Spectral Irradiance Measurements Research Articles

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.

Spectroradiometric Sun Photometry

Abstract This paper presents a method for calculating atmospheric transmittance from direct-beam solar spectral irradiance measurements under cloudless skies by treating spectral irradiance as a multichannel sun photometer. Computing the ratio of the measured spectral irradiance to the extraterrestrial spectral irradiance at the top of the atmosphere produces the atmospheric transmittance as a function of wavelength. Individual band absorber amounts and scattering parameters, based on the LOWTRAN 7 atmospheric transmittance model, are then extracted from the transmittance using iterative fitting over wavelength regions where only a few species are active. Using these parameters to extrapolate the entire terrestrial solar spectrum, the wavelength-integrated spectral irradiance is shown to be within 2% of the total irradiance measured with an absolute cavity radiometer. Instrumentation and procedures that have been used with the method at the National Renewable Energy Laboratory since 1987 are described, al...

Spectral irradiance measurements of tungsten lamps with filter radiometers in the spectral range 290 nm to 900 nm

A method of measuring the absolute spectral irradiance of quartz-halogen-tungsten lamps is described, based on the known responsivity of a filter radiometer, the components of which are separately characterized. The characterization is described for the wide wavelength range essential for deriving the spectrum of a lamp, from 260 nm to 950 nm. Novel methods of interpolation and measurement are implemented for the spectral responsivity of the filter radiometer. The combined standard uncertainty of spectral irradiance measurements is less than 1.4 parts in 102 from 290 nm to 320 nm (ultraviolet B) and 4 parts in 103 from 440 nm to 900 nm (visible to near-infrared). As an example, the derived spectral irradiances of two lamps measured at the Helsinki University of Technology (HUT, Finland) are presented and compared with the measurement results of the National Institute of Standards and Technology (NIST, USA) and the Physikalisch-Technische Bundesanstalt (PTB, Germany). The comparisons indicate that the HUT spectral irradiance scale is between those of the NIST and the PTB in the wavelength range 290 nm to 900 nm. The long-term reproducibility of the spectral irradiance measurements is also presented. Over a period of two years, the reproducibility appears to be better than 1 part in 102.

Quantifying solar spectral irradiance in aquatic habitats for the assessment of photoenhanced toxicity

AbstractThe spectra and intensity of solar radiation (solar spectral irradiance [SSI]) was quantified in selected aquatic habitats in the vicinity of an oil field on the California coast. Solar spectral irradiance measurements consisted of spectral scans (280–700 nm) and radiometric measurements of ultraviolet (UV): UVB (280–320 nm) and UVA (320–400 nm). Solar spectral irradiance measurements were taken at the surface and at various depths in two marsh ponds, a shallow wetland, an estuary lagoon, and the intertidal area of a high‐energy sandy beach. Daily fluctuation in SSI showed a general parabolic relationship with time; maximum structure–activity relationship (SAR) was observed at approximate solar noon. Solar spectral irradiance measurements taken at 10‐cm depth at approximate solar noon in multiple aquatic habitats exhibited only a twofold variation in visible light and UVA and a 4.5‐fold variation in UVB. Visible light ranged from 11,000 to 19,000 μW/cm2, UVA ranged from 460 to 1,100 μW/cm2, and UVB ranged from 8.4 to 38 μW/cm2. In each habitat, the attenuation of light intensity with increasing water depth was differentially affected over specific wavelengths of SSI. The study results allowed the development of environmentally realistic light regimes necessary for photoenhanced toxicity studies.

Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradiance

In the present study, a method is proposed for retrieving UV actinic flux density spectra at ground level from global and direct UV spectral irradiance measurements. The most important parameters to establish the relation between downwelling actinic flux density and global UV irradiance are the ratio of direct to global irradiance and the description of the angular distribution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within ±5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength‐dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in atmospheres with high aerosol content. Various sources of uncertainties, which are involved in the proposed methodology, are discussed and quantified.

Consistency tests on UV spectral irradiance measurements using modeling and a broadband instrument

The UV spectroradiometer used at the University of Lille is presented, and its characterization and calibration are described. In order to detect possible shifts between calibrations, two different consistency tests are regularly performed on cloudless days. The erythemal dose rate obtained by integrating the weighted spectrum is compared to broadband measurements and modeling results. A Sun photometer provides the aerosol optical depth necessary as an input to the radiative transfer model. Results of these tests are presented for some cases; the general agreement is within 5%, which corresponds to the expected uncertainties.

