Annual solar spectral energy distributions in North America

Abstract

Predicting outdoor service life of polymers and coatings requires knowledge of the spectral power distribution (SPD) of the sunlight and the amount of radiant energy they receive. Standard solar SPDs such as ASTM G173 and ASTM G177 are snapshots of the SPD at a single moment under model conditions. A better measure would be average annual spectral energy distributions (SED) for specific locations, especially benchmark outdoor weathering sites. The annual SED is the energy received at each wavelength over the course of a year. A Smithsonian/NIST collaboration placed SR-18 UV spectroradiometers in Miami, FL, Phoenix, AZ, and Madison, WI during the period 1997-2012, collecting data at ca. 2 nm intervals between 290 and 324 nm. Similar data have been collected at Edgewater, MD for many years. The data are not complete for any year, but gaps could be patched using data from other years. These data sets were compared to data available on-line from ongoing U.S Department of Agriculture (USDA)/Colorado State University outdoor spectroradiometer measurements, after similar patching. The USDA data have fewer points in the UV but extend into the visible portion of the spectrum. The data sets give consistent results and show that the SEDs can be fit by SPDs calculated using the SMARTS 2.9.5 model and satellite ozone data. The annual standard deviations are < 5% for wavelengths > 310 nm. SEDs for average annual radiant energy received by horizontal and 45° south exposures are reported for benchmark sites near Miami and Phoenix. The paper provides a general approach for determining an SED for a location based on SMARTS modeling of the autumnal equinox and subsequent scaling based on reported annual total solar radiant energy.