Observations of atmospheric reactive nitrogen (Nr) deposition are severely restricted in spatial extent and type. The chain of processes leading to atmospheric deposition emissions, atmospheric dispersion, chemical transformation and eventual loss from the atmosphere is extremely complex and therefore currently, observations can only address part of this chain.

Modelling provides a way of estimating atmospheric transport and deposition of Nr at the European scale. A description of the different model types is provided.

Current deposition estimates from models are compared with observations from European air chemistry monitoring networks.

The main focus of the chapter is at the European scale; however, both local variability and and intercontinental Nr transfers are also addressed.

Atmospheric deposition is a major input of Nr for European terrestrial and freshwater ecosystems as well as coastal sea areas.

Models are key tools to integrate our understanding of atmospheric chemistry and transport, and are essential for estimating the spatial distribution of deposition, and to support the formulation of air pollution control strategies.

Our knowledge of the reliability of models for deposition estimates is, however, limited, since we have so few observational constraints on many key parameters.

Total Nr deposition estimates cannot be directly assessed because of a lack of measurements, especially of the Nr dry deposition component. Differences among European regional models can be significant, however, e.g. 30% in some areas, and substantially more than this for specific locations.