Measurements Of Diffuse Irradiance Research Articles

Continental aerosol properties inferred from measurements of direct and diffuse solar irradiance

Aerosol particles affect the global energy budget and Earth's climate through the absorption and scattering of both solar and terrestrial radiation. Here we present a study of column‐averaged aerosol optical properties derived from clear‐sky measurements of the direct and diffuse components of the solar downwelling spectral and broadband irradiance during a 10‐day observation period at the Atmospheric Radiation Measurement (ARM) facility in central Oklahoma in the fall of 2001. Using a radiative transfer model and Mie calculations of the optical properties of standard continental aerosol components and assuming a constant aerosol composition with altitude, we determine the phenomenological aerosol model that best fits both the direct and diffuse spectral irradiances. Internal and external homogeneous aerosol mixtures were considered, along with single‐aerosol and coated sphere models. We find that the irradiance data is well fit statistically by at least one of the models for every clear‐sky time considered during the observations period, with no excess gaseous absorption required to fit the data. By far the best fitting model consisted of ultrafine (∼5–30 nm radius) coated spheres with absorbing black carbon cores and nonabsorbing shells of water or a similar substance. The mean value of the 500‐nm aerosol single‐scattering albedos for the entire 10‐day period was 0.76. These results indicate that absorbing carbonaceous aerosols can have a significant radiative impact even in rural settings.

Retrievals of Antarctic aerosol characteristics using a Sun‐sky radiometer during the 2001–2002 austral summer campaign

In order to characterize the Antarctic aerosol and to analyze the effect of katabatic winds on the properties of suspended particles, measurements of solar direct and diffuse irradiance were carried out at the Italian Terra Nova Bay station in Antarctica, during the 2001–2002 austral summer campaign. Measurements were performed by the ground‐based PREDE sky radiometer and processed by using the Skyrad inversion code. Aerosol optical thickness at 500 nm was found to vary between 0.01 and 0.02. The volume size distribution curves showed bimodal features with the two modes located within 0.1–0.3 μm and 5–7 μm radius intervals, respectively. The real part of the refractive index characterizing the Antarctic aerosol was found to have a mean value of 1.40. During the katabatic event the analysis indicated that the advection of larger and drier fresh particles, together with the removal of marine suspended particles, caused the decrease in aerosol optical thickness.

Simultaneous retrievals of column ozone and aerosol optical properties from direct and diffuse solar irradiance measurements

A retrieval technique has been developed to simultaneously determine column ozone amounts and aerosol optical properties using surface observations of solar ultraviolet direct normal and diffuse horizontal irradiance from a multifilter rotating shadowband radiometer. The retrieval consists of a Bayesian scheme involving a tropospheric ultraviolet radiative transfer model. The technique was tested using cloud‐free observations collected during a Mexico City Metropolitan Area air pollution field campaign from April to May 2003. Retrieval results compared favorably to those of independent techniques, including ozone amounts from a direct‐Sun method, Langley‐derived aerosol optical depths, and aerosol single‐scattering albedos from a direct‐to‐diffuse irradiance ratio technique. Further comparisons were performed between the measurements and model simulations when using the retrieval results as inputs, both from the proposed technique and the combined independent methods. Simulations using the results of the new method were found to agree with the observations within the assumed limits of measurement and model uncertainty. It is anticipated that the technique will be applied across a 33‐site network of radiometers maintained by the U.S. Department of Agriculture UV‐B Monitoring and Research Program for development of aerosol climatologies and for providing ground validation for satellite measurements.

Effect of aerosols on the downward shortwave irradiances at the surface: Measurements versus calculations with MODTRAN4.1

