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MacArthur Project Modification - Santa Ana - Air Quality Assessment <br />KPC Environmental, Inc. <br />During spring and early summer days, most of the pollution produced on an average day <br />is lifted by the warm air and moved out through the mountain passes. This effectively <br />creates a flushing of the SCAB of pollutants away from the valleys. During the late <br />summer and winter months, this flushing effect is less pronounced due to the lower wind <br />speeds and early off-shore winds. This stagnation causes the pollutants to be trapped in <br />the regions valleys. <br />3.2.6 Inversions <br />Vertical dispersion of air pollutants in the SCAB is hindered by the presence of a <br />temperature inversion in the layers of the atmosphere near the earth's surface. The height <br />of the base of the inversion is known as the "mixing height." The mixing height changes <br />under atmospheric conditions while the top end of the inversion remains constant. <br />Usually the mixing height is lower in the morning and increases in altitude as the day <br />progresses. The mixing height presents a barrier to the vertical dispersal of air <br />contaminants. During winter months the inversion normally breaks down by mid <br />morning. <br />Pollutants generated by both stationary and mobile sources mix with less contaminated <br />air beneath the inversion layer and will become more concentrated unless the inversion <br />breaks down. On days of no inversion layer or when winds average 15 mph or greater, <br />there will be no significant smog effects. <br />The potential for high concentrations varies with each season. Late spring, summer and <br />early fall, light winds, low mixing height, and increased sunlight combine to produce <br />conditions for the production of photochemical oxidants, e.g. ozone. <br />When strong inversions are formed on cool winter nights with light winds, carbon <br />monoxide generated by automobile exhaust becomes concentrated. CO values are <br />normally at their highest levels from the period of November through February. <br />02/02/11 <br />75B-112