Soil deposition of atmospheric hydrogen constrained using planetary-scale observations

Abstract. Quantifying soil deposition fluxes remains the greatest source of uncertainty in the atmospheric H2 budget. A new method is presented to constrain H2 deposition schemes in global models using observations of the zonal mean H2 distribution and seasonality. A “best-fit” scheme that reproduces the observed zonal-mean seasonality of atmospheric H2 at the planetary scale is found by perturbing a prototype deposition scheme based on soil temperature and moisture dynamics. Comparing the best-fit and prototype schemes provides insight into how the prototype scheme may be improved to better reproduce observed seasonality. The H2 signal driven by the prototype scheme is accurate compared to observations in the Northern Hemisphere extratropics but shows discrepancies in the Southern Hemisphere, with surface mixing ratios that are too high and seasonality that is too weak. A best-fit scheme indicates that the function capturing the soil microbial consumption of H2 requires a shift of +2 to +3 months in the seasonality in the tropics, with peak uptake shifting from February to April in the southern tropics and from August to October in the northern tropics, compared with a prototype scheme sensitive to seasonal soil moisture driven by the shifting of the Intertropical Convergence Zone. New constraints on the H2 surface flux at low latitudes are key to accurately modelling the H2 cycle in the Southern Hemisphere.