Open-ocean aerosol composition obtained during 15 months on a North Sea ferry

Gaseous and particulate nitrogen species had been collected during 15 months of sampling on a ship of duty on the open North Sea. An automated sampling device driven by a geopositioning system ensured that just open-sea values have been obtained. Representative values for five subregions are compared. Spatial patterns of median concentrations are presented for ammonia and ammonium as well as for nitric acid and particulate nitrate. The median of concentrations observed in the middle of the North Sea was found to be for ammonia 24 nmol m−3 and for nitric acid 14 nmol m−3. Reduced nitrogen concentrations fall off rapidly from coastal to open-sea areas. Sea-to-air ammonia fluxes seem to play a minor role for the reduced nitrogen load in the North Sea. Nitric acid concentrations show no large differences between the middle of the North Sea and the German Bight or the Southern North Sea. Production from the gaseous nitrogen oxide pool seems to replace any loss due to deposition in the region. However, the north western North Sea is found to be rather clean and seems to receive much more seldom polluted air masses. The study provides the largest open-sea aerosol composition data set in this area and can now serve for model validation of nitrogen compounds transport to marine regions which are close to the industrialised areas.