An episodic regional chemical transport model (CTM) is described and applied to modeling the vertical chemical structure of the troposphere, as revealed from aircraft flights in the vicinity of Hawaii during the springtime Mauna Loa Observatory Photochemistry Experiment 2c intensive. The CTM simulates the chemistry and transport of 50 species within the North Pacific basin. It is driven by output fields from the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Modeling System. The modeled meteorology is highly correlated with European Center for Medium-Range Weather Forecasts analyses on the large scale but is not accurate enough to correctly simulate the long-range transport of chemical plumes. The simulated total cloud cover is approximately correct when compared with International Satellite Cloud Climatology Project cloud cover. However, the coverage of low-lying clouds is overpredicted, while the upper level cloudiness is underpredicted. Modeled and satellite-derived deep convection are reasonably correlated. The measured concentrations of H2O2, CH3OOH, CO, NOy, and O3 are compared with simulated concentrations from the marine boundary layer to approximately 9 km. Although pronounced chemical plumes and vertical layering complicate the model-measurement comparison, modeled and measured concentrations are generally within a factor of 2, with H2O2 particularly well simulated. A number of measured transient chemical events are examined. This study provides the necessary background for subsequent papers examining the chemistry and transport of the Pacific basin in more detail. ¿ 2000 American Geophysical Union