Direct Normal Solar Irradiance Research Articles

Impact of Chemical Fire Accident on Spectral Solar Irradiance over Urban Environment-A Case Study

Varying levels of atmospheric pollution presently plague many large cities in the world and long-term effect of urban aerosol layer on thermal climate received little attention. Aerosols are important in local and regional climatology through direct and indirect atmospheric processes. The present study deals with the impact of anthropogenic activity associated with chemical factory accident over an urban environment namely Hyderabad, India. Ground based measurements of solar irradiance and aerosol optical depth have been measured by using Multi Filter Rotating Shadow band Radiometer (MFRSR), Multi Wavelength Radiometer (MWR), Quartz Crystal Microbalance (QCM) Impactor and MICROTOPS-Ⅱ sunphotometer respectively. Spatial variation of Aerosol optical depth (AOD) has been showed distinct increase on the accident day and subsequent days compared to normal days. Aerosol size measurements suggest the dominance of accumulation mode particles at distances ~100km away from accident site. Spectral measurements of solar irradiance exhibited variations based on polluted urban atmospheric conditions. We have made an attempt to quantify the percentage attenuation of solar irradiance due to chemical fire accident. In the highly polluted case, relative attenuations of global solar irradiance has been found to be~8% and attenuation of direct normal irradiance is~18% compared to normal days. An enhancement of about~35% diffuse solar radiation has been observed due to fire accident. The diffuse-to-direct ratio of solar irradiance has been found to depend strongly on the atmospheric conditions. The statistical analysis between columnar aerosol optical depth and direct normal solar irradiance showed negative correlation with 83W/m^2 reduction in solar irradiance for 0.1increase in aerosol optical depth at 500nm. The results of the study suggested that the effect of chemical fire accident persisted up to 3 days.

Shortwave radiative forcing efficiency of urban aerosols—a case study using ground based measurements

Aerosols reduce the surface reaching solar flux by scattering the incoming solar radiation out to space. Various model studies on climate change suggest that surface cooling induced by aerosol scattering is the largest source of uncertainty in predicting the future climate. In the present study measurements of aerosol optical depth (AOD) and its direct radiative forcing efficiency has been presented over a typical tropical urban environment namely Hyderabad during December, 2003. Measurements of AOD have been carried out using MICROTOPS-II sunphotometer, black carbon aerosol mass concentration using Aethalometer, total aerosol mass concentration using channel Quartz Crystal Microbalance (QCM) Impactor Particle analyser and direct normal solar irradiance using Multifilter Rotating Shadow Band Radiometer (MFRSR). Diurnal variation of AOD showed high values during afternoon hours. The fraction of BC estimated to be ∼9% in the total aerosol mass concentration over the study area. Results of the study suggest −62.5 Wm −2 reduction in the ground reaching shortwave flux for every 0.1 increase in aerosol optical depth. The results have been discussed in the paper.

NASA Climatological Data for Renewable Energy Assessment

This article presents the latest parameters and delivery methods that provide global solar energy and meteorological resources on the Internet. Over the past several years the number of parameters in the Surface meteorology and Solar Energy Data Set has tripled. The newest parameters include solar geometry, a suite of wind speed and wind direction data, diffuse, direct normal and tilted surface solar irradiance. Requests by users and recommendations by the renewable energy industry have prompted the growth.

Operational method for deriving high resolution direct normal irradiance from satellite data

Ground-based measurements of solar beam irradiance are expensive and rarely available especially in the sunbelt countries, the favourable regions for concentrating solar energy techniques. A method based on satellite data is presented that calculates hourly solar direct normal irradiance for large regions and for many years with high spatial resolution independently from ground measurements. The attenuation of solar irradiance due to scattering and absorption in the cloud-free atmosphere is calculated by a clear-sky broadband parameterisation model. It uses ozone optical depth from the Total Ozone Mapping Spectrometer, aerosol optical depth from the Global Aerosol Climatology Project and water vapour from the reanalysis of the National Centers for Environmental Prediction. Attenuation due to clouds is taken into account by a cloud index that is derived from the infrared and the visible channel of the geostationary Meteosat weather satellites. As clouds have the strongest impact on beam irradiance at ground, the resolution of the cloud parameterisation dominates the overall spatial and temporal resolution. The presented algorithm provides hourly data of direct normal irradiance related to the nominal 5 × 5 km 2 resolution of Meteosat.

