The physico-chemical properties of aircraft engine soot are characterized with respect to their ability to act as CCN. Comparison with laboratory-generated kerosene soot shows a significant influence of combustion conditions on the morphology, microstructure, chemical composition, surface nature, and hygroscopicity of soot. Engine soot particles separate into two components based on composition and structural heterogeneities: a main soot fraction and a fraction of impurities containing an appreciable amount of metal and sulfur. The high concentration of soluble sulfates, of inorganics and of organics in the fraction that contains impurities, explains the engine soot hygroscopicity and its ability to act as CCN at threshold conditions for contrail formation. Laboratory--generated kerosene soot is not able to reproduce the hygroscopicity of engine soot, but we show that it is a good surrogate for the insoluble black carbon fraction of aircraft soot in the upper troposphere.