W.H. White, R.J. Farber, W.C. Malm, M. Nuttall, M.I. Pitchford, and B.A. Schichtel (White et al.) assert that our paper discredits previous studies and their interpretation by regulators. They are wrong on the former and right on the latter. We credit the tracer study component of the Project Mohave report for predicting an outcome similar to what has actually occurred following the closure of the Mohave Power Project (MPP) e very modest visibility improvement at the Grand Canyon at best. We do, however, raise questions about the transport modeling exercises conducted as part of Project Mohave that failed to predict the outcome. There was no consensus between these approaches. The study gave equal weight to tracer and transport modeling studies (Pitchford et al., 1999). In fact, the uncertainties regarding transport modeling should have been more clearly acknowledged at the time. This is an important distinction. As we stressed in our paper, the transport modeling effort reflects a method that continues to be an important tool for developing environmental policy. We think that our paper encourages greater use of tracer studies while raising concerns about the air quality modeling tools.

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