Acid deposition is the end product of a complex, large-scale, tightly coupled system of atmospheric transport processes and chemical reactions. To achieve a credible understanding of acid deposition processes upon which to base policy decisions, the United States began the National Acid Precipitation Assessment Program (NAPAP) in the early 1980s. Throughout the life of NAPAP two key scientific questions have been raised concerning emission of pollutants and acidic deposition: What are the geographic origins of observed levels of deposition; and What will be the effectiveness of emissions reductions for reducing observed levels of acidic deposition. The best hope of answering these questions realistically is to simulate the complex atmospheric system using credible numerical models. As part of the Acid MODES program, instrumented aircraft were used during the summer of 1988 to gather data aloft for diagnostic evaluation of regional Eulerian acid deposition models.