Kurzbeschreibung
(Englisch)
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This project will modify and improve an existing precision sunphotometer (UVPFR) for the measurement of direct solar irradiance at four channels (305 nm, 311 nm, 317 nm, and 332 nm). This UVPFR will be calibrated and operated as travel standard for aerosol optical depth (AOD) in the UV to serve as reference instrument for the AOD Brewer network developed within the COST Action EUBREWNET. The UVPFR will be calibrated twice at the Izana Observatory, Tenerife, Spain to validate its stability over the duration of this project. The UVPFR will participate at two RBCC-E intercomparisons to calibrate all participating Brewer spectrophotometers for AOD and perform Quality Assurance site visits to three Brewer home sites to verify the performance of the Brewer AOD measurements at their home sites during routine operation. The final objective will be to have an operational travel standard for AOD for the European Brewer AOD network, which is one of the three objectives of the COST Action.
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Abstract
(Englisch)
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The absorption and scattering of solar radiation by aerosols has been recognized as an important parameter for climate forcing studies. Furthermore, the absorption of surface ultraviolet (UV) radiation by aerosols has also become of major interest because of the harmful effects of UV radiation on Humans and more generally on the biosphere. Thus, the determination of aerosol properties, especially the aerosol optical depth (AOD) in the UV wavelength region is of great importance to understand the climatological variability of UV radiation as well as to get a better knowledge of AOD itself over a wider wavelength range. Currently, Brewer spectrophotometers measure total column ozone using a well-established measurement procedure for direct solar irradiance at the ozone wavelengths between 303 and 320 nm. The same measurements can be used for retrieving AOD at these wavelengths if the instrument is suitably calibrated. For this reason, AOD retrievals in the UV from the European Brewer Network is one of the three main objectives of COST Action ES1207 (EUBREWNET). However, for the time being, there does not exist a calibration standard for AOD calibrations of Brewer spectrophotometers. The UV Precision Filter Radiometer (UVPFR) has been developed for the measurement of direct spectral UV irradiance at 4 narrow-band channels using custom made interference filters centred between 305 and 332 nm, with bandwidths of less than 2 nm. This instrument is based on the well-known PFR, developed at PMOD/WRC in Davos, which is used in the global WMO/GAW network for AOD in the visible-NIR wavelength range. The main goal of this project is to investigate the suitability of using, and in the end hopefully establish, the UVPFR as a travelling standard for AOD calibrations of the European Brewers. So far, the main activities in the project include modification of the UVPFR to reject out of band stray light, spectral characterization of the instrument in the lab, theoretical modelling of influences of finite spectral bandwidth, a first calibration at the high altitude site Izaña and participation in the 10th Regional Brewer Calibration Centre for Europe (RBCC-E) campaign. Preliminary results of the RBCC-E campaign are promising, indicating that (1) independent calibration results of the travelling standard Brewer from Langley plots and from calibration against the UV-PFR, with FWHM corrections, agree within =1 %, and (2) for a well characterized and well-maintained Brewer AOD can be retrieved with a precision of 0.01. After the final calibration results of Izaña and RBCC-E campaign measurements have been produced and reported Brewer site visits will be made with the UVPFR. The remaining work also includes a detailed uncertainty evaluation and work with the EUBREWNET database team to establish the common AOD calibration and retrieval algorithms. To tie together the project, during 2016 a re-calibration at Izaña will be done and the ATMOZ total ozone measurements intercomparison will be visited with the UVPFR, to assess the stability of both the UVPFR and AOD calibrations of Brewer spectrophotometers.
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