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dominate during winter, however, surface-based radiative inversions often occur. Under these conditions, winds are light and the potential for accumulation/concentration of air pollutants such as PM10 and carbon monoxide are high. Precipitation data show that the rainy season occurs primarily between November and April. Very little rainfall is observed during the rest of the year. The annual average precipitation for the area is approximately 250 millimeters (10 inches). C Wind. Light winds limit the dilution of air pollutants as they disperse downwind from their source.Air pollutants can accumulate, especially in sheltered valleys,when light winds combine with reversals of wind direction between daytime and nighttime air flows, or when calms persist for extended periods. In the project area, the predominant wind direction is from the west. During the winter months, the southerly migration of the Pacific High causes the predominant wind direction to be from the south. The best pollutant dispersion occurs in the summer with the higher velocity winds. The calmest winds, which lead to pollutant stagnation, occur during the fall months. Current Air Quality and Regulatory Background The California Air Resources Board (ARB)maintains air quality monitoring sites in Tracy and Stockton. The monitoring data are generally representative of the proposed project site. In general, the standards for ozone, carbon monoxide, and suspended particulate matter less than 10 microns in diameter(PM10)are often exceeded in the Stockton area and occasionally in the Tracy area. Levels of sulfur dioxide and nitrogen dioxide do not exceed either the State or federal standards. The project is located in San Joaquin County, which lies in the San Joaquin Air Basin. This basin encompasses the Counties of San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare, and Kern.The San Joaquin Valley Air Basin has been designated by the EPA as a non-attainment area for ozone and PM10 federal standards. The San Joaquin County area, which includes the project site, is considered by the EPA as attainment/unclassified for CO. The San Joaquin Valley Unified Air Pollution Control District(SJVUAPCD) is responsible for air pollution control within the San Joaquin Valley Air Basin. The ARB coordinates and oversees the activities of California's many single-county Air Pollution Control Districts (APCDs)and multi-county Unified APCDs (UAPCDs) and Air Quality Management Districts (AQMDs). ARB and the APCDs/UAPCDs/AQMDs operate numerous air quality monitoring stations throughout the State. Data collected at those stations are used to classify areas and air basins as attainment or non-attainment for each criteria air pollutant based on whether ambient air quality standards have been achieved. An area is considered to be in non-attainment for a certain pollutant if violations of the applicable standard have occurred in each of the last three years. One violation per year contributes toward State designation as nonattainment;federal designation occurs with two or more violations per year. For the purposes of considering an air basin as attainment with respect to a standard, ARB and EPA both consider multiple violations of short-term standards on the same day as one violation. The California Clean Air Act (CCAA), which became effective on January 1, 1989, provides a planning framework for attainment of State ambient air quality standards. Local APCDs and AQMDs with areas in violation of State ambient air quality standards are required to prepare plans for attaining the State standards. The CCAA provides for the classification of nonattainment air basins into three classes: moderate, serious, and severe. For each class, the CCAA specifies attainment guidelines that must be followed. For all classes, attainment plans are required to demonstrate a five percent per year reduction in the emissions of 9