Modelling the National and Regional Transport and Deposition of Ammonia
• A range of chemical transport models are used across the Convention to model the emission, transport and deposition of atmospheric ammonia at the national and regional scale. These models have been developed from a range of historical backgrounds and with different purposes in mind.
• Six models ranging from describing the national scale up to the full European scale were considered in the Working Group. The models differ in concepts particularly in their chemical scheme and in scale, ranging from Lagrangian models at a national scale, via Eulerian models at the European scale to nested models coupling the European scale to the local scale.
• Key uncertainties in the modeling of atmospheric ammonia were found to be linked to the emissions (absolute level and spatial and temporal allocation), dry deposition parameterization, spatial resolution of the model and the description vertical diffusion.
• All models on a European scale currently underestimate the measured ammonia concentration whereas the national models in general find a better agreement with the ammonia measurements.
• The concentration of ammonium aerosol was fairly well described by all models. However, both under- and overestimates of measured concentrations were found. The magnitude of the wet deposition of ammonium was in general reproduced well by all models.
• The main reasons for the observed differences between measured and the modeled ammonia concentrations were the spatial resolution of the models and the (parameterization of the) dry deposition process.
• All of the models do not routinely use the compensation point (bi-directional exchange scheme) as a parameterization in the dry deposition process of ammonia. This is thought to be one of the reasons why some models tend to underestimate concentrations particularly in summer. The main reason for not taking this process into account is the lack of a generalized scheme for the compensation point with respect to the main land cover types used in the models. The models should carry out sensitivity tests with implementations of the compensation point (bi-directional exchange) schemes, to estimate the magnitude of the effect on the dry deposition in the model.
• The siting of the measurements plays an important role in the comparison with modeled concentrations; some stations that are situated in agricultural areas should not be used for validation of the Eulerian models with large grid size (50 km), because of the significant contribution of sources close to the measurement stations that cannot be picked up by the models on this spatial scale.
• Currently, differences in the model performance between countries are not fully understood as they may reflect differences in (i) the quality of the emissions inventory, (ii) differences in the model parameterization schemes, (iii) geographical differences (climate, terrain) or (iv) differences in measurement data-sets. Hence, a co-ordinated model intercomparison, using a common model domain, input database and measurement database, is urgently needed to assess relative model performance.
• In many countries, better data (emissions and monitoring data) are available for national modeling efforts than are submitted to EMEP and thus available for other countries. The reporting to EMEP has to be made more flexible to improve data availability.
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