Interagency Program on Ultraviolet Radiation

Ultraviolet (UV) radiation that reaches the lower atmosphere can have severe effects on exposed skin, eyes, plants, etc. Cancers can be caused, some fatal. Recent attention to the changes in stratospheric ozone has generated considerable interest in UV radiation, because as ozone levels decrease the amount of UV reaching the surface will certainly increase. Ozone is a highly efficient absorber of UV radiation. In fact, detailed measurements of UV are one method by which scientists keep track of the total amount of ozone contained in the atmosphere.

There are many reasons for monitoring UV at the surface, corresponding to the concerns about health and the environment. The various US agencies, with their different but interlocking missions, have initiated a number of UV monitoring programs, employing instrumentation falling into three distinctly different categories: broad-band sensors of radiation intensity within a spectral region between two specified levels, narrow-band instruments measuring radiation in several well-defined wavelength intervals, and spectral devices providing detailed information on radiation intensity as a function of wavelength. Different instruments suit different applications. Questions about variations in human exposure are conveniently answered using broad-band instruments. Questions about specific airborne chemicals (e.g. ozone) require spectral information. Narrow band instruments sometimes constitute an optimal practical solution to the desire to satisfy both communities, but these are recent developments that are not yet thoroughly proven in field situations.

The U.S. National UV Network.

The diagram to the right shows the current array of UV monitoring stations operating in the USA (Click here for a listing). The networks shown are operated by the sponsoring agencies, using instruments carefully selected to provide answers as required by the specific agency requirements. The various networks are coordinated through three USGCRP activities ­ (1) the Interagency UV Monitoring Team (chaired NASA and NOAA), (2) the operation of several sites where instruments from the individual agency networks are collocated, and (3) the Central UV Calibration Facility (a joint NOAA and NIST operation in Boulder, Colorado, aimed at assuring that data from the agency networks are comparable and compatible).

Data Availability.

Agency information and data can be obtained at the following addresses:

NOAA/ISIS
NOAA/SURFRAD
NOAA/CMDL
CSU
EPA
EPA/UGA
NSF
SI/SERC
DOE

Work to develop a standardized format for data reporting is continuing. The various data repositories at the sites identified above reflect the reliance on a wide range of sensor types employed and the different agency needs for the data generated. However, in general data are reported as soon as possible after collection, often on a daily basis. The reported data are therefore not quality assured before being posted. Data are reported with an indication of the state of the related quality assurance. Level 1 data are unprocessed, as received from the field. Level 2 data have had recent calibration corrections applied; these are more accurate than level 1 but might still contain errors to be discovered in a thorough quality assurance program yet to follow. Level 3 data are those that have passed all quality assurance steps.

Differences in Instrumentation.

The UV instrumentation that is used in the US network (and in other UV measurement systems) can be considered in three categories: broad-band, narrow-band, and spectral. Broad-band instruments give a single number representing a weighted integration of the irradiance flux over wavelength intervals in the UV region. There are several different weighting functions that can be used, designed to mimic the response of different biological systems to different wavelengths of UV radiation, but most contemporary instruments are designed to have a spectral response function (SRF) that approximates the erythemal action spectrum of human skin, from 280 to 380 nm. They work through the conversion of UV radiation into fluorescence of magnesium tungstate in the green part of the spectrum. The voltage actually measured by the instruments is a convolution of the SRF of the detector and the solar irradiance reaching the sensor. The calibration factor, which transforms the measured voltage into an appropriately weighted spectral irradiance, depends on the solar zenith angle and the amount of ozone overhead because of the dependence of surface UV irradiance on these parameters [e.g. Bodhaine et al., 1998].

Broad-band instruments have advantages of simplicity, reproducibility, and long-term stability (since their behavior is based on a fundamental physical property of the underlying material -- magnesium tungstate). Interpretation of their data is hindered because they are sensitive to a wide range of wavelengths, including some considerably longer than the UV-B wavelengths of interest in measurements designed to interpret UV trends which may be occurring in response to ozone depletion.

The broad-band technique was commonly used in early programs, such as the Robinson-Berger UV monitoring network operated in the US during the 1970s and 1980s (see Hicks et al., 1997). Although the instruments themselves are relatively simple, the use of their data for trend detection is quite complex; indeed, an initial study suggested the existence of a counter-intuitive decrease in surface UV flux at a time when ozone amounts were decreasing [Scotto et al., 1982]. A recent reexamination of these data has revealed that when the calibration histories of the sensors are taken into account the trends previously observed disappear [Weatherhead et al., 1997]. A new generation of broad-band instrumentation has recently been developed to eliminate the sources of errors that affected early devices, such as the lack of temperature stabilization. These newer broadband instruments constitute a common factor among the various networks presently in operation. They are used extensively in the NOAA and USDA programs, for example.

