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Reactive nitrogen (Nr) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth's climate balance in many ways. These include the direct and indirect emissions of nitrous oxide (N2O), atmospheric Nr deposition and tropospheric ozone formation (O3), both of which alter the biospheric CO2 sink, Nr supply effects on CH4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NOx) and ammonia (NH3).
Human production and release of Nr into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an effect European Nr emissions are having on net warming or cooling.
This chapter summarizes current knowledge of the role of Nr for global warming. Particular attention is given to the consequences of atmospheric Nr emissions. The chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric Nr emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe.
The use of Nr fertilizers has major additional effects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect effects are not assessed here.
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.
Key findings/state of knowledge
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.
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