Selection of target loads for acidifcation in emission abatement policy: The use of gap closure approaches

The Abatement Strategies Assessment Model, ASAM, has been used to investigate European emission abatement policies for ammonia and oxides of sulphur and nitrogen. These policies are designed to reduce the deposition of acidifying substances towards critical loads defining the deposition which ecosystems can sustain. Since critical loads are not all attainable the model has been set up to reach various intermediate target loads. This paper uses the model to illustrate the sensitivity of the derived abatement strategies to the choice of target load. Particular attention is paid to the various methods of 'gap closure', designed to calculate target loads. These methods have been discussed within the framework of the UN ECE Task Force on Integrated Assessment Modelling in its preparations for the new UN ECE 'multi-pollutant multi-effect protocol' and involve the reduction of the gap between the situation in 1990 and the critical loads. The 'gap' can be measured using various methods, 'area', 'ratio' and 'accumulated exceedance'. It is shown that the methods of defining target loads have an important bearing on the nature of abatement strategies for pollutants, in terms of the distribution across countries of both costs and benefits. The 'accumulated exceedance' approach reflects both the differing sizes of ecosystem areas indifferent parts of Europe and the different amounts by which their critical loads are exceeded. It probably reflects much more realistically the relationship between deposition levels and damage to ecosystems.