Measurements of aerosol optical depths in the UV-A: A comparison between a USDA Yankee environmental systems UV-Multifilter rotating shadowband radiometer and an EPA Brewer spectrophotometer

This study presents measurements of Aerosol Optical Depth (AOD) spectral dependence in the Ultraviolet (UV) (317-368 nm) from a USDA UV-Multi-Filter Rotating Shadow band Radiometer (UV-MFRSR) and an EPA Brewer spectrophotometer collocated at Boulder, Colorado. The square of the correlation coefficients between the two instruments are: 0.93 for the 317-320 nm wavelength pair, 0.86 for the 332-340 nm wavelength pair, and 0.94 for the 368-360 nm wavelength pair (first wavelength represents the UV-MFRSR, while the second represents the Brewer). The ratio of the UV-MFRSR to Brewer AOD for all wavelengths is 1.01 ± 0.35. The daily-averaged AOD for wavelengths 317/320, 332/340, and 368/360 nm range from 0.112 to 0.373, 0.136 to 0.328, and 0.103 to 0.289, respectively, based on averages from both instruments. The hourly mean Ångström exponents were calculated to be 0.8 ± 1.5 for the UV-MFRSR and 1.3 ± 2.4 for the Brewer (which are not statistically different than zero at the 95% confidence level). The wavelength dependence of the AOD is explained based on reasonable assumptions for the size distribution and optical properties of the aerosol particles using Mie theory.