Decade-long Observations of Hydrogen Soil Uptake and Traffic Emissions in a Suburban Environment (Gif-sur-Yvette, France)

To mitigate climate change, alternative energy carriers are required to replace fossil fuels. Hydrogen is widely regarded as a promising candidate, and the hydrogen economy is currently expanding. However, hydrogen is an indirect greenhouse gas, with estimates of its 100-year global warming potential reaching up to 12.8.  A robust assessment of its climate impact therefore requires a detailed understanding of atmospheric hydrogen sources and sinks. Despite this need, observational studies of atmospheric hydrogen remain relatively limited and most are over short measurement periods. Long-term measurements are essential for identifying trends and characterizing decadal variability. Therefore, this study presents an analysis of ten years (2006–2017) of tropospheric hydrogen measurements conducted at a suburban site in Gif-sur-Yvette, France. The in-situ observations show an average baseline hydrogen concentration of (518 ± 18) ppb, characterized by a seasonal cycle, with maxima between April and June and minima between September and November. Both the baseline levels and the seasonal pattern remain stable over the full decade of measurements. The most uncertain component of the atmospheric hydrogen budget is uptake by soils. Using the radon tracer method applied to nighttime data, soil hydrogen uptake was quantified consistently over the ten-year period. An average deposition velocity of (2.8 ± 0.5) · 10⁻² cm s⁻¹ was obtained, with stronger uptake during the summer. Despite some interannual variability, no significant long-term trend in soil uptake is observed, providing rare observational evidence for the decadal stability of this major hydrogen sink. Diurnal cycles of hydrogen and carbon monoxide exhibit distinct morning peaks associated with traffic emissions. These were used to derive the H2/CO ratio, a key parameter for estimating hydrogen emissions from traffic based on carbon monoxide inventories. An average ratio of 0.56 ± 0.05 was determined, which likewise shows no systematic trend over the decade. Overall, this study provides decade-long observations of hydrogen, demonstrating the long-term stability of baseline concentrations, soil uptake and traffic-related emission ratios.