1997 Air Quality and Trends

In 1997, the annual maximum concentrations of ozone, carbon monoxide and particulate matter (PM10) in the South Coast Air Basin (Basin) exceeded both federal and state standards in some or all areas. No areas of the Basin exceeded standards for nitrogen dioxide, sulfur dioxide, lead or sulfate. In the desert areas of Los Angeles and Riverside counties downwind of the Basin, the standards were exceeded for ozone and PM10.

Maximum Pollutant Concentrations

Maximum pollutant concentrations in the Basin continued to exceed the federal standards for ozone, carbon monoxide and PM10 by a wide margin. Figure 1 shows the 1997 Basin maximum pollutant concentrations as percentages of the federal standards compared to maximum for the previous three years. Maximum 1-hour average and 8-hour average ozone concentrations in 1997 (0.21 ppm and 0.14 ppm) were 168% and 169% of the federal 1-hour and 8-hour standards, lower than the previous three years. The highest 8-hour average carbon monoxide concentration of the year (17.0 ppm) was 179% of the federal standard. Maximum 24-hour average and annual average PM10 concentrations (227 µg/m³ and 64.9 µg/m³) were 150% and 130% of the federal 24-hour and annual standards.

In 1997, for the sixth year in a row, the federal nitrogen dioxide standard was not exceeded, with a maximum concentration (0.0433 ppm) which was 81% of the standard. The more stringent state standard was not exceeded for the fourth consecutive year, with a maximum 1-hour average nitrogen dioxide concentration (0.20 ppm) which was 77% of the standard. Sulfur dioxide concentrations continued to remain well below the federal and state standards. Federal and state lead standards were not exceeded in 1997. State standard for sulfate was not exceeded either. (There is no federal sulfate standard.)

Number of Days Exceeding Standards in 1997

The maximum concentrations and the number of exceedances in different areas of the District are given in the following tables.

Comparison of Air Quality in Different Areas

Ozone (O₃)

Figure 2 shows the number of days on which the 1-hour federal ozone standard was exceeded in different areas of the Basin in 1997. The number of days exceeding the federal standard varies widely between different areas of the Basin. The standard was exceeded most frequently in the Basin's inland valleys in an area extending from the East San Gabriel Valley eastward to the Riverside-San Bernardino area and into the adjacent mountains. The East San Bernardino Valley area recorded the greatest number of exceedances of the federal standard (35 days).

Air Quality Trends Through 1997

In recent years the South Coast Air Basin has been the worst location in the U.S. in terms of the annual number of days exceeding the federal standards. In 1997, there were 86 days on which one or more federal standards were exceeded somewhere in the Basin. Examination of air quality trends through 1997 shows that the concentrations and number of exceedances recorded in 1997 are consistent with a continuation of the downtrends reported in previous years. (Basin trends through 1996 were discussed in the December 1996 issue of the AQSCR, Vol. 9, No. 12.)   Figure 3 shows the trend in percentage number of days exceeding the federal standards in the Basin over the past two decades for ozone, carbon monoxide and PM10.

Ozone Trend

In the past few years, ozone air quality have been the cleanest on record in terms of maximum concentration and number of days exceeding the standards and episode levels. Figure 4 shows the number of days exceeding the state and federal ozone standards and the state health advisory and episode levels for the years 1976-1997 in the Basin. There has been a significant decrease in the number of exceedances at all levels. El Niño contributed to some of the improvement of the ozone air quality in 1997 by bringing unstable weather and increased cloudiness through the summer. (The impact of this weather phenomena on air quality is also evident in 1982 when an El Niño of comparable strength influenced Southern California weather.) There was only one stage I episode in 1997, compared to the record low of 7 days recorded in 1996. Between 1976-1978 and 1995-1997, stage I episodes decreased 94%, health advisories decreased 73%, 1-hour federal standard exceedances decreased 57%, 8-hour federal standard decreased 39% and state standard exceedances decreased 36%.

Trends in exceedances of the ozone federal standards were also examined for each of the individual sites in the District’s ozone monitoring current network which had complete data for the period 1976-1997 (25 sites). Regression analysis showed statistically significant downtrend (0.05 level of significance) for the 1-hour standard at all of the 25 locations. For the 8-hour standard exceedances, 21 of the 25 locations showed significant downtrend, the remaining four sites showed no significant trend. None recorded an uptrend.

Further Information

The maximum concentrations and the number of exceedances in all areas of the District are given in the one page report "1997 Air Quality." The location of the District’s air monitoring stations and cities that are in each area are shown in the map "South Coast Air Quality Management District and Air Monitoring Areas." Both of these documents are available free of charge, and may be obtained by calling (800)242-4666 or by writing to the District, attention of the Public Advisor.

Subscriptions to the monthly "Air Quality Standards Compliance Report" which contains detailed information on the number of days and locations where state and federal ambient air quality standards are exceeded, are available by writing to the South Coast Air Quality Management District, attention of the Public Advisor, or by calling (800)242-4666. Subscription request forms for subscribing to the AQSCR may be obtained by calling (909)396-3720. The annual subscription fee for the calendar year 1997 is $6.00.