Temporal and spatial variability of the stable isotopic composition of atmospheric molecular hydrogen: observations at six EUROHYDROS stations

Despite the potential of isotope measurements to improve our understanding of the global atmospheric molecular hydrogen (H(2)) cycle, few H(2) isotope data have been published so far. Now, within the EUROpean network for atmospheric HYDRogen Observations and Studies project (EUROHYDROS), weekly to monthly air samples from six locations in a global sampling network have been analysed for H(2) mixing ratio (m(H(2))) and the stable isotopic composition of the H(2) (delta(D,H(2)), hereafter referred to as delta D). The time series thus obtained now cover one to five years for all stations. This is the largest set of ground station observations of delta D so far. Annual average delta D values are higher at the Southern Hemisphere (SH) than at the Northern Hemisphere (NH) stations; the maximum is observed at Neumayer (Antarctica), and the minimum at the non-arctic NH stations. The maximum seasonal differences in delta D range from approximate to 18 parts per thousand at Neumayer to approximate to 45 parts per thousand at Schauinsland (Southern Germany); in general, seasonal variability is largest at the NH stations. The timing of minima and maxima differs per station as well. In Alert (Arctic Canada), the variations in delta D and m(H(2)) can be approximated as simple harmonic functions with a approximate to 5-month relative phase shift. This out-of-phase seasonal behaviour of delta D and m(H(2)) can also be detected, but delayed and with a approximate to 6-month relative phase shift, at Mace Head and Cape Verde. However, no seasonal delta D cycle could be observed at Schauinsland, which likely reflects the larger influence of local sources and sinks at this continental station. At the two SH stations, no seasonal cycle could be detected in the delta D data. If it is assumed that the sink processes are the main drivers of the observed seasonality in m(H(2)) and delta D on the NH, the relative seasonal variations can be used to estimate the relative sink strength of the two major sinks, deposition to soils and atmospheric oxidation by the hydroxyl (OH) radical. For the NH coastal and marine stations this analysis suggests that the relative contribution of soil uptake to the total annual H(2) removal increases with latitude.