Basically this Air Departure Duty is just good old Labour envy. It is no different than taxing 4X4 vehicles more.
However we should remember that it was a Tory Chancellor, Ken Clarke, who thought it a good wheeze to introduce ADD.
The nonsense in all this is that transit passengers create just as much pollution as a joining passenger, but do not have this pernicious duty levied on them. This whole 'Green' thing is just rubbish anyway.
Go tell it to the Chinese and Indians.