Nature Gets Credit as Smog Eases in S.D.

San Diego County had less smog in 1991 than any year since 1955, the county’s air district reported Wednesday.

The county also scored its biggest one-year improvement in a decade, according to the annual report of the San Diego County Air Pollution Control District. Smog levels in 1991 exceeded the state clean-air standard 106 days, compared with 139 days in 1990 and 158 days in 1989, APCD officer Richard J. Sommerville said.

But, before county residents and industries start patting themselves on the back, the report said that most of that improvement was caused by temporary gifts of Mother Nature--an El Nino condition and a volcanic eruption--not by cuts in the levels of pollutants emitted.

El Nino conditions cause the inversion layer to rise and increase winds that disperse pollutants, Sommerville said. Last June’s eruption of Mt. Pinatubo in the Philippines created dust clouds that reduced sunlight, which is needed to produce smog.

Smog is produced when hydrocarbons and oxides of nitrogen bake in the sunlight. Inversion layers--layers of warm air close to the ground--keep the pollutants from escaping and cause higher smog levels.

Later this year, as San Diego weather patterns gradually return to normal, Sommerville warned, smog levels might increase.

That was the case in 1982, when an El Nino condition combined with volcanic dust from another eruption and dropped smog levels 37%, Sommerville said. But, by 1983, as the El Nino weakened and the dust cloud dissolved, brown skies returned.

“The combination of industrial and motor vehicle pollution controls are producing a steady and significant improvement in air quality within the San Diego region,” Sommerville said. “However, dramatic improvements in air quality--like the one San Diego experienced in 1991--are usually caused by more favorable meteorological conditions, and the same pattern could happen again.”

Emission controls on industrial and motor vehicles imposed in the early ‘80s have helped decrease the frequency of days with high smog levels, Sommerville said.

From 1978 to 1987, smog-producing emissions in San Diego decreased by an average of 3% a year, Sommerville said. Recent statistics show that industry is responsible for less than 20% of the smog producing particles.

Most of the smog in San Diego coastal communities is produced in Los Angeles and travels to this region via Santa Ana winds blown along the coast, Sommerville said. In order to measure local emissions, smog officials measure levels along the mountain ranges inland, he said.

Sommerville warns that the levels of smog in the county might increase as soon as the population grows and motor vehicle traffic increases.

Since 1980, the number of miles driven in the county each weekday has increased almost 7% per year, and the San Diego Assn. of Governments is predicting an additional 30% increase in the miles driven by the year 2000, Sommerville said.

About 40% of air pollution is caused by people driving to and from work, APCD spokesman Bob Goggin said, and 80% of those people drive alone.

Although vehicles run 90% cleaner than they did in the 1960s because of emission controls, nearly 60% of San Diego’s smog and more than 85% of the area’s carbon monoxide were produced by motor vehicles, Sommerville said.

“The next decade’s focus will be on how people will ride-share and car-pool,” Goggin said. “If we reduce the number of trips we make, we can not only reduce congestion but also improve the air quality, and that has a double value for the community.”