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We compare three different methods for determining an average ‘effective‧ UV albedo. These methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. The first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow-covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present the differences and the correlations between the various methods as well as the results for the different locations.

Seven spectroradiorneters measured simultaneous surface UV irradiances at six different sites in the vicinity of Garmisch-Partenkirchen, Germany, during spring 1999. The measured clear-sky irradiance variability between the sites was analyzed with respect to altitude, aerosol optical depth, solar zenith angle, effective albedo, and tropospheric ozone. For conditions of low aerosol loading the increase of irradiance per 1000 m altitude difference was 9% at 400 nm. 20% at 320 nm, and 30% at 300 nm in this season. Effective albedo differences of 0.15, 0.29, and 0.65 were found between the snow-covered stations and the snow-free ground station with the higher effective albedo values determined at the two mountain stations. Clean continental aerosols with a single-scatter albedo of 0.95 were observed during this campaign. The measurements and the observed variations between the sites should enable more accurate modeling studies to be performed for an Alpine environment.

There exists in the literature many methods of determining the Angstrom parameters for the measurement of the optical thickness of aerosols. This thickness is required in cases such as when the spectral beam solar irradiance at ground level has to be assessed. The present work studies the differences between some of these methods and establishes the relationships between their results. A systematic study investigating 509 direct solar irradiance spectra was undertaken. As a result, a new wavelength pair for filter measurements is proposed to calculate the Angstrom parameters with the Voltz technie, namely, the pair (0.400 μm, 0.750 μm). This new pair is the one which best describes the solar beam attenuation in the atmospheric transmittance models. For the considered atmospheric conditions (continental urban aerosols), the mean value of the Angstrom parameter α is found to be 1.0±0.3, which is relatively close to the conventional α=1.3 if taking into account the variance. The wavelength range explored is from 0.290 μm to 0.900 μm, which concerns many applications in bioscience.

A set of 509 direct solar irradiance spectra, carefully measured over one year, is checked against spectral irradiances computed from five updated transmittance models. The wavelengths under investigation range from 290 to 900 nm, with a 5- or 10-nm step. The parameters explored include the solar altitude angle, with a range from 13° to 68°, and the standard Linke turbidity factor, with a range from 2.0 to 6.0. Measurement devices and experimental processes are described in detail in the paper. The comparison between measured and computed values is carried out by means of the relative mean bias error and the mean absolute relative error. These coefficients are applied to ultraviolet-B and ultraviolet-A total irradiances, and to visible and near-IR spectral irradiances. No clear and systematic sensitivity of the models or measurements to the solar altitude and the turbidity parameters is observed. Of the five models tested, three of them give mean coefficient values between 7% and 16% in UV bands ...

The concept of standardized Linke's turbidity factor TLAM2 allows the conversion of the usual TL values, which exhibit virtual diurnal variations into new ones, which are strictly representative of the atmospheric turbidity caused by both water vapor and aerosol. The TLAM2 are TL values reduced at relative air mass 2 on the basis of a spectral TL model recently developed. The TLAM2 values are used to analyze the variations of atmospheric turbidity in a steep-sided urban alpine valley over a 3-year period. It appears that the dependence of the turbidity on altitude is related to the annual cycle of high and low turbidity periods. The TLAM2 concept is required to detect the instantaneous changes of atmospheric turbidity during the course of a given day, but it can also reveal the general trends in diurnal variations of turbidity, in relation to the season and to the altitude.

In spectral diffuse solar irradiance measurements, when diffusing devices used are neither perfectly Lambertian nor have an ideal cosine response, significant errors may spoil the collected data. An optical method permits the determination of a spectral correction factor (SCF) which fully compensates for the diffusers' imperfection when the sky radiance is isotropic. A study of the errors introduced when using such a isotropic SCF in anisotropic radiance conditions is presented for two common flat-plate diffusers fitted with a shadow-ring or a tracking disk. The wavelength band explored is 0.29–0.90 μm and the clear sky radiance model used is Kittler's. The relevance of several diffuse irradiance spectra measured on clear sky days with isotropic SCF is analysed by comparison with Brine-Iqbal model spectra and total diffuse measurements. A remarkable coherence is found for small solar zenith angles when using a tracking disk and a diffuser with an isotropic SCF smaller than 1.2.

