Nov 1, 2001

The Southland's ozone smog season, which officially ended yesterday, is once again the cleanest on record.

"Air quality continued a long-term improving trend this year due to a number of successful air pollution control programs," said Barry Wallerstein, executive officer for the South Coast Air Quality Management District. "For the third year in a row, we have not had a Stage 1 smog episode."

There were 36 days with unhealthy levels of ozone this year, compared to 40 days last year. Air quality is considered unhealthy when the federal one-hour standard for ozone, 0.12 parts per million, is exceeded. Ozone season officially starts on May 1 and ends on Oct. 31.

Good to moderate air quality is forecast for today and also is likely for the rest of the week. The region has not had unhealthy levels of air quality since late September.

Although ozone smog season has ended, the peak period for particulate pollution is about to begin. Particulate pollution reaches its highest levels during late fall and early winter when stagnant atmospheric conditions favor its formation.

Particulate pollution, technically referred to as PM₁₀, is a complex mix of microscopic particles that obscure visibility and are associated with increased hospitalizations and higher death rates.

While the Southland saw fewer unhealthy ozone days this year, residents did experience a slightly higher peak air pollution level. The ozone peak this year was 0.190 parts per million (on Sunday, Aug. 26 in the east San Gabriel Valley), compared to a peak of 0.184 parts per million last year (on Sunday, May 28 in the northwest San Bernardino Valley).

On and around the last weekend in August, the region experienced stagnant wind conditions and a very strong atmospheric inversion. Atmospheric inversions, common to the Southland during warm months, consist of a layer of cool air topped with warmer air that traps air pollution.

Although temperatures at the ground level were slightly cooler than normal this summer, temperatures aloft were higher than normal, favoring ozone formation. The height of the atmospheric inversion also was lower than normal, favoring the buildup and concentration of air pollution.

"Even though this summer was somewhat cooler than last, the meteorology was more favorable for smog formation," said Joe Cassmassi, AQMD's senior meteorologist.

This year continued a long-term trend of the highest ozone levels occurring on weekends. Air quality scientists believe this so-called "weekend effect" is due to a combination of factors, including the different pattern of driving and emissions on weekends, the carryover of emissions from Friday evening to the weekend, and the increased use of smog-forming consumer products on weekends.

Ozone is a colorless gas with a pungent odor that damages lung cells and can cause chest pain, coughing, headaches, eye irritation and other symptoms. Health research has shown that unhealthy levels of ozone worsen asthma symptoms and may cause permanent lung damage.

Thanks to air pollution control regulations adopted by AQMD and other air quality agencies, smog levels in the Southland have declined steadily since the late 1970s. The number of Stage 1 episodes, when ozone levels reach 0.20 parts per million and everyone is cautioned to avoid vigorous outdoor exercise, has dropped from 121 in 1977 to zero since 1999.

"From year to year, weather conditions cause significant variations in ozone levels," Wallerstein said. "However, the long-term trend is one of steady improvement. We are on track to meet the federal one-hour ozone standard by 2010."

AQMD is the air pollution control agency for Orange County and major portions of Los Angeles, San Bernardino and Riverside counties.

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This page updated: March 03, 2004
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