Impact of ship emissions on the Mediterranean summertime pollution and climate: A regional model study

Ship emissions contribute substantially to atmospheric pollution over the summertime Mediterranean region, thereby modifying the radiation budget through sulfate aerosol forcing. By applying a regional atmospheric-chemistry model and a radiation model we determine the seasonal variability of secondary trace gases and aerosols, their origin and impact on climate. Summer mean sulfate aerosol column burden over the Mediterranean is 7.8 mg m−2 and remarkably higher than the European mean of 4.7 mg m−2. Partitioning SOx emissions into land and water sources allows to investigate their respective impact on the sulfate aerosol concentration, its total burden and direct radiative forcing. 54% of the total sulfate aerosol column burden over the Mediterranean in summer originates from ship emissions. Accordingly, they contribute over 50% to the direct radiative forcing. Running the model without ship emissions significantly reduces near surface concentration of sulfate, ozone, nitric acid, hydroxy radicals and formaldehyde. As the applied ship emission inventory is based on the year 1990 and model results tend to underestimate observed concentrations at Mediterranean locations influenced by ship emissions, we assume that increased ship emissions during the past decade contribute nowadays even stronger to secondary pollution formation and radiative forcing.