H2 is an assumption you've made. When talking about a nuclear explosion. There is hydrogen in a number of compounds (notably water and hydrogen sulfide) present

H2 is an assumption you've made. When talking about a nuclear explosion. There is hydrogen in a number of compounds (notably water and hydrogen sulfide) present in the atmosphere that could be relevant sources. The ppm being incidental is another issue. You've been unnecessarily condescending to an honest question.