Narrow-band instruments make measurements in several separate bands within the UV region. These typically make use of metallic interference filters that transmit radiation in wavelength intervals several nanometers wide. The multi-filter approach allows simultaneous measurement at several wavelengths, which eliminates complications found in scanning instruments that can result from changes in cloud cover during the time period of a spectral scan. Like the broad-band instruments, the narrow-band instruments are relatively inexpensive and easy to operate. The long-term degradation of the optical filters in these instruments must be considered, but the recent development of ion deposition technology has significantly improved filter stability. Many of the narrow-band filter radiometers used today have fairly narrow bandwidths that are close to the resolution of spectrally scanning instruments; recent narrow-band filter instruments use 2 nm bandwidth filters [Bigelow et al., 1998]. With such narrow bands, corrections for effects of column ozone and solar zenith angle are not necessary.

Shadowband UV narrow-band filter radiometers (used in the USDA network) permit determination of the diffuse and direct irradiances. This provides information on the aerosol optical depth, another factor affecting UV. The derived estimates of the solar irradiance at the top of the atmosphere (obtained by extrapolating measured direct irradiances to zero air mass), allow for unattended in-field calibration checks.

Scanning spectroradiometers make continuous spectrally-resolved measurements across the entire UV spectrum, as is required to answer questions about the chemical composition of the atmosphere, or to study effects on receptors with different action spectra. Several different experimental configurations are used for these measurements. Most contemporary instruments employ photomultiplier detectors with single or double monochromators. Double monochromators are preferred because of their improved ability to minimize the contribution of stray light from adjacent wavelengths, which is important given the rapid change of radiation intensity with wavelength for wavelengths < 320 nm. These instruments typically need several minutes to make a measurement, which introduces uncertainty due to temporal variability (for example if a cloud passes overhead). Furthermore, the preservation of accurate long-term calibration of such complex instruments is a challenge. Spectral instruments typically cost more to purchase and operate than do broad-band or narrow-band instruments, and require the presence of highly trained operators.

The USGCRP UV network employs instruments of all three kinds, with broad-band sensors operating at almost all sites. Narrow-band and spectrally-resolved instruments are used at an increasing number of sites. Currently operating sites are shown in the first figure above. The design concept behind the network is that a broad base of relatively simple instruments would be operated over a wide range of geographic locations, with a smaller number of more sophisticated instruments at a subset of locations where more detailed data are desired. In this mode of operation, with different sensors being used to address the specific needs of particular agencies, uniformity of data across the overall array requires that there be a number of sites where the different sensors would be operated side-by-side for purposes of intercomparison and cross-calibration.

To ensure that the various arrays of instrumentation operate in close coordination, the agencies involved have agreed that one instrument of every kind will be operated side-by-side at a single location -- Table Mountain, Boulder, Colorado. The Table Mountain facility is the home of the Central Calibration Facility set up to serve all US UV monitoring activities as a result of earlier interagency agreement.

The Central UV Calibration Facility.

A central UV calibration facility (CUCF) has been established in Boulder, Colorado, where new UV standards provided by the National Institute for Standards and Technology are used to calibrate network instruments. This is as was called for in the U.S. UV-B Interagency Monitoring Strategy. The main purpose is to provide long term, NIST traceable instrument calibrations and characterizations for the U.S. UV-B monitoring Networks which are operated by the supporting agencies (USDA, EPA, NSF, etc.). The success of the U.S. Interagency UV monitoring network requires a strong commitment to calibration. The use of a common calibration facility and standards will provide quality assurance of data for the participating networks. In order to detect trends in UV-B radiation, the measurement base must be stable over the decades of the monitoring effort. The U.S. agencies will, in all probability, continue to deploy instruments of different designs and in different locations to achieve the individual goals of the agencies.

The calibration facility provides the following services, through regular intercomparisons of different instruments at the CUCF [Early et al., 1998].

• Irradiance calibrations and slit scattering/stray light measurements for spectral instruments.
• Absolute irradiance calibration.
• Angular response measurements.
• Linearity measurements for broad band and multi -filter instruments.

In addition, field audits are conducted with a field calibrator for spectral instruments. These measurements are conducted on an annual or semi-annual basis as needed, determined by the stability of each instrument. The central calibration facility also hosts the annual spectoradiometer intercomparisons held at the Table Mountain Test Facility located north of Boulder, Colorado. Further information can be found at the Central UV Calibration Facility page.

Forecasting to Protect the Public -- the UV Index.

