| In 1994 pollutant concentrations in California's South Coast Air Basin (SOCAB) exceeded three of the six federal ambient air quality standards. The annual maximum concentrations of ozone, carbon monoxide, and PM10 exceeded both federal and state standards. The federal standards were exceeded at one or more locations in the SOCAB on 142 days in 1994. Current air quality nevertheless represents substantial improvement over historical air quality. Figure 1 shows the change in exceedances of the standards over the past decade at selected monitoring sites which typify the areas with highest pollutant concentrations and which had complete data for the period. Three year averages were used to reduce the probability of obtaining a weather-induced trend caused by year-to-year variation in weather.
Between 1982-1984 and 1992-1994, the number of days on which the federal ozone standard was exceeded dropped by one third, from 33% to 22% of days, in the East San Gabriel Valley area which exceeded most frequently. Exceedances of the federal carbon monoxide standard decreased from 11% of days in 1982-1984 to 7% of days in 1992-1994. A comparison for the same periods can not be made for PM10 since the first full year of monitoring was performed in 1985. However, between 1985-1987 and 1992-1994, the percent of days exceeding the federal 24-hour standard decreased from 13% to 3%.
Exceedances of the state nitrogen dioxide standard decreased from 1% of days in 1982-1984 to 0.1% of days in 1992-1994. The federal nitrogen dioxide standard has not been exceeded in any area since 1991. There have been no exceedances of lead standards at regular air monitoring stations in the SOCAB since 1982. (The state and federal lead standards, however, were exceeded in some recent years at special monitoring sites immediately adjacent to stationary sources.) The state and federal sulfur dioxide standards were not exceeded in any of the SOCAB monitoring areas during either period. Exceedances of the state sulfate standard decreased from 2% to 0% at the long-term site used in this analysis, though a few sites exceeded in 1994.
The areas of the SOCAB which record the highest pollutant concentrations have shown a significant decrease in exceedances of the federal standards over the past decade. However, the SOCAB's high areas still record far more federal standard exceedances than any other areas of the U.S.. |
