The overwhelming role of soils in the global atmospheric hydrogen cycle

The removal of molecular hydrogen (H2) 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 H2 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 H2 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 H2 has a lifetime of only 1.4 (±0.2) years – significantly shorter than the recent estimate of ~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 H2, 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 H2 helps to explain the fairly homogeneous distribution of H2 in the troposphere.