Multi-model diagnostics and uncertainties of atmospheric H2 chemical production and loss terms within the HYway project

Hydrogen (H₂) is expected to be a crucial substitute for fossil fuels in the ongoing energy transition. However, atmospheric H₂ is recognized as an indirect greenhouse gas, as it can contribute to global warming through its coupling to the atmospheric oxidative capacity, including interactions with methane, ozone and stratospheric water vapour. Yet, large uncertainties still remain in the atmospheric H₂ budget.

Within the framework of the HYway Horizon Europe project, we quantify and diagnose key chemical drivers of uncertainty in simulated H₂ by focusing on two components: chemical production linked to formaldehyde (HCHO) and chemical destruction controlled by the hydroxyl radical (OH). HCHO photolysis is a major source of atmospheric H₂, accounting for more than half of the global total. We firstly assess the HCHO chemical production, loss, and global burden across multiple HYway models. The global HCHO burden range from 0.69 to 1.03Tg, indicating a wide inter-model difference of 50%. Therefore, we evaluate the simulated HCHO by ground- and satellite-based observations. The contribution of biogenic hydrocarbons emissions to the HCHO budget is discussed. Oxidation by OH is the second-largest sink of atmospheric H₂ after soil uptake. However, the modeled OH is typically overestimated by global chemical models. Here, we examine uncertainty in H₂ chemical loss by characterizing the range of OH simulated across HYway models, and by comparing simulated OH with observation-constraint OH fields. Finally, we present a free-running H₂ simulation (driven by emissions rather than prescribed surface H₂ concentrations) and show that the resulting H₂ fields are consistent with observations. Overall, this work provides an integrated evaluation of the chemical controls on H₂ in global models and a basis for improving H₂ projections under future emission scenarios.