Hey guys, I'm writing a story where Earth suffers a catastrophic event, destroying all surface and ocean life. The surface is basically bathed in intense radiation. To be more specific, Earth is attacked by aliens, and ships destroyed during the battle in orbit crash into the planet below, spreading massive amounts of radioactive debris (from reactors, nuclear weapons etc). The aliens might even deliberately "salt the fields" with radioactive materials to ensure complete destruction. My question is this: if a person (who could survive the radiation) returned to Earth, say 30 years after this event, would the atmosphere still be breathable? What would happen to the composition of the atmosphere, especially oxygen and CO2 content? I assume that O2 would decrease, but what would be the main pathways for that to happen, and how long would it take?