County Gets Its 1st Smog Alert Since Last Year : Pollution: The warning comes on summer’s last day after unusual weather creates a pocket of bad air.

Orange County’s first smog alert of the year was declared Sunday after quirky weather patterns left a pocket of foul air over the central portion of the county, while usually smoggy areas of Southern California enjoyed clear skies.

The first-stage alert ironically came on the last day of summer, the season most notorious in Southern California for unhealthy air. It broke the county’s yearlong streak of relatively clear air; the summer’s overcast skies and strong marine breezes have caused the best air quality on record in more than 40 years.

The region’s air-quality agency on Sunday advised central Orange County residents, especially children and the elderly, to avoid strenuous activity outdoors. Residents of that area who suffer from lung or heart disease were warned to stay indoors and keep windows closed.

By 3 p.m. Sunday, air-quality gauges in Anaheim indicated a level of 200 on the pollutant chart for ozone, a caustic, potent chemical that can cause respiratory problems. A ranking of 200, indicating that ozone in the air has reached a concentration of .20 parts per million, triggers a first-stage alert. By 5 p.m., the hazard was over and the health alert ended.

In addition to the late arrival of the season’s first smog alert, the foul air over central Orange County was unusual because it came on a weekend instead of a day with severe traffic, and because it bypassed the region’s usually smoggy cities in the San Gabriel Valley.

Ed Wentworth, a meteorologist with the National Weather Service, explained that sea breezes were very weak on Sunday, and the inversion layer--a condition in which warm air traps cool air near the ground--was so low over Southern California that pollution was trapped over low-lying areas instead of Los Angeles County’s higher-elevation valleys.

“It was a real weak onshore flow. Sea breezes were weak, and (smog) can be pretty nasty in Orange County when it’s like that,” Wentworth said.

The top of the inversion layer, which traps pollutants, was at an elevation of about 500 feet--so low that the valleys of Los Angeles County were 300 feet above it, and skies there were clear, Wentworth said.

“Visibility has been really good here (in Los Angeles) today. But Orange County is lower, so they are underneath it and the smog crunch is down there,” he said.

Along with central Orange County, alerts were posted Sunday in the Corona-Norco area and in Riverside, according to the air-quality district’s daily report. Northern Orange County air quality was rated unhealthful, but not bad enough to trigger an alert. No alerts were issued in Los Angeles County.

The subtle patterns caught air-quality forecasters by surprise, since they had originally forecast a moderate day for the county.

“It’s tricky in Orange County,” Wentworth said. “A situation like this is difficult to forecast. Subtle little changes can make a difference” in air quality.

Smog forecasters at the South Coast Air Quality Management District predicted that air quality would improve today, becoming moderately unhealthy in central and north Orange County, but probably not severe enough to trigger another alert.

Since 1986, Orange County has experienced four to 11 smog alerts each summer, but this season’s abnormally cloudy weather and strong marine layer have kept ozone to a minimum. Ozone is formed when pollutants, mainly from car exhaust and industry, bake under the sun, causing a photochemical reaction.

Last year, the county experienced a record low of four alerts, all in La Habra, the sampling point for northern Orange County. There were no alerts last year in Anaheim, the monitoring station for central Orange County.

The county still has time to catch up with its polluted past, since the region’s smog season, which begins in May, lasts until the end of October. Historically, some of Orange County’s smoggiest days have occurred in September and October.

Air quality officials warn that smog alerts can be deceiving, since ozone is an invisible chemical that causes a familiar brown haze only when combined with other pollutants. Often it can reach unhealthful concentrations without obvious signs, such as poor visibility.

Children, the elderly, asthma sufferers and others suffering from respiratory ailments are especially sensitive to ozone. Scientific studies have shown that exposure to high concentrations can cause premature aging and scarring of the lungs.