CALIFORNIA AIR BASINS INTRODUCTION The observed ability of the winds to carry air pollution from its points of origin over large areas, and the effects of topography and temperature inversions to determine how such transport occurs, long ago gave rise to a concept of "basins" around the major metropolitan areas in California. In 1967 the State Legislature, recognizing that air pollution might be more rationally dealt with by defining the problem in terms of a region containing both the sources and the total area affected, directed the Air Resources Board to divide the State into air basins. The Mulford-Carrell Air Resources Act requires that these basins de fine areas having similar meteorological and geographical conditions, and that the Board in setting the boundaries take into consideration existing political boundaries wherever practicable. The Air Resources Board established the boundaries to the air basins on November 20, 1968. The Act's purposes, as they apply to basins are: (1) to establish air quality standards that may vary from basin to basin; (2) adopt emissions standards for air pollutants for each basin as found necessary; (3) inventory all sources of emissions for each basin; and (4) provide a mechanism for the establishment of regional air pollution control districts within the basins. The criteria on which the basins are defined are to some extent in conflict where meteorological and topographical boundaries depart widely from political boundaries. In these cases a judgment had to be made as to which criterion to follow. The first step in dividing the State into basins was an examination of the physical and meteorological factors that influence the distribution of climatic conditions within the State. Unlike watersheds, it is not possible to define air basins wherein the air pollution problems would be completely isolated from adjacent basins. Winds do not start and stop at political or geographic boundaries. Land breezes from coastal plains or valleys can move offshore and return inland into another valley or basin further up or down the coast. In mountain regions, air moves through gaps and passes from one region to another. Nevertheless, California is peculiarly suited to application of the concept of basins because its large valleys, plains and plateaus are in most instances separated by mountain ranges. Marked differences in topographical and meteorological conditions are found in the several portions of California. The coastal plains, central valley, desert, and mountain areas have different meteorology and topography and hence a different potential for air pollution. As a background for discussing air basins, it is important to have in mind the principal topographical features and meteorological factors in California. TOPOGRAPHY California, the third largest of the 50 states, lies along the Pacific coast between 32° 20' and 42° North latitudes. It is 730 miles long and from 150 to 350 miles wide, with a land area of 159,000 square miles. Elevations range from 14,495 feet at the top of Mt. Whitney to 282 feet below sea level in Death Valley. Conspicuous among the topographic features is the Great Central Valley, which lies along the north-south axis of the State for a distance of about 450 miles and is approximately 50 miles in width. The valley is nearly completely enclosed by the Sierra Nevada and coast ranges which curve toward each other at the northern and southern ends of the valley and merge with the Cascade and Tehachapi Mountains to bound one of the largest mountain-ringed valleys in North America. See Figure 1. Separated from the central valley and from the coast by mountain ranges are two large desert or semidesert areas. In the scuth, a portion of the Great Basin and Southwestern U.S. Desert reaches from the Nevada and Arizona borders to the east side of the mountains. In the northeast corner of the State, a portion of the Intermountain Plateau lies between the Cascade Range and the Nevada and Oregon borders. The balance of the State is essentially mountainous. The Sierra Nevada Range is approximately parallel and close to most of the eastern border of the State from the Cascades at the north end of the central valley to the Tehachapi Mountains at the south. Except where bisected by individual valleys, the coast ranges and the associated southern ranges lie along the coast, often extending to the shore but sometimes set back by narrow coastal plains. River valleys, such as the Salinas, Russian and Santa Clara, penetrate deep into the neighboring mountains. Where the river valleys narrow to become canyons (often at relatively low elevations) they become part of the mountain region. North of the San Francisco Bay, the coast ranges widen and spread inland to blend into the Cascades, while continuing along the coast to form a large mountainous area in the northwestern part of the State. In the numerous valleys and coast plains that extend from the sea into the coast ranges are found many of the major cities and important agriculture areas of California. It is in some of these areas that the most severe air pollution problems occur. Lake Tahoe, Scott Valley, Round Valley and other encircled mountain valleys are small but distinct topographic basins tucked well back into the mountains, where they share in the general climate of their respective mountain regions. METEOROLOGY Throughout the north-south extent of California, climatic conditions vary from subtropical in the southern part, to subtemperate in the northern part. The east-west transverse ranges through Santa Barbara, Ventura, Los Angeles and San Bernardino Counties tend to provide a northern topographic limit for the subtropical climate zone though encroachments of subtropical weather may reach into the central part and the southeastern deserts of the State on some occasions. In the east-west direction across the State there is a sharp contrast in climates. The eastern boundary of California lies in the Great Intermountain Plateau. On the western boundary, the Pacific Ocean provides moist maritime air that profoundly affects the climate of