Why would timing be critical? Consider a jihadi flying a Cessna 182 with a nuke inside. The plane has plenty of carrying capacity and it is too slow to make a getaway after laying down the weapon. It is a nuclear suicide bomber with the ability to fly to the optimum point in space and push a detonation switch.
The airburst might only be viable if the ground security situation was just too tight. Even then, I can't imagine a Cessna getting past fully loaded "high cover" from F16's into a downtown area. Even an airburst at the correct altitude will probably be not as effective radiation wise. But then....a nuke is a nuke is a nuke. Right?
Speaking of high cover. I remember in the first few days after 911, when everything was grounded and the USAF was over every major city and loaded fer bear, an F15 screamed in over my portable classroom at about 1000 feet and kicked in his afterburners. The kids screamed and were absolutely terrified. I yelled at them to run outside and look up, which they did. we were just in time to see the bird scream out of sight in a vertical (ballistic) climb, afterburners just mere bright dots against the blue sky. As the sound rolled over Miami and died away I shouted to them as they craned their necks up:
"Hear that? That's the SOUND OF FREEDOM! They are there, risking their lives to keep YOU safe." I was never prouder to be both an American and a veteran. Most of my students are Haitian immigrants. It made an impression.