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because reaction rates increase with temperature. However, extremely high temperatures can"lift"or"break"an inversion layer, allowing dispersal of these pollutants. The vertical dispersion of air pollutants in the SJVAB is limited by the presence of persistent temperature inversions. Because of expansion-related cooling of the atmosphere, air temperature usually decreases with altitude, and a reversal of this atmospheric state,where the air temperature increases with height, is termed an inversion. Inversions can exist at the surface, or at any height above the ground. The height of the base of the inversion,the level to which pollutants can mix vertically, is known as the"mixing height." There are two principal types of inversions that occur in the SJVAB: surface or radiation inversions, and subsidence inversions. Surface inversions are formed when the ground surface becomes cooler than the air above it during the night;the inversion is dissipated when heat from the sun warms the ground,which in turn heats the lower layers of air,which rise to break up the inversion layer. Summer radiation inversion mixing heights are usually encountered 2,000-2,500 feet above the valley floor; during the winter,radiation inversion heights are typically 500-1,000 feet above the valley floor. Subsidence inversions occur as air is pushed downward by, for example,the movement of air over mountain ranges,or by differential pressure changes in the atmosphere. As this air moves downward, its pressure increases, causing its temperature to increase. The warm layer of air created by this phenomenon will descend to some relatively static elevation above the ground, creating a low inversion layer. This type of inversion is quite persistent,because heat from the ground does not reach the inversion base to break it up. This is common in high- pressure areas along the coast. Precipitation and fog tend to reduce or limit the concentrations of some pollutants. Clouds and fog block the solar radiation needed for ozone generation,reduce the concentrations of water-soluble carbon monoxide(CO), and reduce concentrations of particulate matter.A majority of the precipitation in the SJVAB occurs as rainfall during winter storms. The rare occurrence of precipitation during the summer is in the form of convective rain showers. The amount of precipitation in the SJVAB decreases from north to south primarily because the Pacific storm track often passes through the northern portion of the SJVAB,while the southern portion remains protected by the Pacific high-pressure cell. Stockton in the north receives about 20 inches of precipitation per year,Fresno in the center receives about 10 inches per year, and Bakersfield at the southern end of the valley receives less than 6 inches per year. Average annual rainfall for the entire SJVAB is approximately 9.25 inches on the valley floor (SJVAPCD 2002). Between winter storms,high pressure and light winds allow cold moist air to pool on the SJVAB floor,which creates strong low-level temperature inversions and very stable air conditions leading to the famous tule fog. The formation of natural fog is caused by local cooling of the atmosphere until it is saturated(dew point temperature). Conditions favorable to fog formation are also conditions favorable to high concentrations of CO and respirable particulate matter with an aerodynamic diameter of 10 micrometers or less (PMIO); ozone levels are low during these periods because of the lack of sunlight. Maximum CO concentrations tend to occur on clear, cold nights when a strong surface inversion is present and large numbers of fireplaces are in use. A secondary peak in CO concentrations occurs during morning commute hours when a large number of motorists are on the road and the surface inversion has not yet broken. Aside from the correlation between conditions that contribute to fog formation and those that contribute to high wintertime pollutant concentrations,fog itself can influence airborne pollutant concentrations. The water droplets in fog can act as a sink for CO and oxides of nitrogen(NOx),lowering pollutant concentrations.At the same time,fog helps in the formation of secondary particulates such as ammonium sulfate.These secondary particulates are believed to be a significant contributor to winter-season violations of the particulate matter standards. Summer is considered the ozone season in the SJVAB. This season is characterized by poor air movement in the mornings and by longer daylight hours,which provide a plentiful amount of sunlight to fuel photochemical EDAW Manteca WQCF and Collection System Master Plans EIR Air Quality 4.3-2 City of Manteca