Application of SPCTRAL2 parametric model in estimating spectral solar irradiances over polluted Athens atmosphere

Results obtained using SPCTRAL2 parametric model for the polluted urban atmosphere of Athens, Greece, were analysed and compared with ground level experimental spectral solar irradiance measurements and spectrally integrated solar irradiances in two discrete narrow bands, ultraviolet and visible. For the aerosol characterization, the aerosol optical depth evaluated at 500 nm was used as the basic input parameter. The algorithm used seems to reproduce the experimental solar spectral irradiances adequately depending on the aerosol model used. The results obtained have been explained through mean bias and root mean square statistical deviations and the resultant influence of the aerosol volume spectra.     

Analysis of terrestrial solar radiation exergy

Based on Candau’s definition of radiative exergy, the exergy of the extraterrestrial and the terrestrial solar radiation are computed and compared by using the solar spectral radiation databank developed by Gueymard. The results show that within the spectrum region from 0.28 to 4.0 μm, the total energy quality factor (i.e., the exergy-to-energy ratio) of extraterrestrial solar radiation is about 0.9292, and that of the global terrestrial solar radiation is about 0.9171 under US standard atmosphere condition and zero solar zenith angle. The terrestrial solar spectral radiation exergy flux is large in the near ultraviolet and the visible light region. The reference radiation exergy spectra are obtained under atmospheric conditions consistent with ASTM standard G173-03. The effect of tilt angle on the terrestrial solar radiative exergy for inclined surface, and the effect of air mass on total energy quality factor of the terrestrial solar radiation for horizontal surface are analyzed. With the increase of tilt angle, the terrestrial solar spectral radiation exergy flux initially increases and then decreases, the total energy quality factor of the diffuse part decreases monotonically, while that of the direct part is invariant. The total energy quality factor of the direct, the diffuse and the global terrestrial solar radiation all decrease with the increase of air mass.     

Predicting the spectral and broadband aerosol transmittance in the atmosphere for solar radiation modelling

Calculations of the spectral transmittance of the atmospheric aerosol, using Mie theory, for wavelengths between 0 and 40 μm is presented. The chemical composition of the aerosol particles has been modelled in order to correspond to the atmospheric conditions of medium and large coastal or near coastal cities with important industrial and other anthropogenic emission sources. Individual size distributions and optical properties for each aerosol constituent have been considered.Based on the detailed aerosol model, and using parameterization techniques, analytical broadband aerosol transmission functions for the absorption and total attenuation are obtained. The accuracy of the proposed expressions are verified with various tests, using data from the National Observatory of Athens (NOA). The proposed broadband aerosol transmission functions can be incorporated directly into solar radiation models to predict accurately the beam, diffuse and global solar radiation at a given place.     

SIMULATION OF BROAD-BAND AND SPECTRAL SOLAR ULTRAVIOLET RADIATION

Daily broad-band and spectral measurements of solar ultraviolet radiation reaching the ground at Athens, Greece (38^°N, 24^°E) during 1989-1993 in conjunction with daily total ozone measurements deduced from satellite and ground-based instrumentation were used to investigate their association. Furthermore, employing a new parametric model, the spectral and broad-band solar ultraviolet radiation for die period 1979-1993 were calculated considering the total ozone values, derived from both the satellite Nimbus-7, and the ground-based Dobson spectrophotometer.     

The role of aerosol models of the SMARTS code in predicting the spectral direct-beam irradiance in an urban area

Measurements of the ground-level spectral distribution of the direct-beam solar irradiance between 300 and 1000 nm were made in Athens, Greece, in May 1995. Results obtained using simple model for the atmospheric radiative transfer of sunshine (SMARTS) (version 2.9.2) parametric model for the urban atmosphere of Athens are analyzed and compared to the ground-level experimental spectral solar irradiance measurements obtained by the passive pyrheliometric scanner (PPS) in three discrete bands, UV (320–400 nm), VIS (400–700 nm) and NIR (700–1000 nm). The study uses two different input parameters for the aerosol characterization: the aerosol optical depth at 500 nm, t_α0.5, and the Ångström turbidity coefficient, β. The results clearly show that the nine aerosol models implemented in the SMARTS code lead to quite different predictions of the direct-beam spectrum, strongly depended on the input parameter. In all cases the inadequacies between the measured and the modeled direct-beam spectra are lower and higher when the urban and maritime aerosol models are used, respectively.     

