New Ultraviolet Spectroradiometer measurements at Mauna Loa Observatory

Abstract

A research‐grade scanning UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument, built around a commercially available double monochromator, is interfaced with a PC to provide automatic control and data acquisition. The spectral range sampled by the instrument is 290–450 nm, and the bandpass is about 1 nm. A complete scan requires about 200 seconds and is performed every 5 degrees of solar zenith angle (SZA) during daylight hours. Calibration is performed on site at 6‐month intervals using a 1000‐W standard quartz‐halogen FEL lamp with calibration traceable to NIST. The UV irradiances measured at MLO are much more intense than at low altitude mid‐latitude locations. For observations at a SZA of 45°, the erythemally weighted UV can exceed 18 μW cm−2, which is approximately 15‐20% greater than the maxima seen at Lauder, NeW Zealand, for similar ozone amounts. The difference is primarily due to the higher altitude at MLO. For overhead sun conditions at MLO, erythemal UV can exceed 45 μW cm−2, which to our knowledge is the highest recorded anywhere at the Earth's surface. UV irradiance is strongly correlated (inversely) with Dobson spectrophotometer total ozone measurements at MLO, with higher correlations at shorter wavelengths. The radiative amplification factor (RAF) for erythema at MLO is about 1.44±0.46 at SZA 45°. Using ozone retrievals from the UV spectra themselves, the deduced RAF for erythema is 1.26±0.38. The RAFs for erythema at SZA 60° are similar, and in agreement with other determinations within the limits of experimental uncertainty.