Solar Spectral Irradiance Measurements Research Articles

The Solar Spectral Irradiance from 200 to 2400 nm as Measured by the SOLSPEC Spectrometer from the Atlas and Eureca Missions

The SOLar SPECtrum (SOLSPEC) and the SOlar SPectrum (SOSP) spectrometers are two twin instruments built to carry out solar spectral irradiance measurements. They are made of three spectrometers dedicated to observations in the ultraviolet, visible and infrared domains. SOLSPEC flew with the ATmospheric Laboratory for Applications and Science (ATLAS) while SOSP flew on the EUropean Retrieval CArrier (EURECA) missions. ATLAS 1 and 2 data being already published, this paper is mostly dedicated to the ATLAS 3 and EURECA data in the IR domain. Comparisons between the ATLAS data sets and the Upper Atmosphere Research Satellite (UARS) results are made. EURECA IR data are shown and compared with previous results. Our best UV, visible and IR spectra are finally merged into a single absolute solar irradiance spectrum covering the 200 to 2400 nm domain.

Direct solar spectral irradiance and transmittance measurements from 350 to 2500 nm

A radiometrically stable, commercially available spectroradiometer was used in conjunction with a simple, custom-designed telescope to make spectrally continuous measurements of solar spectral transmittance and directly transmitted solar spectral irradiance. The wavelength range of the instrument is 350-2500 nm and the resolution is 3-11.7 nm. Laboratory radiometric calibrations show the instrument to be stable to better than 1.0% over a nine-month period. The instrument and telescope are highly portable, can be set up in a matter of minutes, and can be operated by one person. A method of absolute radiometric calibration that can be tied to published top-of-the-atmosphere (TOA) solar spectra in valid Langley channels as well as regions of strong molecular absorption is also presented. High-altitude Langley plot calibration experiments indicate that this technique is limited ultimately by the current uncertainties in the TOA solar spectra, approximately 2-3%. Example comparisons of measured and modtran-modeled direct solar irradiance show that the model can be parameterized to agree with measurements over the large majority of the wavelength range to the 3% level for the two example cases shown. Side-by-side comparisons with a filter-based solar radiometer are in excellent agreement, with a mean absolute difference of tau = 0.0036 for eight overlapping wavelengths over three experiment days.

Multivariate analysis of solar spectral irradiance measurements

Principal component analysis is used to characterize approximately 7000 downwelling solar irradiance spectra retrieved at the Southern Great Plains site during an Atmospheric Radiation Measurement (ARM) shortwave intensive operating period. This analysis technique has proven to be very effective in reducing a large set of variables into a much smaller set of independent variables while retaining the information content. It is used to determine the minimum number of parameters necessary to characterize atmospheric spectral irradiance or the dimensionality of atmospheric variability. It was found that well over 99% of the spectral information was contained in the first six mutually orthogonal linear combinations of the observed variables (flux at various wavelengths). Rotation of the principal components was effective in separating various components by their independent physical influences. The majority of the variability in the downwelling solar irradiance (380–1000 nm) was explained by the following fundamental atmospheric parameters (in order of their importance): cloud scattering, water vapor absorption, molecular scattering, and ozone absorption. In contrast to what has been proposed as a resolution to a clear‐sky absorption anomaly, no unexpected gaseous absorption signature was found in any of the significant components.

Interannual variability in solar ultraviolet irradiance over decadal time scales at latitude 55 degrees south.

Ground-based measurements of solar UV spectral irradiance made from Ushuaia, Argentina at latitude 55 degrees S reveal a large degree of variability among corresponding months of different years over the period from September 1990 through April 1998. The magnitude and wavelength dependence of year-to-year changes in monthly spectral UV-B irradiation are consistent with expectations based on the behavior of column ozone and cloudiness. When combined with satellite measurements of column ozone, a regression model fit to the ground-based data set allows estimates of monthly UV-B irradiation over a time frame of two decades, 1978-1998, during several months of the year. Results show a general increase in ground-level irradiation at 305.0 nm from the end of the 1970s to the early 1990s during calendar months from September through December. This is followed by generally smaller irradiances through the middle to late 1990s for all months except November, where the increase continues through the end of the data record. The long-term variability in monthly irradiation over the time period studied is more complicated than can be described by a simple linear trend.

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...

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.

