Monitoring of UV spectral irradiance at Thessaloniki (1990–2005): data re-evaluation and quality control

Abstract

Abstract. We present a re-evaluation and quality control of spectral ultraviolet irradiance measurements from two Brewer spectroradiometers operating regularly at Thessaloniki, Greece. The calibration history of the two instruments was re-examined and data flaws were identified by comparing quasi synchronous measurements. Analysis of the sensitivity of both instruments to variations of their internal temperature revealed that they have temperature coefficients of different sign. These coefficients exhibit small variability during the 15-year period. Using averaged temperature coefficients, we corrected both datasets. Corrections were applied for the angular response error using two different approaches depending on the availability of required ancillary data. The uncertainties associated with the measurements have been estimated and presented. Finally, the two datasets are compared using ratios of irradiance integrals at various bands in the UV, in order to assess any dependencies on the internal instrument temperature, solar zenith angle and wavelength.

Dead time effect on the Brewer measurements: correction and estimated uncertainties

“…The Brewer network provides a variety of products such as the total columns of ozone (TOC) (Kerr et al, 1981), SO 2 (Cappellani and Bielli, 1995) and NO 2 (Cede et al, 2006;Diémoz et al, 2014), the aerosol optical depth (AOD) Gröbner and Meleti, 2004;Meleti and Cappellani, 2000) as well as global and direct irradiance spectra (Bais et al, 1993(Bais et al, , 1996. These measurements have supported scientific research for more than 30 years, enabling the investigation of their shortand long-term variability (Glandorf et al, 2005;Weatherhead et al, 1998;Zerefos, 2002) and interactions among them and among other atmospheric constituents (Bernhard et al, 2007). Additionally, good-quality ground-based measurements are very useful for the validation of satellite products which, under specific conditions, may be highly uncertain (Fioletov et al, 2002).…”

“…More recent studies indicated that the measurements can be largely affected by the individual characteristics of each instrument (Gröbner et al, 2006) and that proper corrections are needed in order to keep the uncertainties within the above mentioned limits or even reduce them further (Garane et al, 2006;Karppinen et al, 2014;Lakkala et al, 2008).…”

Short- and long-term variability of spectral solar UV irradiance at Thessaloniki, Greece: effects of changes in aerosols, total ozone and clouds

“…The present study aims at the quantification of the longterm changes in surface UV irradiance using spectral measurements which have been recorded since 1990 at Thessaloniki (Bais et al, 2001;Garane et al, 2006;Gröbner et al, 2006), one of the longest time series globally (Glandorf et al, 2005). An important aspect is also the attribution of the trends and variability of UV irradiance to changes in the TOC, the aerosol optical depth (AOD) and cloudiness during the same period.…”

In search of traceability: two decades of calibrated Brewer UV measurements in Sodankylä and Jokioinen

“…A review of the literature shows that the methods and algorithms differ amongst the stations maintaining spectrophotometers for measurements of solar spectral UV irradiance (Lam et al, 2002;Kazadzis et al, 2005;Garane et al, 2006, Lakkala et al, 2008Simic et al, 2008;Schneider et al, 2008;Ialongo et al, 2008;Sabburg et al, 2001;Damiani et al, 2012). The measurements with Brewer #037 and Brewer #107 spectrophotometers at Sodankylä and Jokioinen respectively have followed the WMO recommendations, with stability checks every 3-4 weeks and measurements of standard lamps in laboratory (darkroom) every 6-8 weeks.…”

“…The absolute calibration of a spectrophotometer is a crucial part of the measurement chain needed to obtain solar UV spectra with the lowest achievable uncertainty. In fact, maintenance of reliable absolute calibration may be considered as the most important requirement in UV spectroradiometry (Garane et al, 2006). The calibration is a challenging task due to several reasons: it has to be considered that the instrument measures each of the solar irradiance wavelengths separately, the signals at short wavelengths are orders of magnitude smaller than at longer wavelengths, the uncertainties differ at different wavelengths, the calibration lamps themselves are ageing and as a result their spectral output is changing whenever they are used, and multiple transfer standards need to be used.…”