ, emitted insoluble com bustion aerosols, and entrained background aerosols) reveals that only soot must be involved as ice forming nuclei if the visibility criteri on is to be fulfilled. Possible activation pathways of the soot aeroso ls are investigated, including an analysis of their wetting behavior a nd droplet scavenging and heterogeneous nucleation properties. To supp ort these investigations, results of laboratory experiments concerning contact angles of acidic solution droplets on carbonaceous surfaces a nd the freezing probability of sulfuric acid tetrahydrate are presente d. Assuming that the soot particles acquire a liquid coating, heteroge neous freezing rates and their sensitivity on important parameters are studied.