Comparison Of UV Measurements Will Improve Forecasts

A recently completed Commerce Department field test will help environmental scientists worldwide more accurately measure ground-level changes in damaging ultraviolet radiation from the sun.

Such measurements are crucial in assessing effects of ozone depletion in the upper atmosphere on human health, agriculture, fisheries and materials such as concrete, plastics and paints. As ozone levels in the upper atmosphere fall, scientists project that increased levels of harmful ultraviolet radiation, or UV, will reach ground level.

In the Sept. 22 to Sept. 28, 1994, field test on Table Mountain Mesa, 10 miles north of Boulder, Colo., U.S. and Canadian scientists compared readings from seven spectroradiometers, instruments that measure UV radiation.

The intercomparison was led by Ambler Thompson of the Radiometric Physics Division of the National Institute of Standards and Technology. The National UV Calibration Facility of the National Oceanic and Atmospheric Administration's Air Resources Laboratory hosted the intercomparison.

In the week-long test, scientists from NIST, NOAA, the National Science Foundation, the U.S. Department of Agriculture, Colorado State University, the University of Georgia, the Smithsonian Institution, the State University of New York at Albany, the Environmental Protection Agency and the Atmospheric Environmental Service of Canada compared simultaneous UV measurements. Data from the intercomparison will provide a reference base for UV monitoring networks established by the EPA, the USDA and NOAA.

While most UV radiation from the sun is absorbed by the protective ozone layer in the upper atmosphere, some UV does reach the Earth's surface. Scientists and the public are concerned that more UV radiation will reach the Earth's surface due to human-induced depletion of the Earth's stratospheric ozone shield, particularly affecting the middle latitudes where most people live.

It is well known that there is a direct relationship between decreased stratospheric ozone and increased UV at the surface, but presently there are no reliable long-term measurements to ascertain trends in UV at the surface. UV monitoring networks are being established in the United States and worldwide; however, it will take several years or more to develop a record from which meaningful trends can be extracted. This is due to variability in stratospheric ozone as well as clouds and air pollution, which also affect the transmission of UV. Furthermore, UV trends are not likely to be large, so it is extremely important that the measurements be done with high accuracy.

Thompson's goal is to help the various UV monitoring agencies achieve high levels of accuracy in UV measurements so that readings from different networks in different locales will be reliable in calculating global trends. He would like the networks to achieve accuracies of 90 to 95 percent and says the recent intercomparison of instruments in Boulder illustrated that some agencies are making very accurate measurements and others need to improve.

Continuous calibration will be needed to ensure the long- term data quality. To do this, routine intercomparison of spectral instruments, such as the event that just happened in Boulder, will be an integral part of each network's operations.

The spectroradiometers that were tested separate the sun's rays into distinct wavebands, much as a prism does. Most of the sun's radiation is visible light that may be separated into the colors of the rainbow. Only a small part of the solar spectrum is in the ultraviolet, which comprises higher energy wavelengths that are harmful to living things. These harmful high-energy wavelengths cannot be seen by the human eye but can be detected by sensors in these special instruments.

Clouds generally decrease the amount of ultraviolet radiation reaching the Earth's surface. NOAA's experimental ultraviolet forecasts account for likely cloud cover and observed and predicted changes in stratospheric ozone for over 50 U.S. cities.

NOAA, an agency of the U.S. Commerce Department, describes and predicts changes in the Earth's environment and promotes global environmental stewardship.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.