Quality assurance of the Greek UV Network: preliminary results from the pilot phase operation A. Kazantzidis a , A. Bais a , C. Topaloglou a , K. Garane a , M. Zempila a ,C. Meleti a and C. Zerefos b , a Laboratory of Atmospheric Physics, Campus Box 149, Aristotle University of Thessaloniki, 54124, Thessaloniki, Greece b Laboratory of Atmospheric Environment, Foundation for Biomedical Research, Academy of Athens ABSTRACT The stratospheric ozone depletion during the last two decades, the increase of UV-B irradiance levels at the ground and the possible impact on the biosphere has led scientists to develop and use instruments of high accuracy for UV measurements. During the last two years, 9 UV stations have been established in different environments in Greece and Cyprus, with the aim to establish a long-term monitoring network. The instruments of the network (NILU-UV multichannel filter radiometers) can provide measurements of irradiance in the UV and the visible part of the solar spectrum. In this study, first results from the calibration measurements and the quality assurance procedures are presented. The stability of the maximum of spectral response and the full width at half maximum was measured within 0.5 nm. Lamp tests were performed and downward drifts up to 40% in UVA channel sensitivity were observed. Calibration factors derived from lamp measurements could provide measurements of UV dose rate and total ozone with quite good agreement when compared with standard ultraviolet instruments. Keywords: ultraviolet irradiance, multichannel filter radiometer 1. INTRODUCTION The spatial and temporal variability of ultraviolet (UV) irradiance reaching the ground due to clouds, total ozone, aerosol properties and surface albedo has been discussed in detail in scientific literature 1,2,3,4 . Although satellite-derived estimations of UV irradiance could be used to establish a global UV climatology, the weakness to account correctly for aerosol and ozone in the boundary layer and the disadvantage to provide only one measurement per day lead to overestimation of UV irradiance by more than 15% in polluted sites 5,6,7 . The ozone layer depletion and the resultant enhancement of UV irradiance reaching the ground, in combination with the consequences on the ecosystem induced by overexposure to ultraviolet radiation, stimulated the scientific community to establish monitoring programs of solar ultraviolet radiation in different countries 8,9.10 . Substantial efforts have also been made by scientists to produce long- and short-term forecasts of UV radiation levels 11 . Although spectroradiometers are considered to be the appropriate instruments to provide quality assured measurements of solar UV irradiance, they require a lot of attention and effort to maintain them. As a result, multichannel filter radiometers which have the advantages of easy calibration, little effort for maintenance and the appropriate methodology to derive several atmospheric (e.g ozone, cloud transmittance) and biologically effective quantities (e.g erythemal dose) have been extensively used in UV monitoring networks. The results presented in this study refer to the multichannel filter radiometers of Greek UV Network. A short description of the network, the quality assurance procedures and the first results after 1.5 years of pilot operation will be discussed in the next paragraphs. 2. DESCRIPTION OF THE NETWORK