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5.1 AIR QUALITY terrain, such as on the slope of a nearby hill,can cause high ground-level concentrations, especially under stable atmospheric conditions. Another dispersion condition that can cause high ground-level pollutant concentrations is caused by building downwash. Building downwash can occur when wind speeds are high and a building or structure is in close proximity to the emission stack. This can result in building wake effects where the plume is drawn down toward the ground by the lower pressure region that exists in the lee side (downwind) of the building or structure. Fumigation conditions occur when the plume is emitted into a low-lying layer of stable air (inversion) that then becomes unstable,resulting in a rapid mixing of pollutants towards the ground. The low mixing height that results from this condition allows little diffusion of the stack plume before it is carried downwind to the ground. Although fumigation conditions rarely last as long as an hour,relatively high ground-level concentrations may be reached during that period. Fumigation tends to occur under clear skies and light winds, and is more prevalent in the summer. The basic model equation used in this analysis assumes that the concentrations of emissions within a plume can be characterized by a Gaussian distribution about the centerline of the plume. Concentrations at any location downwind of a point source such as a stack can be determined from the following equation: C X z H = Q ) *(e-1/2(y/vr)2) e-1/2(z-H/az)2 + e-1/2(z+H/Gz)z 21b a ya zu where C = the concentration in the air of the substance or pollutant in question Q = the pollutant emission rate ay,az =the horizontal and vertical dispersion coefficients,respectively,at downwind distance x U = the wind speed at the height of the plume center x,y,z = the variables that define the 3-dimensional Cartesian coordinate system used; the downwind,crosswind, and vertical distances from the base of the stack H= the height of the plume above the stack base (the sum of the height of the stack and the vertical distance that the plume rises due to the momentum and/or buoyancy of the plume) Gaussian dispersion models are approved by EPA for regulatory use and are based on conservative assumptions (i.e.,the models tend to overpredict actual impacts by assuming steady-state conditions,no pollutant loss through conservation of mass,no chemical reactions,etc.). The EPA models were used to determine if ambient air quality standards would be exceeded, and whether a more accurate and sophisticated modeling procedure would be warranted to make the impact determination. The following sections describe: • Screening modeling procedures • Refined air quality impact analysis • Existing ambient pollutant concentrations and preconstruction monitoring SAC/371322/082410013(LEC-5.I-AIR-QUALITY.DOC) 5.1-33