The atmospheric module simulates atmospheric chemistry, transport, and deposition, based on the long-range transport model developed previously by EMEP. Impacts are expressed in physical units of deposition on 150-by-150 kilometer grid points. (Grid points will soon be changed to 50-by-50 kilometers, following refinements in the atmospheric models). There is no provision in the model to value impacts, except as is incorporated into the concept of "critical loads"--levels of deposition on a particular grid cell that are deemed the maximum tolerable level in steady state.
Though these critical loads have been widely accepted in policy-making, their definitions create several substantial difficulties. First, as acidity increases different species die at different levels, so the species that defines criticality must be chosen. Second, limits are uncertain and subject to transient fluctuation, such as the large acid flush that comes with spring snow-melt. Third, grid cells are large enough to have substantial internal variation, so defining the most sensitive regions on a scale that large is difficult. Finally, the critical-loads concept was originally advanced to assess sulfur deposition on lakes, and the problems inherent in the concept are even more difficult for forests and for nitrogen deposition. For nitrogen, some researchers argue that no non-zero deposition level exists that is tolerable in steady state.
The model was developed with the explicit intent of providing a tool to assist European policy-makers. Agreement was emerging over transport and deposition of sulfur, but policy-makers wanted a framework to consider impacts together with control costs, and RAINS was designed to meet this need. Officials were involved in the three planning meetings at which the model was designed, and at their suggestion increased detail on emission abatement strategies and parallel submodels for NOx and ammonia were added. The influence of RAINS has in part been attributed to a 1990 IIASA meeting where many heads of delegations interacted intensively with model developers over several days, requesting runs, specifying scenarios, and testing assumptions. By 1990, RAINS was being used by national agencies in four countries to study the effects of control policies. The Working Group on Strategies of the LRTAP Convention makes extensive use of RAINS, requesting analyses of specific control strategies that are under discussion. While an early designers' aspiration that negotiators would actually sit at computers operating the model during negotiations was abandoned, the model has remained accessible enough that several negotiators and advisors use it regularly on their own computers. A 1990 summary of the RAINS model presents six simple "policy-related findings," of which the sixth is, "if you're not sure about points 1 through 5, use the RAINS model yourself." The RAINS analyses have been credited with helping to persuade negotiators to move to nonuniform national emission reductions in the second sulfur protocol, signed in 1994. Once negotiators agreed to reduce divergence between deposition and critical loads by 60 percent over time and RAINS had calculated cost-minimizing national emissions to reach this goal, about half the delegations made the resulting number their final offer, while several others took the emissions level but delayed the date by five to 10 years (Hordijk 1991 ; U.N. Economic Commission for Europe 1991 ; Alcamo, Shaw, and Hordijk 1990; Levy 1995). The RAINS project continues, and current work centers on adding volatile organic compound (VOC) emissions to the model and developing a parallel model for regional air-pollution transport and deposition in Asia.
The next section is Section 3: Design Issues in Integrated Assessment.
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