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1998 Ozone Season Summary
This page updated on November 5, 1999.
1998 had more high ozone events than 1996 and 1997 but this was due to a transition from El Niño to La Niña conditions.
The number of state and national ozone standard exceedance days, number of Stage 1 alerts, and maximum ozone concentrations for the five major air basins in the state from 1996 through 1998 were examined. (The data for 1996 and 1997 are complete and 1998 data is through October 15.) In 1998, each air basin experienced the maximum ozone concentration recorded over the most recent three years. There was also a significant increase in the number of days exceeding the national standard for all air basins but the South Coast. However, in the South Coast Air Basin, the number of Stage 1 alerts jumped from one in 1997 to twelve in 1998. While the number of high ozone events was always greater in 1998 than in 1996 and 1997 for all five air basins, the trend in the number of state standard exceedance days was less consistent, with increases in some air basins, and decreases in others.
California conditions changed from a hot summer in 1996 to a cooler 1997 summer with a strong El Niño influence that lasted into June of 1998. A reversal to La Niña conditions dominated the weather of July, August, and the first half of September 1998. El Niño conditions improve air quality in California, especially in southern California, because these years are influenced by troughs of low pressure that bring in cooler air to the state. This weather pattern deepens mixing layers, allowing pollutants to disperse more rapidly, and produces stronger winds, which dilute pollutants. On the other hand, La Niña tends to build more high pressure ridges. These ridges tend to induce lighter surface winds and lower mixing depths. Air pollution is concentrated more than usual under these circumstances. Thus, weather conditions are partially or wholly responsible for the decrease in high ozone events from 1996 to 1997 and the subsequent increase in 1998. The effect of California's emission control program on air quality is masked by the El Niño and La Niña influences.
While 1998 is not yet over, May to October is the traditional ozone season because high temperatures speed up the photochemical reactions that form ozone, while clear skies provide more ultraviolet energy to the process. The summer months also have several periods where winds are lighter than normal and the mixing depths remain very shallow, thus concentrating ozone and its precursors.
Air quality was much improved in 1998 in comparison to previous La Niña years.
In comparison with four earlier La Niña years (1970, 1973, 1975 and 1988), 1998 has dramatically fewer days with high ozone concentrations in the South Coast Air Basin. The number of state exceedance days has been cut in half. Exceedances of the national ozone standard have dropped to a third and the number of Stage 1 alerts is about one tenth the average of the earlier four La Niña years.
Progress in other regions of the state is also dramatic. In the San Joaquin Valley, significant reductions have occurred since the last La Niña in 1988. State exceedances days are about 60 percent of 1988 levels, and exceedances of the national standard have dropped by 50 percent. There were no Stage 1 days in either year. In San Diego, state exceedance days are about a third of the 1988 total, and national exceedance days are only one fifth of that year. In 1998, there were no Stage 1 days in San Diego, although there were two in 1988. In San Francisco and in the Sacramento Valley, the proportional changes have been very similar to those in the South Coast; only half as many state exceedances and about one third the number of exceedances of the national standard. There have been no Stage 1 days in these air basins since the mid-1980s.
During July 14-19, 1998, California experienced meteorological conditions that were among the most conducive to ozone formation ever observed.
From July 14 to 19, 1998, the South Coast Air Basin experienced five Stage 1 alert days in a row and a peak ozone level of 0.24 ppm, twice the national standard. Air basins as far north as Sacramento saw ozone values above the health advisory level (0.15 ppm). ARB staff conducted am analysis of historical episodes (1980 to 1997) with similar meteorology to the July 1998 event to quantify the effects of emission reduction programs. Eleven episodes were found between 1980 and 1998 that were meteorologically comparable to the July 1998 episode. Maximum ozone levels were in the range of 0.30 to 0.44 ppm during the early 1980's, compared to peaks of 0.20 to 0.24 ppm in the July 13-19, 1998, episode. However, the analysis likely underestimates the downward trend in ozone because the July 1998 episode appears anomalous from the others in two important aspects. First, the six-day duration of the July 1998 episode was equal to the longest observed for the historical period. In addition, the temperatures observed during the July 1998 period at 850 millibar pressure (about one mile), an excellent indicator of high ozone at the surface, were the highest observed in the 1980 to 1998 database. The synoptic weather pattern during this period showed a strong high pressure center over southwest Nevada. This ridge covered all of California and temperatures aloft favored a strong inversion in all of the state, explaining the health advisory levels. However, the strongest core of warm air aloft and shallow inversions were over the South Coast during this period.
Ozone levels increase with temperature and August 1998 was among the hottest months ever observed in California.
Seven Stage 1 episodes occurred in the South Coast during August, 1998. This can be explained by extremely unusual temperature. At the Los Angeles Civic Center, the average daily temperature was almost 5 oF above the average for all other Augusts and was among the hottest months on record. Ozone formation rates and emissions (e.g., evaporative, air conditioning usage) increase with temperature, but hot, rising air tends to blow off the inversion lid over the South Coast. However, the inversion was especially strong during July and August, 1998, with temperatures a mile above the ground at their highest level since 1967.
The long-range forecast is for fewer El Niño events.
In contrast to the summers of the late 1970's that were especially hot and prone to build up of air pollution, there have been four El Niño years during the 1990's of significance. Dr. Gray of Colorado State University, who studies hurricane climatology, expects fewer El Niño's in the future decade. If this prediction is correct, anomalies like that experienced this year are more likely to reoccur. This could temporarily mask some of the progress made by emission reductions. The influence of meteorology needs to be described in examining the effectiveness of air pollution control programs.
Supplemental Analysis for Crestline (Lake Gregory vs Azusa, Glendora, and Upland).
The Air Quality Data Branch staff have compared the ozone trends for 1980 to 1998 at Crestline (now named Lake Gregory) and a group of three sites in the San Gabriel Valley - Azusa, Upland, and Glendora - that have traditionally had the highest ozone readings in the South Coast Air Basin. Regarding the trends of the maximum and the EPDC 1-hour ozone concentrations at Crestline versus the other three sites, there has been steady downward trends in ozone since 1980. Most interesting is the convergence of the Crestline trendline with the trend for the other three sites in recent years. This suggests that the maximum ozone location in the Basin is shifting to the east, perhaps due to emission controls that have reduced hydrocarbons relative to NOX.
Regarding the number of days that exceed the state 1-hour and national 1-hour and 8-hour standards each year from 1980 to 1998 for Crestline versus the maximum of Azusa, Glendora, and Upland, the trend is for Crestline to have less days exceeding the standards in the 1980s, and convergence with the other three sites in 1989. During the first half of the 1990's, Crestline had about the same number of exceedance days of the 1-hour state and national standards as the San Gabriel Valley sites, but significantly more from 1995 to the present. For the national 8-hour ozone standard, the separation between Crestline and the other three sites is more pronounced. Again, these trends are consistent with ozone patterns expected when hydrocarbon emission reductions are greater than NOX emission controls (e.g., Federal and California reformulated gasoline regulations).
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