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The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measurements is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70[degrees]], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310-500nm is within 0.5% [+ or -] 1.1% (for SZA between 20[degrees] and 70[degrees]). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0.02 [+ or -] 0.02 in the range of 315-870 nm. Significant correlation coefficients with a maximum of 0.99 in the range of 340-360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instruments. OCIS codes: 010.0280, 010.1100, 280.4788, 280.1100.

This paper introduces a device that was developed to measure the angular response of UV spectroradiometers in the field. This device is designed to be used at the operating position of spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and -13%, assuming isotropic distribution of the downwelling radiances. OCIS codes: 120.0120, 120.4570, 120.6200.

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences hi irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within [+ or -] 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 [+ or -] 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth (300-365 nm) derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to ~0.3 at 340 nm in winter and ~0.7 in summer. OCIS codes: 120.6200, 010.1110, 010.1290.

To quantify the effect of ambient temperature on the voltage signal of Solar Light UV-Biometers, spectral response functions of two instruments were determined in the laboratory under various external temperature conditions. Despite the biometer's internal temperature stabilization, a temperature increase of 20 [degrees]C at the outside of an instrument's housing resulted in a reduction of the instrument's spectral response by as much as 10% in the UVB range and by as much as a factor of 2 in the UVA range, depending on the individual instrument and on its internal relative humidity. The significance of this effect for outdoor measurements is demonstrated by data from an intercomparison campaign of erythemal radiometers in Thessaloniki, Greece, organized by the Laboratory of Atmospheric Physics (Aristotle University of Thessaloniki), the Cooperation in Science and Technology (European Commission), and the World Meteorological Organization. On 16 September 1999, 12 of 16 Solar Light Biometers showed significant diurnal variation in their sensitivity (as much as 10% for some individual instruments), which can be explained through a heating of the instruments' housings due to direct solar radiation. OCIS codes: 120.0120, 120.3930, 120.5630.

A program for quality assurance of reference standards has been initiated among nine solar-UV monitoring laboratories. By means of a traveling lamp package that comprises several 1000-W ANSI code DXW-type quartz-halogen lamps, a 0.1-[ohm] shunt, and a 6-1/2 digit voltmeter, the irradiance scales used by the nine laboratories were compared with one another; a relative uncertainty of 1.2% was found. The comparison of 15 reference standards yielded differences of as much as 9%; the average difference was less than 3%. OCIS codes: 120.4800, 120.3940, 120.4140, 120.5630, 120.6200, 260.7190.

We present the results of the aerosol measurements carried out over the Aegean Sea during the Photochemical Activity and Solar Ultraviolet Radiation campaign held in Greece during June 1996. Simultaneous observations performed with a lidar and a double-monochromator spectrophotometer allowed us to retrieve the optical depth, the Angstrom coefficient, and the backscatter-to-extinction ratio. The Sun photometric data can be used to improve quantitative aerosol measurements by lidar in the Planetary Boundary Layer. Systematic errors could arise otherwise, because the value of the backscatter-to-extinction ratio has to be supplied. Instead this ratio can be retrieved experimentally by use of an iterative solution of the lidar equation. Key words: Aerosols, optical depth, backscatter-to-extinction ratio, Angstrom coefficient, lidar signal inversion.

A methodology for the absolute calibration of spectral measurements of direct solar ultraviolet radiation, performed with a Brewer spectrophotometer is presented. The method uses absolute measurements of global and diffuse solar irradiance obtained practically simultaneously at each wavelength with the direct-Sun component. On the basis of this calibration, direct-Sun spectra, measured over a wide range of solar zenith angles at a high altitude site, were used to determine the extraterrestrial solar spectrum by applying the Langley extrapolation method. Finally this spectrum is compared with a solar spectrum derived from the airborne tunable laser absorption spectrometer 3 Space Shuttle mission, showing an agreement of better than [+ or -]3%. Key words: Solar ultraviolet, direct-Sun spectra, extraterrestrial spectrum.