PREDICTING THE SPECTRAL AEROSOL TRANSMITTANCE FOR SOLAR RADIATION SPECTRAL AEROSOL MODELS

A model to estimate the spectral transmittance of the atmospheric aerosol for wavelengths λ = 0 to 20 μm is presented. The model is suitable for use in small and medium size cities with important industrial and other anthropogenic emission sources. Individual size distributions and optical properties for each chemical component are assumed. Humidity effects are discussed. The parameters of the model are compared with the existing measurements and are found in close agreement.

The results of the model can be combined directly with solar radiation models to predict the spectral values of the direct, diffuse and total solar radiation.     

A modified algorithm for calculating the cloud index

The cloud index is an important component of the Heliosat algorithm, which estimates solar radiation components from Meteosat High Resolution Visible images. The cloud index quantifies the reflective properties of the atmosphere, and varies from 0 at clear conditions to 1 at overcast. The algorithm is semi-empirical in the way that it includes several constants that need to be tuned. Some of these were removed in the Heliosat-II algorithm where the Meteosat calibration constant was introduced to replace the “pseudo-reflectivity” with a “real reflectivity”. This approach is followed here, and two additional changes are made: (1) An analytical expression is introduced to correct for backscattered radiation from air molecules. (2) A correction is made for non-lambertian reflectivity, removing the time consuming need for determining the ground reflectivity for each month and each slot. The new cloud index is used to calculate global irradiances which are validated against hourly measurements from five European ground stations. The average root mean square deviation is 15.5% for a six-month spring/summer period, of comparable accuracy as using the more time consuming traditional algorithm used in the EU project Satel-Light.     

Recent improvements of the Meteorological Radiation Model for solar irradiance estimates under all-sky conditions

This study presents the recent improvements of the Meteorological Radiation Model (MRM v6) against previous model versions for more accurate estimates of the solar radiation components, i.e. global, diffuse and direct. The MRM v6 follows a different approach for the simulation of the atmospheric conditions by selecting the most appropriate aerosol model (desert, urban, maritime or continental), and usage of look-up tables for the spectral variation of the aerosol optical depth (AOD) and single scattering albedo (SSA). Furthermore, the MRM v6 uses hourly data of sunshine duration in order to achieve better simulations under cloudy skies. The results show that the MRM v6 improves the estimates of the measured global, diffuse and direct solar irradiances at Athens, Greece since the Root Mean Square Error (RMSE) becomes 13.7%, 40.8% and 24.2%, respectively, against 18.0%, 44.5% and 34.1% for the MRM v5. A decrease is also found in Mean Bias Error (MBE), especially for the diffuse (from 26.2% to 19.5%) and direct (from −9.0% to −2.4%) irradiances, indicating that the inclusion of the aerosol properties in MRM v6 significantly improves the estimations. The MRM simulations are very satisfactory on monthly basis indicating that the model is suitable for solar energy applications.     

Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film

Sol–gel nano titanium dioxide (TiO₂) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO₂ lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO₂ thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO₂ thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO₂ photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO₂-coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates.     

Attenuation of solar radiation in the atmosphere

This paper presents the results of measurements of the solar radiant energy absorbed and scattered in the atmosphere. Since much of the information on solar irradiance at the ground is obtained by computation from extraterrestrial radiation data, it is important to know precisely the actual energy that is absorbed and scattered in its passage through the atmosphere for the accurate estimation of the radiant energy received at the ground. Various models exist for the estimation of daily totals of global solar radiation under clear sky and cloudy conditions, taking these effects into consideration and assuming average values for the ozone and water vapour content and the turbidity of the atmosphere. In the present investigation atmospheric attenuation of solar radiation has been calculated from measured values of ozone and water vapour content and turbidity in the atmosphere, at two stations Bangalore (950 metres above sea level) and Nandi Hills (1479 masl). Direct measurements of direct solar radiation for the whole spectrum and various spectral regions were made at Bangalore and Nandi using Ångström pyrheliometers fitted with broad-band pass filters during the clear months January–May 1979. Global solar radiation and sunshine duration measurements were also made at both stations. Using direct measurements of the total ozone and water vapour content and atmospheric turbidity, direct, diffuse and global solar radiation values at the ground were computed from extraterrestrial values of radiation for clear sky conditions. The results are compared with actual measurements and earlier observations of direct solar radiation at other high-level stations. The importance of atmospheric turbidity measurements in the computation of solar radiation is discussed.     

