Reconciliation of Air Quality Models for Control Strategy Decisions

Abstract

Principles of decision theory are used to provide guidelines for regulators in choosing the appropriate source impact allocation when two or more air quality models produce different results. The most important principle of decision theory, as applied to environmental policy issues, is separation of the scientific assessment process from the social-political-institutional- economic management process. In reconciling air quality models, the purpose of the scientific assessment process is to obtain the most accurate source impact allocation, including uncertainty bounds. This can be done by two methods: using the principle of maximum likelihood (determining the composite answer as a weighted average of the individual modeling results, with the weights for each model inversely proportional to its variance), or refining and combining the models together. Both methods often require the use of judgmental estimates of uncertainty. The purpose of the management process is to select the specific source impact allocation that will be used as the foundation for a control strategy. Value judgments—social, political, institutional, and economic considerations—play an important role in this process. There are three general principles in managing the value judgments: consistency of the regulatory approach, decision making at the appropriate managerial level, and open and explicit consideration of value factors.