Weather Clouds Outlook for Cleaner Air : Pollution: The return of summer fog may bring an increase in smog levels after years of improvement.

The return of a typical summer weather pattern--fog-shrouded mornings followed by hazy afternoons--threatens to interrupt a seven-year trend of general improvement in Ventura County smog levels, air quality officials say.

The air over Ventura County has become cleaner since 1983, thanks in part to air quality rules that forced a 25% drop in smog-causing emissions. Last year’s air quality was unusually good, officials say.

But if the summer fog and haze continue, the officials predict that this year’s air will be slightly dirtier than last year’s.

“We’d love to have another year like last year, a model summer” with few days that exceed health standards, said Richard Baldwin, who heads the county’s Air Pollution Control District. But that is not expected, he said.

He said the prospect of more smog this year illustrates how weather remains the dominant force in determining air pollution levels.

Smog increases during foggy summers because a layer of warm air acts as a lid on moisture drawn off the ocean. That prevents the day’s pollutants from dispersing into the atmosphere.

“So the pollutants we have today will be the same ones we have tomorrow,” said Kent Field, air pollution district meteorologist. “The air is stagnant and moist, and that gives the pollution particles something to hang onto.”

When the sun finally does break through, it cooks the gathering pollutants into ozone, the primary component of smog.

Last summer’s weather was unusually favorable, lacking the layers of warm air that trap pollutants near the ground. Foggy weather, though largely absent for the past five years, is more typical, officials said.

“This is a more normal pattern than recent years,” Field said. “That tends to mean that we will have higher smog levels this year than last.”

It is too early to tell whether the heavy marine layer, which has stretched as high as 3,000 feet recently, will persist all summer. “But it’s my guess that it’s going to hang around for a while,” said National Weather Service meteorologist Terry Schaeffer.

On five days already this year, the smoggiest area of the county has exceeded the federal ozone standard of 0.12 parts per million. In all of 1990, the standard was exceeded on only 16 days. The yearly average in the past 10 years is 51 days.

The more stringent state standard of 0.09 ppm has been exceeded on 28 days so far this year compared to 100 days in 1990 and an average of 130 days a year over the last 10 years.

And the smoggiest months are yet to come. Smog levels generally peak in August and begin to fall again in September.

Barring freak weather episodes, such as extreme temperatures or unusual wind patterns, officials this year expect no first-stage smog alerts, which are issued when ozone hits 0.20 parts per million.

But residents can expect to hear 15 to 25 “health advisories” over the next four months warning sensitive people--children, the elderly, and the infirm--to stay indoors. New state regulations effective this year call for such warnings when ozone hits the 0.15-ppm level. The California Air Resources Board set the new advisory level after studies showed that health is affected by lower levels of air pollution than previously believed.

So far this year, no cities in the county have had enough air pollution to warrant an advisory. But there have been several days during the last two weeks when the ozone levels have reached 0.14 ppm in the smoggiest areas, officials said.

“We’re getting awfully close and it’s going to happen soon,” said Douglas Tubbs, manager of the monitoring and technical services division of the air pollution district. Baldwin said he expects the advisories to be issued on 15 to 25 days during the next four months.

Ozone is created when nitrogen oxides react with hydrocarbons in sunlight. Cars, trucks and buses produce more than half of the nitrogen oxides in the air. Southern California Edison’s power plants in Oxnard and the oil industry emit another 40% of the county’s nitrogen oxides.

Motor vehicles are also the largest source of hydrocarbons, producing about 32% of the total. Industries, including oil, manufacturing and agriculture combine to produce another 55% of hydrocarbons.

Hydrocarbons are generally colorless in the air, but nitrogen oxides have a reddish-brown tint. When the two chemically react to form ozone, which is nearly colorless, the horizon sometimes seems to clear, Tubbs said.

“Ozone pollution doesn’t affect visibility, but moisture and nitrogen oxides do,” Tubbs said.

Ventura County’s smoggy months begin when the inland deserts begin to heat up with the warming sun and lengthening days of late spring.

The warming land mass creates a low-pressure area inland, which acts like a vacuum to suck in the denser, cooler air from the coast. That draws the layer of marine fog off the ocean and onto land, where it remains until the ocean and ground overall warm up.

