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Abstract. Excess reactive Nitrogen (Nr) has become one of the most pressing environmental problems leading to air pollution, acidification and eutrophication of ecosystems, biodiversity impacts, leaching of nitrates into groundwater and global warming. This paper investigates how current inventories cover emissions of Nr to the atmosphere in Europe, the United States of America, and China. The focus is on anthropogenic sources, assessing the state-of-the-art of quantifying emissions of Ammonia (NH 3 ), Nitrogen Oxides (NO x ) and Nitrous Oxide (N 2 O), the different purposes for which inventories are compiled, and to which extent current inventories meet the needs of atmospheric dispersion modelling. The paper concludes with a discussion of uncertainties involved and a brief outlook on emerging trends in the three regions investigated is conducted.Key issues are substantial differences in the overall magnitude, but as well in the relative sectoral contribution of emissions in the inventories that have been assessed. While these can be explained by the use of different methodologies and underlying data (e.g. emission factors or activity rates), they may lead to quite different results when using the emission datasets to model ambient air quality or the deposition with atmospheric dispersion models. Hence, differences and uncertainties in emission inventories are not merely of academic interest, but can have direct policy implications when the development of policy actions is based on these model results.The level of uncertainty of emission estimates varies greatly between substances, regions and emission source sectors. This has implications for the direction of future research needs and indicates how existing gaps between modelled and measured concentration or deposition rates could be most efficiently addressed.Correspondence to: S. Reis (srei@ceh.ac.uk)The observed current trends in emissions display decreasing NO x emissions and only slight reductions for NH 3 in both Europe and the US. However, in China projections indicate a steep increase of both.

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A mechanistic, dynamic compensation point model to simulate the vegetatiodatmosphere exchange of ammonia (NH3) is described. The model is applied to long-term micrometeorological measurements of NH3 exchange obtained over moorland in southern Scotland . The model describes the gaseous bi-directional exchange between the atmosphere, leaf surface water films, plant stomata and apoplast. A simple chemistry module is included to simulate the exchange of water-soluble atmospheric pollutants at the air-water interface on wet plant surfaces. Initialization of the chemistry module is achieved during rain events using measured rain chemical composition. The exchange of NH3 with stomata is based on the concept of a stomatal compensation point parametrized by means of the apoplast ammoniumhydronium r ratio. The trans-cuticular transfer of ammonium may also constitute a sink for dissolved ammonia on plant cuticular water films, and is parametrized using a trans-cuticular resistance and the concentration difference between leaf surface water and the apoplast. The leaching of base cations from the inside of the plant to foliar surfaces is simulated in a similar fashion to ammonium transfer, and the heterogeneous oxidation of sulphur dioxide (SO2) in thin water films is also treated. Numerical iterative procedures at each time-step allow the calculation of pH and dissolved ion concentrations.Modelled NH3 fluxes were compared with 3259 half-hourly micrometeorological measurements over moorland, and with two existing modelling approaches, the static canopy compensation point model and the canopy resistance model. The dynamic and static canopy compensation point models both gave long-term estimates of the NH3 dry deposition flux to moorland within 10% of actual measurements, while the canopy resistance approach overestimated deposition by about 30%. The dynamic model performed best during wet conditions for which it was designed, and performed reasonably well during dry conditions using a more empirical resistance approach.The model was also capable of simulating SO2 dry deposition fluxes to within 20% of measured fluxes. The model provides a tool that may also be used to simulate scenarios, whereby NH3/S02 concentration ratios in the atmosphere vary, and examine 'co-deposition' interactions of these two species.