RESEARCHERS AIR THEORIES ON WEEKEND SMOG.

Byline: Dana Bartholomew Staff Writer

Even though cars and trucks spew more smog on weekdays, scientists said Tuesday that a new study has begun to explain why air actually is worse on Saturdays and Sundays.

In an unprecedented analysis of this weekend effect, researchers said they have focused on six theories about how airborne chemicals react to create ozone and about the times people drive during the week.

The preliminary report, scheduled to be released this week by the California Air Resources Board, also defends a 30-year anti-smog strategy and counters auto-industry claims that the weekend effect is a reason not to reduce some car emissions.

The state's strategies of controlling emissions of nitrogen oxides and volatile organic compounds have been working, said board spokesman Richard Varenchik. ``We've seen that the control strategies that we have . . .,'' he said, ``have reduced ozone for every day of the week for 30 years.''

Of 16 ozone health advisories this year, most occurred on the weekends, according to the Southcoast Air Quality Management District, which also contributed to the study.

What has surprised researchers is that, while smog has generally plummeted on both weekdays and weekends, it is decreasing faster on weekdays. For the past 30 years, peak ozone periods have gradually shifted from Monday through Thursday to Saturday and Sunday.

``The weekend effect is very strong in L.A. and San Francisco, and virtually nonexistent in Sacramento,'' said Bart Croes, chief researcher for the state board that conducted the study. ``On average, ozone is about 20 percent higher on the weekend than during weekdays, and the day of the week that is highest is Sunday.''

Ozone, the main component of smog, makes the eyes and lungs burn and is suspected of causing long-term lung damage.

Yet, according to the study, pollutants such as carbon monoxide, particulate matter and other contaminants were higher during the week.

``Just the fact they have documented the effect is something not a lot of lay people are aware of; it's news to the general public,'' said Sam Atwood, spokesman for the South Coast district. ``The good news is that levels of ozone have declined.''

After its study of smog and freeway traffic, state board researchers posed six hypotheses, some of them more plausible than others, for the weekend effect:

--The nitrogen oxides reduction hypothesis - that lower weekend traffic reduces emission of nitrogen oxides, one of two key ingredients in smog. But reducing this smog component increases the ratio of evaporating paint, gasoline and other volatile organic compounds, thereby producing more ozone.

This theory is supported by General Motors, which has argued that a reduction in nitrogen oxide in cars can increase, rather than decrease, ozone. Researchers consider this plausible.

--The nitrogen oxides timing hypothesis - that, because motorists tend to drive later in the day on Saturday and Sunday, during peak hours of sunlight, ozone production is greater on weekends. This also is deemed plausible.

--The hypothesis of carryover near the ground - that more drivers are out on Friday and Saturday nights than on weekday nights. The pollutants their cars emit lurk near the ground and are converted to ozone by sunlight the following day. This was deemed marginally plausible.

--The carryover-aloft hypothesis - that large reservoirs of ozone-causing pollutants, from Friday and Saturday night driving, are carried offshore on sea breezes, only to return the following day and create more ozone. Researchers said this is plausible.

--The hypothesis that total emissions of ozone-causing pollutants actually increase on the weekend as homeowners rev up their lawn mowers and recreational vehicles, such as motorcycles and Jet Skis. Researchers said this is not plausible.

--The soot-and-sunlight hypothesis - that soot, caused by vehicles such as diesel trucks, is higher during the week and shields ozone-causing pollutants from the sun, while higher ultraviolet weekend light creates more ozone. This was declared theoretically plausible.

These theories are also being studied by the Department of Energy's National Renewable Energy Laboratory and the University of Nevada's Desert Research Institute. And the Coordinating Research Council has launched a similar study, funded by the auto and petrochemical industries.

State board officials were quick to discount the theory supported by General Motors and the theory that there are more actual pollution emissions on the weekend.

``There is something at stake here,'' said a state board researcher who asked not to be identified. ``What the report says is we have not found a smoking gun, but we are looking at all the potentials (for weekend smog).''

A few facts are known. Despite a population boom and a 150 percent increase in car miles across Los Angeles since 1975, ozone levels have been slashed by two-thirds - with Stage I ozone smog alerts down from almost 200 a year to zero. And less serious advisories of unhealthful air have been reduced by 90 percent.