The 1997 North American interagency Intercomparison of Ultraviolet Spectroradiometers including narrowband filter radiometers - Statistical Data Included

Journal of Research of the National Institute of Standards and Technology, Jan, 2002 by Kathleen Lantz, Patrick Disterhoft, Edward Early, Ambler Thompson, John DeLuisi, Jerry Berndt, Lee Harrison, Peter Kiedron, James Ehramjian, Germar Bernhard, Lauriana Cabasug, James Robertson, Wanfeng Mou, Thomas Taylor, James Slusser, David Bigelow, Bill Durham, George Janson, Douglass Hayes, Mark Beaubien, Arthur Beaubien

The fourth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 15 to 25, 1997 at Table Mountain outside of Boulder, Colorado, USA. Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NIST and NOAA, and included participants from the ASRC, EPA, NIST, NSF, SERC, USDA, and YES. The UV measuring instruments included scanning spectroradiometers, spectrographs, narrow band multi-filter radiometers, and broadband radiometers. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity. The spectral irradiance responsivity was determined two to three times outdoors to assess temporal stability. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST traceable standard lamp, and a simple convolution technique with a Gaussian slit-scattering function to account for the different bandwidths of the instruments, the measured solar irradiance from the spectroradiometers excluding the filter radiometers at 16.5 h UTC had a relative standard deviation of [ or -]4 % for wavelengths greater than 305 nm. The relative standard deviation for the solar irradiance at 16.5 h UTC including the filter radiometer was [ or -]4 % for filter functions above 300 nm.

Key words: environmental monitoring; intercomparison; solar ultraviolet; spectroradiometers.

1. Introduction

Concern over the effects of changes in solar ultraviolet (UV) radiation on biological ecosystems, humans, and materials has prompted several government agencies in North America to develop UV Monitoring Networks and research programs to address the current and long-term impacts of these changes (1, 2). Detecting long-term trends in solar ultraviolet irradiance requires accurate measurements of the absolute irradiance, for individual instruments, for an entire network, and between networks (3).

The North American Interagency Intercomparisons of Ultraviolet Monitoring Spectroradiometers are performed near Boulder, Colorado, to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance and to compare these results between instruments of different monitoring networks on an annual or bi-annual basis. The first such Intercomparison was held September 19 to 29, 1994; the second Intercomparison was held June 12 to 23, 1995; and the third Intercomparison was held June 15 to 25, 1996. The experimental details and results from these efforts have been described previously (4, 5, 6). Results from the fourth Intercomparison, held September 17 to 25, 1997, are presented here. This Intercomparison was coordinated by the Optical Technology Division of the National Institute of Standards and Technology (NIST) and the Surface Radiation Research Branch (SRRB) of the National Oceanic and Atmospheric Administration (NOAA). The following agencies and organizations participated: the Natio nal UV Monitoring Center (NUVMC) at the University of Georgia which administers the Environmental Protection Agency's (EPA) UV Network, the National Institute of Standards and Technology (NIST), Biospherical Instruments which administers the National Science Foundation's (NSF) CV Monitoring Network for Polar Regions, the Smithsonian Environmental Research Center (SERC), the Natural Resource Ecology Laboratory (NREL) of Colorado State University (CSU) which administers the Department of Agriculture's (USDA) UV Monitoring Network (7), the Atmospheric Sciences Research Center (ASRC) of the State University of New York (SUNY) which represents the USDA monitoring program, and Yankee Environmental Systems, Inc. (YES) which manufacturers a variety of UV instruments used in UV monitoring networks and for atmospheric UV research. A list of attendees is given in Appendix A.

As in previous years, the goal of the Intercomparison was to track the comparison of UV irradiance measured by instruments that are part of UV Monitoring Networks. Unlike other years, there was an emphasis on new prototype instruments for measuring and monitoring of UV irradiance, which precluded a true "blind" comparison of solar irradiance data because of a lack of current participant calibration data for several of the instruments. Synchronized solar scans from each instrument were compared based upon responsivity calibrations performed by NOAA. Measurements of the spectral irradiance responsivity checked the absolute irradiance scales used by the networks and provided a common scale for the synchronized measurements of solar irradiance. As with the other Intercomparisons, these synchronized solar irradiance measurements were the most important aspect of the fourth Intercomparison as they allow assessment of the present limits to which irradiance determined by different instruments can be compared. Instru ment parameters characterized included wavelength uncertainty as a function of wavelength, stray-light rejection, the slit-scattering function, bandwidth as a function of wavelength, and spectral irradiance responsivity. A field calibration unit was used for the spectral irradiance responsivity measurements which has been shown to reduce uncertainties of the responsivity for this experiment (8). Other instruments determined the atmospheric conditions during the Intercomparison, which is useful for correlating these conditions with the measured solar ultraviolet irradiance. A list of all the instruments present at the Intercomparison is given in Table 1.1. Please note the times given in this paper are in Universal Coordinated Time (UTC), 6 h ahead of Mountain Daylight Time, the local time.