A detailed analysis of measurements and model calculations of clear‐sky shortwave irradiances at the surface is presented for a set of 18 cases collected during 3 cloudless days in the Netherlands in 2000. The analysis is focused on the influence of the optical and physical properties of aerosols on simulations of direct and diffuse downward solar irradiance at the surface. The properties of aerosols in the boundary layer are derived from surface measurements, under the assumption that all aerosol is confined to a well‐mixed atmospheric boundary layer. The simulations of the irradiances are performed with the radiative transfer model MODTRAN 4, version 1.1. The analysis reveals no discernable differences between model and measurement for the direct irradiance, but several significant differences for the diffuse irradiance. The model always overestimates the diffuse irradiance measurements by 7 to 44 Wm−2 (average: 25 Wm−2). On the basis of an estimated uncertainty in the differences of 18 Wm−2, it appears that for 13 out of 18 cases the model significantly overestimates the measurements. This number decreases if instrumental errors (e.g., pyranometer zero‐offset) and assumptions on the model input (e.g., wavelength‐independent surface albedo) are considered. Nevertheless, the analysis presented here points to a persistent and significant positive model‐measurement difference for the diffuse irradiance, which typically amounts to 1–4% of the top‐of‐atmosphere irradiance, and does not depend on the solar zenith angle. The reason for the discrepancy may be found in the presence of ultrafine absorbing aerosol particles that were not detected by the surface instrument for measuring aerosol absorption. It is also possible that these particles are not present near the surface, due to dry deposition, but do contribute to the total extinction if they are situated higher up in the boundary layer.

Method for estimation of refractive index and size distribution of aerosol using direct and diffuse solar irradiance and aureole by means of simulated annealing

An estimation method for refractive index and size distribution of aerosols using measurements of direct and diffuse solar irradiance as well as the solar aureole by means of a modified simulated annealing is proposed. The proposed method is based on simulated annealing modified to acceralate a learning process by using a gradually decreasing oscillation function of temperature of annealing. By a using Gauss Seidel based atmospheric code, simulation data of direct and diffuse solar irradiance are generated together with estimated aureole measurements by means of an empirical method derived from experimental data. A comparison between the existing method proposed by P Romanov et.al. based on a linear inversion method and the proposed method is made. The results show double improvement of the estimation accuracy for both the aerosol size distribution and refractive index.

Cloud Coverage Based on All-Sky Imaging and Its Impact on Surface Solar Irradiance

In Lauder, Central Otago, New Zealand, two all-sky imaging systems have been in operation for more than 1 yr, measuring the total, opaque, and thin cloud fraction, as well as indicating whether the sun is obscured by clouds. The data provide a basis for investigating the impact of clouds on the surface radiation field. The allsky cloud parameters were combined with measurements of global, direct, and diffuse surface solar irradiance over the spectral interval from 0.3 to 3 mm. Here, the results of ongoing analysis of this dataset are described. As a reference for the magnitude of the cloud influence, clear-sky irradiance values are estimated as a simple function of solar zenith angle and the earth‐sun distance. The function is derived from a least squares fit to measurements taken when available cloud images show clear-sky situations. Averaged over a longer time period, such as 1 month, cloud fraction and surface irradiance are clearly negatively correlated. Monthly means in the ratio of the measured surface irradiance to the clear-sky value had a correlation coefficient of about 20.9 with means of cloud fraction for the months from July 2000 to June 2001. In the present work reductions in the surface irradiance and situations in which clouds cause radiation values to exceed the expected clear-sky amount are analyzed. Over 1 yr of observations, 1-min-averaged radiation measurements exceeding the expected clearsky value by more than 10% were observed with a frequency of 5%. In contrast, a reduction of more than 10% below estimated clear-sky values occurred in 66% of the cases, while clear-sky irradiances (measured irradiance within 610% of estimated clear-sky value) were observed 29% of the time. Low cloud fractions frequently lead to moderate enhancement, because the sun is often unobscured and the clouds are brighter than the sky that they hide. As cloud fraction increases the sun is likely to be obscured, causing irradiance values to fall well below clear-sky values. However, in the case of unobscured sun, there is a tendency for strongest enhancements when cloud fractions are highest. Enhancements, especially at high solar zenith angle, are also often observed in association with thin clouds.

Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model

Aerosol single scattering albedo (ω), the ratio of aerosol scattering coefficient to total aerosol extinction coefficient, at UV wavelengths is an important aerosol radiative parameter in determining surface UV irradiance. Surface measurements of total and diffuse UV irradiance in the summer and fall of 1999 at the seven narrowband wavelength channels of an UV multifilter rotating shadowband radiometer (UVMFR‐SR) at Black Mountain, N. C., were coupled with a tropospheric ultraviolet radiative transfer model to produce values of ω. Its value ranged from 0.65 to 0.91 at 300 nm, 0.71 to 0.96 at 305.5 nm, 0.73 to 0.97 at 311.4 nm, 0.74 to 0.91 at 317.6 nm, 0.76 to 0.96 at 325.4 nm, 0.77 to 0.97 at 332.4 nm, and 0.80 to 0.99 at 368 nm. Error in this procedure decreases with increasing aerosol optical depth (AOD), from ±0.63 at AOD = 0.05 to ±0.04 at AOD = 1.0 averaged over the seven wavelengths. The current values of ω have a slightly wider variation than values reported from a previous study at the same site. The lower values of ω could indicate that, over the site, preferential absorption of UV radiation by black carbon aerosols could be occurring. More values of ω in the UV spectrum will allow for better estimation of this parameter for UV radiative transfer modeling and will lessen error in estimation of surface UV irradiances.

Results from the first ARM diffuse horizontal shortwave irradiance comparison

The first intensive observation period (IOP) dedicated exclusively to the measurement of diffuse horizontal shortwave irradiance was held in the Fall 2001 at the central facility of the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site with the cooperation of the Baseline Surface Radiation Network (BSRN) community. The purpose of the study was to compare diffuse irradiance measurements among most commercial pyranometers and a few prototypes calibrated independently using current practices. The hope was to achieve a consensus for this measurement with the goal of improving the uncertainty of shortwave diffuse irradiance measurements. All diffuse broadband measurements were made using the same type of two‐axis tracker with the direct beam blocked by shading balls. Tracking was excellent during the IOP with no lost data associated with tracker problems. Fourteen simultaneous measurements were obtained over a two‐week period under mostly clear skies with low to moderate aerosol loading. Totally overcast data were obtained during the morning of one day. Five of the measurements are reproducible to about 2 W/m2 at the 95% confidence level. Three more agree with the mean of these five to about 4 W/m2 at the 95% confidence level after correction for thermal offsets.

A method to calibrate a solar pyranometer for measuring reference diffuse irradiance

Accurate pyranometer calibrations, traceable to internationally recognized standards, are critical for solar irradiance measurements. One calibration method is the component summation, where the pyranometers are calibrated outdoors under clear sky conditions, and the reference global solar irradiance is calculated as the sum of two reference components, the diffuse and subtended beam solar irradiances. The beam component is measured with pyrheliometers traceable to the World Radiometric Reference, while there is no internationally recognized reference for the diffuse component. In the absence of such a reference, we present a method to consistently calibrate pyranometers for measuring the diffuse component with an estimated uncertainty of ±(3% of reading+1 W/m 2). The method is based on using a modified shade/unshade method, and pyranometers with less than 1 W/m 2 thermal offset errors. We evaluated the consistency of our method by calibrating three pyranometers four times. Calibration results show that the responsivity change is within ±0.52% for the three pyranometers. We also evaluated the effect of calibrating pyranometers unshaded, then using them shaded to measure diffuse irradiance. We calibrated three unshaded pyranometers using the component summation method. Their outdoor measurements of clear sky diffuse irradiance, from sunrise to sundown, showed that the three calibrated pyranometers can be used to measure the diffuse irradiance to within ±1.4 W/m 2 variation from the reference irradiance.

Evaluation of a New Photodiode Sensor for Measuring Global and Diffuse Irradiance, and Sunshine Duration

A new integrated device (called the BF3) has been developed, which enables the simultaneous measurement of horizontal global and diffuse irradiance as well as sunshine presence at any time. The sensor needs no specific polar alignment or routine adjustment, and works at any latitude. To evaluate the performance of this new device, a BF3 sensor was installed on the roof of a six-story building in the Merchiston Campus of Napier University, Edinburgh from February 22–July 3, 2001. Horizontal global and diffuse irradiance data were collected from the BF3. To enable a cross check, two Kipp and Zonen CM11 sensors, one with a shade ring, have also been installed beside the BF3 sensor on the same roof. These were used to give a reference measure of the horizontal global and diffuse irradiance. To evaluate the BF3 sunshine duration performance, the direct beam normal irradiance was calculated from the CM11 global and diffuse readings, and compared with a threshold of 120W.m−2 to give sunshine presence according to the WMO definition. This was compared against the BF3 output, and also with data from two Campbell-Stokes sunshine recorders on the same site. The results show a stable performance on the part of the BF3 sensor for the measurement of horizontal global and diffuse irradiance. The global irradiance measured by the BF3 showed values 4.7% high, with a standard error of 16.5W.m−2 compared to the Kipp and Zonen sensors. Diffuse values were 1.4% high with a standard error of 13.4W.m−2. The BF3 sunshine duration was within 2% of that calculated from the WMO definition over the study period, with a typical daily error of less than 20 min. This is well within the WMO requirements for a sunshine recorder. In comparison, the Campbell-Stokes recorders gave readings up to 7% different from the WMO values, with a typical daily error of almost an hour.

