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.
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.
The stratospheric ozone depletion during the last two decades, the increase of UV-B irradiance levels at the ground and the possible impact on the biosphere has led scientists to develop and use instruments of high accuracy for UV measurements.
During the last two years, 9 UV stations have been established in different environments in Greece and Cyprus, with the aim to establish a long-term monitoring network. The instruments of the network (NILU-UV multichannel filter radiometers) can provide measurements of irradiance in the UV and the visible part of the solar spectrum.
In this study, first results from the calibration measurements and the quality assurance procedures are presented. The stability of the maximum of spectral response and the full width at half maximum was measured within 0.5 nm. Lamp tests were performed and downward drifts up to 40% in UVA channel sensitivity were observed. Calibration factors derived from lamp measurements could provide measurements of UV dose rate and total ozone with quite good agreement when compared with standard ultraviolet instruments.
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.
The objectives of the COST action 726 are to establish long-term changes of UV-radiation in the past, which can only be derived by modelling with good and available proxy data. To find the best available models and input data, 16 models have been tested by modelling daily doses for two years of data measured at four stations distributed over Europe. The modelled data have been compared with the measured data, using different statistical methods. Models that use Cloud Modification Factors for the UV spectral range, derived from co-located measured global irradiance, give the best results.
A sensitivity analysis is performed for the expected changes to the photodissociation rates of various key species that are relative to tropospheric chemistry, due to changes in the radiative transfer, caused by contrails and clouds generated from the use of hydrogen as fuel. The results are compared to those which correspond to contrails from conventional aircraft. For the determination of cirrus clouds as a model input the cirrus clouds climatology developed in the frame of the ISCPP has been used. The optical parameters of the contrails generated by conventional fuel and hydrogen fuel are adopted from the recent literature. The mean total ozone field, which corresponds to present status and has been used as an input is based on both TOMS and ground-based data, while for the future projections the IPCC 1999 scenario has been used. The Tropospheric Ultraviolet and Visible (TUV) radiative transfer model has been used, to calculate the actinic flux in the UV and visible part of the spectrum, at 3-km height steps in the troposphere. For this purpose, the model has been initially tested against spectral measurements, and an agreement of +/- 10% has been achieved when the appropriate input parameters are well documented. The different optical properties of the aerosols and clouds have been considered in the model calculations. For the calculation of the photodissociation rates the latest available chemicokinetical data have been incorporated into the model. It was found that the additional perturbations induced from contrails are larger for the kerosene contrails compared to the ones induced by LH2 contrails, since they are expected to have smaller optical depths.
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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