A model of Linke's turbidity factor, TL, is developed by means of updated spectral extraterrestrial irradiances and extinction coefficients of gaseous absorbers. It is shown that the new values of TL are clearly different from those obtained by Kasten's formula which parameterizes the optical thickness of the clean dry atmosphere. The model is used to investigate the dependence of TL on the relative optical air mass and to elucidate the relationships linking TL to Angstrom's turbidity coefficient and to the water vapor content. For any TL, the corresponding value related to the air mass 2.0 can be determined. Such a standardized value is independent of solar elevation and is therefore strictly representative of the atmospheric turbidity. It can be linked to Angstrom's turbidity coefficient. Practical procedures and algorithms for computing the standard Linke's turbidity factor and determining Angstrom's turbidity coefficient are described. A relationship for converting the TL values obtained by Kasten's formula into the new values is proposed.

Simultaneous measurements of horizontal global solar irradiance were done over three years, at altitudes ranging from 200 m to 1,700 m in an industrial alpine valley, for studying the clearness index variations versus height. Estimated values of the Angstrom coefficient suggested that the main cause of depletion of the clear sky clearness index k c at the lower levels was the scattering by the aerosols. For irradiances, a depletion term D = 1 − k c was empirically assumed to be a function of the altitude z , as simple as D = ( Mz + N ) −1 . A study carried out over 8,000 10-minute sequences of measurement, during clear sky periods, showed that the coefficient M was sensible to the season only, while N was mainly a function of the sun elevation h such as N = A ( h − C ) 0.5 + B ; from 240 m to 1,100 m, k c was found increasing by about 12% for h = 30° and 7% for h = 60°. The yearly global horizontal irradiation, although including many cloudy sequences, appeared linearly related to ln( z ), its relative increase between 200 m and 1,700 m rising 10%; whereas, for shorter periods, the monotony of the vertical profiles of the clearness index for irradiation was often significantly affected by local stationary clouds or seasonal mists.

As a result of sky radiance anisotropy, significant errors spoil diffuse solar irradiance data obtained by means of noncosine-true instruments, which happens in spite of knowledge of the relative cosine response. To reduce these errors we recommend a method that involves an additional measurement of diffuse irradiance from a horizon band. The efficiency of the so-called double-measurement method is found to be optimum for a 25° horizon band elevation. We confirm the feasibility of the method with relevance tests that use diffuse solar spectral measurements carried out on cloudless skies by means of an UV spectroradiometer.

Routine measurements of global and diffuse UV irradiances at Briançon station (1310 m a.s.l.) are used to retrieve the direct solar irradiance and the aerosol optical depth (AOD), for cloudless days. Data of three years (2003, 2004, 2005) are analyzed; the results confirm those of a preliminary analysis for 2001, 2002. The atmosphere is very clear in winter, with AODs between 0.05 and 0.1. The turbidity increases slowly in spring, starting end of February, with AODs around 0.2–0.3 in mid summer, some values reaching 0.4. A similar behaviour is observed for all years, with somewhat higher values in late summer for the year 2003.

Ultraviolet spectral irradiances are systematically recorded in Briancon (at an altitude of 1310 m), in the French Southern Alps. Up to now, aerosols optical depth (AOD) had never been routinely measured in Briancon, so it has appeared of great interest to take advantage of the many spectra recorded to learn more about aerosols over the spectral range: 320 nm - 448 nm. To retrieve AOD, one only has to remove the contribution of ozone and molecule from the total optical depth, obtained from the direct irradiance. The method has been implemented for every spectra acquired during clear sky days selected among the period from July 2004 to March 2005. Data recorded show that at a given wavelength, AOD is roughly constant during the day. Lowest values for AOD at 12:00 U. T. are found to be around 0.05 at 440 nm and 0.15 at 320 nm, and may increase threefold for some days. AOD range between 320 and 448 nm is larger in summer than in winter. This agrees with the way α, the Angstrom parameter, varies from one season to the other. Unfortunately, one can only make a restricted use of α, as its determination over the ultraviolet range is subject to high AOD uncertainties. The main source of AOD error rests in the 10% of uncertainty on direct irradiance measured. This gives rises to relatively high uncertainties on AOD, which increase all the more when aerosols load in the atmosphere is low. In Briancon, renowned for its air goodness, true AOD values are likely to be reached within 45% - 50% as the best in a majority of cases.

