Influence of the oxidation of non-methane volatile organic compounds on tropospheric hydrogen: A STOCHEM-CRI global Lagrangian model study

In this study, a global Lagrangian chemistry-transport model, STOCHEM-CRI, was employed to study the formation of hydrogen (H₂) from the oxidation of twenty anthropogenic and natural biogenic non-methane volatile organic compounds (VOCs). The base case STOCHEM-CRI model was perturbed by applying a step-change to the emissions of each VOC in turn. Differences between each perturbed case and the base case were used to derive the responses in the fluxes through the formaldehyde photolysis source of H₂ and in the oxidation rate of each VOC. H₂ yields were largest for the natural biogenic VOCs: isoprene, α- and β-pinene and least, close to zero, for ethyne and benzene. Differences in yields were driven by the number of carbon atoms, by the chemical mechanisms of the hydroxyl radical and ozone reactions and by the involvement of long-lived oxidation products. These responses were then used to estimate the global H₂ source strengths from the oxidation of anthropogenic and natural biogenic VOCs. Ethene oxidation dominated the global H₂ source from the oxidation of anthropogenic VOCs, totalling 1.6 Tg yr⁻¹. Isoprene oxidation dominated the natural biogenic VOC oxidation source, totalling 20 Tg yr⁻¹, over ten times higher than the anthropogenic source. Uncertainties in the VOC oxidation source of H₂ make a considerable contribution to the uncertainties in the global warming potentials for H₂ estimated with current chemistry-transport models.