Solar energy outside the Earth's atmosphere

Extensive measurements have been made in different countries and by many research groups to determine the solar constant and the spectral distribution of solar energy. Earlier determinations were made from ground-based observations. The extrapolation to zero air mass under the highly variable conditions of the atmosphere led to large uncertainties in the proposed values.In recent years renewed interest in this topic has arisen due to the direct effect of solar radiation on thermal balance of spacecraft, electrical output of solar panels, and other satellite-related problems. Measurements of the solar constant have been made by total irradiance detectors from high altitude research aircraft, balloons, and space probes. Their data have produced converging evidence that the value of the solar constant commonly accepted in earlier years, 2·00 cal cm⁻² min⁻¹, was too high.Spectral irradiance measurements at high altitude were made using a variety of monochromators and filter radiometers. The results of all recent high altitude measurements have been critically examined by an ad hoc committee on “Solar Electromagnetic Radiation.” The committee has proposed standard values for engineering use for the solar constant and the solar spectrum. These values will be presented and also the reasons for recommending their acceptance. Detailed comparisons have been made between the revised spectral irradiance values and those which had been widely accepted in earlier years. New values have been computed for frequently-cited astrophysical quantities which are derived from the solar constant and the solar spectrum. The spectrum has been extended to 10A in the X-ray range and to 10 m in the microwave range. The revised value of the solar constant is 1·94 cal cm⁻² min⁻¹. The proposed standard solar spectrum shows significant differences from other curves throughout the spectral range.     

A simple solar radiation model for computing direct and diffuse spectral fluxes

A solar spectral model that describes the solar radiation flux on a clear day at any given location was developed and tested. The model computes spectral fluxes of global, global photon, direct and diffuse solar radiation incident at the surface. Input parameters describe location and atmospheric characteristics. Location is described by latitude, altitude, slope orientation and surface albedo. Atmospheric characteristics described are turbidity, precipitable water vapor and total ozone content. The model was constructed using a one-layer homogeneous atmosphere with refinements, which are:

1. (1) Use of climatological data to predict the total ozone content, if it is not known.

2. (2) A more advanced treatment of infrared solar radiation (0.8–4.5 μm) absorption.

3. (3) A more complex scheme for predicting diffuse radiation.

4. (4) The capability of handling a spectral albedo.

5. (5) Inclusion of albedo dependence on zenith angle.

Input parameters are minimized and several simplifying features are incorporated for ease of handling variables not routinely measured. Turbidity and total ozone content are treated as climatological estimates if specific location measurements are not available. Precipitable water vapor can be predicted using surface vapor pressure, since the sounding network is not dense. These features allow researchers outside the field of solar radiation to use the model.Because complete measurements with needed location and atmospheric characteristics could not be found, the validity of the model was tested by comparing it with a more complex, multilayered atmosphere model by Dave et al. [3]. Calculated fluxes of total direct, diffuse, and global radiation from the model presented were 11.5, 20.1 and 13.2 per cent lower, respectively, when they were adjusted for differences in extra-terrestrial solar radiation fluxes. Direct spectral fluxes closely agreed in spectral composition, with slight exception in the 0.8–0.95 μm region. Diffuse spectral flluxes were slightly higher in the UV region lower in the rest of the spectrum than were those in the multilayered atmosphere model. A sensitivity analysis of the model was also conducted: the most influential inputs were found to be latitude, slope orientation and turbidity, and the least influential was total ozone content.The hourly integrated values for the model compared very well with measured values for clear days at Davis. Discrepancies between predicted and measured values were due to lack of local turbidity coefficients.     

