The tropospheric cycle of H₂: a critical review

Abstract

A B S T R A C TThe literature on the distribution, budget and isotope content of molecular hydrogen (H 2 ) in the troposphere is critically reviewed. The global distribution of H 2 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 H 2 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 H 2 yr −1 . The other approach, referred to as top down, relies on inverse modelling or analysis of the deuterium budget of tropospheric H 2 . These provide a global turnover of about 105 Tg H 2 yr −1 . The difference is due to a much larger sink strength by soil uptake and a much larger H 2 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 H 2 production from VOC. An update of the major source and sink terms yields: fossil fuel use 11 ± 4 Tg H 2 yr −1 ; biomass burning (including bio-fuel) 15 ± 6 Tg H 2 yr −1 ; nitrogen fixation (ocean) 6 ± 3 Tg H 2 yr −1 ; nitrogen fixation (land) 3 ± 2 Tg H 2 yr −1 ; photochemical production from CH 4 23 ± 8 Tg H 2 yr −1 and photochemical production from other VOC 18 ± 7 Tg H 2 yr −1 . The loss through reaction of H 2 with OH is 19 ± 5 Tg H 2 yr −1 , and soil uptake 60Tg H 2 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 H 2 yr −1 combined with a tropospheric burden of 155 Tg H 2 yields a tropospheric H 2 lifetime of 2 yr. Besides these major sources of H 2 , there are a number of minor ones with source strengths <1 Tg H 2 yr −1 . Rough estimates for these are also given.

Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

“…Storage fluxes of H 2 were calculated, but were typically small (<| 1 nmol m −2 s −1 |), and were therefore not included in the comparison. The midday summertime H 2 uptake rates correspond to H 2 deposition velocities of 0.04 to 0.10 cm s −1 , which were within the range of previously reported soil H 2 deposition velocities, so our results support the previously reported values that typically range between 0.01 and 0.10 cm s −1 (Ehhalt and Rohrer, 2009).…”

“…The major sources and sinks are nearly balanced so atmospheric H 2 concentrations are stable. Although the global atmospheric H 2 budget has been derived through a variety of methods, it remains poorly constrained at the regional level and disputed at the global level, and a process-based understanding is lacking (as reviewed by Ehhalt and Rohrer, 2009). Therefore, there are large uncertainties in the estimated impact of changes to the H 2 biogeochemical cycle that might arise from changes in energy use, land use, and climate.…”

Impact of a possible future global hydrogen economy on Arctic stratospheric ozone loss

“…The current production (76 Tg yr À1 ) and loss rates (79 Tg yr À1 ) yield a tropospheric H 2 lifetime of 2 years. 44,45 Today about 50% of the total tropospheric H 2 production can be considered anthropogenic. 44 The average vertical distribution of H 2 in the lower and middle stratosphere proves to be relatively uniform as in the troposphere.…”

“…The reaction of H 2 with OH and soil uptake are the major sinks of H 2 in the troposphere. The strength of each term is given in the literature [38][39][40][41][42][43][44] and compiled in Ehhalt and Rohrer 44 (2009). The current production (76 Tg yr À1 ) and loss rates (79 Tg yr À1 ) yield a tropospheric H 2 lifetime of 2 years.…”

Accurate measurements of carbon monoxide in humid air using the cavity ring-down spectroscopy (CRDS) technique

“…Atmospheric carbon monoxide (CO) plays an important role in tropospheric chemistry (Logan et al, 1981), and acts as a useful tracer for emissions of CO 2 , CH 4 , and H 2 from biomass and fossil fuel burning (Andreae and Merlet, 2001;Levin and Karstens, 2007;Ehhalt and Rohrer, 2009;Turnbull et al, 2009). A considerable number of techniques have been employed to perform atmospheric measurements of CO, such as nondispersive infrared spectroscopy (NDIR) (Dickerson and Delany, 1988), vacuum ultraviolet resonance fluorescence (VURF) (Gerbig et al, 1999), tunable diode laser absorption spectroscopy (TDLAS) (Sachse et al, 1987), closed path Fourier transform infrared (FTIR) absorption (Griffith et al, 2012), gas chromatography combined with a mercuric oxide detector or a flame ionization detector (GC/HgO or GC/FID) (Novelli, 1999) and, more recently, quantum cascade laser (QCL) (McManus et al, 2008), integrated cavity output spectroscopy (ICOS) (O'Keefe, 1998), and cavity ring-down spectroscopy (CRDS) (Crosson, 2008).…”