Variability of the downwelling diffuse attenuation coefficient with consideration of inelastic scattering.

In situ time-series measurements of spectral diffuse downwelling irradiance from the Bermuda Testbed Mooring are presented. Averaged diffuse attenuation coefficients of downwelling irradiance, Kd,and their elastic and inelastic components are investigated at seven wavelengths. At shorter wavelengths (<510 nm), Kd is weakly dependent on the solar zenith angle owing to the prevailing scattering effect and therefore can be considered a quasi-inherent optical property. At longer wavelengths (>510 nm), Kd shows a strong dependence on the solar zenith angle. As depth increases, inelastic scattering plays a greater role for the underwater light field at red wavelengths.

Evaluation of an innovative sensor for measuring global and diffuse irradiance, and sunshine duration

Delta-T Device Limited of Cambridge, UK have developed an integrated device which enables simultaneous measurement of horizontal global and diffuse irradiance as well as sunshine status at any given instance in time. To evaluate the performance of this new device, horizontal global and diffuse irradiance data were simultaneously collected from Delta-T device and Napier University's CIE First Class daylight monitoring station. To enable a cross check a Kipp & Zonen CM11 global irradiance sensor has also been installed in Currie, south-west Edinburgh. Sunshine duration data have been recorded at the Royal Botanical Garden, Edinburgh using their Campbell-Stokes recorder. Hourly data sets were analysed and plotted within the Microsoft Excel environment. Using the common statistical measures, Root Mean Square Difference (RMSD) and Mean Bias Difference (MBD) the accuracy of measurements of Delta-T sensor's horizontal global and diffuse irradiance, and sunshine duration were investigated. The results show a good performance on the part of Delta-T device for the measurement of global and diffuse irradiance. The sunshine measurements were found to have a lack of consistency and accuracy. It is argued herein that the distance between the respective sensors and the poor accuracy of Campbell-Stokes recorder may be contributing factors to this phenomenon.

Aerosol optical depth measurements and their impact on surface levels of ultraviolet‐B radiation

Surface measurements of total and diffuse UV irradiance at the seven narrowband wavelength channels of the ultraviolet multifilter rotating shadow‐band radiometer (UVMFR) were used to determine total column ozone and aerosol optical depth for two 6‐month periods in 1997 and 1999 at a site in the Blue Ridge Mountains of North Carolina. The retrieved column ozone displayed a seasonal dependence and consistent agreement with the Total Ozone Mapping Spectrometer (TOMS). The mean ratio of retrieved ozone to TOMS ozone was 0.98 with standard deviations of 0.02 and 0.01 for 1997 and 1999, respectively. Aerosol optical depth at 317, 325, 332, and 368 nm was derived for a 6‐month period of 1999. The seasonal trend exhibited is influenced by the persistent summertime haze that occurs in the region. The retrieved aerosol optical depths are used as input in a radiative transfer model to investigate the effect of their realistic values on the calculation of the UV index (UVI) forecasted by the National Weather Service. The percentage change in calculated surface erythemally weighted UV (versus calculations using the standard UVI aerosol inputs) ranges from a 4% increase to a nearly 50% decrease, dependent upon the aerosol optical depth and amount of absorption by aerosols. Based on our measurements, it was found that during the summertime the UV index can deviate by up to −5 index units from the forecast using the standard aerosol inputs.