Diffuse and global solar spectral irradiance under cloudless skies

A simple empirical model to calculate solar spectral diffuse and global irradiance under cloudless skies is presented here. This formulation takes into account absorption of radiation by molecules such as O₃, H₂O and the uniformly-mixed gases. Attenuation by Rayleigh scattering and aerosol extinction are included. Aerosol attenuation is calculated through Ångström's turbidity parameters α and β. The diffuse radiation is assumed to be composed of three parts: (1) Rayleigh-scattered diffuse irradiance; (2) aerosol-scattered diffuse irradiance; and (3) irradiance arising out of multiple reflections between the atmosphere and the ground. The global irradiance is the sum of these three components of diffuse irradiance plus the direct irradiance. The input parameters include an extraterrestrial spectrum, zenith angle, turbidity coefficient β, wavelength exponent α, ground albedo _g, water vapor content and ozone content. The model is shown to yield very good results up to air mass two when compared to accurate theoretical calculations. No comparisons with measured spectra are presented because of a lack of accurate specifications of input parameters. Results are presented to show the effect of variation of certain of the input parameters.     

On the choice of the angle of tilt for south facing solar collectors in the Athens basin area

This paper seeks to furnish a fundamental piece of information to the solar engineer or scientist regarding the proper choice of the solar collector's tilt angle. Global radiation monthly average daily values are analyzed by a simple correlation into diffuse and beam components. The diffuse and beam components are then used to calculate monthly average hourly and daily values on an inclined surface. The results of these calculations are tabulated and plotted against the angle of tilt for summer, winter and all-year-round intended use. Global radiation measurements used in this work come from (a) the Climatological Bulletin of the National Observatory of Athens, years 1957–1981, (b) the unpublished records of the National Weather Service of Greece (referred to as EMY), years 1977–1982, and (c) the Scientific Publications of the Public Power Co. (referred to as PPC) on measurements of solar potential of Greece, years 1982–1983; the latter was the source of solar diffuse data as well.     

Impact of Pinatubo aerosols on the seasonal trends of global, direct and diffuse irradiance in two northern mid-latitude sites

In June 1991, Mt Pinatubo's eruption in the Philippines ejected a staggering 20 million metric tons of SO₂ into the stratosphere which resulted in an aerosol cloud covering most of the Earth within a few months after the eruption. In this article we illustrate how the seasonal trends of global, direct and diffuse solar radiation were modified by the eruption in two mid-latitude sites in Europe and the U.S., totalling about 12 yr of hourly data. A spectacular increase in the diffuse fraction of solar radiation as opposed to a decrease in direct radiation which extended from late 1991 to mid 1993 was observed in both sites and for clear sky conditions. Global radiation was not altered significantly from these data sets. Sunphotometer measurements in one of the sites show the volcanic aerosols tripled the total atmospheric aerosol optical depth at 1 μm and approximately doubled the aerosol optical depth at 0.5 μm.     

Spectral solar irradiance before and during a Harmattan dust spell

Measurements of the ground-level spectral distributions of the direct, diffuse and global solar irradiance between 300 and 1100 nm were made at Akure (7.15°N, 5.5°E), Nigeria, in December 1991 before and during a Harmattan dust spell employing a spectroradiometer (LICOR LI-1800) with 6 nm resolution. The direct spectral solar irradiance which was initially reduced before the dust storm was further attenuated by about 50% after the spell. Estimated values of the Ångström turbidity coefficient β indicated an increase of about 146% of this parameter while the Ångström wavelength-exponent α decreased by about 65% within the 2-day study period. The spectral diffuse-to-direct and diffuse-to-global ratios suggest that the main cause of the significant reduction in solar irradiance at the surface was the scattering by the aerosol which led to an increase in the diffuse component. The global irradiance though reduced, was less sensitive to changing Harmattan conditions. It is recommended that solar energy devices that use radiation from Sun and sky be used under fluctuating Harmattan conditions. There are some deviations from the Ångström formula under very turbid Harmattan conditions which could be explained by the relative increase of the particle sizes.     