Effect of row spacing, growth stage, and nitrogen rate on spectral irradiance in winter wheat

Soil reflectance affects spectral irradiance measurements taken in winter wheat at early stages of growth when percent cover is low. The objective of this study was to determine the critical percent vegetation coverage needed for forage nitrogen (N) uptake calibration with indirect spectral irradiance measurements. Two field experiments were conducted at Tipton and Perkins, OK in October 1996. The effect of row spacing (15.2, 19.0, 25.4, and 30.5 cm) and growth stage (Feekes 4 and 5) under various N fertilizer rates (0, 56, 112, and 168 kg N ha‐1) on spectral irradiance measurements from wheat was evaluated. The normalized difference vegetative index (NDVI) was used to characterize wheat canopy irradiance. In general, NDVI decreased with increasing row spacing and increased with N fertilizer rate at Feekes growth stage 4. Row spacing and N rate were independent of each other since no significant interaction was found. High correlation (r=0.81–0.98) was observed between NDVI and vegetation coverage. Percent vegetation coverage was a good predictor of the other dependent variables including forage dry matter, and total N uptake, which could indirectly be determined using NDVI. The coefficients of variation (CV's) from NDVI values decreased with increasing vegetation coverage suggesting that less variable NDVI values (CV less than 10%) might be obtained from plots where vegetation coverage exceeds 50%.

A new inversion algorithm to retrieve instantaneous values for the aerosol optical depth from spectral irradiance measurements

A new algorithm to determine both the instantaneous values of the aerosol optical depth /spl tau//sub /spl alpha//spl lambda// and the daily atmospheric ozone content Oz is presented. The algorithm is based on inversion methods and uses instantaneous experimental data of the total atmospheric optical depth /spl tau/T/sub /spl lambda//. A description of the method and an analysis of the Oz and /spl tau//sub /spl alpha//spl lambda// values for clear days in Valencia (Spain) is presented. The values estimated by means of this algorithm have been compared with those obtained by the algorithm proposed by Flittner et al. (1993) and by an algorithm, based on Flittner but employing the covariance matrix associated with the experimental errors in its calculations. Experimental measurements of direct spectral irradiance at normal incidence have been used as input parameters in these algorithms. The error analysis shows that the errors introduced by this method are small. Furthermore, the proposed method has the additional advantage of showing the variations of the aerosol optical depth with the optical air mass.

Modelling short-term spectral irradiance variations

On time-scales of the solar rotation most of the solar irradiance variations are caused by the changing distribution of solar surface magnetic features. We model these short-term irradiance variations using calculations of sunspot and facular contrasts as a function of wavelength and limb angle on the Sun. The position of active regions on the solar disc is derived from the MDI magnetograms. The reconstructed irradiance variations are compared with total and spectral irradiance measurements obtained by the VIRGO experiment on SOHO.

All-weather ultraviolet solar spectra retrieved at a 05-Hz sampling rate

A measurement scheme and an algorithm have been developed to retrieve global irradiance ultraviolet solar spectra (290-400 nm) at a sampling rate of 0.5 Hz. The algorithm combines spectral irradiance measurements performed with a slow (a few minutes) scanning spectroradiometer (Optronic Model OL752) and a moderate bandwidth multichannel radiometer (Biospherical ground-based ultraviolet radiometer Model 541). The filter radiometer instrument allows for continuous observations of global UV radiation at five channels (approximately 10-nm bandwidth), performed simultaneously with spectral measurements. Information about changing cloud conditions during a spectral scan was retrieved from filter measurements and applied to spectral data, hence estimated spectra without cloud variations could be constructed. The quality of the estimated spectra depends on data quality from both instruments. The method works well in all kinds of weather conditions, as long as the Sun is above the horizon and none of the instruments are hampered by measurement errors.

Uncertainty of measurements of spectral solar UV irradiance

Most investigations on the nature and effects of solar ultraviolet (UV) radiation at the Earth's surface require measurements of high accuracy combined with well‐defined procedures to assess their quality. Here we present a general evaluation of all relevant errors and uncertainties associated with measurements of spectral global irradiance in the UV. The uncertainties are quantified in terms of dependence of the characteristics of the spectroradiometer, the uncertainty of calibration standards, the solar zenith angle, and atmospheric conditions. The methodologies and equations presented can be applied to most spectroradiometers currently employed for UV research. The sources of error addressed include radiometric calibration, cosine error, spectral resolution, wavelength misalignment, stability, noise, stray light, and timing errors. The practical application of the method is demonstrated by setting up a complete uncertainty table for the mobile spectroradiometer of the Fraunhofer Institute for Atmospheric Environmental Research (IFU). This instrument has successfully participated in several international intercomparisons of UV spectroradiometers. The expanded uncertainty (coverage factor k = 2) for measurements of global spectral irradiance conducted with this instrument varies between 6.3% in the UVA and 12.7% at 300 nm and 60° solar zenith angle. The expanded uncertainties in erythemally and DNA weighted irradiances are 6.1% and 6.6%, respectively. These expanded uncertainties are comparable to uncertainties at the 2σ level in conventional statistics. A substantial reduction of these uncertainties would require smaller uncertainties in the irradiance standards used to calibrate the instrument. Though uncertainties caused by wavelength misalignment and noise become prominent in the shortwave UVB, which is the most important spectral range for UV trend detection, the results indicate that the accuracy of the IFU radiometer is sufficient to detect long‐term trends in UV arising from a 3% change in atmospheric ozone. The detection of trends caused by a 1% change in ozone may be beyond the capabilities of current instrumentation.

