Most of what you see is water vapor condensed in the cold air.
But, when you think of it, there has got to be more. Each airliner takes off with tons of jet fuel. What comes out of the tailpipe? I suspect the only way you would end up with
only water vapor would be to burn only hydrogen and oxygen. Anyone here know the combustion efficiency of a jet engine? What else is in the exhaust gas when cruising along at flight level 38? Oxides of nitrogen? Unburned hydrocarbons? Any sulfur in the mix? I don't know.
How long before California decides to put catalytic converters on airliners?
Best
Les