The most common, reactive stuff in the atmosphere are OH radicals, i believe [(wikipeda)][1]. So i guess they're the natural prey for H2 molecules. Btw., as soon as it's diluted in the surrounding air,it will no longer rise. Only bulk masses have buoyancy. The g gradient in the atmosphere is much to small to seperate gases.

  [1]: https://en.wikipedia.org/wiki/Hydroxyl_radical#Importance_in_Earth.27s_atmosphere