The most important aerosol properties for determining aerosol effect in the solar radiation reaching the earth's surface
are the aerosol extinction optical depth and the single scattering albedo (SSA). Most of the latest studies, dealing with
aerosol direct or indirect effects, are based on the analysis of aerosol optical depth in a regional or global scale, while
SSA is typically assumed based on theoretical assumptions and not direct measurements. Especially for the retrieval of
SSA in the UV wavelengths only limited work has been available in the literature.
In the frame of SCOUT-O3 project, the variability of the aerosol SSA in the UV and visible range was investigated
during an experimental campaign. The campaign took place in July 2006 at Thessaloniki, Greece, an urban environment
with high temporal aerosol variability. SSA values were calculated using measured aerosol optical depth, direct and
diffuse irradiance as input to radiative transfer models. The measurements were performed by co-located UV-MFRSR
and AERONET CIMEL filter radiometers, as well as by two spectroradiometers. In addition, vertical aerosol profile
measurements with LIDAR and in-situ information about the aerosol optical properties at ground level with a
nephelometer and an aethalometer were available.
The ground-based measurements revealed a strong diurnal cycle in the SSA measured in-situ at ground level (from 0.75
to 0.87 at 450nm), which could be related to the variability of the wind speed, the boundary layer height and the local
aerosol emissions. The reasons for SSA differences obtained by different techniques are analyzed for the first time to
provide recommendations for more accurate column SSA measurements.
Spectral measurements of direct solar ultraviolet irradiance are very important for many applications in the field of atmospheric sciences. Despite its usefulness, few UV monitoring sites include such measurements in their regular observational programs. Standardization of measurement methodologies and calibration techniques is required in order to reach the quality standard of global irradiance measurements. This study presents preliminary results from an intercomparison campaign of seven UV spectroradiometers of different types that took place at the high altitude site of Izana (28.3°N, 16.5°W, 2367 m above sea level), in Tenerife, Canary Islands in June 2005. The campaign is focused primarily on spectral measurements of direct solar irradiance. Among the objectives is to improve the quality of direct solar irradiance spectral measurements, through instrumental modifications and standardization of calibration techniques, as well as to assess the significance of the differences in the field of view of the spectroradiometers with respect to aerosols and to solar zenith angle. Under the low aerosol conditions prevailing during this campaign, we aimed to establish the differences among the various instruments under "ideal" conditions. Moreover, continuous measurements under stable total ozone and aerosol optical depth will be used to determine the extraterrestrial solar flux, through the application of the Langley extrapolation method. A first comparison of sky radiance measurements of the zenith light and of various directions on the sky show effects of sensitivity to polarization of one type of instruments and the variability of the provisional radiance calibration of 4 instruments.
Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.
On 29 March 2006, a total eclipse of the Sun was visible on the Greek island Kastelorizo (36.150°N, 29.596°E). An extended set of instruments was installed in order to measure the variability of different components of the radiation field during the eclipse. Seven spectroradiometers (two scanning double monochromators measuring especially in the UV range, 4 photo diode array instruments and one CCD-spectrograph for the UV and visible wavelength range) performed measurements during 28 and 29 March. A narrow band multi-filter radiometer and two broadband erythemal and UVA radiometers were operated with about 1 sec temporal resolution. Two sun-photometers were used to measure ozone column and aerosol optical depth. The weather conditions on March 28 were almost perfect, whereas on 29 March thin cirrus clouds were occasionally present in front of the sun. Details about the observed changes in the radiation field on the eclipse day are presented and compared with model calculations of the change in extraterrestrial solar irradiance. The results show an underestimation of the model calculations compared with measurements, with respect to the effect of the limb darkening on the spectral behavior of the solar irradiance during the eclipse. The absolute changes in the global and direct irradiance and their wavelength dependencies are discussed. Finally, the decrease in total ozone retrieved during the course of the eclipse from direct irradiance measurements is investigated with respect to the effect of the limb darkening and the influence of the diffuse radiation entering the field of view of the Brewer spectroradiometer.
Measurements of sky radiance have the potential to derive optical characteristics of tropospheric aerosols as aerosol optical depth, complex refraction index and aerosol size distribution. However, if the amount of aerosols is not very high, then the effect of polarization of diffuse sky radiance has to be considered, otherwise the derived aerosol parameters become very uncertain. The extension of these retrieval algorithms to the UV range provides additional information, but requires sophisticated radiative transfer models which account for polarization effects on molecular and aerosol scattering as well as for multiple scattering processes in the earth's atmosphere. Measurements of the degree of polarization in the UV and visible range under conditions with very low amounts of aerosols provide data for detailed model validation and for further model development. In 2004 measurements of polarization of diffuse sky radiance between 310 nm and 450 nm have been performed under cloudless conditions. Very low amounts of aerosols have been found in Lauder (New Zealand, 380 m above sea level). Scans of the sky in the vertical plane of the sun and in the horizontal plane through the sun with a 1.5° aperture were carried out with a polarization filter oriented at angles to the vertical between 0° and 135° every 45°. From these measurements, the Stokes-parameters for linear polarization are derived. The measurements at different solar zenith angles are compared with the results of radiative transfer modelling (libradtran), where no aerosols are considered. Reasonable agreement between measurements and modelling is found.
NILU has developed an accurate, reliable and robust filter instrument for measuring irradiances at ultraviolet (UV) and visible wavelengths. The NILU-UV instrument has been thoroughly tested through comparisons with well calibrated spectral radiometers over extended time periods with significant variations in ozone and cloud cover. The objective of this work is to present the instrument and to derive UV doses, total ozone abundances and cloud effects from the NILU-UV instrument, and compare the results with similar results from a double monochromator Bentham spectroradiometer and a Brewer ozone spectrophotometer.
