Zuko smiles when he's finally recognised, although it's tinged with sadness. It really has been a long time. He returns the bow, though, trying to remain focused on the tasks at hand.

"I'm very glad to see you again, Aang... but I'm afraid your presence here is one of those issues." Then he gestures to the barrier. "This is another. It's preventing anyone entering or leaving the city, and there seems to be no way through it."