This paper addresses the use of critical loads in optimized emission abatement strategies. As deposition targets, critical loads can not be satisfied at all receptors. In Europe, consequently, there is a need for alternative criteria which still relate to ecological indicators, yet which are feasible, consistent and equitable. Two criteria are suggested: the relative critical load coverage and the relative deposition reduction. These criteria permit deposition goals to be set which guarantee that a specified fraction of ecosystems will attain critical loads, and thus be protected from adverse environmental impacts. In areas which can not achieve critical loads with the best available control measures, deposition is reduced to a specified fraction of the unabated level.
After presenting examples which demonstrate their derivation, strengths and weakness of these criteria are discussed. The criteria have been implemented in the RAINS optimization model. Some preliminary examples show the sensitivity, interactions and utility of the criteria. Results obtained indicate that optimized emission strategies based on critical loads are similar to emission strategies based on deposition reductions, at certain levels of the criteria. This suggests that it may not be necessary to utilize critical loads to formulate deposition targets. A second example shows the effect of excluding countries from European cost minimization. A country's participation can save costs with moderate deposition targets, however, significant costs can be imposed with low (stringent) deposition targets. These preliminary results have significant implications for negotiations and multilateral negotiations. Suggestions for future analyses conclude the paper