Physical features of the atmospheric aerosol determined with an aureolemeter and a FSSP probe in the Mediterranean Lampedusa island

In order to investigate the influence of the atmospheric aerosol on the ultraviolet radiation on earth, the measurement campaign Photochemical Activity and Ultraviolet Radiation (PAUR II) Modulation was carried out in the central Mediterranean Sea during the period May–June 1999. Two sites were chosen for measurements: the island of Crete (Greece), and the island of Lampedusa (Italy). The aerosol features over the Lampedusa island, as well as the dust coming from Sahara desert, were investigated by measurements of direct and diffuse solar irradiance carried out with an aureolemeter. The columnar volume size distributions of the aerosol showed a four-modal shape in a less turbid atmosphere when the aerosol optical depth was less than 0.2 at λ=500 nm, and a tri-modal shape in a turbid atmosphere when the aerosol optical depth at the same wavelength was greater than 0.5; the background aerosol turned out to be mainly composed of sea salt. The increase of the aerosol optical depth and of the particles density with radius about 1 μm has been found to be strictly related to the passage of Saharan dust in the time periods 14–22 May and 1–3 June, 1999. The columnar volume of particles obtained by the aureolemeter has been compared with the columnar volume of particles retrieved by in situ measurements carried out with a forward scattering spectrometer probe (FSSP) aboard a light aircraft flying over the island. Although the above two techniques refer to aerosol columns of different height and operate with different resolutions, their relevant results are in good agreement, especially during days with lower aerosol content. The two volume radius distributions have been also compared and their behaviours show a satisfactory agreement, mainly for particles with radius greater than 1 μm.

A comparative analysis of broadband global and diffuse irradiance methods to determine Ångström's turbidity coefficient

SYNOPSIS Two methods to determine Ångström's Turbidity Coefficient, using horizontal global and diffuse irradiances in cloudless skies are compared. It is shown that the method referred to as “Global and diffuse” (GDIM), which does not require other measurements than irradiances, is preferable when horizontal global and diffuse broadband irradiance measurements are available by the side of temperature and relative humidity, necessary measurements to apply the second method. Global and diffuse irradiance measurements are often available, but they are less often accompanied by the other measurements. In these circumstances only GDIM can be applied. GDIM's advantage was taken to determine monthly mean values of Ångström's turbidity coefficient between 1989 and 1998, in Valencia, Spain. It was observed that the turbidity coefficient decreased throughout the period analysed.

Consistency tests applied to the measurement of total, direct, and diffuse shortwave radiation at the surface

Motivated by recent studies suggesting that the clear‐sky atmosphere absorbs more shortwave (solar) radiation than the theoretical models, we have performed two consistency tests on the data used in several of these studies. These data consist of broadband measurements of shortwave irradiance to the surface (total, direct, and diffuse) taken in Oklahoma. In the absence of aerosols, Rayleigh scattering is the sole source of diffuse radiation and thus without any unknown source of atmospheric SW absorption, the measured diffuse irradiance should not be less than that produced by a model incorporating both Rayleigh scattering and conventional atmospheric absorption. The measurements of broadband diffuse irradiance, however, exhibit considerable sub‐Rayleigh behavior. On the other hand, measurements of the diffuse irradiance in narrow spectral bands, centered at 415, 500, and 608 nm, indicate no sub‐Rayleigh behavior, suggesting that exhibited by the broadband measurements is probably unrealistic. Related to this is the finding that the total surface SW irradiance, when evaluated as the sum of the direct‐beam irradiance (pyrheliometer) and the diffuse irradiance (shaded pyranometer), differs considerably and diurnally from the single measurement of the total irradiance by the unshaded pyranometer under conditions in which possible cosine response errors of the unshaded pyranometer have been minimized. This indicates that the pyranometer daytime offsets differ from each other, suggesting their daytime offsets likewise differ from their nighttime offsets, which are nearly identical. We emphasize that these conclusions apply solely to the data for Oklahoma, and they are focused upon obtaining a better understanding of the clear‐sky absorption problem that analyses of these data have raised.

