Origin of nitrogen deposition in Germany: Source allocation and influence of local emissions

Eutrophication is one of the main reasons threatening biodiversity in Germany. To quantify the risk for biodiversity caused through nitrogen deposition, in Germany national data on the exceedance of critical loads for eutrophication are used as indicators for the National Strategy on Biodiversity and for the National Sustainability Development Strategy. In addition, to prevent ecosystems from unwanted further excessive nitrogen deposition, according to the German emission control and nature protection legislation, nitrogen deposition has to be assessed when new nitrogen emitting projects are submitted. To support the development of the national nitrogen strategy and devise regional emission targets an effort was made to provide insight into the geographic regions of origin of the nitrogen deposited within the federal states in Germany.

The atmospheric dispersion modelling was performed with chemistry transport model LOTOS-EUROS. Simulations over Germany were run with a resolution of 0.10° longitude by 0.05° latitude, nested in a European simulation. In addition to species concentrations and deposition fluxes, the contributions of predefined source categories were calculated and tracked using a labelling approach for the year 2019. The labels applied in this simulation encompass all emissions from the 16 individual German federal states, all 9 neighbouring countries of Germany and countries further away. Additionally, labels were attached to international shipping emissions, natural emissions, and boundary conditions to cover intercontinental transport. This result in a full source-receptor matrix with contributions to N-deposition in all federal states with contributions from all states and neighbouring countries. To assess contributions of local sources, an additional simulation was performed with emission reductions on three surface and two stack point sources.

Largest average depositions in Germany are found in Hamburg and Bremen followed by Niedersachsen, Nordrhein-Westfalen and Schleswig-Holstein. Hamburg and Bremen are small city states with relative high NOx emission densities. However, local emissions only contribute approximately 20%, main contributors are neighbouring areas Niedersachsen and Schleswig-Holstein due to their large agricultural sector with high ammonia emissions. A general trend can be spotted that states with dominant NHx emissions also have the highest local deposition. This holds for Baden-Württemberg, Bayern, Niedersachsen, Nordrhein-Westfalen and Schleswig-Holstein, where on average 40% of the nitrogen deposition originates from the sources within each state. For many states, there is also a considerable contribution of countries surrounding Germany. Emissions from the Netherlands contribute mainly in Bremen, Hamburg, Niedersachsen, Nordrhein-Westfalen and Schleswig-Holstein. The other main contributor is France, affecting mainly the states in southern Germany. The assessment of local contributions from the surface point sources shows that about 18-22 % of the emitted mass is deposited within a radius of 20 km from the source In contrast, for two the stack point sources, NHx deposition within 20 km of the source is only about 5% of the emission strength.