Evaluation of new photochemical sources of molecular hydrogen using two atmospheric photochemical models.

Abstract:

Rising interest in the possible use of H₂ as an alternative energy source has driven renewed focus on understanding the atmospheric H₂ budget and the implications of increased emissions for atmospheric chemistry and composition (Popa et al., 2015; Ehhalt & Rohrer, 2009; Price et al., 2007). Ground-based measurements from around the world collected since the late 1980s onwards have shown an average H₂mixing ratio of 530 ppbv(Dlugokencky et al., 2018). Most research used photochemical transport models to explain these observed values. The modelling exercises included the known anthropogenic CO sources as a proxy for H₂ sources, considering the formation of H₂ from the photolysis of H₂CO as the principle photochemical source of H₂. However, H₂ is also produced from the photochemical degradation of larger VOCs and these sources are not included in present atmospheric models. In this study, we use two atmospheric photochemical models in order to account for additional photochemical sources of H₂ that have been found recently (Harrison et al., 2019). The first is a box model using the nearly explicit Master Chemical Mechanism (MCM) that considers over 4000 organic species. We use the box model to test the effect of the new photochemistry and identify the most relevant reactions to include in reduced chemical mechanisms. These are then integrated in a new H₂ simulation in the GEOS-Chem chemical transport model (modified from v12.5.0). We provide a baseline simulation of H₂ and explore the impact of the new photochemical sources on global and regional budgets of H₂. The baseline serves also to test possible new anthropogenic sources, as well as other emission scenarios and assess uncertainties in other sources and understudied sinks of H₂.