Molecular hydrogen in the near‐surface atmosphere and dissolved in waters of the tropical North Atlantic

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Abstract
The concentration of H2 has been measured in near‐surface air and at fifteen hydrographic stations across the tropical North Atlantic. For both air and seawater samples H2 was analyzed in the gas phase by the HgO technique. Air samples taken 10 m above the ocean surface gave a mean value of 0.65 ± 0.02 ppmv at mid‐ocean and showed higher values near Africa. A composite of all seawater analyses showed dissolved H2 varying in the upper 50 m from 4 to 73 nl H2/l H2O and averaging 18.5 nl/l. Below the mixed layer, H2 dropped quickly to near the equilibrium value (9.8 nl/l), but subsurface structure allowed occasional supersaturation to 20 nl/l. In the deepest water, below 1500 m, H2 was found to be undersaturated with respect to the present atmosphere. The northern hemispheric oceanic source strength has been calculated to be 0.33 × 1012 g H2/yr, which is only one quarter the previous estimate. Subsurface maxima within the pycnocline, reminiscent of methane maxima, were twice observed. The data indicate H2 dissolved in the open ocean may be much less variable than previously suspected from existing measurements.