The Operational Priority Substances model

The model is set up as a universal framework supporting the modelling of a wide variety of pollutants including fine particles but the main purpose is to calculate the deposition of acidifying compounds over the Netherlands at a high spatial resolution. Previous versions of the model have been used since 1989 for all the atmospheric transport and deposition calculations in the State of the Environment reports and Environmental Outlook studies in the Netherlands.

An extensive model validation exercise was carried out using observations from the National Air Quality Monitoring Network over the past twenty years. Good agreement was found for both SO_x and NO_y species in the spatial patterns, just as in trends over the past ten years. An exception is formed by the NHx species, which are, in general, underestimated by approximately 25%. This discrepancy has for some time been known as the 'ammonia gap'. The total uncertainty for deposition to a nationally distributed ecosystem is estimated at 20%, 25 and 30% for SO_x, NO_y and NH_x, respectively. For a specific ecosystem (size: 500 × 500m to 5000 × 5000m), the uncertainties will be much higher: 50, 60, 100% for SO_x, NO_y and NH_x deposition, respectively. Included in these figures are the uncertainties in current emission estimates. Uncertainties in dry deposition velocities dominate the total uncertainty.