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conditions associated with winter storms result in periods of low air pollution and excellent visibility. However, between winter storms,high pressure and light winds lead to the creation of low-level temperature inversions and stable atmospheric conditions,resulting in high levels of CO concentrations and particulate matter(PM). The orientation of the wind-flow pattern in the SJVAB is parallel to the valley and mountain ranges. Summer wind conditions promote the transport of 03 and precursors to the SJVAB from the San Francisco Bay Area through the Carquinez Strait,a gap in the Coastal Mountain Range,and low mountain passes, such as Altamont Pass and _ Pacheco Pass. The College Park site is located in the northern portion of the SJVAB. The climate is semiarid, and the annual nominal precipitation is approximately 14 inches. January temperatures range from a normal minimum of 37°F to ` a normal maximum of 53°F. July temperatures range from a normal minimum of 61'F to a normal maximum of 95°F(NOAA 1992). The predominant wind speed is 10 mph. Atmospheric Stability and Inversions Stability describes the resistance of the atmosphere to vertical motion and is dependent on the vertical distribution of temperature with height. When the temperature decreases vertically at 10°Celsius (°C)per 1,000 meters,the atmosphere is"neutral." When the lapse rate(change in temperature with respect to height) is greater than 10°C per 1,000 meters,the atmosphere is"unstable." When the lapse rate is less than 10°C per 1,000 meters,the atmosphere is"stable." Stability categories range from Extremely Unstable(Class A),through Neutral(Class D), to Stable(Class F). Conditions are unstable during daytime hours when solar heating warms the lower atmospheric layers sufficiently. The dispersive power of the atmosphere decreases with progression through the categories. Stability categories D through F in the SJVAB are predominant during late fall and winter because of cool temperatures and entrapment of cold air near the surface. March and August are transition months,with equally occurring percentages of categories F and A. During spring(April-May)as well as summer(June-July), category A is predominant. Fall(September-November)has comparable percentages of category A and category F. An inversion is a layer of warmer air over a layer of cooler air. Inversions influence the mixing depth of the atmosphere,which is the vertical depth available for diluting air pollution near the ground,thus significantly affecting air quality conditions. The SJVAB experiences surface-based and elevated inversions. The shallow surface-based inversions are present in the morning and are often broken by daytime heating. The deep elevated inversions occur less frequently than the surface-based inversions but generally result in more severe stagnation. The surface-based inversions occur more frequently in the fall, and the stronger elevated inversions usually occur during December and January. CRITERIA AIR POLLUTANTS v The ARB and the EPA focus on the following air pollutants as indicators of ambient air quality: 03,CO,nitrogen dioxide(NOA sulfur dioxide(SO2), PM, and lead. Because these are the most prevalent air pollutants known to ` be deleterious to human health and extensive health-effects criteria documents are available,they are commonly referred to as"criteria air pollutants." A brief description, including adverse health effects and formation processes of each criteria air pollutant, is provided below. Ozone 03 is a photochemical oxidant and the primary component of smog. 03 is not directly emitted into the air,but formed through complex chemical reactions between precursor emissions of organic compounds and oxides of nitrogen in the presence of sunlight. Organic compounds and oxides of nitrogen are emitted by mobile _ (transportation) and stationary(industrial) sources. 03 in the upper atmosphere(stratosphere)acts in a beneficial EDAW College Park at Mountain House Specdic Plan III Draft EIR Air Quality 4.12-2 San Joaquin County