Ammonia concentrations in the Netherlands: spatially detailed measurements and model calculations

From September 2000 to September 2001 the ammonia concentrations in air were measured on a monthly basis at 159 locations in the Netherlands using passive samplers. Eight of the locations were situated in the RIVM's National Air Quality Monitoring Network (LML), which have a continuous monitoring station. The yearly averaged concentrations of ammonia in the atmosphere in the Netherlands found from the passive samplers vary from 1 to 18μgm−3 and is on average 6.6μgm−3. Measurements taken at the eight LML stations represent the ammonia concentration level in the Netherlands reasonably well; the yearly average of ammonia concentrations at the LML stations is 18% higher than the average based on the passive sampler sites. The concentrations at the sites were calculated with the OPS model using ammonia emission inventories on 500- and 5000-m resolutions. The OPS model describes the spatial pattern well. The model calculations based on emissions at a 500-m resolution are more in agreement with the measurements than the calculations based on the emissions at a resolution of 5000m. The reason for this is that at 500m the sources are more exactly located than in the 5000-m inventory. The agreement increases if individual source locations within the 500-m grid cell are used. However, the OPS model based on the 500m resolution emissions underestimates the measured concentrations by about 30%. The reason for this is not yet clear, but is most probably due to a combination of the uncertainties in emissions and the description of the dry deposition process in the model. The model comparison is discussed in terms of results of ammonia concentration modelling (with the OPS model) in other countries.