Hydration properties of original engine soot produced by burning TC1 aviation kerosene in aircraft engine combustor are studied under simulated plume conditions. Engine-generated soot demonstrates a high level of water uptake due to the existence of fraction of impurities originating within an engine. It contains 13.5 wt% water soluble compounds including 3.5 wt% organic and inorganic sulfates. Total amount of water vapor absorbed by engine soot particles increases with the temperature decrease in the exhaust plume and reaches 18 wt% at threshold contrail formation conditions. Chemical processing of soot particles in the young plume through the coagulation with sulfate aerosols and H2SO4/H2O heterogeneous nucleation leads to 0.2 wt% of H2SO4 accumulating at high fuel sulfur content. Such a small amount of additional H2SO4 cannot result in the increase of water uptake by the fraction of impurities of engine-generated soot particles but may activate the hydrophobic fraction that originally contains a negligible amount of water soluble compounds. HNO3 processing may compete with H2SO4 in respect to increase of water uptake. It is inferred that the fraction of impurities of engine-generated soot plays a key role in the CCN formation in the aircraft plume.