The tropospheric cycle of H₂: a critical review

The literature on the distribution, budget and isotope content of molecular hydrogen (H²) in the troposphere is critically reviewed. The global distribution of H² is reasonably well established and is relatively uniform. The surface measurements exhibit a weak latitudinal gradient with 3% higher concentrations in the Southern Hemisphere and seasonal variations that maximize in arctic latitudes and the interior of continents with peak-to-peak amplitudes up to 10%. There is no evidence for a continuous long-term trend, but older data suggest a reversal of the interhemispheric gradient in the late 1970s, and an increase in the deuterium content of H² in the Northern Hemisphere from 80 standard mean ocean water (SMOW) in the 1970s to 130 today. The current budget analyses can be divided in two classes: bottom up, in which the source and sink terms are estimated separately based on emission factors and turnovers of precursors and on global integration of regional loss rates, respectively. That category includes the analyses by 3-D models and furnishes tropospheric turnovers around 75 Tg H² yr−1. The other approach, referred to as top down, relies on inverse modelling or analysis of the deuterium budget of tropospheric H². These provide a global turnover of about 105 Tg H² yr⁻¹. The difference is due to a much larger sink strength by soil uptake and a much larger H² production from the photochemical oxidation of volatile organic compounds (VOC) in the case of the top down approaches. The balance of evidence seems to favour the lower estimates—mainly due to the constraint placed by the global CO budget on the H² production from VOC. An update of the major source and sink terms yields: fossil fuel use 11±4 TgH² yr⁻¹; biomass burning (including bio-fuel) 15 ± 6 Tg H² yr⁻¹; nitrogen fixation (ocean) 6 ± 3 Tg H² yr−1; nitrogen fixation (land) 3 ± 2 Tg H² yr⁻¹; photochemical production from CH₄ 23 ± 8 Tg H² yr−1 and photochemical production from other VOC 18 ± 7 Tg H² yr⁻¹. The loss through reaction of H² with OH is 19 ± 5 Tg H² yr⁻¹, and soil uptake 60+30 −20 Tg H² yr⁻¹. All these rates are well within the ranges of the corresponding bottom up estimates in the literature. The total loss of 79 Tg H² yr⁻¹ combined with a tropospheric burden of 155 Tg H² yields a tropospheric H² lifetime of 2 yr. Besides these major sources of H², there are a number of minor ones with source strengths > 1 Tg H² yr⁻¹. Rough estimates for these are also given.