Lately a number of studies related with UV irradiance estimates from satellite data based on the Ozone Monitoring
Instrument (OMI) have shown a high correlation with ground-based measurements but a positive bias in many locations,
the satellite derived UV being higher. One of the key factors that this bias has been attributed to is the boundary layer
aerosol absorption not taken into account in the current OMI UV algorithm. In this work we have used a correction
procedure based on climatological global aerosol absorption data taken from AeroComm aerosol initiative. This dataset
includes aerosol optical depth and aerosol single scattering albedo assembled by combining, ground-based aerosol
measurements from AERONET and information from several global aerosol models. The results of this correction were
compared with synchronous ground-based measurements from 9 UV monitoring stations. The results generally showed a
significantly reduced bias of 7-20%, a lower variability, and an unchanged, high correlation coefficient.
Modern polymeric materials possess an ever increasing potential in a large variety of outdoor objects and structures
offering an alternative for many traditional materials. In outdoor applications, however, polymers are subject to a
phenomenon called weathering. This is primarily observed as unwanted property changes: yellowing or fading, chalking,
blistering, and even severe erosion of the material surface. One of the major weathering factors is UV radiation.
In spring 2005, the Finnish Meteorological Institute with its research and industrial partners launched a five-year
material research project named UVEMA (UV radiation Effects on MAterials). Within the framework of the project, a
weathering network of seven European sites was established. The network extends from the Canary Islands of Spain
(latitude 28.5°N) to the Lapland of Finland (latitude 67.4°N), covering a wide range of UV radiation conditions. Since
autumn 2005, the sites of the network have been maintaining weathering platforms of specimens of different kinds of
polymeric materials. At the same time, the sites have been maintaining their long-term monitoring programmes for
spectrally resolved UV radiation. Within UVEMA, these data are used for explaining the differences between the
degradation rates of the materials at each site and for correlating the UV conditions in accelerated ageing tests to those
under the Sun.
We will present the objectives of the UVEMA project aiming at deeper understanding of the ageing of polymers and
more reliable assessments for their service life time. Methodologies adopted within the project and the first results of the
project will be summarized.
Monitoring of the terrestrial solar ultraviolet irradiance by using a radiometer is often considered as expensive
and laborious or the data collected as insufficient in spatial coverage and in some cases in its temporal
resolution, too. Therefore, alternative methods, all relying on modelling in one way or the other, have been
developed. They differ in which input they receive, either standard meteorological information, space-based
radiance measurements or ground-based irradiances from broadband or multiband UV radiometer or from
pyranometer. A comparison of performance is presented between three methods during a 15-month period.
The ground reference instrument is the Brewer Mk-III #107 spectroradiometer of the Observatory of
Jokioinen, Finland. Compared to the reference, the space-based method overestimates the UV irradiance at
noon by 14.6% and the pyranometer-based by 0.9% with root-mean-square differences of 35.5% and 10.4%,
respectively. Daily erythemal doses agree by 3.8% for the space-based and 0.4% for the pyranometer-based
method with a scatter of 16.5% and 4.6%, respectively. Spectral irradiances generated by the pyranometerbased
model agree within 0.4% on average with a standard deviation of 17%. A rough estimate on the cost of
each approach suggests that none of them is clearly superior to the others and the actual nature of the data
needed may be used in decision making concerning monitoring strategies.
An Antarctic UV-monitoring network established in 1999 as a Spanish-Finnish-Argentinian co-operation consists of multiband filter radiometers located at Belgrano, Marambio, and Ushuaia. To provide with quality controlled and assured calibrated groundbased Antarctic UV data, bi-weekly lamp tests were used on every site and visits of travelling reference instruments on two of the sites. Along the six years of operation, the sensitivity in some of the instrument channels was found to drift up to 61%. In both stations, always the same channels showed the best stability or worst instability. The rigorous quality assurance programme ensured that reliable time series of solar data could be produced, however. The most recent Antarctic ozone depletion period of 2005/2006 was studied by comparing OMI satellite-based erythemally weighted daily doses with the measured polynomial corrected data for August 2005-March 2006 for Ushuaia and Marambio. The root mean square (RMS) of difference between the groundbased and satellite-retrieved daily doses was on monthly basis smaller for Ushuaia (19 - 28 %) than for Marambio (17-58 %), possibly due to e.g. bigger heterogeneity of the ground albedo, and variability of the cloudiness. Our final task of combining the polynomial corrected lamp calibration factors and the traveling reference calibration factors, to produce the final calibrated Antartic UV data, is discussed, too.