Aerosol optical properties during Asian dust storm episodes in South Korea

¶Aerosol optical depth (τaλ) is the most comprehensive variable to characterize aerosol, assess atmospheric pollution and make atmospheric corrections to satellite remotely sensed data from ground-based instruments. The aerosol optical properties in Korea peninsula observed at both Kwangju and Anmoyondo during the Asian dust period (March–May) have been presented in this study. Aerosol optical depths at all wavelengths showed a sharp increase during major dust outbreak in spring when compared with the average for the season. For example at Kwangju, aerosol optical depths increase from the spring average value of 0.43±0.02 at 501 nm to values >0.70 during major dust event days in 1999 and 2001. Daily mean Angstrom exponents (α) computed at both sites show dramatic changes from high values to low values on dust day periods with a sharp increase in the single scattering albedo. The study also revealed that the observed direct normal solar irradiance and diffuse horizontal irradiance during days with increased dust pollution levels is capable of reducing direct solar radiation by as much as 28% and a corresponding increase of 14% in diffuse solar irradiance during cloudless and windless days.

Comparison of model‐estimated and measured diffuse downward irradiance at surface in cloud‐free skies

Diffuse downward shortwave irradiance at the surface arises from the scattering of radiation by molecules and aerosol particles. Recently, we reported that pyranometer‐measured diffuse solar irradiance in cloud‐free atmospheres is overestimated by radiative transfer models at two low‐altitude sites by an amount that exceeds modeling and measurement uncertainties but is correctly estimated within these uncertainties at two high‐altitude sites [Halthore et al., 1998]. Here we explore this phenomenon in detail, with more cases and improved uncertainty analysis, confirming that the excess in modeled diffuse irradiance cannot be explained by uncertainties in measurements or aerosol‐scattering properties that are input into the radiative transfer models or by errors in multiple‐scattering schemes. The phenomenon is observed for all comparisons with data obtained intermittently over a 5‐year period at the low‐altitude sites. Model computations also exceed radiometer‐measured sky radiance along the solar almucantar. Despite the inconsistencies between measured and modeled diffuse irradiance, atmospheric transmittance models correctly compute direct normal solar irradiance at all sites. These results indicate that at low altitudes a continuum atmospheric absorption process accounting for 0.022±0.01 in vertical optical thickness at 550 nm, corresponding to 4±2% absorptance, may need to be included in radiative transfer models and in models that retrieve aerosol optical thickness from extinction measurements. This is a substantial excess absorption with major implications for climate modeling and weather forecasting. In remote sensing studies, neglect of this excess absorption would lead to substantial errors in satellite sensor calibration and satellite inferred top‐of‐atmosphere flux. An agent or process for this absorption has not yet been identified.

Technical note: Simple models for estimating the total, diffuse, direct and normal solar irradiation in Bahrain

Very simple models have been made to estimate the monthly and daily average total, diffuse and beam solar irradiation on a horizontal surface in Bahrain. The direct normal solar irradiation has also been measured and modeled using the rotating shadow band pyrometer. The monthly average estimation was found to be more accurate than the daily average, and modeling the direct solar irradiation is the most inaccurate followed by the diffuse; since these two are very susceptible to atmospheric pollution and weather conditions. The highest actually measured monthly average total for direct, diffuse and direct normal solar irradiation was found to be 585, 383, 343 and 716 W m −2, respectively, while the lowest actually measured monthly average values were 373, 242, 96 and 342 W m −2, respectively. The results of the total solar irradiation in four different sites in Bahrain were found to have similar values (the maximum hourly values ranged from 820 to 1000 W m −2 at mid-day in June) which is expected since the area of Bahrain is nearly 700 km 2.

