Atmospheric molecular hydrogen budget
Atmospheric Molecular Hydrogen (H2) Budget
Atmospheric molecular hydrogen budget refers to the inventory of processes introducing or removing H2 from the atmosphere. H2 is the second most abundant reduced gas of the atmosphere, after methane. Even with its low mixing ratio (530 ppbv, which corresponds to 0.000053%), H2 is involved in the complex reactions that are maintaining the atmosphere in its current oxidation state. Opposite to the atmospheric concentrations of carbon dioxide , methane and nitrous oxide, there is currently no consensus for long-term atmospheric H2 trend, but it is generally assumed that H2 sources and sinks are at equilibrium. Once emitted in the atmosphere, H2 as a lifetime of 1 – 2 years before being either oxidized by the hydroxyl radicals or consumed through a microbial-mediated soil uptake. Controversial modelling studies proposed that a future H2-based economy could alter the steady state of H2 in the atmosphere. Indeed, loss of H2 during its large-scale production, storage and transport could enhance the global emission factor and the atmospheric burden of H2. Although the main sources and sinks of atmospheric H2 are relatively well constrained (see the accompanying figure), the potential impact of global change on H2 atmospheric burden remains to be ascertained.
Methane and non-methane hydrocarbons oxidation (NMHC)
Methane and NMHC photochemical oxidation is the most important source of H2. In the atmosphere, generation of H2 from methane and NMHC oxidation occurs within a complex set of reactions leading to the formation of formaldehyde. Once generated, formaldehyde has a very short atmospheric lifetime because of its fast reaction with hydroxyl radicals or its photodissociation in H2 and carbon monoxide.
Industries and fossil fuel
H2 is utilized in the chemical and petrochemical industries to produce ammonia, refined petroleum products, but also in metallurgic, electronic and pharmaceutical industries. H2 monitoring realized in urban areas suggested that combustion of fossil fuel represented the main source of H2. In urban areas, atmospheric H2 concentrations follow a bimodal diurnal cycle, correlated with the morning and afternoon traffic peaks. No assessments about the contribution of industries to the H2 budget are currently available, but quantities of H2 originating from industrial and fossil fuel are estimated based on carbon monoxide emission inventories and their corresponding H2/CO emission ratios.
Chemical characterization of experimental forest fire air masses suggested that for each emitted carbon dioxide molecule, 0.033 H2 molecules were emitted, corresponding to a global H2 emission factor around 9 to 21 Tg(H2) yr-1 due to biomass burning. Uncertainties in that term of the H2 budget arise from the measured H2:CO2 ratio which is dependent of the combustion efficiency, a factor determined by the type of biomass and the nature of the combustion; smoldering or flaming combustions.
Nitrogen fixation by-products
Production of H2 as nitrogen fixation by-products has been noticed during enzymatic assays involving purified nitrogenases. For each nitrogen molecule fixed by the nitrogenase, one H2 molecule is emitted. Importance of nitrogen fixation for atmospheric H2 budget has been assessed by measuring the H2 soil to air exchanges in a variety of ecosystems. Contrary to grassland, forest and desert, fields of nitrogen-fixing leguminous plants are a net source of H2.
Water samples collected at different stations located in the Atlantic Ocean and in the Mediterranean Sea revealed that surface waters were generally supersaturated with H2. Vertical profiles of dissolved H2 concentrations typically decreased with depth, but should also contain distinct areas enriched with H2, corresponding to biomass-rich zones. Depending on the studied sites, it was proposed that cyanobacteria or anaerobic microorganisms were responsible of the elevated dissolved H2 concentrations detected.
Oxidation by hydroxyl radicals
Because of its hydroxyl radicals-mediated oxidation reaction, H2 is seen as an indirect greenhouse gas. Indeed, H2 oxidation exerts indirect incidences on methane and ozone concentrations, the latter being two greenhouse gases. Having a hydroxyl radicals-mediated oxidation rate similar to methane, the addition of H2 in the atmosphere would diminish the availability of the hydroxyl radicals to oxidize methane, resulting to an increase of its atmospheric lifetime and then, its global warming potential.
Microbial soil uptake
Microbial-mediated H2 soil uptake is the most important sink for atmospheric H2. H2 soil uptake has been discovered in the 1970’s by transferring soil samples in a closed system into which a first-order decrease of the H2 headspace concentrations was observed. H2 soil uptake activity was inhibited by heat sterilization of the soil, the absence of oxygen or the addition of antimicrobial agents. However, considering that soils chloroform, acetone or toluene fumigations inhibited only partially H2 consumption, the activity was attributed to free soil hydrogenases. This concept has however been challenged recently following the isolation of specialised actinobacteria demonstrating the ability to consume atmospheric H2.
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