Solar spectral irradiance measurements: visible to near-infrared regions

A spaceborne spectral irradiance monitor (SIM) is being developed at the Laboratory for Atmospheric and Space Physics, University of Colorado, to measure solar spectral irradiance and its variation for the NASA Earth Observing System (EOS) SOLar STellar Irradiance Comparison Experiment (SOLSTICE). The SIM consists of two independent and identical prism spectrometers. Each channel is equipped with an electrical-substitution radiometer and additional photodiodes to provide spectral coverage from 0.3 µm to 2.0 µm with a resolving power greater than 30. One of the spectrometers is used daily, while the other is used on a lower duty cycle to account for instrument degradation caused by solar exposure. The prism is rotated on a flex pivot and driven with a voice-coil motor. Closed-loop drive control and wavelength calibration are achieved by using a concave mirror on the prism table to focus a second solar beam onto a linear charge-coupled device (CCD) located at the focal plane of the prism. Radiation-induced changes in prism transmission are measured in flight by using one of the spectrometers to transfer monochromatic light to the other spectrometer. Visible and infrared photodiodes on a flip-arm measure the ratio of the beam into and out of the prism, thus measuring the prism transmission as a function of wavelength.

New procedure for interpolating NIST FEL lamp irradiances

The Shuttle Solar Backscatter Ultraviolet (SSBUV) programme uses 1000 W, quartz-halogen, tungsten coiled filament (FEL) lamps as the primary radiometric calibration source in the 250 nm to 405 nm wavelength region. The National Institute for Standards and Technology (NIST) recommends using the product of a fifth-order polynomial and a Planck function as a fitting function for interpolating the NIST-calibrated FEL lamp irradiances to users' wavelengths. The NIST fitting procedure can result in more than a 1% overshoot in some data sets. This problem is related to the unconstrained properties of the fifth-order polynomial. Overshooting during lamp irradiance interpolation gives rise to corresponding errors in the SSBUV calibration and solar spectral irradiance measurements. Errors of this magnitude are unacceptably large for the SSBUV, and we have therefore developed a new interpolation scheme. A new fitting procedure has been developed to replace the NIST procedure in the SSBUV calibrations. Although it uses fewer fitting parameters, the new fitting function provides a better fit than the NIST fitting function does. The overshoot problems experienced with the NIST procedure do not exist in the newly interpolated lamp irradiances. We have also investigated the properties of the NIST FEL lamp interpolation scheme in the visible and near-infrared ranges, finding that the overshoot problem experienced in the ultraviolet extends into these regions, with potential errors of the order of 0.5% to 1%.

Calibration of the SOLSPEC spectrometer to measure the solar irradiance from space

The SOLSPEC instrument has been built to carry out absolute solar spectral irradiance measurements from space. It was first flown in December 1983 on mission STS 9 (Spacelab 1), from August 1992 to June 1993 on board the free-flying EURECA platform with mission STS 46 (deployment) and STS 57 (retrieval), then flown three times more with the ATLAS missions STS 45, 56 and 66. Further applications are foreseen in combination with the International Space Station Alpha (ISSA). SOLSPEC covers a wavelength range from 180 nm to 3000 nm split into three separate spectrometer channels (ultraviolet, visible and infrared) and has been calibrated pre- and post-flight using a black body at the Landessternwarte Heidelberg with temperatures up to 3300 K. In this paper the calibration concept is presented, together with a short instrument description and suggestions for improvements in order that higher black-body temperatures can be reached.

NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 2. Results, validation, and comparisons

Accurately measuring long‐term solar UV variability is an experimental challenge because instrument response degradations are typically large enough to obscure solar change. For satellite instruments, one solution is a series of regular comparisons with a well‐calibrated reference. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument made solar spectral irradiance measurements between 170 and 400 nm from December 1988 to October 1994, covering the maximum and most of the decline of solar cycle 22. The NOAA 11 irradiance data were corrected for long‐term instrument sensitivity changes using comparisons with coincident flights of the Shuttle SBUV (SSBUV) instrument. The NOAA 11 data show a decrease of 7.0(±1.8)% in smoothed 200–208 nm irradiance from Cycle 22 maximum in mid‐1989 to October 1994, near solar minimum. The long‐term decrease in solar irradiance at 250 nm was ∼3.5(±1.8)%. Longward of 300 nm, no solar variations were observed to within the 1% accuracy of the data. The NOAA 11 measurements overlap observations from the Upper Atmosphere Research Satellite (UARS) Solar Stellar Irradiance Comparison Experiment (SOLSTICE) and Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instruments from October 1991 to October 1994, providing the first opportunity to compare three coincident long‐term solar UV irradiance data sets. We find reasonable agreement between the NOAA 11, SOLSTICE, and SUSIM results at all wavelengths in the 170–400 nm region. Power spectral analysis gives consistent results for all three instruments on solar rotational timescales, and reveals the evolution of solar rotation periodicity and strength during a solar cycle. We find significant differences between instruments in both period and spectral location when the spectral irradiance data are analyzed on intermediate (50–250 days) timescales. The NOAA 11 spectral irradiance data provide a valuable complement to the UARS solar data, and capture the entire maximum of solar cycle 22.