Received 18 March 2005; revised 25 July 2005; accepted 26 September 2005; published 17 December 2005. (1) A differential absorption method, developed by the Laboratoire d'Optique Atmospherique (LOA), is applied to retrieve the total ozone column from UV global irradiance spectra under clear and cloudy sky conditions. Contrarily to clear sky, cloudy sky generates a high uncertainty in the retrieved ozone column. This study shows that under cloudy conditions the daily ozone mean is a rather good estimation of the true value. The standard deviation allows us to estimate the relative uncertainty of this mean value, i.e., about 7%. Results for all conditions from 3 years at Villeneuve d'Ascq are presented, as well as a comparison with TOMS (Total Ozone Mapping Spectrometer) data. The results are very similar to those obtained for clear days: In 75% of cases both values agree to within 5%; however, some cases present larger relative differences. Over the 3 years considered, there is a bias less than 3% (LOA > TOMS). This method is applied to five other European sites involved in the EDUCE (European Database for UV Climatology and Evaluation) project. Three of them are Brewer stations. The results of the comparison between the LOA-retrieved ozone and TOMS data show that a relative difference smaller than 5% is obtained in 63-80% of the cases, depending on site and year. Biases smaller than 3% are observed (LOA > TOMS). The comparison between the LOA-retrieved ozone and total ozone data from direct sun and zenith sky observations of the Brewer instruments shows better agreement. For more than about 80% of cases, except at one site, the relative difference is smaller than 5%, and the biases are smaller than 1%. The LOA method allows us to obtain a complementary data set and hence to provide time series of reliable measurements of total ozone column under all sky conditions.

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Briancon; the data of years 2001 and 2002 are analyzed in this paper. Comparison with modeled values is used for cloud screening, and an average UV-A aerosol optical depth is used as an index of turbidity; it is found to be around 0.05 for the clear winter days and around 0.2 in summer. Langley plots are used to verify the instrument calibration; they confirm the expected uncertainty smaller than 5%. The ozone total column amount is estimated with an uncertainty between -3 and +5 Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.

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%.

Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.

An investigation is made of the dark-current conduction processes in Cu2S–CdS single crystal heterojunctions. The reverse currents are multistep tunneling currents as in thin film Cu2S–CdS photocells, but the number of required steps is smaller. For the forward currents, thermal currents over the barrier are dominant above 300 K, and tunneling currents appear only at lower temperatures. It follows that crystallographic defects enhance the tunneling currents, and that high efficiency solar photocells have to be made with recrystallized CdS layers. A band profile is given, and a photovoltaic mechanism is proposed for the Cu2S–CdS heterojunctions. L'etude des courants d'obscurite dans des heterojonctions Cu2S–CdS monocristal, montre que les courants inverses proviennent d'une conduction par effect tunnel assiste par des recombinaisons sur des niveaux localises dans la bande interdite du CdS. Pour les courants directs, il y a competition entre les courants thermiques conventionnels qui sont predominants au dessus de 300 K et les courants tunnels qui apparaissent a basse temperature. Il s'ensuit que les defauts cristallographiques propres aux couches minces polycristallines favorisent les courants tunnels et que des photopiles solaires performantes doivent ětre faites a partir de couches de CdS recristallisees. Un modele est proprose pour la structure de bande de la jonction et le fonctionnement des photopiles au Cu2S–CdS.

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