Episodic modeling of the chemical structure of the troposphere as revealedduring the spring MLOPEX 2 intensive

An episodic regional chemical transport model (CTM) is described and applie d to modeling the vertical chemical structure of the troposphere, as reveal ed from aircraft flights in the vicinity of Hawaii during the springtime Ma una Loa Observatory Photochemistry Experiment 2c intensive. The CTM simulat es the chemistry and transport, of 50 species within the North Pacific basi n. It is driven by output fields from thc Pennsylvania State University/Nat ional Center for Atmospheric Research Mesoscale Modeling System. The modele d meteorology is highly correlated with Europe an Centre for Medium-Range W eather Forecasts analyses on the large scale but is not accurate enough to correctly simulate the long-range transport of chemical plumes, The simulat ed total cloud cover is approximately correct when compared with Internatio nal Satellite Cloud Climatology Project cloud cover. However, the coverage of low-lying clouds is overpredicted, while the upper level cloudiness is u nderpredicted. Modeled and satellite-derived deep convection are reasonably correlated. The measured concentrations of H2O2 CH3OOR, CO, NOy, anti O-3 are compared with simulated concentrations front the marine boundary layer to approximately 9 km, Although pronounced chemical plumes and vertical lay ering complicate the model-measurement comparison, modeled and measured con centrations are generally within a factor of 2, with H2O2 particularly well simulated. A number of measured transient chemical events are examined. Th is study provided the necessary background for subsequent papers examining the chemistry and transport of the Pacific basin in more detail.