The following Class I areas are in the Southern California region:
Agua Tibia Wilderness Area
Cucamonga Wilderness Area
San Gabriel Wilderness Area
San Gorgonio Wilderness Area
San Jacinto Wilderness Area

Terrain of southern California is characterized by coastal air basins, principally the South Coast Air Basin, and surrounding mountain ranges. Generally the area is bounded on the north by the San Gabriel Mountains. Several openings lead to the more interior valleys and the Mohave Desert. Under some conditions, these openings provide transport corridors for air pollution even under severe stagnation conditions that prevent vertical mixing and upper level transport.

The climate of this region is heavily influenced by the semi-permanent combination of Pacific anticyclone and ‘thermal’ low pressure that extends up from Mexico into the California Valley in summer and begins to break down in fall. Typically, by late September, mid-latitude transient weather systems start to invade the California coastline – affecting the northern California coast first and later in the season the southern California coastline. These transient systems continue to impact the coast until early April. Yet, the very cold/dry penetrations that affect the southern California coast do not typically begin until early December (late fall). These cold/dry penetrations are usually associated with northwesterly flow, and the South Coast Air Basin is usually scoured of pollutants during these episodes.

The synoptic weather situations that favor increases in pollution are the quiescent periods when steady-state high pressure (anticyclones) cover the southern half of California. During such periods, especially within the Los Angeles area almost completely enclosed by mountains on the north and east, the vertical temperature structure (inversion) tends to prevent vertical mixing of the air through more than a shallow layer (1,000 to 2,000 feet deep). The geographical configuration and the southerly location of the Basin permit a fairly regular daily reversal of wind direction—offshore at night and onshore during the day. This is especially true close to the coastline, less pronounced at more interior areas. Winds during these anticyclonic situations are typically quite low in speed, on the order of 5 mph or less.

With the concentrated population and industry, pollution products tend to accumulate and remain within this circulation pattern until ventilated. When transported to distant regions, either through vertical mixing and upper level transport, or through low level transport through passes and corridors into the Mojave interior, the region is a major source region for pollutants including particulate matter

References
1997 Southern California Ozone Study (SCOS97) Study Plan
Weekend/Weekday Nitrate Study
State Climatologist (through National Climatic Data Center)