Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H₂ forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H₂ formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H₂ and a H₂ formation mechanism on PAHs. Since H₂ formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T> 200 K). At these temperatures the conventional route of H₂ formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H₂ formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H₂ forms very efficiently on PAHs, and that this process can reproduce the high H₂ formation rates derived in several PDRs.

A prototype harbour design toolbox has been developed, with a Graphical User Interface, incorporating simple model instruments to rapidly assess the impact of a harbour construction on a coastal environment, such as expected siltation rates and impact on the coastline evolution. The toolbox can be useful for different stakeholders, particularly in the early phases of a harbour development project, to explore order of magnitude impacts, relevant design parameters and dominant processes. This paper will set out the current status of the harbour design toolbox and investigate the benefits of use for specific stakeholders. The ambition is to further extend the harbour design toolbox towards an integral coastal design tool, facilitating the implementation of other type of interventions and linking to other disciplines such as marine ecology and navigation. It is intended that the integral coastal design tool will facilitate a multi-disciplinary approach to aid in the feasibility, design, construction or maintenance phase of coastal development project.

Molecular hydrogen is the most abundant molecule in the universe. A large fraction of H₂ forms by association of hydrogen atoms adsorbed on polycyclic aromatic hydrocarbons (PAHs), where formation rates depend crucially on the H sticking probability. We have experimentally studied PAH hydrogenation by exposing coronene cations, confined in a radio-frequency ion trap, to gas phase atomic hydrogen. A systematic increase of the number of H atoms adsorbed on the coronene with the time of exposure is observed. Odd coronene hydrogenation states dominate the mass spectrum up to 11 H atoms attached. This indicates the presence of a barrier preventing H attachment to these molecular systems. For the second and fourth hydrogenations, barrier heights of 72 ± 6 meV and 40 ± 10 meV, respectively, are found, which are in good agreement with theoretical predictions for the hydrogenation of neutral PAHs. Our experiments, however, prove that the barrier does not vanish for higher hydrogenation states. These results imply that PAH cations, as their neutral counterparts, exist in highly hydrogenated forms in the interstellar medium. Due to this catalytic activity, PAH cations and neutrals seem to contribute similarly to the formation of H₂.