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Ill. ENVIRONMENTAL SETTING,UY&ACTS AND MITIGATION MEASURES B. AIR QUALITY This section evaluates the air quality impacts of construction and operation of the expanded Stockton Scavenger Transfer Station. Air emissions are measured against standards provided in the San Joaquin Valley Unified Air Pollution Control District's Guide for Assessing and Mitigating Air Quality Impacts. B.1 SETTING CLIMATE AND METEOROLOGY The primary factors that determine air quality are the locations of air pollutant sources and the amounts of pollutants emitted. Meteorological and topographical conditions, however, also are important. Atmospheric conditions such as wind speed, wind direction,and air temperature gradients interact with the physical features of the landscape to determine the movement and dispersal of air pollutants. The project site lies within the San Joaquin Valley Air Basin(SJVAB), a basically flat area bordered on the east by the Sierra Nevada mountains;on the west by the Coast Ranges; and to the south by the Tehachapi mountains. Airflow in the SJVAB is primarily influenced by marine air that enters through the Carquinez Straits where the San Joaquin-Sacramento Delta empties into the San Francisco Bay (SJVUAPCD, 1998a). The region's topographic features restrict air movement through and out of the basin. As a result,the SJVAB is highly susceptible to pollutant accumulation over time(SJVUAPCD, 1998x). Frequent transport of pollutants into the SJVAB from upwind sources also contributes to poor air quality. Wind speed and direction play an important role in dispersion and transport of air pollutants. During summer periods,winds usually originate out of the north end of the San Joaquin Valley (SJV)and flow in a south-southeasterly direction through the SJV, through the Tehachapi pass and into the neighboring Southeast Desert Air Basin. During winter months, winds occasionally originate from the south end of the SJV and flow in a north-northwesterly direction. Also,during winter months,the SJV experiences light, variable winds, less than 10 mph. Low wind speeds, combined with low inversion layers in the winter,create a climate conducive to high concentrations of certain air pollutants. The SJVAB has an inland Mediterranean climate that is characterized by warm,dry summers and cooler winters. Summer high temperatures often exceed 100 degrees Fahrenheit, averaging from the low 90s in the northern part of the valley to the high 90s in the south. The daily summer temperature variation can be as high as 30 degrees Fahrenheit. Winters are for the most part mild and humid. Average high temperatures during the winter are in the 50s, while the average daily low temperature is about 45 degrees Fahrenheit. The vertical dispersion of air pollutants in the SJV is limited by the presence of persistent temperature inversions. Air temperatures usually decrease with an increase in altitude. A reversal of this atmospheric state, where the air temperature increases with height, is termed an Stockton Scavenger Transfer Station Expansion III.B.1 ESA 1990190