the coastal sections. The coast ranges and the Sierra Nevada Range, separated by the Great Central Valley, are oriented parallel to the State's eastern and western borders. These mountains and the intervening valleys amplify and define the gradations of climates which lie between the contrasting border climates. These general climatic regions are considerably modified by the vertical differentation within the State. Major and minor mountain ranges not only bound the climate regions of the valleys, but also are climatic zones in their own right because of the differences produced by elevation. Cooler climates with more wind are, in general, characteristic of the mountains as contrasted with the nearby valleys. This change with increase of elevation is a gradual one, but, due to temperature inversion, conditions may be sharply defined on a day-to-day basis. Temperature inversions--atmospheric layers in which temperature increases with height rather than decrease, as is the general condition--form an effective barrier against the vertical interchange of air and, incidentally, the dispersion of air pollution upward. Inversions in California are due to three major causes: the chilling of the air in contact with the ground due to the radiation of ground heat into space at night which produces radiation inversions; the intrusion of cold marine air beneath the warmer air over the land along the coast; and the large-scale, inversion produced by the general tendency for the air along the entire coast and several hundred miles inland to sink toward the earth from higher altitudes. This latter inversion, called a "subsidence inversion", is present over almost all of California during me? of the spring, summer and fall. The base of the subsidence inversion layer is the boundary between mountain climatic zones and the adjacent valley climatic zones throughout the spring, summer and fall. The inversion base slopes upward from the coastline to the interior. Also, the inversion layer along the coast slopes gently upward both north and south from about San Luis Obispo County. As a result, the elevations that bound the mountain climatic zones tend to slope upward from west to east and upward toward both the north and south from the central part of the State. The zones above and below the inversion layer in the coastal areas are fairly well described by the 1,000 to 2,000 foot contours. On the eastern side of the coastal ranges the boundary is about the 2,000-foot contour as far north as Colusa County. From there northward, it slopes up to about 4,000 feet. On the west side of the southern Cascades and the Sierra Nevada, the boundary is at about 4,000 feet; on the east side it lies at about 6,000 feet. The Tehachapi Mountains and the eastern transverse ranges zones are bounded by about the 4,000-foot elevation. The 1,000-foot level marks the western transverse ranges mountain zone near the coast. The boundary rises to about 2,000 feet along the peninsular ranges in Riverside and San Diego Counties. During the winter season, the boundary between valley and mountain zones descends to somewhat lower elevations due to ground-based inversions created by nocturnal radiation cooling and cold air drainage from the upper mountain slopes. None of the mountain climatic zones can be classed as basins in the topographic or meteorological sense of the word. They do, however, have similar weather and air pollution potential. During the warm portions of the year, wind circulation in the mountain zones is generally up-slope with only brief periods of down-slope winds at night. During the cold season, wind circulation in the absence of storm activity is generally down-slope with brief periods up-slope winds on south-facing slopes. Mountain climatic zones, thus, are characterized by considerable vertical wind motion and by winds and temperatures different from those in the valleys and on the plains. The mountain climatic zones are shown as shaded areas in Figure 2. A number of lower elevation areas, more or less flat valleys, plains and plateaus, also constitute distinct climatic zones. These are the areas that give rise to the "basin" concept, and these are the regions upon which the "air basins" required by the Mulford-Carrell Air Resources Act were developed. The major ones are: The Great Central Valley, the Southeastern Desert and Great Basin Region, the portion of the great intermountain plateau in the northeastern corner of the State and the coastal area. INTERIOR CLIMATIC ZONES Central Valley The Great Central Valley is the most distinctly bounded, large topographic basin in the State. The valley is more complex climatically than it is topographically. Because of the temperature contrast much of the year between the valley and the Pacific waters, air from the coast enters the valley, primarily through the gap at San Francisco Bay, and undergoes rapid modification in temperature and relative humidity. Part of the flow turns northward into the Sacramento valley and part southward into the San Joaquin valley. Thus, the two valleys differ climatically in that each has a separate and distinct system of wind circulation, although, from the standpoint of temperature and humidity, they differ rather little during the warm seasons. A wind divergence zone is created by the splitting of the airflow through the coastal range. This divergent zone separates the valleys as far as summer wind patterns are concerned. The mean summer position of this divergent zone lies at about the Sacramento-San Joaquin and the Amador-El Dorado County borders. During the late fall and winter, cold air drains off the mountain slopes into the two valleys. This often results in airflow toward the north in the San Joaquin valley and southward in the Sacramento valley, creating a zone of wind convergence that fluctuates to the north and south of the delta area according to the relative strengths of the airflow out of each valley. The central valley, therefore, while topographically a single large basin, is meteorologically two distinct basins whose common boundary fluctuates with the seasons. |