Measurements using specially instrumented aircraft were obtained during August and September, 1988 as an integral part of the ACID MODES (Model Operational and Diagnostic Evaluation Study) field study. Specialized flights, each designed to diagnose different aspects of the performance of the Regional Acid Deposition Model (RADM), were performed. Briefly, zig-zag type horizontal transects (ZIPPER) and vertical up-and-down saw tooth transects (CURTAIN) during fair weather conditions provide an evaluation of the of the skill by which RADM predicts the overall magnitude, as well as the horizontal and vertical gradient and spatial pattern of all the major acidic species and precursers on a regional scale. The data provide a reference for testing the model's ability to distribute primary pollutants from the source regions and to predict the correct magnitude of the gas phase conversion rates exemplified by the secondary species. The Frontal Passage flights were performed to test the RADM's ability to simulate conditions associated with a synoptic front representing a major scavenging event. This is reported in Spicer et al., (1991). Horizontal transects in and above the mixed layer over 1000 km distances were flown parallel to a cold front prior to, soon after, and then during pollutant build up. This series of flights took nearly one week to accomplish. Finally, a set of flights designed to characterize the diurnal behavior of the primary and secondary species over approximately four RADM cells provides the means to evaluate the performance of the gas phase chemistry and the daytime mixing aspects of the RADM. A case study is discussed in Schaller et al., (1991). Individually, each type of flight pattern is designed to emphasize model performance for conditions in which different sets of processes are in control. Collectively, the tests provide a powerful measure of the RADM's overall skill and scientific credibility.