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.
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.
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.
This study describes the atmospheric aerosol load encountered over a number of sites from the Southern Balkan region
with a relatively well-known air quality factor. Using the aerosol optical depth AOD, retrieved from the two Moderate
Resolution Imaging Spectroradiometers, MODIS, on board the Terra and Aqua NASA satellites, the aerosol content of
numerous sites is investigated under the scope of local pollution sources, inter-regional transport and large scale dust
and/or biomass burning events. The wide time range of seven full years of MODIS/Terra measurements permits the
discussion of possible climatological aspects as well. The MODIS AOD is further validated using ground-based Brewer
spectrophotometer measurements over a metropolis of Northern Greece, Thessaloniki. Thessaloniki is situated in a
unique sea-side location which inflicts it with high humidity and sea-salt particles, and is furthermore frequently affected
by biomass burning and desert dust aerosols arriving from surrounding sources. Local and regional pollution further
influences the quality of the local air and the observed tropospheric optical depth. The air masses responsible for either
transporting polluted air into the free troposphere or circulating boundary layer aerosol load around the region of Norther
Greece have been identified and discussed in detail.
Measurements performed with a backscatter and Raman lidar at Thessaloniki, Greece were used to characterize cirrus
clouds and aerosol layers by determining their optical properties. This is achieved through the application of different
post-processing algorithms. We retrieved the cirrus cloud's optical properties by using three independent mathematical
methods. In the first method, an iterative procedure was used based on the criterion that forward and backward
integration coincide to the desired degree of accuracy. In the second method, the optical depth of the cirrus cloud can be
determined by comparing the backscattering signals just bellow and above the cloud if the lidar signals are correctly
represent the scattering medium. The third method, the well known Raman method, is applicable to night time
measurements and is capable for determining the vertical profile of lidar ratio. The results are considerably influenced by
multiple scattering effects, that not taken into account and this leads to a significant underestimation of calculated cirrus
optical depths and lidar ratios. To estimate and correct this effect we have applied a radiative transfer model that
calculates the multiple scattering contributions for each cirrus case analyzed. We have compared the resulting optical
depths and lidar ratios and found a good agreement between these methods. The comparison has been performed to the
effective values of optical depth and lidar ratio.
The temporal variability of global ultraviolet solar spectral irradiance measured regularly at Thessaloniki, Greece during the last 15 years is presented. The measurements were conducted by a single- and a double-monochromator Brewer spectroradiometers which operate at the Laboratory of Atmospheric Physics since 1989 and 1993, respectively. Recently the entire series of measurements was re-evaluated and quality controlled, by revising the calibration history of the two instruments and by comparing these measurements with those obtained by a collocated erythemal radiometer and a pyranometer. In addition, the spectral measurements were corrected for the angular response error of each instrument and for the effect of temperature variations.
The longest of the re-evaluated series, which was obtained by the single monochromator, was statistically analyzed to derive estimates of the long-term changes and variability of UV irradiance radiation. Daily integrals were derived with the aid of broadband measurements which were used to simulate the diurnal variation of the spectral irradiance at one minute increments. The effect of clouds and solar zenith angle on the log term variability of UV irradiance are also investigated. Finally, signals of inter-annual natural variations and oscillations on this data set are explored and removed in an attempt to attribute the observed variability to different factors or mechanisms and investigate their effects on the long term changes of UV irradiance at the ground. All long term changes that were calculated have positive signs and vary according to wavelength solar zenith angle and the period of data. Monthly erythemal irradiance increases in the 1990's by about 6%, possibly as a result of reduction of clouds and aerosols.
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.
KEYWORDS: Ultraviolet radiation, Clouds, Ozone, Aerosols, Atmospheric modeling, Electronic filtering, Solar radiation models, Solar radiation, Data modeling, MODIS
The decrease in stratospheric ozone observed during last decades and the possible relationship with the observed increase UV-B radiation in biosphere has been extensively discussed in relevant studies. However, the detection of UV trends is difficult due to lack of long-term measurements of UV radiation at the Earth's surface, and the large variability introduced by changes in cloudiness, aerosols and surface albedo.
Recently, several methods for reconstruction of UV radiation levels for the past at single sites have been proposed. In most of these studies, measurements of total ozone and empirical or model derived relations for the impact of clouds and surface albedo on UV transmittance have been used. In this study, a method for estimating erythemal (CIE) UV doses using measurements of total ozone and total solar radiation is presented for Thessaloniki, Greece (40.5°N, 23°E). Measurements of total solar radiation and UV erythemal dose rates for five years period (2000-2004) were used to estimate the effect of clouds and aerosols as a function of solar zenith angle (SZA). The method is then tested, when compared with measurements from previous years. The mean difference (%) between measured and modeled cloud modification factors for UV dose rates was calculated between -1.3% and 2.4% for all SZA groups.
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.
TOMS UV algorithm is capable of taking into account the scattering aerosols via its scene reflectivity. It also accounts for absorbing aerosols in free troposphere (dust and smoke plumes) via aerosol index correction. On the other hand, the effects of aerosol absorption in the boundary layer are not properly taken into account, because they do not appear as absorbing aerosols in the TOMS AI data (positive AI). This additional error has been claimed to be the reason for the observed positive bias between TOMS derived UV and ground-based measurements. We compared TOMS overpass irradiances against the Brewer measurements in NASA/GSFC site in USA and Thessaloniki, Greece with the main objective of evaluating the effect of absorbing aerosols with the measurements of aerosol optical properties. We found that the bias between TOMS UV and ground-based data depends on the aerosol absorption. In other words, the bias was increasing with the increasing aerosol absorption, τabs. A simple correction to account for this effect is proposed, assuming that the climatology of τabs is known.
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 our major concern on the stratospheric ozone depletion is possible adverse effects on the biosphere, it is important to establish the way to determine biologically effective doses of solar UV radiation. The spore dosimetry system measuring the lethality of dry bacterial spores on membrane filters has been developed to meet this purpose. The methodology to evaluate experimental correlation with spectral measurements based on the effectiveness calculation has been applied in several field comparisons carried out at Nea Michaniona (Greece), Brussels (Belgium), and Sao Martinho (Brazil). When plotted against UVB irradiance (total energy below 320 nm), the calculated values of MED (minimal erythema dose), SID (spore inactivation dose) and DND (DNA damage dose) exhibited increasing exponents in power regressions, while the exponents from spore dosimetry exceeded those of the calculated values. The results of calculated versus observed values of SID indicate a general convergence at low to modest dose rates, but at high dose rates the calculated ones tended to yield lower values than those obtained from direct biological measurements.
Spectral measurements of solar global and direct UV irradiance are routinely conducted at Thessaloniki, Greece with a double monochromator Brewer spectroradiometer. From absolutely calibrated direct and global irradiance spectra, aerosol optical depth and diffuse irradiance spectra between 300 nm and 365 nm are derived. The modification of the global and diffuse irradiance and of the relationship between direct and diffuse irradiance by the aerosol single scattering albedo (SSA) is investigated with respect to aerosol optical depth and solar zenith angle using radiative transfer model calculations. Model calculations were used also to investigate the effect of SSA on the radiance distribution in the ultraviolet. The model-derived relations are compared with measurements, with the aim to establish an indirect method of deriving an effective single scattering albedo from spectral measurements of the direct and global irradiance. The uncertainties introduced by different sources into the SSA estimates are discussed. Finally the effective SSA is determined for two days with different aerosol amounts and composition. The overall accuracy in determining indirectly the SSA depends strongly on the amount of aerosols varying between 0.1 and 0.2 units of SSA.
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