NOAA 11 Solar Backscattered Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 1. Observations and long‐term calibration

Measurements of solar ultraviolet (UV) irradiance are a valuable diagnostic of physical processes in the solar atmosphere and a key component in characterizing the external forcing of the Earth's atmosphere. However, the deleterious effects of solar UV radiation on satellite instrument components have complicated efforts to determine the magnitude of long‐term solar variations for almost 30 years. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument, primarily designed to measure stratospheric ozone, also made daily spectral scan measurements of solar UV irradiance in the 160–405 nm region from February 1989 to October 1994. An onboard calibration system and comparisons with coincident Shuttle SBUV (SSBUV) measurements were used to correct for long‐term NOAA 11 instrument sensitivity changes. Time series of the NOAA 11 solar irradiance data indicate a long‐term accuracy of approximately ±0.9–2.3% (2σ) over the 5.5‐year data record. Long‐term solar irradiance changes of approximately −3% are observed between 215 and 250 nm, increasing in magnitude to approximately −7% at 200–205 nm. Additional discussion of solar variations observed by NOAA 11 SBUV/2 are presented by DeLand and Cebula [this issue].

Direct Solar Spectral Irradiance Measurements and Updated Simple Transmittance Models

Abstract A set of 509 direct solar irradiance spectra, carefully measured over one year, is checked against spectral irradiances computed from five updated transmittance models. The wavelengths under investigation range from 290 to 900 nm, with a 5- or 10-nm step. The parameters explored include the solar altitude angle, with a range from 13° to 68°, and the standard Linke turbidity factor, with a range from 2.0 to 6.0. Measurement devices and experimental processes are described in detail in the paper. The comparison between measured and computed values is carried out by means of the relative mean bias error and the mean absolute relative error. These coefficients are applied to ultraviolet-B and ultraviolet-A total irradiances, and to visible and near-IR spectral irradiances. No clear and systematic sensitivity of the models or measurements to the solar altitude and the turbidity parameters is observed. Of the five models tested, three of them give mean coefficient values between 7% and 16% in UV bands ...

Outdoor measurement of multi-junction solar cells and modules based on the reference sunlight method

To evaluate the indoor measurement accuracy of multi-junction solar cells and modules experimentally, we have successfully used natural sunlight whose spectral irradiance is close to the reference sunlight. The reference sunlight for evaluating the performance of solar cells can be realized by choosing natural sunlight incident on a sun-facing 37° tilted surface at air mass of 1.5, when the atmosphere has turbidity of 0.27, precipitable water of 1.42 cm and so on. The measurement of atmospheric parameters was performed based mainly on the measurement of direct solar spectral irradiance and additionally on the measurements of irradiance and illuminance. In addition, measured values were compared with aerological observation data. The comparison of I–V characteristics measured outdoors and indoors shows that our pulsed-light solar simulator is capable of measuring multi-junction amorphous silicon alloy solar cells and modules accurately enough when its irradiance is set by an amorphous silicon reference cell. This fact is also confirmed by the examination of spectral irradiance on the test plane of the solar simulator. Moreover, we showed that irradiance (or illuminance) can be used instead of atmospheric turbidity and also water vapor content on a surface of the Earth can be used as a rough estimate of precipitable water for finding the reference sunlight conditions.

