UC Irvine

Understanding the global hydrogen (H2) budget is critical as H2 is expected to play an important role in future energy systems. Tropospheric H2 sources include direct emissions and atmospheric production via chemical reactions, while sinks are soil uptake and removal by hydroxyl radical (OH). Large uncertainties remain in quantifying the atmospheric production and loss of H2 due to large uncertainties in H2 uptake in the soil and the lack of global-scale knowledge of the abundance of OH. We use a suite of global three-dimensional Atmospheric Chemistry Models (ACM) to evaluate key reactive species involved in atmospheric production and loss-formaldehyde (HCHO), nitrogen dioxide (NO2), and carbon monoxide (CO). A box model is then used to simulate the evolution of global mean tropospheric H2 from pre-industrial to present day; to test different relative contributions in atmospheric production from methane and Volatile Organic Compounds (VOC); and to assess atmospheric loss with different OH concentrations. Isotopic compositions of the different sources and sinks are further used to constrain these terms and assess the potential importance of geological sources to the H2 budget. Models generally match each other for HCHO, though model diversity exists for NO2 and CO. From model evaluations and box model constraints, we estimate atmospheric H2 production of 37–60 Tgyr^-1, and atmospheric losses of 15–30 Tgyr^-1, suggesting that some top-down literature estimates may overestimate production. Box model results suggest an upper bound of 9 Tgyr^-1 for geological sources, considerably lower than the 23 Tgyr^-1 proposed previously. We recommend more isotopic observations and targeted measurement campaigns to further refine the budget.