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6. ` AERMOD model is the appropriate model for this analysis based on the coverage of simple, intermediate,and complex terrain. It also predicts both short-term and long-term(annual) average concentrations. The model was executed using the regulatory default options (stack-tip downwash,buoyancy-induced dispersion, final plume rise), default wind speed profile categories, default potential temperature gradients, and no pollutant decay. Building wake effects were also addressed. The model is based on the Industrial Source Complex-3 (ISC3)model and PRIME r downwash algorithm. ` Special features of AERMOD include its ability to treat the vertical inhomogeneity of the planetary boundary layer, special treatment of surface releases, irregularly-shaped area sources,a three plume model for the convective boundary layer, limitation of vertical mixing in the stable boundary layer, and fixing the reflecting surface at the stack base. A treatment of dispersion in the presence of intermediate and complex terrain is used that improves on that currently in use in ISCST3 and other models. The selection of the appropriate dispersion coefficients used in the modeling depends on the land use within three kilometers (km)of the Port of Stockton. The land use typing was based on the classification method defined by Auer(1978);using pertinent United States Geological Survey (USGS) 1:24,000 scale(7.5 minute) topographic maps of the area. If the Auer land use types of h.. heavy industrial, light-to-moderate industrial, commercial, and compact residential account for 50%or more of the total area,the Guideline on Air Quality Models recommends using urban dispersion coefficients; otherwise, the appropriate rural coefficients were used. Based on ` observation of the area surrounding the Vernalis,rural dispersion coefficients were applied in the analysis. r RECEPTOR LOCATIONS Receptors were placed at locations within residential areas surrounding the Proposed Project. Receptor base elevations were determined based on USGS mapping programs. Receptors were also positioned as flagpole receptors at a height of 1.8 meters above the ground(breathing height). METEOROLOGICAL DATA The rate at which emissions are dispersed in the atmosphere depends upon the intensity of the ` ambient turbulence,the velocity of the wind,the position relative to obstacles in the now field, and any dilutions attributable to the source itself. The most important factor leading to plume spread in the atmosphere is the amount of ambient turbulence. In a stable atmosphere, the horizontal and vertical turbulence is very limited. The plume remains near its emission height and undergoes minimal mixing. This situation is common during the nighttime and early morning hours. If the layer below the plume height becomes neutral to unstable,the plume mixes r rapidly to the surface. This is known as a fumigation condition and can cause high concentrations. This occurs for short duration during the early morning. As heating of the r H i L