Comparison of spectral direct and diffuse solar irradiance measurements and calculations for cloud‐free conditions

Ground‐based spectral measurements of direct and diffuse solar irradiance from the Rotating Shadowband Spectroradiometer, taken in cloud‐free conditions in Oklahoma in the fall of 1997, are compared over the spectral range 10000–28500 cm−1 to corresponding calculations by an accurate multiple‐scattering radiative transfer model. For each case analyzed, the aerosol optical depths used in the calculation were determined by fitting an Angstrom relation based on the ratio of the direct‐beam measurements to a direct‐beam calculation with no aerosols present. Also used in the calculation was a spectrally‐independent aerosol single‐scattering albedo chosen to provide agreement with the diffuse measurements. The spectral agreement between the measurements and calculations for the direct and diffuse irradiances is very good, providing strong evidence that in this spectral range there are no unmodeled molecular absorbers of significance to the atmospheric energy balance. Especially notable is the correspondence between the observations and calculations for a case characterized by a large amount of water vapor in the direct‐beam path, directly contradicting the suggestion that water vapor absorbs more shortwave radiation than is represented in radiative transfer models.

Identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects

We present an automated method to identify periods of clear skies for a 160° field of view using only 1‐min measurements of surface downwelling total and diffuse shortwave irradiance. The clear‐sky detection method is verified using Whole Sky Imager and lidar data, observer reports, and model comparisons. Identified clear‐sky irradiance measurements are then used to empirically fit clear‐sky irradiance functions using the cosine of the solar zenith angle as the independent variable. These fitted functions produce continuous estimates of clear‐sky total, diffuse, and direct component shortwave irradiances. While this method ignores diurnal changes in such variables as column water vapor and aerosol amounts and changes between clear‐sky days, it is shown that the resultant clear‐sky irradiance estimates have RMS uncertainty comparable to the uncertainty of the measuring instruments themselves. The estimated clear‐sky irradiances are used to estimate the effect of clouds on the downwelling shortwave irradiance as a difference between the measured and clear‐sky amounts. We show that the cloud effect calculations from this method appear to decrease the uncertainty due to systematic pyranometer offsets and cosine response errors. Thus any data set that includes downwelling diffuse and total shortwave measurements can be processed to identify clear‐sky periods and produce estimates of clear‐sky irradiance and cloud effects.

Estimating the diffuse component from daily and monthly measurements of global radiation

In this study ≈83 000 measurements of daily global ( R s) and diffuse ( R d) solar irradiance from 25 sites in Australia and Antarctica (12°S–67°S, 1971–1995) are used to develop correlations between the diffuse fraction ( R d/ R s) and the clearness index ( R s/ R o) where R o is the extra-terrestrial solar irradiance. A single set of parameters which are slightly dependent on latitude are derived for use with daily irradiance data (typical R 2 ≈ 0.90). Comparisons with previously published regressions from the northern hemisphere indicate that the model and parameters are also suitable for use in that hemisphere. A theoretical approach is used to scale the daily model to a monthly basis and subsequent testing established the validity of the theoretical approach. However, for practical applications, empirical relationships are necessary to estimate the variances and covariances which are part of the theory but which are not normally available in databases. A final empirical scaling model was developed to estimate the monthly average daily diffuse radiation (RMS = 0.91 MJ m −2 day −1, R 2 = 0.86, N = 2780). The methods are intended for use in broad scale carbon balance models but should be equally applicable to a broad range of solar engineering tasks.

Calibration and data elaboration procedure for sky irradiance measurements

The problems encountered in the elaboration of measurements of direct and sky diffuse solar irradiance are the following: (1) to carry out the calibration for the direct irradiance, which consists in determining the direct irradiance at the upper limit of the atmosphere; (2) to carry out the calibration for the diffuse irradiance, which consists in determining the solid viewing angle of the sky radiometer; (3) to determine the input parameters, namely, ground albedo, real and imaginary parts of the aerosol refractive index, and aerosol radius range; and (4) to determine from the optical data the columnar aerosol optical depth and volume radius distribution. With experimental data and numerical simulations a procedure is shown that enables one to carry out the two calibrations needed for the sky radiometer, to determine a best estimate of the input parameters, and, finally, to obtain the average features of the atmospheric aerosols. An interesting finding is that inversion of only data of diffuse irradiance yields the same accuracy of result as data of both diffuse and direct irradiance; in this case, only calibration of the solid viewing angle of the sky radiometer is needed, thus shortening the elaboration procedure. Measurements were carried out in the Western Mediterranean Sea (Italy), in Tokyo (Japan), and in Ushuaia (Tierra del Fuego, Argentina); data were elaborated with a new software package, the Skyrad code, based on an efficient radiative transfer scheme.