The US EPA Brewer Spectrophotometer Network and the USDA UVB Monitoring and Research Program: Data comparison from co-located instruments

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 US EPA's Brewer Spectrophotometer network and the USDA's UVB Monitoring network, with a combined instrument total of 52 sites throughout the mainland USA, US territories and some overseas locations. The Brewer records full sky spectra from 287-363nm with 0.5nm resolution whereas the USDA instrument is a broadband device that measures broadband erythemally weighted UV data over a number of bands in the UV. To date, limited comparisons of data collected from these networks have been analyzed for comparative and quality assurance (QA) purposes. The data used in this paper is taken from sites where instruments from each program are co-located, namely, BigBend National Park, Texas and Everglades National Park, Florida. In order to reduce the contribution of errors in the Brewer-based instruments, the raw data has been 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 also ±5%. Uncertainty is comprised of i) standard lamp measurement errors, ii) spectral response determination and iii) the angular response of the diffuser. In this paper, we perform spectrally resolved comparisons between the Brewer UV irradiance measurements and the data collected by the broadband erythemal UV meters at co-located sites between 1997 through to 2002.