The environmental consequences of changes in UV radiation are well recognized, especially those related to erythema. Public interest is high, and has generated need for a new forecast product now widely disseminated by the National Weather Service -- the UV Index. (Click here for the current UV forecast from ARL's READY web site) The UV Index is a number from one to ten, with low values corresponding to little sunburn danger and with high numbers being warnings to take protective action. The UV Index has quickly become one of the most popular US forecast products. It is generated in collaboration with the USGCRP program; it is the data generated by the national monitoring network that are used to evaluate and improve the predictions.

Each year the National Weather Service performs a validation of the summer-time UV Index forecasts. Generally, the UV Index forecast coincides with the observations. But the spread of actual observations is greater than that forecast. This inability to match the large dynamic range of the actual observations is a short coming of the current forecasting technique. However, for the majority of cases, the probability of making a correct forecast is quite good. The adjacent diagram conveys this in the form of a histogram of all the differences between the observation sites and the forecasts. The histogram shows that 26% of the time the UV Index is exactly correct. 65% of the time the UV Index forecast is within ± 1 UV Index unit. And 84% of the time the UV Index is within ± 2 UV Index units.

Biological Response Studies.

There is a large research effort to explore how living organisms respond to UV radiation. Under sponsorship of the National Institutes of Health and the National Cancer Institute, a large program addressing effects on humans is under way. Under the leadership of the Department of Agriculture, studies are investigating effects on vegetation, especially crops. Marine ecosystems are being studied by NOAA researchers, under the leadership of the National Undersea Research Program. The Environmental Protection Agency, Office of Research and Development, supports extramural research and intramural research on biological effects of UV exposure. The EPA STAR (Science to Achieve Results) Program provides the primary EPA mechanism for engaging the non-federal scientific community in addressing the potential outcomes of UV irradiance to biota. Peer-reviewed grants currently support UV research on photosynthesis, oxidative stress in plants, DNA-repair in amphibians, mutations in human genes, and impacts on phytoplankton, blue green algae, coral, and other aquatic species. The EPA intramural research focuses on the interactive effects of climate change and UV radiation on nutrient and carbon cycles in coastal waters of the Southeast and the role of UV exposure in amphibian deformities and declines through laboratory and field studies. The EPA intramural research program involves collaborations on research supported by NSF, ONR, NOAA, NPS, and USGS.

Reading Materials.

D. S. Bigelow, J. R. Slusser, A. F. Beaubien, and J. H. Gibson, The USDA Ultraviolet Radiation Monitoring Program, Bull. Amer. Met. Soc., 79, 601-619, 1998.

Bodhaine, B. A., E. G. Dutton, R. L. McKenzie, and P. V. Johnston, Calibrating Broadband UV Instruments: Ozone and Zenith Angle Dependence, J. Atmos. Ocean. Tech., 15, 916-925, 1998.

Diffey, B. L. Solar ultraviolet radiation effects on biological systems. Physics in Medicine and Biology, 36, 299-328, 1991.

Early, E. A., A. Thompson, C. Johnson, J. DeLuisi, P. Disterhoft, D. Wardle, E. Wu, W. Mou, Y. Sun, T. Lucas, T. Mestechkina, L. Harrison, J. Berndt, and D. Hayes, The 1995 North American Interagency Intercomparison of Ultraviolet Monitoring Spectroradiometers, J. Res. Nat. Inst. Stand. Tech., 103, 15-62, 1997.

Herman, J. R., P. K. Bhartia, J. Ziemke, Z. Ahmad, and D. Larko, UV-B increases (1979-1992) from decreases in total ozone, Geophys. Res. Lett. 23, 2117-2120, 1996.

Hicks, B. B., J. J. DeLuisi, and D. R. Matt, The NOAA Integrated Surface Irradiance Study (ISIS) - A New Surface Radiation Monitoring Program, Bull. Amer. Met. Soc. 77, 2857-2864, 1996.

Long, C., A. J. Miller, H.-T. Lee, J. D. Wild, R. C. Przywarty, and D. Hufford, Ultraviolet index forecasts issued by the National Weather Service., Bull. Amer. Met. Soc. 77, 729-748, 1997.

Scotto, J., G. Cotton, F. Urbach, D. Berger, and T. Fears, Biologically effective ultraviolet radiation-surface measurements in the United States, 1974 to 1985, Science, 239, 762-764, 1988.

USGCRP, The U. S. Interagency UV-Monitoring Network Plan, USGCRP-95-01, Washington, DC, 1995.

Weatherhead, E. C., G. C. Tiao, G. C. Reinsel, J. E. Frederick, J. J. DeLuisi, D. S. Choi, and W. K. Tam, Analysis of long-term behavior of ultraviolet radiation measured by Robinson-Berger meters at 14 sites in the United States., J. Geophys. Res., 102, 8737-8754, 1997.

WMO, Scientific Assessment of Ozone Depletion: 1994, World Meteorological Organization/United Nations Environment Programme, Geneva, Switzerland, 1995.