The overwhelming role of soils in the global atmospheric hydrogen cycle
The removal of molecular hydrogen (H2) from the atmosphere is dominated by the up- take 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 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 1 recent estimate of ∼2 years – which is expected to decrease in the future. In addition, 5 variations of H2 and its D/H isotopic ratio in the northern hemisphere allow an inde- pendent, better constrained estimate. We derive that 82% of the annual turnover of 0 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 com- pounds 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 1 to explain the fairly homogeneous distribution of H2 in the troposphere.