2011-022 - Addendum to the FInal Environmental Impact Report No. 2004-02

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MacArthur Project Modification -- Santa Ana - Air Quality Assessment KPC Environmental, Inc. During spring and early summer days, most of the pollution produced on an average day is lifted by the warm air and moved out through the mountain passes. This effectively creates a flushing of the SCAB of pollutants away from the valleys. During the late summer and winter months, this flushing effect is less pronounced due to the lower wind speeds and early off -shore winds. This stagnation causes the pollutants to be trapped in the regions valleys. 3.2.6 Inversions Vertical dispersion of air pollutants in the SCAB is hindered by the presence of a temperature inversion in the layers of the atmosphere near the earth's surface. The height of the base of the inversion is known as the "mixing height." The mixing height changes under atmospheric conditions while the top end of the inversion remains constant. Usually the mixing height is lower in the morning and increases in altitude as the day progresses. The mixing height presents a barrier to the vertical dispersal of air contaminants. During winter months the inversion normally breaks down by mid morning. Pollutants generated by both stationary and mobile sources mix with less contaminated air beneath the inversion layer and will become more concentrated unless the inversion breaks down. On days of no inversion layer or when winds average 15 mph or greater, there will be no significant smog effects. The potential for high concentrations varies with each season. Late spring, summer and early fall, light winds, love mixing height, and increased sunlight combine to produce conditions for the production of photochemical oxidants, e.g. ozone. When strong inversions are formed on cool winter nights with light winds, carbon monoxide generated by automobile exhaust becomes concentrated. CO values are normally at their highest levels from the period of November through February. 02/02/11 4