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Computers track course of acid rain

Armed with computer models, scientists are beginning to decipher the patterns of acid rain.

Meteorologists hope to use the models to find satisfactory areas for industrial plants.

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Major study to see if computers could track long-range air pollution started about ten years ago. Dr. James W. S. Young of Canada's Atmospheric Environmental Service explains that computers track pollutants by modeling physical and chemical atmospheric conditions.

Various models emerged. Scientists found that major pollutants, such as oxides of sulfur and nitrogen, require different models, since these families of chemicals undergo different reactions.

The most advanced models to date, says Dr. Nels Laulainen, a Battelle Northwest Laboratory physicist, look at where sulfur oxides travel.

Most sulfur oxide emissions come from midwest America, particularly the Ohio River Valley, western Ontario, and Tennessee, says Dr. George M. Hidy, chief scientist and vice-president of Environmental Research & Technology Inc. Dr. Hidy, who recently completed modeling studies for the Electric Power Research Institute, found that almost 60 percent of the emissions come from utility power plants. Many are in the Midwest.

Sen. Daniel P. Moynihan (D) of New York disagrees with this figure. He maintains that the electric industry's contribution is closer to 80 percent.

Air masses with sulfur oxide emissions constantly drift above the eastern United States, says Dr. Young. They form visible clouds for a third of the year , according to Dr. Hidy, which can be thousands of miles long.

Hidy says that computer models show that sulfur pollution's seasonal variations follow known weather patterns. Clouds with sulfur oxides normally form in June, July, and August, he continues. They usually hang over a region for 3 to 11 days before rain washes the sulfur to the ground or they are carried off by changing winds.

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Dr. Laulainen points out that ''there is some reason to believe that the pollutants get circled and circled around'' eastern North America and the adjoining Atlantic Ocean. Since the polluted air masses can travel thousands of miles, he says, ''It is not all that rare for pollutants to drift by a second time'' before depositing. Occasionally pollutants even waft over a region three times. But he says that ''they have to land somewhere'' eventually.

Dr. Young has traced the travels of sulfur oxide pollutants to and from his country and the US. Because of prevailing air currents, he says, sulfur oxide pollution from the US is maximum in summer and minimum in winter. The US receives the most Canadian air pollution in spring and the least in summer. Dr. Young says that, according to American and Canadian research findings, the US sends about three times as much sulfur oxide to Canada as Canada sends to the US.

Dr. Hidy says modeling of localized travels of sulfur oxide emissions indicates that several ''hot spots'' exist, areas with greater concentrations of airborne pollution. ''These probably reflect increased deposit rates,'' he says. One large hot spot includes western Pennsylvania, western New York, eastern Ohio, and northwestern Virginia, according to Hidy. Some smaller regions are southern Connecticut above New York City; Philadelphia; northern Maryland; an area south of Washington, D.C.; and Birmingham, Ala.

Although a decade of computer modeling has led to these findings, it has not yielded a useful forecasting instrument. Scientists don't know how well models can predict the pollution routes, although they can simulate previous pollution events accurately, according to Dr. Young.

Dr. Laulainen explains that today's models include many assumptions about weather. For example, models do not look at various atmospheric conditions from terrains such as mountains, cities, and coastlines. Therefore, he says, the models can ''only forecast where the average wind will take a blob of polluted air.''

Furthermore, he says that the chemical reactions programmed in the models are very simple. Scientists using the models assume that the same percentage of sulfur oxides put into the air always return to the ground.

''But it turns out that chemistry in nature doesn't always abide by such simple-minded equations,'' Laulainen explains.

Dr. Young adds that complex interactions between various chemicals in the atmosphere can change the proportion of sulfur fallout to emissions.

To increase model accuracy, scientists began using new meteorological theories around 1977, according to Dr. Laulainen. New models use much more data. But this means greatly increased computer costs: tracing the movement of one polluted air mass may cost thousands of dollars.

Therefore, scientists anticipate using the more sophisticated models to determine how the less costly, older models can be improved. Dr. Laulainen says that hybrid versions will probably be used for future regulation and planning.

Scientists expect advanced models will eventually be used to find acceptable regions for industrial plants. Air modelers disagree as to when they will use models for this. Dr. Glenn Hilst, air modeling program manager for the Electric Power Research Institute, gives a conservative estimate, saying that practical forecasting will be possible in ''less than several decades.'' But Drs. Young and Laulainen think useful model predications will be available within two to five years.

The time it takes to refine the models depends on whether or not the researchers run into unexpected problems. Dr. Laulainen says, ''I really expect that (air modelers) are going to find some surprises. . . . We're on the very forefront of meteorological research today.''