Modelling the UK reduced nitrogen budget

Currently, more deposition of reduced nitrogen species to the UK is measured than can be accounted for from the calculated emissions. A newly developed long-range transport model was used to study this budget. Changes in horizontal resolution of model and an improved parameterization of dry deposition of ammonia gave an improved agreement between measured and modelled deposition. However, the model still estimated only 65% of the measured deposition with a UK emissions term of 283 ktonnes N yr⁻¹. Scaling the emissions by factors of × 1.2 and × 1.3 still resulted in the model underestimating of reduced nitrogen deposition. Modelled aerosol ammonium and ammonia gas concentrations for the base-case were compared with observations. The ammonia gas concentrations appear to be modelled reasonably well but the aerosol ammonium concentrations were underestimated by a factor of at least 2. This is an important finding as this indicates that other parameters related to wet deposition simulation are unlikely to be in gross error, implying that the UK emissions term is too small. It was concluded that an emission source term of the order 400 ktonnes N yr⁻¹ is required to reconcile the model with the measurements.