O2‐O2 absorption band identification based on optical depth spectra of the visible and near‐infrared

The first implementation of the rotating shadowband spectroradiometer allows the routine collection of direct normal solar spectral irradiance data at 512 wavelengths. These data are used on clear days to calculate spectra in optical depth. A persistent absorption band near 477 nm in the first optical depth spectra collected led to the eventual identification of six bands in the 400 to 1080 nm range resulting from absorption by O2 collision pairs. All of these O2‐O2 bands were discovered earlier using atmospheric long pathlength or laboratory high pressure techniques, however, it is notable that this method's sensitivity permitted their detection at one air mass. These data suggest that there are no unexplained narrowband absorption features in the visible/near‐infrared, that excess optical depth measured near 1000 nm may now have an explanation, and that the linearity of the O2‐O2 absorption may be exploited to investigate mean pathlengths in the atmosphere.

Comparison of model estimated and measured direct‐normal solar irradiance

Direct‐normal solar irradiance (DNSI), the energy in the solar spectrum incident in unit time at the Earth's surface on a unit area perpendicular to the direction to the Sun, depends only on atmospheric extinction of solar energy without regard to the details of the extinction, whether absorption or scattering. Here we report a set of closure experiments performed in north central Oklahoma in April 1996 under cloud‐free conditions, wherein measured atmospheric composition and aerosol optical thickness are input to a radiative transfer model, MODTRAN 3, to estimate DNSI, which is then compared with measured values obtained with normal incidence pyrhe‐liometers and absolute cavity radiometers. Uncertainty in aerosol optical thickness (AOT) dominates the uncertainty in DNSI calculation. AOT measured by an independently calibrated Sun photometer and a rotating shadow‐band radiometer agree to within the uncertainties of each measurement. For 36 independent comparisons the agreement between measured and model‐estimated values of DNSI falls within the combined uncertainties in the measurement (0.3–0.7%) and model calculation (1.8%), albeit with a slight average model underestimate (−0.18±0.94%) for a DNSI of 839 W m−2 this corresponds to −1.5±7.9 W m−2. The agreement is nearly independent of air mass and water‐vapor path abundance. These results thus establish the accuracy of the current knowledge of the solar spectrum, its integrated power, and the atmospheric extinction as a function of wavelength as represented in MODTRAN 3. An important consequence is that atmospheric absorption of short‐wave energy is accurately parametrized in the model to within the above uncertainties.

Measurements of the spectral components of direct normal solar radiation over Athens

Abstract The spectral composition of direct solar radiation and its long-term trends for the period 1963–1990 over Athens have been studied. Measurements were made with the use of QRT, OG1, RG2 and RG8 Schott glass fillers, allowing the division of the solar spectrum into 4 bands. The percentage of direct normal solar irradiance was found to increase in the wavelength regions under 630 nm and to decrease in the regions with longer wavelengths.

Calculated Ångströim's Turbidity Coefficients for Fairbanks, Alaska

Angstrom`s turbidity coefficient, {beta}, was determined from measurements of direct normal solar irradiance (broadband) at Fairbanks, Alaska (latitude, 64.82). The frequency distribution and seasonal changes of derived values were similar to those reported for rural sites in other parts of the world, whereas magnitudes were at the lower end of the reported range. The method was sensitive enough to detect a seasonal pattern along with annual deviations associated with hemispheric scale disturbance created by the El Chicon volcanic eruptions of 1982. Daily deviations from the seasonal pattern were associated with antecedent precipitation events and the occurrence of local or regional forest fires in summer and possible episodes of Arctic haze in winter. Knowledge of Angstrom turbidity values will improve representations of solar irradiance in micro- and mesoscale ecological and weather models. 38 refs., 3 figs., 1 tab.

