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APPENDIX B 1., air Quality 'San Joaquin County has been designated as an Air Qui..lity Mainte- nance Area for photochemical oxidents and for particulates be- cause national ambient air quality standards (NAAQS) are not expected to be attained by 1980 and maintained through 1985. x _ either because of existing air quality or projected qr=�nvth and development in the area. " Air quality in San Joaquin County is affected by a combination of natural and man-made factors. Natural factors include thermal atmospheric inversions which prevent dispersion of pollutants and wind borne dust. Man-made factors include mobile and stationary sources of hydrocarbon and other pollutants, agricultural burning and generation of dust. San Joaquin Valley is an area of frequent thermal inversions which include radiation inversions and subsidence inversions. Radiation inversions occur most often in the fall and winter months, when the horizontal winds are lessened and when the nights are longer. The process for a radiation inversion is based on nightly loss of heat from the earth' s surface cooling the thin layer of air nearest the surface. As a result this layer of cool air is sandwiched between the earth and the warmer higher layers of air. Tule fogs are the result of radiation inversions combined with a high atmospheric humidity and a temperature below the dew point. These fogs and inversions remain until the sun warms the lower layer of air suffi- ciently to evaporate the moisture and produce the mixing of air. Subsidence inversions are the result of the compression and heating of a descending layer of upper air. This thermal inversion layer, which usually exists at an altitude of 7,000 or more feet, descends during the summer and fall, creating a canopy at 1,000 and 2,000 feet. Vertical and horizontal air movement is greatly constrained. The heating effect of the sun is much less effective in dispelling subsidence inversions than radiation inversions. The effect of these inversions is the restriction of dispersion of pollutants into the atmosphere causing ground levels of pollution to rise substantially. During early summer months westerly winds pass through the Carquinez Straits into the San Joaquin Valley. These winds carry peat dust and other soil particles throughout the Delta and into central portions of San Joaquin County. Wind borne particles are also attributed to agricultural planning, lack of adequate vegetative groundcover, and poor construction grading and excavation practice. Man-made sources of air pollution are the subject of air quality monitoring and standards administered by the Air Resources Board and the San Joaquin County Air Pollution Control District. E1