Direct deposition of air pollutants in the wake of container vessels: The missing term in the environmental impact of shipping

Abstract

Understanding the deposition of pollutants produced by the operation of vessels is necessary to quantify air-water pollution interactions. This work develops and presents a novel model for calculating the amount of pollutants deposited on the sea water at the wake of vessels. Deposition depends on meteorological conditions, and vessel operation and design. The work was repeated for several subcategories of vessels included within the broader category of container ships and considered the impact of scrubber operation if such a system were installed on the vessel. Computational fluid dynamics (CFD) modelling was utilized, employing the open-source code OpenFOAM, to calculate the concentration of pollutants in the wake of the vessel. The sensitivity of deposition to different parameters was studied. The key parameters, including the wind relative speed and angle, the main engine power as a proxy for vehicle size, and the exhaust gas velocity, were combined to a single equation that can be used to predict deposition from sea-going container vessels. Direct deposition of pollutants (e.g., SO2) in the wake of vessels was found to be as much as 24 % of the total pollutant quantity produced, under favourable conditions. Direct deposition is so far neglected by air quality models that only report regional level deposition, starting from background ambient levels of pollution. This creates a bias to the predicted air and water pollution impacts. The use of the model proposed to whole vessel fleets, taking stock of the local weather conditions, will enable understanding the global impacts of direct deposition.