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UV radiation decreasing over U.S.?

UV radiation decreasing over U.S.?

Long-term measurements of ultraviolet (UV) radiation from the sun are not living up to scientific expectations, according to researchers who have been monitoring this radiation at eight U.S. airports. Between 1974 and 1985, they report, the amount of UV radiation appears to have decreased. However, some scientists familiar with the instruments used in this project are questioning the accuracy of the findings.

Atmospheric scientists have anticipated an increase in the UV radiation reaching the earth's surface because of recent observations that the global ozone layer has thinned since the late 1970s (SN: 1/9/88, p.20). Stratospheric ozone protects life on earth by absorbing dangerous UV radiation, and scientists have estimated that for every 1 percent drop in ozone, there should be a 2 percent increase in the amount of UV radiation reaching the earth's surface.

"That's the confusing part about this finding. The UV should be increasing," says Gerald Cotton of the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md. Cotton worked with Joseph Scotto and Thomas Fears of the National Cancer Institute and Frederick Urbach and Daniel Berger at Temple University in Philadelphia. The researchers published their results in the Feb. 12 SCIENCE.

The experiment relied on Robertson-Berger meters that were stationed along the edge of the contiguous United States at cities of various altitudes. These instruments detect radiation wavelengths ranging from 280 to 330 nanometers.

Previous studies have shown that a wavelength in the neighborhood of 297 nm is most effective for producing sunburn in typical Caucasian skin, and the same range of wavelengths is known to cause changes in DNA that can lead to skin cancer.

Over the 12-year period, the researchers found a 2 to 7 percent drop in the amount of UV radiation reaching the ground. According to Cotton, these are the longest records for UV radiation measured at the ground.

This decrease does not necessarily call into question the observed loss in stratospheric ozone, says Cotton. Within the earth's troposphere -- the layer directly below the stratosphere -- there are many factors that absorb UV radiation, such as clouds, air pollution and even debris from volcanic eruptions.

Studies have shown that the United States has become more cloudy in recent years. In addition, volcanic eruptions, like El Chichon in 1982, send fine particles into the atmosphere, where they can remain for years.

"Maybe there's something going on in the troposphere that's more important than we expected," says Cotton.

However, he adds, "I'm not ruling out that there might be something going on with the instruments, too." Cotton notes that there has been some controversy ove the reliability of measurements taken with the Robertson-Berger meter.

According to physicist Jon Geist, the meters are not accurate enough to monitor the small, long-term trends in UV radiation. Moreover, the instruments tend to lose accuracy over time and need to recalibrated, a process that can often introduce error. Geist and his colleagues at the National Bureau of Standards in Gaithersburg, Md., have analyzed the design of the Robertson-Berger meter in an attempt to improve the machine.

"I seriously question whether this instrument is capable of providing data that are meaningful at this level of accuracy," says Geist. "I don't think you can tell a 10 percent change over 10 years."

Berger, who worked on the study and designed the meter used during the project, acknowledges that there were "major problems" with the machines. But he believes that the meters were accurate enough for this study.

"I think," he says, "that the long-term variations are probably correct."
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Title Annotation:ultra-violet radiation
Author:Monastersky, Richard
Publication:Science News
Date:Feb 20, 1988
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