A strong sea breeze develops, usually by noon, and gradually progresses further inland. After sundown, the sea breeze usually dies down, and late at night a reverse, land breeze develops, although usually it is much weaker than the sea breeze.

Occurrences of restricted visibility are most frequent in the summertime. Subsidence associated with the Pacific High as well as cold water upwelling leads to a very stable atmospheric layer near the ocean surface. Condensation of moisture leads to low clouds and fog (stratus). Low clouds and fog move into the coastal zone with the sea breeze, but usually quickly dissipate farther inland.

Visibility is least restricted in the spring and most restricted in summer and early fall. In late summer, when visibility tends to be lowest, the frequency of visibility equal to or less than 3.6 km (2 nm) ranges from less than 5 percent in the areas farthest offshore to more than 20 percent in some areas in the Santa Barbara Channel and Santa Maria Basin (USDOI, BLM, 1979a). The frequency of dense fog, visibility equal to or less than 0.5 km (0.25 nm), is only about 1 to 2 percent in most areas.

Data on temperature inversions and atmospheric stability for offshore areas are very scarce. The information given here is based on data from coastal stations. The MMS has been augmenting the database through specific field studies. Temperature inversions exist along the Southern California coast persistently in the summer, and somewhat less frequently in the winter. An inversion is a condition in which temperature increases with height. Atmospheric mixing is greatly restrained within an inversion layer leading to a high air pollution potential. The inversion layer is deepest over the offshore area and the coastline. Over land, the inversion weakens with distance from the coastline due to rapid heating.

(2) Air Quality

Regional air quality depends upon the prevailing weather conditions, local topography, and the amount of pollutants being emitted into the air. The Federal Government has established limits for the airborne levels of seven pollutants and the frequency that these limits may be exceeded. These individual air pollutant criteria are used to determine the air quality designation (attainment status) of onshore areas for each of those pollutants. The States also have air pollution standards that are as stringent or more stringent than those of the Federal Government. Some States, such as California, have set standards for pollutants not regulated by Federal law. Table III-5 lists both Federal and the most recent California air quality standards.

Table III-5. Federal And California Ambient Air Quality Standards

1.

2.

3.

4.

5.

6.

7.

Federal standards, other than ozone and those based on annual averages or annual geometric means, are not to be exceeded more than once per year. The ozone standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above the standard is equal to or less than one.

California standards for ozone carbon monoxide, sulfur dioxide (1 hour), nitrogen dioxide and particulate matter (PM10), are values that are not to be exceeded. The sulfates, lead, hydrogen sulfide, vinyl chloride, and visibility reducing particles standards are not to be equaled or exceeded.

Concentration expressed first in units in which it was promulgated. Equivalent units given in parentheses are based upon a reference temperature of 25°C and a reference pressure of 760 mm of mercury. All measurements of air quality are to be corrected to a reference temperature of 25°C and a reference pressure of 760 mm Hg (1013.2 millibars); ppm in this table refers to ppm by volume, or micromoles per mole of gas.

Federal Primary Standard: Levels necessary to protect public health with an adequate margin of safety.

Federal Secondary Standards: Levels necessary to protect the public from known/anticipated pollutant adverse effects.
Applies only at locations where the State standard for ozone and/or total suspended particulate matter are violated.

In sufficient amounts to reduce the prevailing visibility to less than 10 miles when the relative humidity is less than

The USEPA has long had the responsibility for enforcing the Federal standards over land and for 3 miles off the coasts. The 1990 Clean Air Act extended the USEPA jurisdiction into Federal waters for all OCS areas except those located in the Central and Western Gulf of Mexico. This mandate effectively rescinded the USDOI's jurisdiction over OCS air quality on September 4, 1994, except in the previously mentioned areas of the Gulf of Mexico.

The coastal areas of Washington and Oregon generally exhibit some of the best air quality in the Nation due to prevailing weather conditions experienced in the Pacific Northwest. This opinion is generally held despite very few ambient air monitoring stations located in coastal areas. The majority of available coastal air quality data comes from locations along the Washington coast. Air data from these locations show that the only exceedance of the State standards was observed in Hoaquiam, Washington which exceeded the 1-hr SO2 standard. All other coastal monitoring stations exhibited ambient air measurements well below State standards. Measurements of existing air quality in the North Coast Air Basin of California are limited primarily to observations of O, and 10-micron particulate matter (PM10). The air monitoring site in Ukiah monitors all criteria pollutants with the exception of PM10. Monitoring sites in Humboldt, Mendocino, and Sonoma counties within the North Coast Air Basin reported exceedances of the 24-hour average California PM10 standard. No exceedances of the Federal PM10 standards have occurred this decade. These locations have never measured O, above the Federal standards. Measurements of Q, at various locations along the coast of California are presented in Table III-6.

