Understanding and quantifying chemical uncertainties in the hydrogen budget

The potential transition to a hydrogen-based economy, requires a comprehensive understanding of hydrogen's atmospheric behaviour for well-informed decision-making. Among the uncertainties surrounding the atmospheric fate of hydrogen, the chemical processes governing its formation and transformation are pressing.

This study employs STOCHEM-CRI, a global 3D tropospheric chemical transport model, to explore the chemical uncertainty associated with atmospheric hydrogen. The primary objective is to improve our understanding of the hydrogen distribution, sources, and sinks on a global scale. Addressing the significant role of formaldehyde (HCHO) as a chemical source, we update its photolysis parameterisation in accordance with recent recommendations (JPL 2020 and IUPAC 2013) and assess its variability. Furthermore, we evaluate the atmospheric burden of HCHO as a function of its sources to identify key photochemical contributors to the present hydrogen budget.

The study undertakes preliminary studies of the major sink of atmospheric hydrogen, namely uptake by soil, to gauge its impact. Through a meticulous examination of model outputs against observational data, various scenarios are systematically assessed for their ability to accurately replicate global hydrogen distribution and seasonal variations.

Preliminary results show updates to the photochemical parameters of HCHO significantly reduce the hydrogen burden by between 50 and 90 ppb globally. This is namely due to updates to the quantum yield of the molecular (H2 producing) photolysis channel which varies significantly when compared to previous recommendations. There is limited variation between the two updates (JPL 2020 and IUPAC 2013) of up to 5 ppb. Additionally, minor updates relating to the temperature dependence of the soil sink result in significant improvement in the models replication of observational data, including seasonal variation.