Meanwhile, a warm layer of air called an inversion layer forms over the county, holding pollutants near the ground and preventing them from dispersing. Consequently, the warmest inland areas of the county, like Simi Valley and Fillmore, also tend to be the smoggiest.

Winds also play a large role in determining how polluted Ventura County air becomes, officials say.

Simi Valley, the county’s smoggiest city with a five-year average of 106 days annually that fail state standards, creates much of its own pollution problem from industry and motor vehicles. But Simi Valley’s unlucky geographical location in the east end of the county also makes it the recipient of a double dose of smog from its neighbors.

Light morning breezes push Simi Valley’s emissions toward the west. But when the predominant afternoon breezes kick up from the west over the ocean, they push industrial pollutants, tailpipe exhaust from the Ventura Freeway and Oxnard power plant emissions into hot Simi Valley. There they are trapped against mountains, held down by the inversion layer, and cooked by the sun.

The city sometimes receives an additional influx of emissions in the morning, when pollutants creep in from the San Fernando Valley.

Fillmore and Piru, which are farther inland than Simi, also receive pollutants from the coast as ocean winds push emissions up the Santa Clara River Valley. That rural area of citrus groves and small communities is generally the second-smoggiest spot in the county, with an average of 54 days in which air is considered unhealthy by state standards.

Ojai, the small arts community at the northwest end of the county, is also a smog pocket for pollutants traveling with the wind at their backs. The emissions from the Ventura Freeway and California 33 move up the Ventura River Valley and settle on the Ojai Valley, where they are trapped by the mountains to the north. Ojai averaged 48 smoggy days a year by state standards during the last five years.

The coastal area of the county is typically the least polluted zone, with an average of only 13 days a year that the state says are unhealthful. But the coast also occasionally receives pollutants from neighboring Los Angeles County. Wind currents carry the ozone-causing emissions up along the coast until ocean breezes blow them inland.

“That’s when we see the ozone levels really shoot up,” Field said.

Such emissions from Los Angeles, joining with Santa Ana winds blowing from the east, combined two years ago to create Ventura County’s only first-stage alert since 1983.

In that freak episode in April, 1989, the winds pushed a bank of pollutants out over the ocean, where they churned for days. When the west winds picked up, they blew the mass back over land, meeting warm weather that heated them into the highest ozone levels the county had experienced since 1983.

By comparison, Los Angeles County had an annual average of 67 first-stage alerts over the last three years, compared to seven in Orange County, 20 in Riverside County and 51 in San Bernardino County.

Ventura County has never had a second-stage smog alert, set at 0.35 ppm under both state and federal standards.

Ozone is no longer the county’s only air pollution problem. Particles of dust, debris and liquid in the air also pose a significant health and air pollution problem, officials say. The county’s air passes federal standards for particulates but not the more stringent state regulations.

The tiny particles include dust, debris stirred up by farming, construction and demolition, pieces of vehicle tires sloughed off along the road, ash from wildfires and salt from sea spray.

“It’s going to be a very difficult problem to deal with,” said William Mount, chief of planning for the air pollution district. Mount said the district will develop a plan to battle particulate pollution that may include more intensive road cleaning or more frequent applications of water during construction and greater incentives to leave cars at home in favor of public or pooled transportation or bicycles.

County Smog Average For five years, 1986-1990

Number of Days That County Air Fails Health Standards for Ozone

Fillmore/ Thousand Simi El Rio Piru Ojai Oaks Valley Ventura Greater than 22 54 48 41 106 13 0.09 ppm ozone Calif. standard Greater than 3 6 5 7 36 2 0.12 ppm ozone federal standard Greater than 2 0.20 2 2 12 0.60 or equal to 0.15 ppm ozone state advisory

Ventura County wind and smog

Prevailing afternoon winds from offshore push emissions east through the county’s inland valleys, trapping pollutants against the mountains. Polluted air also creeps from the San Fernando Valley over the Santa Susana Pass and into Simi Valley. Occasionally, wind carries smog up the coast from Los Angeles County, where it moves onshore into Ventura County with the afternoon breeze.