Clearness index estimation for spectral composition of direct and global solar radiations

Measurements of cloudless direct, global, and diffuse solar radiations, taken over a one year period at Helwan, are analyzed in terms of global index and diffuse fraction for clear-sky conditions. The dependence of the diffuse fraction on the global index was represented by a polynomial. These results support the use of routine instantaneous surface meteorological data to calculate global and diffuse radiations on a horizontal surface in the absence of any other radiation measurements. The spectral composition of the global solar-radiation was found to be 4.3% UV band, 32.5% band range 250–630 nm, 13.74% red band, 52.75% infrared band and 29.7% diffuse solar-radiation. The spectral distribution of direct solar-radiation ratio of the extraterrestrial solar radiation was found to be: 0.69% green and blue band, 47.5% yellow and orange band 45% red band, and 52.7% infra-red band.     

Interactions between air pollution and solar radiation

The interaction between air pollutants and solar radiation is discussed in terms of three examples: measurements of the direct solar beam to determine atmospheric turbidity; the role of solar radiation in initiating photochemical smog processes; and urban-rural differences of incident solar energy. Turbidity measurements from several non-urban stations, particularly in the south-central and south-eastern U.S., show a slight increase during the past 10–15 yr, with most of the change occurring during summer. Measurements at certain urban stations indicate a decreasing turbidity trend. Coincident measurements of incident solar radiation at urban and rural sites are presented for St. Louis and Los Angeles. The urban-rural differences of incident total and ultraviolet (UV) solar energy ranged from near zero to more than 50 per cent. The solar depletion was much larger over Los Angeles than St. Louis. The impact of solar radiation on photochemical smog over Los Angeles is shown be discussing the sensitivity of ozone concentrations to variations of UV radiation. The results of a numerical photochemical diffusion model indicate that the seasonal decrease of UV energy from July to November has a substantial effect on ozone levels, especially over the western part of the Los Angeles Basin.     

Monthly average solar radiation in panama—Daily and hourly relations between direct and global insolation

Regressions are developed to estimate daily global and direct radiation and the hourly distribution of direct radiation for Barro Colorado Island, Panama from monthly mean values observed 35 km away at Chiva-Chiva. The ratio model of Liu and Jordan and the logarithmic model of Anderson for estimating direct from global radiation are compared. Both gave satisfactory results after accounting for “seasonal” variation, but the ratio model was preferred in this case for the smaller number of separate regressions required. The ratio model fitted for diffuse radiation at Chiva-Chiva agreed closely with regressions for stations at similar latitude. For a given value of the clearness index, the direct component of solar radiation was relatively (but not absolutely) reduced during the dry season compared with the wet season. A likely explanation for this unexpected result is increased marine and terrestrial aerosol during the dry season when offshore winds are stronger and burning of crop and wasteland occurs. The models of Whillier and of Garnier and Ohmura, which assume constant atmospheric transmittance throughout the day, gave unsatisfactory fits to the hourly distribution of direct radiation. They were also unable to mimic an observed morning/afternoon asymmetry that was strongest in the wet season. Hourly direct radiation was accurately estimated from hourly global radiation by quadratic polynomials fitted separately to the morning and afternoon data. The resulting regressions will enable estimation of radiation in forest understory from measurements of insolation in the open by computerized image analysis of hemispherical canopy photos.     

MODELING THE ALTITUDE EFFECT ON SOLAR UV RADIATION

A simple numerical model that describes the altitude effect of solar UV flux under cloud-free conditions was developed and tested. The model computes the direct and diffuse UV irradiances for the wavelength range 290–385 nm, at any sub-polar location and time (restricted to arid areas). The input parameters include extraterrestrial solar irradiance, ozone content and vertical distribution, aerosol amounts and size distribution, SO₂ and NO₂ contents, surface albedo and solar zenith angle. Model results were compared with measurements made in the tropical Chilean Andes for altitudes up to 5500 m above sea level. The model and data show good agreement. For the measured direct component a linear increase with altitude was assumed, whereas model results, computed up to 15 km altitude, exhibit a non-linear behavior. However, in the lowest few kilometers a linear regression was adequate for both model and measurements. As for the diffuse component, the variation with altitude strongly depends on wavelength and solar zenith angle. At short wavelengths and large solar zenith angles, a pronounced maximum occurs at a level which depends on these parameters. The maximum cannot be observed when integrating over the UV-A range. This behavior can be understood by taking into account the sources of the diffuse flux at any given level in the atmosphere.