In the San Francisco Bay Area Air Basin, O, concentrations exceed both Federal and State standards with an observed 1-hour high average of 0.13 ppm measured in 1993. Concentrations of NO2 and SO2 are well within both the Federal and State standards. PM10 and CO concentrations have exceeded both Federal and State standards this decade.

The North Central Coast Air Basin measures O, at several coastal sites, one of which also measures CO, NO2, and SO2. O, exceeded the more stringent State standard in 1993, but was within the Federal standard. PM10 concentrations exceeded both the State 24-hour and annual standards. CO, NO2, and SO2 are within Federal and State standards.

The South Central Coast Air Basin contains San Luis Obispo, Santa Barbara, and Ventura Counties. San Luis Obispo is considered in compliance with the Federal O, standard. Both Santa Barbara County and Ventura County are in violation of the Federal standard. Measurements of O, at various locations along the coast in the South Central Coast Air Basin are presented in Table III-6. The highest 1-hour average O, concentrations measured in San Luis Obispo, Santa Barbara, and Ventura Counties in the past few years are 0.11, 0.14, and 0.17 ppm, respectively. Concentrations of NO2 and CO are within both Federal and State standards. SQ concentrations at a select site in San Luis Obispo exceeds the 1-hour and 24-hour State standards.

1. State standard is 0.09 ppm; Federal standard is 0.12 ppm.

Source: California Air Quality Data, California Air Resources Board (1992, 1993, 1994).

III-141

The PM10 concentrations are within the Federal standard for all three counties in the air basin. The State 24hour average PM10 standards are exceeded in many locations in San Luis Obispo, Santa Barbara, and Ventura Counties, with all three counties designated nonattainment for the state PM10 standard.

The entire South Coast Air Basin is classified nonattainment for O3. The highest measured 1-hour average O, concentration in the past few years was 0.32 ppm in 1991 at the Glendora site. The average annual number of days per year when the Federal O, standard is exceeded range from about 10 days in the coastal areas to more than 120 days at some of the inland locations.

The maximum 1-hour average NO2 concentration measured in the South Coast air Basin ranges from 0.13 to 0.26 ppm, with the highest concentration occurring at Pico Rivera. The 1-hour average State standard for NO2 was exceeded at many locations in Los Angeles County. The annual average NO2 concentrations ranged from 0.022 to 0.050 ppm, with the highest concentrations occurring at Pomona. The Federal annual average NO2 standard was exceeded at many locations in Los Angeles County. The South Coast Air Basin is therefore classified nonattainment for NO2.

The maximum 1-hour average CO concentration ranged from 6 to 21 ppm. This is within the Federal standard of 35 ppm; however, the State standard of 20 ppm was exceeded. The maximum 8-hour average CO concentration ranged from 3.9 to 14.6 ppm mg/m3. The Federal and State CO standards for the 8-hour average were exceeded at numerous locations in Los Angeles and Orange Counties. The South Coast Air Basin is classified nonattainment for CO.

The South Coast Air Basin is in attainment of the Federal SO, standards. The maximum 1-hour average SO2 concentrations ranged from 0.01 to 0.08 ppm. The maximum 24-hour average SO2 concentrations ranged from 0.001 to 0.013 ppm. The highest measured concentrations occurred at North Long Beach. These concentrations are within the Federal and State standards.

The Federal PM10 standard was exceeded at many locations in the South Coast Air Basin. The State PM standard was exceeded at all monitoring locations.

The San Diego Air Basin is classified nonattainment for O3. The highest measured 1-hour average O, concentration was 0.21 ppm in 1991, and occurred in Escondido. The average annual number of days that exceed the Federal O, standard ranges from 2 to 3 days at the coastal monitoring sites.

The maximum 1-hour average NO2 concentration was a 0.13 ppm and occurred at both San Diego and Escondido. This concentration is in compliance with the California 1-hour standard of 0.25 ppm. The highest annual average NO2 concentration was 0.024 ppm, and occurred at San Diego. The Federal standard is 0.053 ppm. The San Diego Air Basin is in attainment of the Federal NO2 standard.

The western portion of San Diego County is classified nonattainment for CO. The maximum 1-hour average CO concentration is 11 ppm. The maximum 8-hour average CO concentrations measured was a 7.5 ppm at San Diego. The Federal CO standards have not been exceeded this decade.

The San Diego Air Basin is in attainment of the Federal SO2 standard. The highest measured 1-hour and 24-hour average SO2 concentrations were 0.07 ppm and 0.13 ppm, respectively. These levels are well with Federal and State standards.

The maximum 24-hour average PM10 concentrations ranged from 56 to 127 ug/m3. The annual geometric mean PM10 concentrations ranged from 24.7 to 41.1 ug/m3. The concentrations measured at all monitoring stations were within the Federal PM10 standard, however, the State standard was exceeded at several locations.

III-142