Deposition of nitrogen into the North Sea

The flux of nitrogen species from the atmosphere into the ocean, with emphasis on coastal waters, was addressed during the ANICE project (Atmospheric Nitrogen Inputs into the Coastal Ecosystem). ANICE focused on quantifying the deposition of atmospheric inputs of inorganic nitrogen compounds (HNO3, NO3-, NH3 and NH4+) into the North Sea and the processes governing this deposition. The Southern North Sea was studied as a prototype. Because the physical and chemical processes are described, as opposed to empirical relations, the results can potentially be transferred to other regional seas like the Mediterranean, the North Atlantic continental shelf area and the Baltic. Two intensive field experiments were undertaken, centred around the offshore tower Meetpost Noordwijk and the Weybourne Atmospheric Observatory in East Anglia (UK). Long-term measurements were made on a ferry sailing between Hamburg and Harwich/Newcastle. These measurements provided data for sensitivity studies of a variety of problems associated with the coastal region that are not easily evaluated with larger scale models, to constrain models and to test model results. Concentrations of nitrogen compounds over the North Sea and the resulting deposition presented in this paper were obtained with the Lagrangian transport-chemistry model ACDEP. The average annual deposition in 1999 was 906kg Nkm-2. The results are compared with experimental data from the ferry. Effects of temporal and spatial variations are evaluated based on experimental results and small-scale model studies. In particular, effects of the aerosol size distribution on the nitrogen deposition are discussed.