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<br /> The conditions that form high ambient air concentrations of 03 are sunshine, early morning rs
<br /> stagnation in source areas, high surface temperatures, strong and low morning inversions, L•�
<br /> greatly restricted vertical mixing during the day, and daytime subsidence that strengthens
<br /> the inversion layer. Because of its long formation time in the atmosphere, 03
<br /> concentrations are most affected by transport patterns. The highest concentrations of 03
<br /> tend to occur downwind from Valley emission sources; however, the CARB indicates that
<br /> some of the 03 found in the Valley originates in other air basins (SJVUAPCD, 1994). The
<br /> most frequent 03 transport route is from source areas in the populated rim of the San
<br /> Francisco Bay area, to inland receptor areas downwind and those areas to the south.
<br /> In the winter, temperature inversions dominate during the night and early morning hours
<br /> but frequently dissipate by afternoon. At this time, the greatest pollution problems are from
<br /> CO and NO,. High CO concentrations occur on winter days with strong surface inversions
<br /> and-light winds (SJVUAPCD, 1992). As a result, CO transport is extremely limited.
<br /> 2
<br /> High NO levels usually occur during the autumn or winter on days with summer-like {
<br /> weather conditions. These conditions include low inversions, limited daytime mixing, and
<br /> stagnant windflow conditions. Although days are clear, sunlight is limited in duration and
<br /> intensity, therefore, photochemical reactions necessary to form 03 are incomplete.
<br /> Atmospheric particulates are made up of fine solids or liquids such as soot, dust, aerosols,
<br /> fumes, and mists. A large portion of the total suspended particulate matter (TSP) in the
<br /> atmosphere is PM,,). These small particulates cause the greatest health risk since they can
<br /> more easily penetrate the defenses of the human respiratory system. Peak concentrations
<br /> of PM,a occur downwind of precursor emission sources.
<br /> Attainment Status Designations
<br /> The California Air Resources Board (CARB) is required to designate areas of the state as
<br /> attainment, nonattainment, or unclassified for any state standard.. An "attainment"
<br /> designation for an area signifies that pollutant concentrations did not violate the standard
<br /> for that pollutant in that area. A "nonattainment" designation indicates that a pollutant
<br /> concentration violated the standard at least once, excluding those occasions when a
<br /> violation was caused by an exceptional event, as defined in the criteria. An "unclassified"
<br /> designation signifies that data do not support either an attainment or nonattainment status.
<br /> The CCAA divides districts into moderate, serious, and severe air pollution categories, with
<br /> increasingly strict control requirements mandated for each (SJVUAPCD, 1992, 1994).
<br /> The EPA designates areas for 03, CO, and NO2 as either "does not meet the primary
<br /> standards," or "cannot be classified," or"better than national standards." For SO2, areas
<br /> are designated as "does not meet the primary standards," "does not meet the secondary
<br /> standards," "cannot be classified," or "better than national standards." In 1991, new
<br /> "nonattainment" designations were assigned to areas that had previously been classified
<br /> as Group I, II, or III based on the likelihood that they would violate the national PM,,)
<br /> standards. All other areas are designated "unclassified" (CARB, 1994b). The attainment
<br /> status designations of the SJVAB are presented in Table 4.5-3.
<br /> Draft Environmental Impact Report 4-66 ER-96-3
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