The field calibrator on the Brewer
The operational UV spectrometer at Lampedusa is Brewer MK III 123. The instrument was installed in December 1997. The Brewer spectrophotometer was originally designed for total ozone measurements. Total ozone is derived by comparing direct Sun signals measured at the wavelengths of 306.3, 310.1, 313.5, 316.8, and 320.1 nm. The retrieval scheme is similar to that used for the Dobson spectrophotometer; the derived total ozone values are independent of the aerosol amount and of the absolute instrumental responsivity. The Brewer spectrophotometer is also capable to measure spectral ultraviolet irradiance, detected through a teflon diffuser enclosed in a quartz dome.
The Brewer MK III is a double monochromator spectrometer: a substantial improvement in the quality of UV measurements below 305 nm with respect to the single-monochromator measurements, due to the better stray light suppression, has been achieved with this spectrometer. The instrument measures hemispheric UV irradiances betwen 286 and 363 nm through the teflon diffuser collector; spectral resolution is around 0.55 nm, and measurements are recorded at every 0.5 nm interval. The uncertainty on the observed irradiance is estimated to be around 4–5%. The uncertainty on total ozone, for cloud-free conditions, is around 1%. A method to derive aerosol optical depth from the Brewer direct Sun regular ozone measurements has been also implemented.
To obtain reliable measurements of the UV irradiance, regular calibration of the instrumental wavelength and spectral absolute responsivity is mandatory. The wavelength calibration is regularly performed by the instrument by means of an internal mercury lamp. The absolute spectral responsivity is determined every approximately 40 days by means of a field calibrator, developed by NIST and NOAA in the USA, which uses NIST traceable 1000 W FEL lamps [Early, E.A., E.A. Thompson, and P. Disterhoft, Field calibration unit for ultraviolet spectroradiometers. Appl. Opt. 37, 6664-6670, 1998].
Brewers #123 (left) and 17 (right) during the ozone calibration in February 2002
A 1000 W FEL lamp