QASUME is a European Commission funded project that aims to develop and test a transportable unit for providing quality assurance to UV spectroradiometric measurements conducted in Europe. The comparisons will be performed at the home sites of the instruments, thus avoiding the risk of transporting instruments to participate in intercomparison campaigns. Spectral measurements obtained at each of the stations will be compared, following detailed and objective comparison protocols, against collocated measurements performed by a thoroughly tested and validated travelling unit. The transportable unit comprises a spectroradiometer, its calibrator with a set of calibration lamps traceable to the sources of different Standards Laboratories, and devices for determining the slit function and the angular response of the local spectroradiometers. The unit will be transported by road to about 25 UV stations over a period of about two years. The spectroradiometer of the transportable unit is compared in an intercomparison campaign with six instruments to establish a relation, which would then be used as a reference for its calibration over the period of its regular operation at the European stations. Different weather patterns (from clear skies to heavy rain) were present during the campaign, allowing the performance of the spectroradiometers to be evaluated under unfavourable conditions (as may be experienced at home sites) as well as the more desirable dry conditions. Measurements in the laboratory revealed that the calibration standards of the spectroradiometers differ by up to 10%. The evaluation is completed through comparisons with the same six instruments at their homes sites.
Theoretical and experimental investigations of the effects of atmospheric aerosols on diffuse sky radiance in the visible and infrared range of the solar spectrum show that the retrieval of physical aerosol properties from intensity measurements of diffuse skylight leads to non-unique solutions for aerosol optical depth, complex refraction index and aerosol size distribution. Additional photopolarimetric radiance measurements have shown to add valuable information to intensity data, thus allowing a more specific determination of aerosol parameters. The extension of these retrieval algorithms to the UV range provides additional information, but requires the development of sophisticated radiative transfer models which account for polarization effects on molecular and aerosol scattering as well as for multiple scattering processes in the earth’s atmosphere. In order to provide a reference for these models, radiance measurements in the UV and visible range of the solar spectrum have been performed with a high resolution Bentham DTM300 double monochromator, equipped with a linear sheet polarizer. The measurements show strong differences between the directional distributions of horizontally and vertically polarized diffuse sky radiance of the upper hemisphere. Comparison between data taken at the Jungfraujoch (Switzerland, 3576 m a.s.l.) and in Thessaloniki (Greece, 20 m a.s.l.) under different atmospheric aerosol conditions reveals that aerosol Mie scattering effects horizontally and vertically polarized radiance in different ways, which confirms that photopolarimetric radiance data contain more information about aerosol properties than intensity measurements alone.
The vast majority of radiation measurements, including UV, refer to the radiation incident on a flat horizontal plate. However, this may not be the most appropriate way to specify radiation for bodies affected by UV, since they are rarely flat or horizontal. In particular the target molecules involved in atmospheric chemistry are approximately spherical and the actinic flux would be a better measure of the incident radiation. The ADMIRA project is addressing the issue of converting spectral UV irradiances to spectral actinic fluxes that can then be weighted with any required cross-section or action spectrum to give photolysis rates or biologically effective radiation incident on a sphere. The success with which this conversion can be made will depend on the prevailing atmospheric conditions and the knowledge of such at the time the irradiance measurements were made. Several different approaches to the conversion are being assessed, together with their associated uncertainties. These range from the simple empirical method to more complex radiative-transfer model based algorithms. Here we report on a coordinated campaign of simultaneous irradiance and actinic flux measurements supported by a wide range of ancillary measurements and their application to a simple empirical approach to converting irradiances to actinic fluxes.
Within the frame of the Austrian UV Monitoring Network, repeated recalibrations of Solar Light Sunburn Meters between December 1997 and March 2000 have shown significant temporal changes in the instruments' relative spectral response function as well as in their absolute calibration. Therefore, laboratory investigations of the effects of ambient temperature and internal relative humidity on the behavior of two Sunburn Meters have been performed. Despite internal temperature stabilization, both instruments show significant dependence of their spectral response function on ambient temperature. When the outside temperature of the detector's housing varies between 13 degree(s)C and 44 degree(s)C, spectral sensitivity changes by up to 10% in the UVB range and by up to a factor of 2 in the UVA range, depending on internal relative humidity. As a consequence, output voltage variations of 10% are observed when the detector is mounted in front of a 1000 W halogen lamp and its internal relative humidity is changed while its ambient temperature is kept constant. Whereas temperature effects take place within several hours, instabilities due to variations in internal relative humidity show typical time constants in the order of several days.
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.
Because of the very steep decrease of ozone absorption in the wavelength-range between 290 nm and 350 nm it is important to achieve a very high wavelength accuracy for high-resolution spectral measurements of solar UV-radiation. A wavelength error of 0.1 nm may result in an intensity error up to 10%. Therefore, a method is needed for determining the wavelength shift of measured solar UV-spectra in relation to a reference spectrum. By a comparison of the Fraunhofer structures of the measured spectrum and the extraterrestrial solar radiation, a wavelength correction with an accuracy of about 0.02 nm is possible.
Spectral measurements of solar ultraviolet irradiance carried out near Thessaloniki, Greece, are analyzed with respect to very strong variations of aerosol optical depth while the other influencing parameters are about constant. When aerosol optical depth is increased about twofold, global UV irradiance (from sun and sky) decreases less than 10%. In contrast, direct irradiance (from sun only) is reduced about twofold. If spectral measurements of global irradiance only are made, it is not possible to distinguish between situations with highly different aerosol contents. Measurements of both, global and direct irradiance, appreciate additional information to adapt radiation transfer models to actual atmospheric aerosol conditions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.