The problem with using the bomb in Europe is that it would have crossed into other parts of Europe and killed a lot on non-Germans.
I don’t think they had a real handle on fallout that early in the scheme of things.
I mean, we dosed a good portion of the country with the Nevada tests.
If an atomic bomb had been dropped on Nazi Germany (as on Japan), it would probably have killed few non-Germans (except for any non-Germans around Ground-Zero). The two bombs dropped on Japan exploded around 2000 feet, and resulted in much of the fission products going into the stratosphere where the short-lived fission products decayed before the dispersed and diluted fallout eventually came down throughout the world.
Any local fallout around Ground Zero would have landed on people already exposed to the blast, fireball, or lethal levels of prompt radiation from the explosion.