We have developed the three-dimensional global University of Maryland Chemical Transport Model (UMD-CTM), which can operate on a uniform horizontal grid or operate with a stretched-grid feature that allows transport and chemistry to be computed with mesoscale resolution in a region of interest. The model is suitable for computing photochemical air quality over a specific region, as well as addressing interregional and intercontinental transport issues. The model contains options for a uniform grid or a stretched-grid advection scheme and contains a fast chemical solver and schemes for convective transport, eddy diffusion, emissions, dry deposition, wet scavenging, and stratospheric influx. The model was run on a uniform grid for a full year, and results were evaluated with a variety of surface, airborne, balloon-borne, and satellite observations from many regions of the world. The evaluation was quantified by means of an evaluation index, which compares the model versus observation differences with the variance in the measurements. For most species no systematic biases were found in the results. Results of a simulation with the stretched-grid version of the model are reported in part 2 of this series of papers <Park et al., 2004>.