Clarifying Information for Interpreting Air Quality Trends

This page last reviewed January 14, 2010

Air quality trends represent a summary of data collected at a number of monitoring sites. Whenever data are summarized, the results can be influenced by a number of factors. Identifying these factors and understanding their impact provides important information which helps in interpreting the trends.

An air quality trend statistic is generally used to characterize the air quality in a particular region (i.e., a county or an air basin). However, the air quality data used to calculate the trend statistic may not be consistent from year-to-year. Among the factors than can influence a trend statistic are: the number monitoring sites in the network, the opening and closing of critical sites, and the completeness of the data. The following tables provide information that is helpful for interpreting air quality trends -- in particular, the air quality trends presented in the Air Resources Board’s California Almanac of Emissions and Air Quality. The information is organized first by air basin [for the State’s major (most populated) air basins] and then by county within the air basins. It is important to note that not every area requires clarifying information. As a result, not all air basins and/or counties within an air basin are included in the following tables.

Clarifying Information for the Major Air Basin Summaries

San Diego Air Basin

PM ₁₀

1993 - 4 new sites opened

San Joaquin Valley Air Basin

Carbon Monoxide

1990 - Fresno-Olive Street site closed

1993 - Fresno-Fisher Street site opened

Sacramento Valley Air Basin

Carbon Monoxide

1981 - Sacramento-El Camino & Watt site opened

Clarifying information for the County Summaries

Great Basin Valleys Air Basin

Inyo County

Ozone

1994 - new site in Death Valley National Monument opened

Mojave Desert Air Basin

Kern

PM₁₀

1990 - Mojave site opened

Los Angeles County

Ozone

1990 - Lancaster site relocated

Carbon Monoxide

1990 - Lancaster site relocated

PM₁₀

1990 - Lancaster site relocated

Riverside County

Ozone

1988 - no sites operating

1992 - no data from high site (Joshua Tree National Monument)

San Bernardino County

Ozone

1981 - Cajon Pass site opened for less than one year only

1985 - Hesperia site opened

Carbon Monoxide

1983 - Victorville site closed

Mountain Counties Air Basin

Mariposa County

Ozone

1981 - only one site operating, incomplete data

Nevada County

Ozone

1993 - Nevada City site closed

1994 - Grass Valley site opened

Carbon Monoxide

1991 - incomplete data

1993 - incomplete data

1994 - incomplete data

Plumas County

Carbon Monoxide

1987 - incomplete data

Tuolumne County

Ozone

1992 - Sonora site opened

North Central Coast Air Basin

San Benito County

Ozone

1987 - Pinnacles site opened

Santa Cruz County

Ozone

1983 - Scotts Valley site closed

1993 - new Scotts Valley site opened

1995 - Scotts Valley site relocated

PM₁₀

1993 - Davenport site opened

North Coast Air Basin

Mendocino County

Carbon Monoxide

1984 - data incomplete

1986 - data incomplete

1988 - data incomplete

1992 - data incomplete

Northeast Plateau Air Basin

Siskiyou County

Carbon Monoxide

1987 - data incomplete

Sacramento Valley Air Basin

Butte County

Carbon Monoxide

1981 - Chico-Salem site opened

1992 - incomplete data at high site (Chico-Salem)

Placer County

Carbon Monoxide

1991 - data incomplete

Sacramento County

Carbon Monoxide

1981- Sacramento-El Camino & Watt site opened

PM₁₀

1993 - 5 new sites opened

Shasta County

Carbon Monoxide

1982 - data incomplete

1983 - data incomplete

1985 - data incomplete

1987 - data incomplete

1988 - data incomplete

1989 - data incomplete

Solano County

Carbon Monoxide

1980 - data incomplete

1988 - data incomplete

Sutter County

Ozone

1993 - Sutter Buttes site opened

Yolo County

Carbon Monoxide

1991 - data incomplete

1993 - data incomplete

1994 - data incomplete

1995 - data incomplete

Salton Sea Air Basin

Imperial County

Ozone

1981 - Calipatria site closed

1992 - Calexico site opened

San Diego Air Basin

San Diego County

PM₁₀

1993 - 4 new sites opened

San Francisco Bay Area Air Basin

Sonoma County

Carbon Monoxide

1981 - Santa Rosa site relocated

San Joaquin Valley Air Basin

Fresno County

Ozone

1989 - Fresno-Cal State #2 closed

Carbon Monoxide

1990 - Fresno-Olive Street site closed

1993 - Fresno-Fisher Street site opened

PM₁₀

1989 - Fresno-Drummond site opened

Kern County

Ozone

1981 - Edison site opened

Merced County

Ozone

1981 - only one site operating, incomplete data

1991 - only one site operating, incomplete data

Carbon Monoxide

1981 - incomplete data

1982 - incomplete data

1991 - incomplete data

1992 - incomplete data

San Joaquin County

Carbon Monoxide

1983 - Stockton-Claremont site opened

Stanislaus County

Carbon Monoxide

1982 - Modesto-J Street site closed

Central Coast Air Basin

San Luis Obispo County

Ozone

1989 - Atascadero site opened

PM ₁₀

1992 - Arroyo Grande site opened

Santa Barbara County

Ozone

1986 - 8 new sites opened

Carbon Monoxide

1983 - Santa Barbara-State Street site closed

Ventura County

Carbon Monoxide

1980 - incomplete data

1985 - Simi Valley site closed

1986 - new Simi Valley site opened

South Coast Air Basin

Orange County

Ozone

1981 - Santa Ana site closed

San Bernardino County

Carbon Monoxide

1982 - Fontana-Foothill site closed

1986 - San Bernardino-4th Street site opened

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