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Appendix J <br /> Air Quality Technical Information <br /> Carbon Monoxide Modeling <br /> Dispersion Modeling <br /> Predicting the ambient air quality impacts of pollutant emissions requires an <br /> assessment of the transport,dispersion,chemical transformation, and removal <br /> processes that affect pollutant emissions after their release from a source. <br /> Gaussian dispersion models are frequently used for such analyses. The term <br /> "Gaussian dispersion"refers to a general type of mathematical equation used to <br /> describe the horizontal and vertical distribution of pollutants downwind from an <br /> emission source. <br /> i Gaussian dispersion models treat pollutant emissions as being carried downwind <br /> in a defined plume, subject to horizontal and vertical mixing with the <br /> surrounding atmosphere. The plume spreads horizontally and vertically with a <br /> reduction in pollutant concentrations as it travels downwind. Mixing with the <br /> surrounding atmosphere is greatest at the edge of the plume, resulting in lower <br /> pollutant concentrations outward(horizontally and vertically) from the center of <br /> y, the plume. This decrease in concentration outward from the center of the plume <br /> is treated as following a Gaussian("normal') statistical distribution. Horizontal <br /> and vertical mixing generally occur at different rates. Because turbulent motions <br /> in the atmosphere occur on a variety of spatial and time scales,vertical and <br /> horizontal mixing also vary with distance downwind from the emission source. <br /> The CALINE4 Model <br /> The ambient air quality effects of traffic emissions were evaluated using the <br /> CALINE4 dispersion model (Benson 1989). CALINE4 is a Gaussian dispersion <br /> model specifically designed to evaluate air quality impacts of roadway projects. <br /> r. <br /> Each roadway link analyzed in the model is treated as a sequence of short <br /> segments. Each segment of a roadway link is treated as a separate emission <br /> source producing a plume of pollutants which disperses downwind. Pollutant <br /> concentrations at any specific location are calculated using the total contribution <br /> y <br /> y� DeSilva Gates Quarry Project <br /> Draft Environmental Impact Report J_1 <br /> AS85105M <br />