1998 Air Quality and Trends

Despite significant long-term progress in air quality in the South Coast Air Basin (Basin), in 1998 pollutant concentrations continued to exceed federal and/or state standards for ozone, carbon monoxide, particulate matter (PM10) and nitrogen dioxide. In the desert areas of Riverside County downwind of the Basin (Salton Sea Air Basin), the standards for ozone and PM10 were exceeded.

Maximum Pollutant Concentrations

Figure 1 shows maximum pollutant concentrations in 1998 for the Basin compared to other urban areas in the U.S. Maximum concentrations in these other large U.S. urban areas exceeded the federal ozone and PM10 standards in some instances, but did not exceed the carbon monoxide standard. (Other areas in the country did report exceedances of the carbon monoxide standard, but to a lesser degree than the Basin.)

Maximum ozone concentrations in the Basin continued to exceed the federal standards for ozone by a wide margin. The maximum 1-hour and 8-hour average ozone concentrations (0.24 ppm and 0.206 ppm, both recorded in the Central San Bernardino Mountains area) were 192% and 242% of the 1-hour and 8-hour federal standard, respectively. The highest 8-hour average carbon monoxide concentration (13.5 ppm, recorded in the South Central Los Angeles County area) was 142% of the federal standard. Maximum 24-hour average and annual average PM10 concentrations (116 µg/m³ and 56.2 µg/m³, recorded in the Metropolitan Riverside County area) were 77% and 112% of the federal 24-hour and annual standards, respectively.

Figure 1
Maximum Pollutant Concentrations as Percent of Federal Standards
South Coast Air Basin Compared to U.S. Metropolitan Areas

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The federal nitrogen dioxide standard was not exceeded in 1998. The maximum annual average concentration of 0.0433 ppm (recorded in the Pomona/Walnut Valley area) was 81% of the standard. The more stringent state standard was exceeded on one day at one location in the Basin, with a maximum 1-hour average concentration (0.26 ppm recorded in the Banning area) which was 102% of the standard. Sulfur dioxide, sulfate and lead concentrations remained well below the state and federal standards in 1998.

Number of Days Exceeding Standards in 1998

Comparison of Air Quality in Different Areas

Ozone (O₃)

The Basin exceeded the federal ozone standards far more frequently than any other area of the U.S. In 1998, the seven highest U.S. locations in terms of number of days over the 1-hour federal ozone standard were located in the Basin (maximum 57 days). The 21 highest locations were in California. Other areas with the greatest number of exceedances outside California were located near or in Houston, Texas (maximum 12 days) and Atlanta, Georgia (11 days).

The number of days exceeding the federal standard also varies widely between different areas of the Basin. Figure 2 and Figure 3 show the number of days on which the 1-hour and 8-hour federal ozone standards were exceeded in different areas of the Basin in 1998. The ozone standards were exceeded most frequently in the Basin's Central San Bernardino Mountains and the adjacent areas. The federal standards were not exceeded in the coastal areas.

Figure 4 shows the 1994-1996 3-year average of the annual fourth-highest 8-hour average ozone concentrations at locations in the Basin (for comparison to the new federal ozone standard). The 8-hour average concentration was also lowest at the coastal areas, increasing to a peak in the Basin’s inland valleys and adjacent mountains of the Riverside-San Bernardino areas.

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Carbon Monoxide (CO)

The Basin was also highest in the nation in terms of number of exceedances of the carbon monoxide standard (10 days). Areas with the greatest number of exceedances outside the Basin in 1998 were Calexico, California (8 days) and Weirton, West Virginia (6 days).

Figure 5 shows the number of days exceeding the federal carbon monoxide standard in various areas of the Basin in 1998. The federal carbon monoxide standard was exceeded only in the South Central Los Angeles County area. Exceedances of the more stringent state standard were also limited to Los Angeles County areas, where vehicular traffic is most dense. Again, the South Central Los Angeles County area exceeded the standard most frequently.

Particulate Matters (PM10)

In 1998, the Basin was among the few areas in the U.S. exceeding the federal annual PM10 standard. Phoenix (Arizona) and New York were among other urban areas exceeding the federal annual PM10 standard.

Figure 6 shows the 1998 annual average PM10 concentrations at locations in the Basin. The federal annual PM10 standard exceedance in 1998 was limited to the Metropolitan Riverside County area. The much more stringent state annual PM10 standard was exceeded in about 50% of the areas of the Basin. The federal 24-hour PM10 standard was not exceeded anywhere in the Basin in 1998. The far more stringent state 24-hour standard, however, was exceeded in almost all areas of the Basin (except in the Central San Bernardino Mountains area). In the Coachella Valley desert area of Riverside County, 24-hour average PM10 concentrations exceeded the federal standard level on one day that was attributed to a high wind event and the data was excluded according to the EPA's Natural Events Policy.

Figure 5
Carbon Monoxide - 1998
Number of Days Exceeding the Federal Standard

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Figure 6
Suspended Particulate Matter (PM10) - 1998
Annual Arithmetic Mean, µg/m³

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Ozone (O3)

With the weather pattern rebounding in 1998 from the unusual ly clean conditions observed during the 1997 El Niño weather phenomenon, air pollution levels in some areas of the Basin were slightly higher than those observed in 1997. The overall long-term trend, however, showed continued improvement in air quality despite significant increases in economic growth and use of motor vehicles in the region. Figure 9 shows the number of days exceeding state and federal ozone standards, health advisory and episode levels in the Basin for the years 1976-1998. Between the periods 1976-1978 and 1996-1998, stage I episodes decreased 94% (from a mean of 113 days per year in 1976-1978 to a mean of 7 days per year in 1996-1998), health advisories decreased 76% (from 174 to 41 days). Exceedances of 1-hour and 8-hour federal standards decreased 63% (from 196 to 73 days), and 41% (from 210 to 123 days), and state standard exceedances decreased 42% (from 232 to 135 days). (Three-year averages were used to minimize the effect of year-to-year variations due to changes in meteorological conditions.) Some eastern areas of the Basin, including the San Bernardino Mountains area, experienced ozone air quality improvement at a slower rate. This was the result of eastward transport of emissions and pollutants originating in western portions of the Basin.

Figure 9
OZONE, 1976-1998
Basin-days Exceeding Standards and Episode Levels

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Carbon Monoxide (CO)

Figure 10 shows thfe number of days exceeding the federal carbon monoxide standard in the South Central Los Angeles County, the area with the most frequent exceedances of the standards, for the years 1976-1998. Between 1976-78 and 1996-98, the three-year average number of exceedances decreased by 84% for the federal standard.

Figure 10
Carbon Monoxide
Days Exceeding 8-Hour Federal Standard, 1976-1998

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Particulate Matter (PM10)

Figure 11 shows the annual average PM10 concentration in Metropolitan Riverside County, the area frequently recording the highest concentration in the Basin, for the period 1985-1998. Although year-to-year variation occurred, due in part to weather, a decrease in PM10 concentration was observed over the period. Between 1985-1987 and 1996-1998, the three-year average PM10 concentration decreased by 33%.

Figure 11
pm10
Annual Average Concentration, 1985-1998

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Pollutant Tables

Gas (Ozone,Carbon Monoxide,Nitrogen Dioxide,Sulfor Dioxide)
Particulates (PM10,Total Suspended Particulates,Lead,Sulfates)