One of the main objectives of the European Brewer Network (EUBREWNET; COST Action ES1207) is the retrieval of AOD in the UV spectral range. A UV PFR has been developed by PMOD/WRC and is used as a travelling standard to facilitate the calibrations of EUBREWNET field instruments.
The UV-PFR has been found to be a stable high quality radiometer for AOD determination in the UV. However, due to the ~1 nm wide spectral response functions of the UV-PFR, the calibration constants from Langley plot calibrations underestimate the true extra-terrestrial signals, especially at the shortest wavelengths. The filter bandwidths also result in an apparent decrease in ozone optical depth with increasing airmass. An adjusted formula for AOD, with correction terms for the filter bandwidth influence, has therefore been introduced (Carlund et al., 2017).
For a reference instrument, calibration accuracy and stability are issues of major importance. The UV-PFR has therefore been absolutely calibrated by Langley plots at the high altitude station Izaña Observatory, Tenerife, Spain, where stable aerosol atmospheric conditions and columnar ozone concentrations often occur. Calibrations performed during two periods about 14 months apart showed that the UV-PFR sensitivity changed by ≤1 % per year at all four wavelengths. The UV-PFR has also been found to give stable and consistent AOD values compared to a reference standard PFR for over a 12-month measurement period at PMOD/WRC.
Figure 1. The PMOD/WRC UV PFR measuring at Izana, Tenerife.
In 2015, the UV-PFR participated in the 10th Regional Brewer Calibration Center for Europe (RBCC-E) campaign at the El Arenosillo station in southern Spain. During this campaign 21 Brewer spectrophotometers were calibrated for AOD against the UV-PFR. Within a 14-month period after this campaign, visits were performed with the UV-PFR to two of the participating Brewer home sites. AOD differences at the RBCC-E campaign and at a following site visit are shown for one of the Brewer wavelengths in Figure 2.
The stability of Brewer #163 was confirmed during two months in autumn 2015 and two months in spring 2016 at PMOD/WRC. During a one week visit to Brewer #186 at AEMET in Madrid, it became clear that the sensitivity of Brewer #186 had increased by 2 – 4 % since the 10th RBCC-E campaign about 14 months earlier. These findings both highlight the suitability for Brewers to measure AOD at a high precision, as well as the necessity of regular calibration.