Comparison of Ultraviolet Data From Colocated Instruments from the US EPA Brewer Spectrophotometer Network and the U.S. Department of Agriculture UV-B Monitoring and Research Program

Abstract

Several ground-based ultraviolet (UV) monitoring networks exist in the United States, each of which is unique in the instrumentation employed for measurements. Two of these UV networks are the U.S. Environmental Protection Agency's (EPA's) Brewer Spectrophotometer Network and the U.S. Department of Agriculture's (USDA's) UV-B monitoring network, with a combined instrument total of 52 sites, with 32 sites located in the mainland United States. The Brewer records full sky spectra from 287 to 363 nm with 0.55-nm resolution, whereas the USDA instrument is a broadband device that measures broadband erythemally weighted UV data. To date, limited comparisons of data collected from these networks have been analyzed for comparative and quality assurance (QA) purposes. The data we use is taken from sites where instruments from each program are colocated, namely, Big Bend National Park, Texas, and Everglades National Park, Florida. To reduce the contribution of errors in the Brewer-based instruments, the raw data is corrected for stray light rejection, the angular response of the full sky diffuser, the temperature dependence of the instruments, and the temporal variation. This reduces the estimated errors of the absolute irradiance values of each Brewer spectral measurement to approximately ±5%. The estimated uncertainty of the USDA instruments is approximately ±6% with a systematic bias of (–13 to 5% depending on the total ozone) and is comprised of (1) standard lamp measurement errors, (2) spectral response determination, and (3) the angular response of the diffuser. We perform comparisons between the Brewer spectrally integrated and erythemally weighted UV irradiance measurements and the data collected by the broadband erythemal UV meters at colocated sites between 1997 through to 2002. ©2005 Society of Photo-Optical Instrumentation Engineers