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surface persists,a fully unstable mixing layer develops, and the plume loops up and down in <br /> response to large-scale convective eddies. A neutral stability atmosphere yields moderate <br /> amounts of turbulence and results in a cone-shaped plume. Finally, if an inversion is present <br /> below the emission height,a lofting condition exists and the plume is cut off from ground level <br /> impacts. <br /> Stability class frequencies were calculated from the deviation of the horizontal wind direction _ <br /> (sigma theta). The Sigma Theta method was used to categorize stability class as a function of <br /> wind speed and time of day. Stability classes range from extremely unstable(A) to moderately <br /> stable(F). These classes are used in dispersion models to estimate how much a plume will spread — <br /> over time and space. In general, the more stable the atmosphere is,the less potential for plume <br /> spread,creating higher plume concentrations. <br /> Surface meteorological data and upper air meteorological(mixing height) data from Fresno and <br /> Sacramento, California, respectively,were used for the modeling analysis <br /> (http://www.valleyair.orwbusind/pto/Tox Resources/toxics resources.htm). Meteorological data <br /> were obtained from SJVUAPCD and used for modeling impacts of the Proposed Project. Data <br /> from 1986 through 1990 was used and the worst case year of analysis was reported. <br /> SOURCE RELEASE CHARACTERISTICS <br /> Onsite equipment was treated as area sources located within the phases of operations. The area <br /> source was modeled with a release height of 3 meters. Table AQ-9 provides the exhaust <br /> parameters(based on engineering judgment and other similar facilities)for the asphalt batch _ <br /> operations. The loadout, silo filling, and storage tank sources were treated as a volume source <br /> with a height of 4.5 meters and a length of 20 meters. <br /> TABLE AQ-9 <br /> ASPHALT BATCH EXHAUST PARAMETERS <br /> Height Diameter Velocity Temperature <br /> Source (m) (m) (m/s) (K) <br /> Dryer/Heater 12.2 1.44 17.4 422 <br /> m=meters. <br /> m/s=meters per second. <br /> K=Kelvin. <br /> Health Risk Assessment <br /> The SJVUAPCD has a significance threshold for health risk exposure to diesel emissions of 10 _ <br /> cancers per million for 70-year exposure; indicating the primary concern from diesel engine <br /> exhaust emissions is a potential long-term health risk to sensitive receptors. <br />