Solar spectral UV-monitoring data for 8 European sites with 5-10 years of data, and covering a latitudinal range from 41 degrees North to 67 North have been re-evaluated and resubmitted to the European UV-database (EUVDB) in Finland as part of the EU-project SCOUT-O3. All resubmitted spectra (420000) were quality checked, flagged, and corrected with respect to wavelength scale errors and spectral distortions using the SHICrivm software package. Additional data products provided by the software are standardized spectra, spectral atmospheric transmissions, and biologically weighted UV-irradiances for a wide variety of biological action spectra. The resubmitted spectra were shown to have improved based on the EUVDB quality flagging criteria. Spectral and effective irradiances were integrated and summed in a standardized way to obtain daily, monthly, and seasonal UV-doses. The measured summer sums varies from 478 kJ/m2 for Thessaloniki to 228 kJ/m2 for Sodankyla. Clouds reduced the exposure during summer time by 30% on average, in Bilthoven this was 35%, while in Thessaloniki only 17% was reduced. Using co-located ozone and pyranometer measurements results of a generic UV-modelling approach, derived in a specific low albedo and low surface elevation environment, are systematically compared to the UV-doses obtained for all sites. Generally, a good agreement is found, measured and modelled total UV-doses agree within a few percent with a standard deviation of 15 typically. Deviations with respect to the application in a high surface albedo and high altitude environment have been identified and handles to improve the modelling have been assigned.
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.
Since October 1995 a global daily forecast of the UV index and the daily dose, as the irradiance of the biologically effective ultraviolet radiation, for clear sky is calculated. The Austrian model as well as the input parameters are described. By connecting the daily dose with the sensitivity of the photobiological skin types, a recommendation is given to select an appropriate sun protection factor of a sunscreen to avoid overexposure of the skin. The validation of the Austrian forecast model is done by long-term measurements of the biologically effective ultraviolet radiation. Measurements were taken from 6 different sites at 4 continents (Antarctica, Australia, America and Europe) covering the latitudinal range from 67 degree(s)N to 60 degree(s)S. By using the underestimation as criteria in the sense of radiation protection, the Austrian model shows less than 12% underestimation over the whole period for the UV index and less than 10% for the daily dose. The evaluation shows further that the forecast of the daily dose is much more influenced by the attenuation due to clouds than the UV index.
Solar calibrations and laboratory tests were performed for erythemally weighted Robertson- Berger type UV radiometers. The test instruments included four Solar Light Model 500 and two Model 501 meters. The solar calibrations were performed at different elevation angles with relatively clear skies. The absolutely calibrated Optronic 742 spectroradiometer was used as a reference instrument and the measurement results were normalized to the temperature of 25 degree(s)C. The uncertainty of the spectroradiometric measurements is estimated to be +/- 8%. The absolute average calibration factors obtained varied from 1.05 to 1.16 for Model 500 meters and from 0.93 to 0.98 for two Model 501 meters. The spectral responsivity and cosine responses were found to be satisfactory for both meter types. To obtain better knowledge of the thermal properties of the Solar Light Model 500 meters a temperature sensor was installed at the underside of the green filter located just below the phosphor layer. The obtained temperature coefficient based on the green filter temperature measurements was 0.80%/ degree(s)C. The overall uncertainty of Solar Light Model 501 measurements is estimated to be +/- 11% and according to the phosphor temperature corrected SL 500 measurements +/- 14%. Without temperature correction the uncertainty of Model 500 measurements increases up to +/- 19%.
Exceptionally low total ozone up to 40% below the normal level has been measured over Northern Europe during winter and spring in 1992 and 1993. The increase in the UV exposure of the Finnish population associated with the combined effects of ozone depletion and snow reflection was examined in this study with the aid of broadband measurements and theoretical calculations. The theoretical calculations were verified with spectral and broadband measurements. The calculations show that the annual horizontal doses in Helsinki (60.2 degree(s)N, 25 degree(s)E) are about 30% higher than in Saariselka (68.4 degree(s)N, 27.5 degree(s)E), but the difference is only 12% for vertical doses owing to the stronger contribution to vertical (facial) surfaces of the reflection of UV from snow. In Saariselka, the maximum vertical irradiance at the end of April approaches the midsummer values. The ozone depletions had no significant effect on the biologically effective UV in 1992 since the total ozone returned to normal at the end of March before the UV increased to biologically significant level. In contrast, in 1993 low ozone levels were measured still at least up to mid May resulting in an average theoretical increase of 8% during a period from 14 April until 22 May in biologically effective UV.
Spectral ultraviolet measurements in the wavelength range 290 to 325 nm made at Sodankyla (67.4 degree(s)N, 26.6 degree(s)E) in 1990 - 1992 were used to study the effects of surface albedo, atmospheric total ozone, and atmospheric aerosol content on UV irradiance on the surface of the Earth. Clear sky observations with optical air masses 1.4 and 2.0 were used in the analysis. The annual course of surface albedo between 0.1 and 0.8 was found to be responsible for a change of 10% in irradiance at 325 nm, but no effect was seen at lower wavelengths or in CIE-weighted integrated values.
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