This figure from Wikipedia's Atmosphere of Earth shows a hydrogen fraction of 0.000055 percent by volume.

Question: Where does molecular hydrogen in the atmosphere come from?

Does this come directly from biological production like much of the CO2 and CH4, or is this coming from geological sources or breakdown of methane by atmospheric chemistry for example, or someplace else?

Related: What are the sources of molecular hydrogen in human breath?

Gas proportions in the Earth's lower atmosphere; "The proportion of water vapor (H₂O) is variable so it is not represented in this chart, however it averages about 1% in the troposphere."

Atmosphere gas proportions

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    $\begingroup$ I can't give a per-centage but there is some contribution from refineries and chemical plants. H is difficult to contain and small leaks at gaskets are common . These are generally small leaks that require test equipment to detect. Several different processes use high pressure H so a great number of flanges. It can also diffuse through steel at temperatures above 500F and process temperatures to 900 F are common. Also welding must release some H but I am unaware of any measurements; The only H from the welding that is of concern is the H that enters the steel an may embrittle it. $\endgroup$ – blacksmith37 Mar 25 at 21:24
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  • $\begingroup$ @KeithMcClary do you think there is enough there for an answer? So far no other explanations have been proposed. $\endgroup$ – uhoh Jun 20 at 7:37
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    $\begingroup$ I don't think I will write it (I would just copypaste from wiki). I see there is recent interest in geological sources: h2euro.org/latest-news/… , researchgate.net/publication/… , greeleytribune.com/news/… (the latter has some good info between the crackpotty bits). $\endgroup$ – Keith McClary Jun 21 at 3:13

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