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Environmental & Resource Protection
Air Quality

The County of Santa Cruz is located in the North Central Coast Air Basin (NCCAB). The NCCAB is comprised of Monterey, Santa Cruz and San Benito Counties. The basin lies along the central coast of California and covers an area of 5,159 square miles. The northwest sector of the basin is dominated by the Santa Cruz Mountains. The Diablo Range marks the northeastern boundary, and together with the southern extent of the Santa Cruz Mountains forms the Santa Clara Valley which extends into the northeastern tip of the Basin. Farther south, the Santa Clara Valley evolves into the San Benito Valley which runs northwest-southeast and has the Gabilan Range as its western boundary. To the west of the Gabilan Range is the Salinas Valley, which extends from Salinas at its northwestern end to King City at its southeastern end. The western side of the Salinas Valley is formed by the Sierra de Salinas, which also forms the eastern side of the smaller Carmel Valley. The coastal Santa Lucia Range defines the western side of the Carmel Valley.

The semi-permanent high pressure cell in the eastern Pacific is the basic controlling factor in the climate of the air basin. In the summer, the high pressure cell is dominant and causes persistent west and northwest winds over the entire California coast. Air descends in the Pacific High forming a stable temperature inversion of hot air over a cool coastal layer of air. The onshore air currents pass over cool ocean waters to bring fog and relatively cool air into the coastal valleys. The warmer air aloft acts as a lid to inhibit vertical air movement.

The generally northwest-southeast orientation of mountainous ridges tends to restrict and channel the summer onshore air currents. Surface heating in the interior portion of the Salinas and San Benito Valleys creates a weak low pressure which intensifies the onshore air flow during the afternoon and evening.

In the fall, the surface winds become weak, and the marine layer grows shallow, dissipating altogether on some days. The air flow is occasionally reversed in a weak offshore movement, and the relatively stationary air mass is held in place by the Pacific High pressure cell, which allows pollutants to build up over a period of a few days. It is most often during this season that the north or east winds develop to transport pollutants from either the San Francisco Bay area or the Central Valley into the NCCAB

During the winter, the Pacific High migrates southward and has less influence on the air basin. Air frequently flows in a southeasterly direction out of the Salinas and San Benito Valleys, especially during night and morning hours. Northwest winds are nevertheless still dominant in winter, but easterly flow is more frequent. The general absence of deep, persistent inversions and the occasional storm systems usually result in good air quality for the basin as a whole in winter and early spring.

In Santa Cruz County, coastal mountains exert a strong influence on atmospheric circulation, which results in generally good air quality. Small inland valleys such as Scotts Valley with low mountains on two sides have poorer circulation than at Santa Cruz on the coastal plain. In addition, Scotts Valley is downwind of major pollutant generating centers, and these pollutants have time to form oxidants during transit to Scotts Valley. Consequently, air pollutants tend to build up more at Scotts Valley than at Santa Cruz.

Monterey Bay is a 25-mile wide inlet, which allows marine air at low levels to penetrate the interior. The Salinas Valley is a steep-sloped coastal valley which opens out on Monterey Bay and extends southeastward with mountain ranges of two to three thousand feet elevation on either side. The broad area of the valley floor near the mouth is twenty five miles wide, narrowing to about six miles at Soledad, which is forty miles inland, and to three miles wide at King City, which is about sixty miles from the coast. At Salinas, near the northern end of the Valley, west and northwest winds occur about one-half the time during the entire year. Although the summer coastal stratus rarely extends beyond Soledad, the extended sea breeze, which consists of warmer and drier air currents, frequently reaches far down the Salinas Valley. In the southern end of the Valley, which extends into the South Central Coast Air Basin to Paso Robles, winds are generally weaker most of the year except during storm periods.

Hollister, at the northern end of the San Benito Valley, experiences west winds nearly one-third of the time. The prevailing air flow during the summer months probably originates in the Monterey Bay area and enters the northern end of the San Benito Valley through the air gap through the Gabilan Range occupied by the Pajaro River. In addition, a northwesterly air flow frequently transports pollutants into the San Benito Valley from the Santa Clara Valley.

Attainment Status of the NCCAB

Designations in relation to the state standards are made by the (California Air Resources Board (CARB) while designations in relation to the National standards are made by Environmental Protection Agency (EPA). State designations are reviewed annually while the national designations are reviewed when either the standards change, or when an area requests that they be re-designated due to changes in the area’s air quality. Designations are made by air basin, and in some cases, designations are made at the county level. Designations are made by pollutant according to the following categories:

Attainment – Air quality in the area meets the standard.

Nonattainment Transitional – Air quality is approaching the standard (state only).

Nonattainment – Air quality in the area fails to the applicable standard.

Unclassified – Insufficient data to designate area or designations have yet to be made.

Nonattainment designations are of most concern because they indicate that unhealthy levels of the pollutant exist in the area, which typically triggers a need to develop a plan to achieve the applicable standard.

Current State and National designations are shown below:

Attainment Status for the North Central Coast Air Basin – January 2009
Pollutant State Standards National Standards
Ozone (O3) Nonattainment1 Attainment2
Inhalable Particulates (PM10) Nonattainment Attainment
Fine Particles (PM2.5) Attainment Unclassified/Attainment3
Carbon Monoxide (CO) Monterey Co. - Attainment San Benito Co. - Unclassified Santa Cruz Co. - Unclassified Attainment
Nitrogen Dioxide (NO2) Attainment Attainment
Sulfur Dioxide (SO2) Attainment Attainment
Lead Attainment Unclassified/Attainment4
  1. Effective July 26, 2007, the ARB designated the NCCAB a nonattainment area for the State ozone standard.
  2. On March 12, 2008, EPA adopted a new 8-hour ozone standard of 0.075 ppm, while temporarily retaining the existing 8-hour standard of 0.08 ppm. EPA is expected to issue new designations by March 2010.
  3. In 2006, the Federal 24-hour standard for PM2.5 was revised from 65 to 35 µg/m3.
  4. On October 15, 2008 EPA substantially strengthened the national ambient air quality standard for lead by lowering the level of primary standard from 1.5 µg/m3 to 0.15 µg/m3.Initial recommendations for designations are to be made by October 2009 with final designations by January 2012.
  5. Nonattainment pollutants are highlighted in Bold.

Source: Monterey Bay Unified Air Pollution Control District, 2009.

Additional information and State and National designations and ambient air quality standards can be obtained at: