The Impacts of Hydrogen on Tropospheric Ozone and their Modulation by Background NOx

A shift in our energy production is crucial to the control of global warming. This will occur as fossil fuels are phased out, following legislation created to reach the targets set out in the Paris Agreement. One of the possible sources for a low carbon energy landscape is renewable hydrogen. Whilst hydrogen represents an alternative energy store, it can leak from the system. Understanding the fate of leaked hydrogen is vital to quantify the implications of this energy transition. This study uses the atmospheric version of the United Kingdom Earth System Model to analyse the impact of hydrogen on the atmosphere. The model indicates that increased atmospheric hydrogen leads to an increase in tropospheric ozone concentrations. Ozone is a greenhouse gas and therefore there is an indirect atmospheric warming due to hydrogen emission through ozone. Understanding the relationship between hydrogen and the chemical ozone budget is therefore required to dissect how this warming occurs. We find that hydrogen increases ozone production, governed by the increased flux through the reaction of HO₂ with NO. Future atmospheric nitrogen oxide concentrations are expected to decrease in the coming decades, under most climate scenarios. Understanding the relationship between hydrogen and background NO_x concentrations is therefore crucial in determining the mechanisms of how hydrogen is expected to impact future atmospheres. We use the model to calculate the tropospheric global warming potential of hydrogen and how this is altered by changing background NO_x. We find that this tropospheric GWP will stay relatively constant alongside decreases in ground level anthropogenic NO_x.