COMPARISON OF ARESE CLEAR SKY SURFACE RADIATION MEASUREMENTS

A variety of broadband and spectral surface irradiance measurements from three instrument platforms show significant agreement with one another during three clear sky days (11, 15 and 18 October 1995) during the ARM Enhanced Shortwave Experiment (ARESE). This agreement is only possible when carefully considering absolute and angular calibration issues and applying the recommended correction procedures to the raw signals. By removing systematic measurement errors associated with the various instruments, it is possible to achieve consistencies in the various broadband data sets good to about 1–2% accuracy. It is through these careful comparisons that the measurement techniques currently used in determining downwelling surface flux radiances reflect the highest degree of accuracy and certainty.

Characterisation of optical detectors using high-accuracy instruments

The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1 σ level. The uncertainty is caused almost completely by the multipoint measuring system.

Retrieval of the aerosol size distribution from spectroradiometer measurements at a coastal site in the Mediterranean Sea

Values of aerosol optical depth,tau a lambda, have been obtained from spectral irradiance measurements registered at Valencia (Spain) between July 1993 and June 1995. Aerosol size distribution functions were calculated by considering these tau a lambda values and by using the inversion method proposed by King. The results show that such functions are bimodal, with a primary mode that is not apparent and a clearly defined secondary mode in the range 0.5-0.8mu m. Generally, the shape of the distribution function does not depend on the complex refractive index m assigned to the aerosols, although an increase in the imaginary part of m produces a decrease in the function's minimum.

Measurements of irradiance attenuation and estimation of aerosol single scattering albedo for biomass burning aerosols in Amazonia

Investigation of the effects of biomass burning aerosols on the surface irradiance were conducted as a part of the Smoke, Clouds, and Radiation ‐ Brazil (SCAR‐B) experiment during August–September 1995. Measurements of broadband and spectral irradiance, in conjuction with measurements of aerosol physical and optical properties (optical depth, phase function, and size distribution) were made under varying conditions of aerosol loading during the SCAR‐B field campaign. Estimates of aerosol single scattering albedo (ω0) were made from matching of the measured irradiance values to the model computed irradiances by varying ω0, for observations made under cloudless conditions. Values of ω0, at approximately 550 nm, estimated from this technique using broadband 400–700 nm irradiance measurements, ranged from approximately 0.82 to 0.94 for the dates and times of these SCAR‐B measurements. Utilizing spectral irradiance data, the model retrieved values of ω0decreased with increasing wavelength, with the change of ω0as a function of wavelength differing on different days. Reductions in photosynthetically active radiation (PAR; 400‐700 nm) incident at the surface were computed to range from about 20 to 45% compared to background aerosol conditions for the 2 month biomass burning season at several locations in the southern Amazon Basin. These large reductions in incident PAR at the surface due to the heavy aerosol loadings could have implications for primary production of sensitive ecosystems. In addition, reductions of total incident solar radiation from aerosol direct radiative effects may have significant impact on reducing surface heating and increasing aerosol layer heating from absorption.

Primary productivity of the Palmer Long Term Ecological Research Area and the Southern Ocean

