High-resolution measurements of atmospheric molecular hydrogen and its isotopic composition at the West African coast of Mauritania

Abstract

Oceans are a net source of molecular hydrogen (H₂) to the atmosphere, where nitrogen (N₂) fixation is assumed to be the main biological production pathway followed by photochemical production from organic material. The sources can be distinguished using isotope measurements because of clearly differing isotopic signatures of the produced hydrogen. <br><br> Here we present the first ship-borne measurements of atmospheric molecular H₂ mixing ratio and isotopic composition at the West African coast of Mauritania (16–25° W, 17–24° N). This area is one of the biologically most active regions of the world's oceans with seasonal upwelling events and characterized by strongly differing hydrographical/biological properties and phytoplankton community structures. The aim of this study was to identify areas of H₂ production and distinguish H₂ sources by isotopic signatures of atmospheric H₂. For this more than 100 air samples were taken during two cruises in February 2007 and 2008. During both cruises a transect from the Cape Verde Islands towards the Mauritanian Coast was sampled to cover differing oceanic regions such as upwelling and oligotrophic regimes. In 2007, additionally, four days were sampled at high resolution of one sample per hour to investigate a possible diurnal cycle of atmospheric H₂. <br><br> Our results indicate the influence of local sources and suggest the Banc d'Arguin as a pool for precursors for photochemical H₂ production, whereas oceanic N₂ fixation could not be identified as a source for atmospheric H₂ during these two cruises. The variability in diurnal cycles is probably influenced by released precursors for photochemical H₂ production and also affected by a varying origin of air masses. This means for future investigations that only measuring the mixing ratio of H₂ is insufficient to explain the variability of an atmospheric diurnal cycle and support is needed, e.g. by isotopic measurements. Nevertheless, measurements of atmospheric H₂ mixing ratios, which are easy to conduct online during ship cruises, could be a useful indicator of production areas of biological precursors such as volatile organic compounds (VOCs) for further investigations

Pulses of labile carbon cause transient decoupling of fermentation and respiration in permeable sediments

“…H 2 isotopic composition shows a depleted source; the source signature of À779‰ (calculated using a Keeling plot, Supporting Information Fig. S2) indicating high confidence of a H 2 O H 2 equilibrium (often interpreted as microbial origin), though it is not possible to differentiate between sources (Walter et al 2012(Walter et al , 2013.…”

The Under-Explored Possibilities of Ground State Carbonyl Photochemistry

“…[73][74][75] Contemporary models underestimate the concentration of H 2 in the atmosphere compared to field observations by ∼30-50%, indicative that photolytic sources of H 2 are missing from atmospheric models. 76,77 Photolysis of VOCs in the troposphere is the main source of H 2 in the atmosphere, and is estimated to account for 40-70% of total atmospheric H 2 . [78][79][80][81] Formaldehyde is known to account for ∼50% of this photolytic H 2 , 82 while the mechanism(s) generating the other half of photolytic H 2 is unknown.…”

Isotopic evidence for biogenic molecular hydrogen production in the Atlantic Ocean

“…Molecular hydrogen (H 2 ) is the second most abundant reduced compound in the atmosphere after methane (CH 4 ). H 2 is not a radiatively active gas itself, but -via its role in atmospheric chemistry -it indirectly influences the lifetime of the greenhouse gas CH 4 and several air pollutants (Prather, 2003;Schultz et al, 2003;Tromp et al, 2003;Warwick et al, 2004;Jacobson, 2005Jacobson, , 2008Feck et al, 2008;Ehhalt and Rohrer, 2009;Popa et al, 2015). The main H 2 sources are photo-oxidation of CH 4 and non-methane volatile organic compounds (NMVOCs) in the atmosphere and combustion processes at the surface, whereas soil deposition and oxidation by hydroxyl radicals (HO • ) are the main sinks.…”