The overwhelming role of soils in the global atmospheric hydrogen cycle

The removal of molecular hydrogen (H-2) from the atmosphere is dominated by the uptake in soils. Notwithstanding, estimates of the magnitude of this important process on a global scale are highly uncertain. The CARIBIC aircraft observations of the seasonal variations of H-2 and its D/H isotopic ratio in the Northern Hemisphere allow an independent, better constrained estimate. We derive that 82% of the annual turnover of tropospheric H-2 is due to soil uptake, equaling 88 (+/- 11) Tg a(-1), of which the Northern Hemisphere alone accounts for 62 (+/- 10) Tg a(-1). Our calculations further show that tropospheric H-2 has a lifetime of only 1.4 (+/- 0.2) years - significantly shorter than the recent estimate of similar to 2 years - which is expected to decrease in the future. In addition, our independent top-down approach, confined by the global and hemispheric sinks of H-2, indicates 64 (+/- 12) Tg a(-1) emissions from various sources of volatile organic compounds by photochemical oxidation in the atmosphere. This estimate is as much as up to 60% larger than the previous estimates. This large airborne production of H-2 helps to explain the fairly homogeneous distribution of H-2 in the troposphere.