Science / Medicine : L.A.’s Dirty Air Visits Grand Canyon : Pollution: Researchers trace industrial chemicals from Southland into Nevada and Arizona deserts. Concentrations there tend to mimic five-day work week of metropolitan area.
Where do air pollutants go to “get away from it all?”
In Los Angeles, they head for the desert.
Researchers from St. Louis, Reno and San Diego have found that industrial chemicals released in the Los Angeles Basin show up in the Nevada and Arizona deserts one to two days later. The chemicals, which serve as tracers to monitor large-scale air movements, indicate that pollutants from Los Angeles make a significant contribution to the haze and smog that frequently obscure vision at the Grand Canyon.
In a novel finding, the researchers showed that the average concentrations of the tracer chemicals, called halocarbons, in the desert exhibited a seven-day cycle that mimics the Los Angeles work week, with five days of elevated readings and two days of lower readings.
“The sun sets every night, and the Earth goes around the sun every year, but no geophysical variable takes weekends off,” said chemist Warren White of Washington University, who participated in the research. “Diurnal (daily) and seasonal patterns in pollutant levels may be driven by natural cycles in winds and radiation. But only activities that reflect the human work cycle show a clear pattern of five days on, two days off. Nature simply doesn’t work this way.”
The findings could affect the U.S. Environmental Protection Agency’s highly controversial plan to force the installation of more than $1 billion worth of pollution controls on the 2,250-megawatt Navajo Generating Station in Page, Ariz., only 80 miles northeast of the main visitor’s center at Grand Canyon National Park.
The plant, the largest in the West, emits sulfur dioxide that the EPA says is the major contributor to haze in the canyon. The haze reduces visibility from 150 miles on a clear day to less than 10 miles--not enough to see from one rim of the canyon to the other--on a few really bad days.
Although the researchers are not yet able to precisely quantify how much of the haze problem at the park is due to Los Angeles’ emissions, the city’s contribution may be large enough to reduce the pressure to install controls on the Navajo Generating Station.
When the haze problem is at its worst at the Grand Canyon, said chemist David F. Miller of the Desert Research Institute in Reno, “it’s always the result of L.A.-polluted air being there. It seems that the more severe the conditions, the higher the fraction of Los Angeles air.”
White added: “It looks like, in the early summer, Los Angeles is responsible for quite noticeable, severe haze on one day in four.”
The new results are among the first from the Southern California Air Quality Study, an $8.5-million research project begun in 1985. In the summer and winter of 1987, more than 200 scientists and technicians gathered in Southern California to track the origin and fate of air pollutants.
Identifying the source of polluted air is normally a difficult problem because pollutants from any one city are much like those from any other city, including auto exhaust, power plant emissions and the like.
“In the middle of the desert,” Washington University chemist Edward S. Macias said, “it is hard to distinguish between effluent from Los Angeles and that from Phoenix, Salt Lake City (and) Tucson.”
To get around this problem, the team studied a halocarbon called methylchloroform. The chemical, which is not a contributor to air pollution, is used by the aerospace and electronics industries in Los Angeles but not in the other cities, and thus provides a unique marker to identify air that was once in the Los Angeles Basin.
The researchers established monitoring stations in Claremont; Cajon Pass (an outlet for air from the basin); Spirit Mountain, Nev., and Meadview, Ariz., about 12 miles from the mouth of the canyon.
In Claremont, the concentrations of the chemicals rose each day, peaking in the afternoon. In Cajon Pass, the peaks occurred during the evening, reflecting the time required for movement of the air. In Arizona and Nevada, the peaks would occur one to two days later.
Concentrations of methylchloroform measured at Spirit Mountain were often more than 10% of the concentration measured in Claremont, indicating that air from Los Angeles was not diluted nearly as much as researchers had believed.
The researchers found that the concentrations of the tracers in the air also varied rapidly through the day at the distant sites, reflecting the peaks and valleys of normal use.
“The data shows . . . that you have pollution fronts coming in,” White said. “You may have fairly low concentrations of methylchloroform one hour and then much higher ones the next hour.”
White said that the average summertime concentration of ozone at Spirit Mountain was about 60 parts per billion (p.p.b.). When there is little movement from Los Angeles, as indicated by the tracers, he said, the average concentration is 50 p.p.b. But when concentrations of methylchloroform were high, the ozone concentrations typically rose above 85 p.p.m.
“The picture is one of a summertime desert awash in urban effluents,” White said.
As researchers have continued their study of the Colorado River Basin, they have expanded it to include transport of air pollutants northward into the Owens Valley. They have found patterns similar to those observed at Spirit Mountain as far north as Mt. Whitney. The main difference is that the biggest episodes at the Grand Canyon occur in spring and summer, while in the Owens Valley they occur in the late summer and fall.
According to White, one of the most startling findings of the study is that “even when you’ve left L.A. far behind, you haven’t escaped the L.A. air. You can’t escape it. . . . That realization, to me, has a very visceral impact.”
THE PATTERNS OF POLLUTANTS
Throughout much of the year, polluted air from Los Angeles flows out of the basin, through the Cajon Pass, and travels at least as far as the Grand Canyon, sometimes affecting visibility in the park. Officials are trying to determine just how much of the Grand Canyon haze is due to L.A. emissions and how much is produced by the Navajo Generating Station located 80 miles from the park.
Measurements of halocarbons at Cajon Pass and Spirit Mountain exhibit a pattern of five days of high emissions, followed by two days of low concentrations, reflecting the work week of L.A.-based industries that produce the chemicals. Measurements at Cajon Pass, just beyond the city, shows a dip in emissions on Sunday and Monday. The full reaches Spirit Mountain about a day later.
Figures in parts per billion Cajon Pass, California Spirit Mountain, Nevada
