The South Coast Air Basin (Basin) recorded its lowest ozone concentration on record in 1999, and for the first time since ozone monitoring began, the Basin made it through a summer without experiencing a stage 1 episode.  Also, the year 1999 was the first year in the history of ambient air monitoring that the Basin was not the location recording the highest ozone concentration in the nation. Figure 1 shows the number of days exceeding the stage 1 episode level in the Basin for the period 1976-1999, and also shows the maximum 1-hour average ozone concentration (in ppm) for the same period.
 
The monthly distribution of the number of days exceeding the federal standards in the Basin shows not only that the number of exceedances has been significantly reduced in the past few decades, but also that the period of the year that high ozone concentration occurs (smog season) has also diminished significantly.  Although weather conditions greatly contribute to the lower ozone concentration, weather-adjusted trend studies have indicated that the significant downtrend in ozone concentration and shorter duration of "smog season" in the Basin are mainly attributed to emission reductions and reduced reactivity of emitted organic compounds in the region. 

Maximum Pollutant Concentrations

In a continuing trend of significant long-term improvement in air quality in the Basin, the year 1999 recorded a new in ozone concentrations.  Nonetheless, maximum pollutant concentrations in the region still exceed the federal standards for ozone, carbon monoxide and particulate matter (PM10 and PM2.5) by a wide margin. Figure 2 shows maximum pollutant concentrations in 1999 for the Basin compared to other metropolitan areas in U.S.  Maximum concentrations in these large U.S. urban areas exceeded the federal ozone and PM10 standards in some instances, but did not exceed the carbon monoxide standard.

Maximum 1-hour average and 8-hour average ozone concentrations in 1999 (0.17 ppm and 0.143 ppm) were 136% and 168% of the federal 1-hour and 8-hour standards, respectively.  The highest 8-hour average carbon monoxide concentration of the year (11.7 ppm) was 123% of the federal standard.  Maximum 24-hour average and annual average PM10 concentrations (183 µg/m³ and 72.3 µg/m³) were 121% and 144% of the federal 24-hour and annual standards, respectively.  PM2.5 concentrations were regularly monitored in the District in 1999 and the federal standards were exceeded at almost all sites monitored.  Maximum 24-hour average and annual average PM2.5 concentrations (122 µg/m³ and 30.9 µg/m³) were, respectively, 185% and 205% of the federal 24-hour and annual standards.

In 1999, the federal nitrogen dioxide standard was not exceeded, with a maximum concentration (0.0503 ppm) which was 94% of the standard.  The more stringent state standard, however, was exceeded on one day at one location in the Basin.  The maximum 1-hour average nitrogen dioxide concentration (0.31 ppm) was 119% of the state standard.  State standard for sulfate was also exceeded on one day at one location.  The maximum 24-hour concentration (25.6 µg/m³ ) was 102% of the state standard.  (There is no federal sulfate standard.)  Sulfur dioxide and lead concentrations continued to remain well below the federal and state standards in 1999.

Number of Days Exceeding Standards in 1999

The maximum concentrations and the number of exceedances in different areas of the District for the year 1999 are presented in Tables 1 through 9 .
 

Ozone	Particulate Matter (PM10)	Lead
Carbon Monoxide	Particulate Matter (PM2.5)	Sulfates
Nitrogen Dioxide	Total Suspended Particulates (TSP)	Sulfur Dioxide

Air Quality Trends

Examination of air quality trends through 1999 shows that the number of exceedances recorded in 1999 is consistent with a continuation of the downtrends reported in previous years. Figure 3 shows the trend in percent number of days exceeding the federal standards in the Basin over the past two decades for ozone, carbon monoxide and PM10.

Between 1976-1978 and 1997-1999, the three-year average number of days exceeding the 1-hour and 8-hour ozone federal standards decreased by 71% and 44%, respectively.  The three-year average number of days exceeding the carbon monoxide federal standard was reduced by 91% between 1976-1978 and 1997-1999.  The three-year average number of days exceeding the federal 24-hour standard decreased by 79% between 1985-1987 and 1997-1999. 

Monthly distribution of Ozone Concentration

Figure 4 shows the 5-year average of number of days per month exceeding the federal ozone standard for the period 1976-1999.  Up until late 1980's it was common to have days exceeding the federal ozone standard as early as February and during late fall as late as November and December.  In late 1990's (since 1996) there have been no federal standard exceedances recorded in the months of January-March and November -December.  Also, the frequency of exceedances in fall (September and October) has been reduced significantly in the recent years
.

Comparison of Air Quality in Different Areas

Ozone (O3)

The South Coast Air Basin still has some of the worst air quality in the nation in terms of the annual number of days exceeding the federal standards.  In 1999, there were 51 days on which one or more federal standards were exceeded somewhere in the Basin, most of which (41 days) were for ozone alone.  The highest U.S. location in terms of number of days over the federal ozone standard was located in the Basin (Central San Bernardino Mountains, 30 days).  Other areas with the greatest number of exceedances outside California were located near or in Houston, Texas (18 days) and Atlanta, Georgia (13 days).

Figure 5 shows the number of days exceeding the 1-hour federal ozone standard in different areas of the Basin in 1999.  The standard was exceeded most frequently in the Basin's Central San Bernardino Mountains and adjacent areas.  The coastal areas of Los Angeles and Orange Counties did not exceed the federal ozone standard.  The more stringent state standard was exceeded at all locations monitored.

Figure 6 shows the distribution of the three-year average of the annual fourth-highest 8-hour average ozone concentrations in the Basin for the years 1997-1999 (for comparison to the 8-hour federal ozone standard).  The three-year average concentrations exceeded the 8-hour average standard level in most areas of the Basin.  The 8-hour average concentration was also lowest at the coastal areas, increasing to a peak in the Basin's mountains of the San Bernardino and adjacent areas. 

Carbon monoxide (CO)

The Basin was the second highest location in the nation in terms of the number of days exceeding the federal carbon monoxide standard (8 days).  The location with the most frequent exceedances was Calexico in Imperial County, California.  Figure 7 shows the number of days exceeding the federal 8-hour standard for carbon monoxide in the various areas of the Basin in 1999.  The standard was exceeded only in Los Angeles County areas, in the South Central Los Angeles County area. 

Particulate Matters (PM10)

Figure 8 shows the annual average PM10 concentrations at locations in the Basin in 1999.  The federal annual PM10 standard was exceeded at more than one third of the locations monitored in the Basin, most of which located in the Southwest and Central San Bernardino Valley and Metropolitan Riverside County areas.   

Particulate Matters (PM2.5)

Figure 9 shows the annual average PM2.5 concentrations in different areas of the Basin.  PM2.5 concentrations were also higher in the inland valley areas of San Bernardino County and Metropolitan Riverside County areas.  However, in contrast to PM10 concentration, PM2.5 concentrations were also high in the metropolitan areas of Los Angeles and Orange Counties.  The high PM2.5 concentrations in these areas are due to the secondary formation of smaller particulate resulted from mobile and stationary source activities.