Assessing the impacts of international emissions reduction scenarios on the acidification of freshwaters in Great Britain with the First-order Acidity Balance (FAB) model and the Hull Acid Rain Model (HARM)

Critical loads models for acidity underpin international negotiations for the reduction of acid deposition through emissions controls. In Great Britain and Scandinavia, critical loads for freshwater ecosystems are calculated with the First-order Acidity Balance (FAB) model, which can provide a catchment based estimate of deposition reduction requirements of sulphur and nitrogen species in order to protect any aquatic target organism for which a critical chemical threshold is defined. The FAB model is applied to a national freshwaters database for Great Britain using three deposition scenarios generated with the Hull Acid Rain Model (HARM). Critical load exceedance and changes in three important chemical indicators (non-marine sulphate, nitrate and acid neutralising capacity) are assessed for 1990 baseline deposition levels, planned emissions reductions under existing international commitments (REF scenario), and a potential stringent emission reduction scenario under a multi-pollutant, multi-effect strategy (E10 scenario). Model outputs indicate that the number of sampled sites exceeding their critical load would be reduced by 60% and 73% respectively under the two future deposition scenarios. There is a clear need for a strategy to reduce both S and N deposition from 1990 levels if British freshwaters in sensitive areas are to be protected.