S. Kazadzis

Portable device for characterizing the angular response of UV spectroradiometers

This paper introduces a device that was developed to measure the angular response of UV spectroradiometers in the field. This device is designed to be used at the operating position of spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and - 13%, assuming isotropic distribution of the downwelling radiances.

Ozone and Spectroradiometric UV Changes in the Past 20 Years over High Latitudes

Abstract This study analyzes changes in solar ultraviolet (UV) irradiances at 305 and 325 nm at selected sites located at high latitudes of both hemispheres. Site selection was restricted to the availability of the most complete UV spectroradiometric datasets of the past twenty years (1990–2011). The results show that over northern high latitudes, between 55° and 70°N, UV irradiances at 305 nm decreased significantly by 3.9% per decade, whereas UV irradiance at 325 nm remained stable with no significant long-term change. Over southern high latitudes (55°–70°S), UV irradiances did not show any significant long-term changes at either 305 or 325 nm. Changes in solar UV irradiances are discussed in the context of long-term ozone and other atmospheric parameters affecting UV variability at ground level.

Variability of spectral UV irradiance at Thessaloniki, Greece, from 15 years of measurements

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 1990s by about 6%, possibly as a result of reduction of clouds and aerosols.

COMPARISONS OF SATELLITE DERIVED AEROSOL OPTICAL DEPTH OVER A VARIETY OF SITES IN THE SOUTHERN BALKAN REGION AS AN INDICATOR OF LOCAL AIR QUALITY

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.

Comparisons of spore dosimetry and spectral photometry for measurement of biologically effective doses of solar UV radiation

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.

Effects of aerosol optical depth and single scattering albedo on surface UV irradiance

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.

Estimation of the Solar Energy Potential in Greece Using Satellite and Ground-Based Observations

We report on estimation of the solar energy potential in Greece using satellite- and ground-based observations in conjunction with radiative transfer model (RTM) simulations. Pyranometers of the Hellenic Network for Solar Energy and the National Observatory of Athens provide accurate insolation measurements and are used to verify 1- and 2-day ahead forecasts provided by the mesoscale model MM5. The total energy potential for each ground station is found to range from 1.5 to 1.9 MWh/m2 with cloudiness causing increments in the forecast error of the order of 10 %. We also present a model for generating instantaneous and accurate gridded surface solar radiation spectra and budgets via a synergy of large (2.5 M record) RTM look-up tables and neural networks (NN). We demonstrate that NNs fed with cloud inputs retrieved from the Spinning Enhanced Visible and Infrared Imager onboard the Meteosat Second Generation 3 satellite are able to produce maps of the Earth disk at high resolution (1 nm, 0.05 × 0.05 degrees, 15-min) and we cross-validate them with other models to guarantee the quality and accuracy of the irradiance products. We show that these complimentary approaches are ideal for correct assessments of the solar energy potential and for providing accurate solar energy applications in real-time.

Introducing a Cloud Screening Detector Using Global Horizontal Irradiances in UV and PAR in Thessaloniki, Greece

In Thessaloniki, Greece, a NILU-UV multi-filter radiometer has been in regular operation since 2005 providing photosynthetically active radiation and ultraviolet (UV) measurements at 1-min intervals. In this study, a new approach to detect cloud presence based on such measurements, is evaluated with reference to all sky camera images taken at 15 min intervals during the year 2014. Daily channel measurements sampled at 1-min intervals are analyzed using singular spectrum analysis (SSA) and provide daily trend cycle, periodic and noise components for each day of the study period. A set of statistical limitations and threshold criteria are then applied to the extracted SSA components in order to detect the presence of clouds via their attenuation effect. Cloud detections are cross-referenced against all sky images for a visual confirmation and overall assessment of the algorithm.