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San Joaquin County Community Air Quality Technical Information <br /> Development Department <br /> from overlapping pollution plumes originating from the sequence of roadway <br /> segments. <br /> When winds are essentially parallel to a roadway link,pollution plumes from all <br /> roadway segments overlap. This produces high concentrations near the roadway <br /> (near the center of the overlapping pollution plumes),and low concentrations <br /> well away from the roadway (at the edges of the overlapping pollution plumes). <br /> When winds are at an angle to the roadway link,pollution plumes from distant <br /> roadway segments make essentially no contribution to the pollution <br /> concentration observed at a receptor location. Under such cross-wind situations, <br /> pollutant concentrations near the highway are lower than under parallel wind <br /> conditions(fewer overlapping plume contributions),while pollutant <br /> concentrations away from the highway may be greater than would occur with <br /> parallel winds (near the center of at least some pollution plumes). <br /> The CAUNE4 model employs a "mixing cell' approach to estimating pollutant <br /> concentrations over the roadway itself. The size of the mixing cell over each <br /> roadway segment is based on the width of the traffic lanes of the highway <br /> (generally 12 feet per lane)plus an additional turbulence zone on either side <br /> (generally 10 feet on each side). Parking lanes and roadway shoulders are not <br /> counted as traffic lanes. The height of the mixing cell is calculated by the model. <br /> Pollutants emitted along a highway link are treated as being well mixed within <br /> the mixing cell volume due to mechanical turbulence from moving vehicles and <br /> convective mixing due to the temperature of vehicle exhaust gases. Pollutant <br /> concentrations downwind from the mixing cell are calculated using horizontal r <br /> and vertical dispersion rates which are a function of various meteorological and <br /> ground surface conditions. <br /> Modeling Procedures <br /> Roadway and Traffic Conditions <br /> Traffic volumes and operating conditions used in the modeling were obtained <br /> from the traffic analysis prepared for this project by Omni Means. Free flow <br /> traffic speeds were adjusted to reflect congested speeds using methodology from <br /> the Highway Capacity Manual(Highway Research Board 1965). CO modeling <br /> was conducted on the intersections of SR 132 EB &WB Ramps/Bird Road, <br /> Blewett Road/Bird Road, SR 132/Koster Road,Mine Entrance/Koster Road, <br /> Blewett Road/Koster Road <br /> DeSilva Gates Quarry Project <br /> Draft Environmental Impact Report J-2 <br /> As 05105 05 <br />