Atmospheric turbidity and transmittance of solar radiation in Riyadh, Saudi Arabia

During the last two decades, the urban areas in the city of Riyadh—the capital of Saudi Arabia—were increasing at an exceptionally high rate through a series of development plans. The major plans had been completed by the end of 1982. Some other big utility projects were started and completed during 1987. As a consequence, the air quality has deteriorated markedly and air pollution episodes recorded during these activities showed that particulates were present in the atmosphere at high concentrations. Later in January 1991 the Gulf war started and the firing of the oil fields in Kuwait soon followed. It was estimated that soot particulates were emitted at a rate of 600 ton d −1 along with high rates of other gases. This event has led to significant air quality and visibility problems. Direct normal solar radiation has been measured during the summer months of July and August which were characterized by very dry and cloudless weather for the period between 1982 and 1992. A year-to-year trend of the transmittance of direct normal solar irradiance was then determined. The atmospheric fine aerosol (<2 μm diameter) loading data during the same period were used to establish a correlation between the aerosol concentration and the extinction coefficient. The total horizontal and direct normal solar radiation measurements during some days when the dark smoke emitted from the oil field fires in Kuwait were passing over Riyadh are presented. The reduction in solar irradiation reflects the intensity of dark smoke at a distance of 500 km from Kuwait.

Ground- and satellite-based meteo-data for solar thermal power stations in Jordan

Meteorological Parameters play an important role in the design and operation as well as in the economic assessment of solar thermal power stations. Beside the usual climatic data, quantity and quality of the direct normal solar irradiation are of particular importance, because they considerably affect the electricity production cost. An extensive literature search for Jordan resulted in vast quantities of conventional meteorological data, such as global horizontal irradiation, air temperature, rainfall, humidity, sunshine duration and wind speed. All these data are highly valuable but of limited use for the design and operation of solar power stations, because: - No direct normal irradiation data are available (there are only some measured data available on the global and very few on the diffuse irradiation, which have been converted into direct normal data) - For quality analysis the time resolution is not sufficient. - The measured meteodata in general do not cover well potential sites for solar power stations. Therefore, within the frame of the PHOEBUS-project (30 MW-solar power station), a specific meteo measurement campaign was started in the middle of 1989 in the South of Jordan. The preliminary equipment installed in 1989 was completed in January 1990 by adding the main equipment (PC-data acquisition system, extra sensors, etc.) At the time one meteo-station (reference station) developed by Paul Scherrer Institute, PSI and two smaller, so-called satellite-meteostations developed by Deutsche Forschungsanstalt für Luft- und Raumfahrt, DLR are in operation. Parallel to these three ground-stations, an evaluation of other potential sites in Jordan based on METEOSAT-data was performed. First measurements of the direct-normal irradiation in Quwairah confirm the high values calculated for Jubeiha-Amman station of the Royal Scientific Society, RSS. For the periode July 89 to June 90 a yearly sum of 2668 kWh/m 2 for the direct-normal irradiation and 3501 kWh/m 2 for the global-normal irradiation was registered. The direct normal monthly sums correspond well to irradiation data measured at Barstow, California. For a reliable site evaluation it is important to know the spatial solar irradiation distribution. However, the few meteo stations do not sufficiently cover the territory of Jordan. One promising way to get information about the spatial distribution is the analysis of image data taken by earth remote sensing satellites. Based on 1985 data of the visible channel of the geostationary satellite METEOSAT, it was possible to estimate monthly values for the global-horizontal and the diffuse-horizontal irradiation with a longitude and latitude resolution of 0.5°, respectively (i.e. the spatial resolution for Jordan is about 45km × 55km). The model used has been developed by the Institute for Geophysics and Meteorology of the University of Cologne (Germany) in the framework of the European research program SUNSAT. These data have been combined with irradiation data of the meteo station at Jubeiha-Amman leading to the monthly and yearly spatial distribution of the direct normal component. The contour plots show that favourite sites can be found in the south of the country as well as south-east of the mountain Jabal al Arab whereas in the Wadi as Saharan the conditions are less promising.

Determination of Ångström's turbidity coefficient from direct total solar irradiance measurements

A method of determining Ångström's turbidity coefficient from the measured total (broadband) direct normal solar irradiance is described here. This direct normal irradiance can be expressed in terms of the individual transmittances of the various atmospheric attenuators, such as ozone layer thickness, precipitable water vapor thickness, and α and β of the Ångström's turbidity formula. From the resulting parameterization equation, an explicit expression for β is obtained. By assigning a fixed value to α, the value of β is determined. Use of this method is demonstrated with data from Ajaccio (France), a mediterranean coastal station.

Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres

Abstract In a previous work, we described a simple model for calculating direct normal and diffuse horizontal spectral solar irradiance for cloudless sky conditions. In this paper, we present a new simple model (SPCTRAL2) that incorporates improvements to the simple model approach and an algorithm for calculating spectral irradiance on tilted surfaces. The model was developed using comparisons with rigorous radiative transfer codes and limited outdoor measurements. SPCTRAL2 produces terrestrial spectra between 0.3 and 4.0 μm with a resolution of approximately 10 nm. Inputs to the model include the solar zenith angle, the collector tilt angle, atmospheric turbidity, the amount of precipitable water vapor and ozone, surface pressure, and ground albedo. A major goal of this work is to provide researchers with the capability to calculate spectral irradiance for different atmospheric conditions and different solar collector geometries using microcomputers.

Spectral solar irradiance data sets for selected terrestrial conditions

Direct normal and global spectral solar irradiance data sets are presented for selected terrestrial conditions, along with a brief review of previous data sets. The new data sets presented cover the 0.305 μm to 4.045 μm region and were generated with the rigorous BRITE Monte Carlo radiative transfer code, the revised Neckel and Labs extraterrestrial solar spectrum, the U.S. standard atmosphere model, and a rural aerosol model. The data for the 2.45 μm to 4.045 μm region were taken from existing ASTM standards (E891-82 and E892-82). Tabular and graphical data presentations include irradiance versus wavelength, photon flux density versus wavelength, and photon flux density versus photon energy.

Effects of the El Chichon Volcanic Cloud on Direct and Diffuse Solar Irradiances

Abstract Direct normal and diffuse solar irradiances and 500 nm aerosol optical depths measured at the University of Michigan departed far from normal on 26 October 1982, when it is concluded that the main stratospheric cloud from the El Chichon volcanic eruption arrived at the 42°N latitude of the radiation measurement facility. For clear-sky data analyzed through 19 January 1983, direct solar is about 25% less than normal and diffuse solar is about 85% greater. For the same aerosol optical depths and solar zenith angles, the ratio of diffuse to direct is about 30% greater for about 0.3 cm of precipitable water but nearly the same for 0.9 cm. Aerosol optical depths are nearly three times greater for wind directions that naturally advect the cleanest air. The effect of circumsolar irradiance on the methods used to measure direct normal and diffuse irradiances cause the former to be overestimated and the latter to be underestimated.

Effects of the El Chichon volcanic dust cloud on insolation measurements at Corvallis, Oregon (U.S.A.)

Insolation measurements made on clear days at Corvallis, Oregon, since the last week of November, 1982, have indicated a decrease of about 14% in the atmospheric clearness index, an increase by a factor of about 2.3 in the ratio of the diffuse sky irradiation to the global irradiation and a decrease of about 28% in the transmittance of the atmosphere for the direct normal solar irradiation compared to the normal, clear‐day values. It is felt these anomalies are caused by the increased atmospheric turbidity due to the El Chichon volcanic dust cloud.

Effects of the Mount St. Helens Volcanic Cloud on Turbidity at Ann Arbor, Michigan

Measurements of turbidity were made at the University of Michigan irradiance and meteorological measurement facility just prior to, during and after the passage of the volcanic cloud from the 18 May 1980 eruption of Mount St. Helens. They were made with a Volz sunphotometer at wavelengths of 500 and 880 nm. The volcanic cloud gradually moved over the measurement facility on the afternoon of 20 May and remained over it until 2300 GMT 21 May. Its effects on the turbidity coefficient and wavelength exponent in Ångström's turbidity equation are evaluated by comparing results for 20 and 22 May, both of which were cloudless days, with those on 21 May. The volcanic cloud increased the turbidity coefficient by a factor of about 8 and decreased the wavelength exponent by a factor of about 3 compared to average values for 20 and 22 May. Some uncertainty in the results is caused by cirriform clouds in addition to the volcanic cloud on 21 May. These results and effects of the volcanic cloud on recordings of global, direct normal and diffuse solar irradiances are described.