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L Draft Supplemental Environmental Impact Report Page IV.D-2 Forward Inc.Landfill 2014 Expansion Project originate from the south end of the valley and flow in a north-northwesterly direction. Also, during winter months, the valley experiences light, variable winds, less than 10 miles per hour (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 (°F), L 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 OF. 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 approximately 45 degrees OF. Precipitation in the Stockton area falls almost exclusively from mid-November to mid-April from the fringes of mid-latitude storms. The Stockton area averages 13.5 inches of rain annually. ►� The vertical dispersion of air pollutants in the valley 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 inversion. Air above and below an inversion does not mix because differences in air density thereby restrict air pollutant dispersal. Regulatory Framework Air quality issues are under the jurisdiction of the United States Environmental Protection Agency (EPA), the California Air Resources Board (CARB), and the SJVAPCD. Regulation of air pollution is achieved through both Federal and State ambient air quality standards and emission limits for individual sources of air pollutants. An"ambient air quality standard" represents a level of an air pollutant in the outdoor (ambient) air that is necessary to protect public health. The ambient standards do not apply to indoor environments. Again, as required by the federal Clean Air Act (CAA), the EPA identified criteria pollutants and established National Ambient Air Quality Standards (NAAQS,or national standards) to protect the public health and welfare. There are NAAQS for ozone (03), carbon monoxide (CO),nitrogen dioxide (NO,),sulfur dioxide (SO2), respirable particulate matter equal to or less than 10 microns in diameter (PM,o),fine particulate matter equal to or less than 2.5 microns in diameter(PM2.5), and lead (Pb). These pollutants are known as "criteria" air pollutants because standards have been established to meet specific public health and welfare criteria. The NAAQS are defined as the maximum acceptable concentration that may be reached,but ` not exceeded more than once per year. California has adopted more stringent ambient air quality standards for most of the criteria air pollutants (known as CAAQS, or State standards). The pollutants of greatest concern in the area are 03 and PM,o. The State and National Ambient ft• Air Quality Standards are summarized in Table IV.D-1, which also provides a brief discussion of the related health effects and principal sources for each air pollutant. Criteria Air Pollutants The following provides a brief summary of the potential health and welfare effects and typical sources of each of the criteria air pollutants. Ozone. Ozone is a respiratory irritant and an oxidant that increases susceptibility to respiratory infections and that can cause substantial damage to vegetation and other materials. .. Ozone is not emitted directly into the atmosphere,but is a secondary air pollutant produced in the atmosphere through a complex series of photochemical reactions involving volatile organic