A major objective of the Palmer Long Term Ecological Research (Palmer LTER) project is to obtain a comprehensive understanding of the various components of the Antarctic marine ecosystem. Phytoplankton production plays a key role in this so-called high nutrient, low chlorophyll environment, and factors that regulate production include those that control cell growth (light, temperature, and nutrients) and those that control cell accumulation rate and hence population growth (water column stability, grazing, and sinking). Sea ice mediates several of these factors and frequently conditions the water column for a spring bloom which is characterized by a pulse of production restricted in both time and space. This study models the spatial and temporal variability of primary production within the Palmer LTER area west of the Antarctic Peninsula and discusses this production in the context of historical data for the Southern Ocean. Primary production for the Southern Ocean and the Palmer LTER area have been computed using both light-pigment production models [Smith, R.C., Bidigare, R.R., Prézelin, B.B., Baker, K.S., Brooks, J.M., 1987. Optical characterization of primary productivity across a coastal front. Mar. Biol. (96), 575–591; Bidigare, R.R., Smith, R.C., Baker, K.S., Marra, J., 1987. Oceanic primary production estimates from measurements of spectral irradiance and pigment concentrations. Global Biogeochem. Cycles (1), 171–186; Morel, A., Berthon, J.F., 1989. Surface pigments, algal biomass profiles and potential production of the euphotic layer—relationships reinvestigated in view of remote-sensing applications. Limnol. Oceanogr. (34), 1545–1562] and an ice edge production model [Nelson, D.M., Smith, W.O., 1986. Phytoplankton bloom dynamics of the western Ross Sea ice edge: II. Mesoscale cycling of nitrogen and silicon. Deep-Sea Res. (33), 1389–1412; Wilson, D.L., Smith, W.O., Nelson, D.M., 1986. Phytoplankton bloom dynamics of the Western Ross Sea ice edge: I. primary productivity and species-specific production. Deep-Sea Res., 33, 1375–1387; Smith, W.O., Nelson, D.M., 1986. Importance of ice edge phytoplankton production in the Southern Ocean. BioScience (36), 251–257]. Chlorophyll concentrations, total photosynthetically available radiation (PAR) and sea ice concentrations were derived from satellite data. These same parameters, in addition to hydrodynamic conditions, have also been determined from shipboard and Palmer Station observations during the LTER program. Model results are compared, sensitivity studies evaluated, and productivity of the Palmer LTER region is discussed in terms of its space time distribution, seasonal and interannual variability, and overall contribution to the marine ecology of the Southern Ocean.

The parameterisation of the atmospheric aerosol optical depth using the Ångström power law

We have analysed the ability of the Ångström power law to model the spectral aerosol optical depth, τ a λ , for the 400–670 nm band, obtained from spectral direct irradiance measurements at normal incidence. The spectra were registered at ground level in Valencia, Spain, using a Li-cor 1800 spectroradiometer. The results obtained showed that the fitting method that introduces lower errors in the determination of the Ångström power law coefficients is to adjust directly the spectral experimental data. In this way the errors obtained for the turbidity coefficient, β, were about 0.004 and for the wavelength exponent, α, 0.07. The correlation coefficient was always greater than 0.95. These values of the correlation coefficient could be improved by parameterisation of τ a λ using an alternative function of the wavelength to the Ångström power law. But this may not be justified for the turbidity values attained in Valencia.

A comparative study of SPCTRAL2 and SMARTS2 parameterised models based on spectral irradiance measurements at Valencia, Spain

Abstract Results obtained using the parametric models SPCTRAL2 and SMARTS2 for the urban area of Valencia, Spain, have been analysed and compared with experimental measurements at ground level obtained with two Li-cor 1800 spectroradiometers with a 6 nm resolution. The study used two different input parameters in both models for the aerosol characterisation: the aerosol optical thickness at 0.5 μ m, τ a λ (0.5) , and the Angstrom turbidity coefficient β . The results obtained show that both algorithms reproduce quite correctly the spectral irradiance experimental values when an urban aerosol model parameterised by the τ a λ (0.5) value is considered. In all the cases the deviations are lower when SMARTS2 code is used.

Cloud flagging of UV spectral irradiance measurements

Cloud flags can be used as identifiers to classify measured spectra of UV irradiance as to the conditions of cloudiness during the time of the spectral measurement. That classification is helpful for quality checks of spectra as well as for their further analysis and comparisons with model calculations. A method has been developed to derive cloud flags for UV spectra. It uses readily available ancillary measurements such as direct (or global) and diffuse radiation, relative sunshine duration and hourly observations of cloud cover and visibility as input data. Application of the cloud flagging algorithm to 16,217 UV spectra measured with Brewer spectroradiometers MKII and MKIII at the station Potsdam in the years 1995 and 1996 classified 807 UV spectra (5%) as `clear sky', with almost all of the remaining data, i.e., 15,384 (95%) being classified as `cloudy', and—due to missing part of corresponding ancillary data—26 spectra classified as `unknown' with referring to cloud conditions. Among the combinations of ancillary data tested, both direct/diffuse ratios and sky brightness turned out to be the most useful parameters for cloud detection including Cirrus clouds. They can also be used to select those spectra distorted by moving clouds. In addition to the simple `clear sky vs. cloudy' classification, direct/diffuse ratios together with sky brigthness have been tentatively used as potential classifiers to separate five different conditions of cloudiness as to their optical effect. The cloud flagging algorithm is neither confined to UV spectra nor to meteorological data of a specific site, but it can be applied to any meteorological data at any other site.

Detector-based transfer standards for improved accuracy in spectral irradiance and radiance measurements

This paper discusses the performance of a range of new detector-based transfer standards for the dissemination of source-based scales. The transfer standards presented include both detector-stabilized and -monitored sources of spectral irradiance and radiance as well as a more radical absolute transfer standard spectrometer. The primary aim in developing the new transfer standards is to improve traceability and accuracy to the International System of Units (SI) while at the same time reducing the cost. Some generic remote-sensing applications are considered as examples.