What typically happens to H2 molecules released into atmosphere (with no spark source)? I'm wondering what happens when $\ce{H2}$ leaks from say a transmission pipe in an unenclosed area. No immediate source of ignition, I know it rises more quickly than helium (x2 I believe) and dissipates in concentration quickly but does it react with gases in atmosphere to form methane or other compounds? 

Short of ignition and burning causing more $\ce{H2}$ to dissociate what can happen, especially in upper atmosphere with more UV and so on. Methane has reaction in the upper atmosphere giving it a half life of ~7 years for example.


  The covalent radius of a neutral hydrogen atom is 0.0371 nm, smaller
  than that of any other element. Because small atoms can come very
  close to each other, they tend to form strong covalent bonds. As a
  result, the bond dissociation enthalpy for the $\ce{H-H}$ bond is relatively
  large (435 kJ/mol). $\ce{H2}$ therefore tends to be unreactive at room
  temperature. In the presence of a spark, however, a fraction of the $\ce{H2}$ molecules dissociate to form hydrogen atoms that are highly reactive. Source: chemed.chem.purdue.edu
The most common, reactive stuff in the atmosphere are $\ce{-OH}$ radicals (Wikipedia). So I guess they're the natural antagonist for $\ce{H2}$ 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 too small to separate gases.
In the end, hydrogen however does diffuse upwards and is ultimately lost into space. Hydrogen and helium are light enough to be able to reach escape velocity in the upper atmosphere.