Stelios Kazadzis

Correcting global solar ultraviolet spectra recorded by a brewer spectroradiometer for its angular response error.

We present a methodology for correcting the global UV spectral measurements of a Brewer MKIII spectroradiometer for the error introduced by the deviation of the angular response of the instrument from the ideal response. This methodology is applicable also to other Brewer spectroradiometers that are currently in operation. The various stages of the methodology are described in detail, together with the uncertainties involved in each stage. Finally global spectral UV measurements with and without the application of the correction are compared with collocated measurements of another spectroradiometer and with model calculations, demonstrating the efficiency of the method. Depending on wavelength and on the aerosol loading, the cosine correction factors range from 2% to 7%. The uncertainties involved in the calculation of these correction factors were found to be relatively small, ranging from ~0.2% to ~2%.

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements

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 in 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.

Quality assurance of spectral solar UV measurements: results from 25?UV monitoring sites in Europe, 2002 to 2004

With the transportable reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) routine quality assurance of spectrally resolved solar ultraviolet irradiance measurements were successfully performed at 25 UV monitoring sites in Europe. The absolute scale carried by the QASUME reference spectroradiometer is traceable to the primary irradiance standard of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, and has proved to represent the average scale in use at 25 independent European laboratories; it can thus be taken as a European irradiance reference. Out of the 27 instruments 13 showed deviations relative to the QASUME reference spectroradiometer of less than 4% in the UVB (15 instruments in the UVA) for solar zenith angles below 75 ◦ . The results so far have shown the unique possibilities offered by this transportable reference spectroradiometer for providing on-site quality assurance of solar ultraviolet irradiance measurements. (Some figures in this article are in colour only in the electronic version)

Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval.

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 +/- 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 +/- 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 approximately 0.3 at 340 nm in winter and approximately 0.7 in summer.

Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse C. S. Zerefos, E. Gerasopoulos, I. Tsagouri, B. Psiloglou, A. Belehaki, T. Herekakis, A. Bais, S. Kazadzis, C. Eleftheratos, N. Kalivitis, et al.

Comparisons of Spore Dosimetry and Spectral Photometry of Solar-UV Radiation at Four Sites in Japan and Europe¶

Abstract In order to develop monitoring and assessment systems of biologically effective doses of solar-UV radiation, concurrent measurements of spectral photometry and spore dosimetry were conducted in summer months at four sites in Japan and Europe. Effectiveness spectra were derived by multiplying spectral irradiance in 0.5 nm steps between 290 and 400 nm with the inactivation efficiency of the spores determined using monochromatic radiation of fine wavelength resolution. Shapes of the effectiveness spectra were very similar at the four sites exhibiting major peaks at 303.5, 305.0, 307.5 and 311.0 nm. The dose rates for spore inactivation from direct survival measurements and from calculations by the integration of the effectiveness spectra were compared for 174 data points. The ratios (observed/calculated) of the two values were concordant with a mean of 1.26 (±0.24 standard deviation [SD]). The possible causes for the variations and slightly larger observed values are discussed.

Solar dimming and brightening over Thessaloniki, Greece, and Beijing, China

This work presents evidence that ultraviolet (UV)-A solar irradiances show increasing trends at Thessaloniki, Greece, where air quality has been improving because of air pollution abatement strategies. In contrast, over Beijing, China, where air quality measures were taken later, solar brightening was delayed. It is shown that until the early 1990s, UV-A irradiances over Thessaloniki show a downward trend of –0.5% yr−1, which reverses sign and becomes positive in the last decade (+0.8% yr−1). This brightening is related to a decreasing trend in local aerosol amounts. Both the negative rate of change (dimming) and the positive rate of change (brightening) are amplified in the UV-A solar irradiances, compared with the total solar irradiance, by a factor of 2.6. Satellite derived short-wave radiation over Beijing showed negative changes of –0.4% (1984–1991) and –0.1% yr−1 during 1994–2006. The negative trend in solar radiation continued even during 2000–2006. Satellite-derived aerosol optical depth (AOD) increased by +1.0% yr−1 during 2000–2006, in agreement with in situ measurements of increasing AOD. Therefore, a statistically significant change from dimming to brightening in Beijing could not be seen in the last decade, but it is expected to occur in the near future.

Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradiance

In the present study, a method is proposed for retrieving UV actinic flux density spectra at ground level from global and direct UV spectral irradiance measurements. The most important parameters to establish the relation between downwelling actinic flux density and global UV irradiance are the ratio of direct to global irradiance and the description of the angular distribution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within ±5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength-dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in atmospheres with high aerosol content. Various sources of uncertainties, which are involved in the proposed methodology, are discussed and quantified.

Comparison of Model Calculations with Spectral UV Measurements during the SUSPEN Campaign: The Effect of Aerosols

Spectral measurements of global solar irradiance, obtained under cloud-free conditions during the SUSPEN campaign (July 1997) in Thessaloniki, Greece, are compared with radiative transfer model calculations, showing an agreement to within 65% for wavelengths higher that 305 nm. The uncertainties in the modeled spectra were analyzed with respect to the aerosol-related model input parameters (single-scattering albedo and asymmetry factor), which were not derivable from measurements. A range of single-scattering albedo values was used to investigate its impact on surface UV irradiance through comparison of measurements with model calculations. It was found that a difference in the single-scattering albedo of 0.1 changes the model‐measurement ratio by 7%‐14%, depending on solar zenith angle. Finally, an attempt was made to relate the estimated values of singlescattering albedo to wind direction and relative humidity, which control the origin and type of the aerosols in the area.

Charge-coupled device spectrograph for direct solar irradiance and sky radiance measurements

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% +/- 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 +/- 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.