OBSERVATION OF THE UV SOLAR SPECTRAL IRRADIANCE BETWEEN 200 AND 350 nm DURING THE ATLAS I MISSION BY THE SOLSPEC SPECTROMETER

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from space. It consists of three spectrometers designed to measure the solar spectral irradiance from 180 to 3000 nm. It flew for the first time in December 1983 with the SpaceLab 1 mission (SL1) and later with the ATLAS missions after significant improvement of the instrument optics and calibration procedures. For the ATLAS 1 mission in March 1992, the thermal conditions encountered during the measurements were better than those of SL1, leading to better data quality. Furthermore, other Sun spectrometers, two on the same platform and two others on board the Upper Atmosphere Research Satellite, have also carried out UV absolute spectral measurements at the same time. These opportunities allowed comparisons of solar irradiance determinations. The UV part of the measurements made during that mission is presented here as well as its calibration and accuracy analysis.

Transformation of irradiance measurements into spectral actinic flux for photolysis rates determination

The actinic flux is the only radiometric quantity suitable for photolysisfrequency determination. It is derived from solar spectral irradiancemeasurements performed by a portable spectroradiometer in the 300–850nm wavelength range. The spectral irradiance is first divided into a directpart and a diffuse part, according to the atmospheric conditions, and thenconverted into the corresponding actinic flux quantity. As an intermediary,the ratio $$r_{dd} $$ of diffuse actinic flux to diffuseirradiance is calculated by the spherical harmonics radiative code withrespect to wavelength, solar zenith angle, surface albedo, and aerosolproperties.The results of extensive sensitivity studies of $$r_{dd} $$ asa function of the main atmospheric parameters are discussed and lead to theconclusion that aerosol optical depth is the major critical value for aprecise $$r_{dd} $$ determination. The global algorithm totransform spectral irradiance into actinic flux is finally applied forphotodissociation rate calculations by convolution of the obtained actinicflux spectra with the absorption cross-sections and quantum yields of themolecule studied. Photolysis rates of different atmospheric photooxidants havebeen measured with this spectroradiometric method during the summers of 1993and 1994 in Brittany and in Portugal. The ozone and nitrogen dioxidephotodissociation rates obtained present a good agreement with thej(O3) and j(NO2) actinometerresults, for the same experimental conditions in Brittany.

Radiometry at the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Centre

The Physikalisch-Meteorologisches Observatorium Davos and World Radiation Centre (PMODIWRC) is responsible for the worldwide homogeneity of measurements of the meteorologically important radiation: the <i>short-wave</i> radiation that originates from the sun and is manifested as total and spectral radiation in the direct solar beam and the diffuse radiation scattered by the atmosphere, and the long-wave radiation, which is infrared radiation emitted by the atmosphere and the Earth's surface. The total solar irradiance measurements are performed by electrically calibrated absolute radiometers of high accuracy. Improved methods have to be developed to transfer the scale of irradiance accurately to space to ensure the necessary continuity of solar monitoring programs. Results of such transfers by comparison with highly accurate cryogenic radiometers are presented. For the measurements of the direct solar spectral irradiance, filter radiometers-so-called sun photometers (SPMs)-are used. Highly stable SPMs have been developed at PMOD/WRC to measure the solar spectral irradiance from the ultraviolet to the near infrared. Results from the space experiments are presented, with special emphasis on the problem of degradation. A third field of research is the accurate measurement of the surface radiation budget and the determination of its dependence on the elevation in Alpine regions and on increasing concentration of greenhouse gases. The implementation of a network in the Swiss Alps is described, and some results are presented.

Results of solar spectral irradiance measurements by SOVA2 on EURECA

The solar irradiance has been measured at 3 wavelengths (335, 500 and 862nm) by the experiment SOVA2 from the European Retrievable Carrier (EURECA). The dataset, corrected for instrumental effects and degradation, is examined for solar variability on timescales from minutes to days.

Variability of solar UV‐B radiation at high and middle latitudes during EASOE 1991/92

Measurements of ultraviolet solar spectral irradiance from ground‐based observations made at Reykjavik (64°N) and Thessaloniki (40°N) are used to calculate erythemally‐weighted UV‐B dose during EASOE. It is shown that the largest values of UV‐B dose observed were about double those expected with an unperturbed ozone amount under cloudy conditions. At middle latitudes UV‐B doses were within expectation throughout the period of the campaign.

A differential absorption technique in the near infra-red to determine precipitable water

Abstract The analysis of an improved version of a radiometric method is described by which the total water amount over the entire atmospheric column may be determined by the use of a moderate resolution grating spectrometer. The method requires transmission measurements of solar spectral irradiance at differentially absorbing channels located in the wing of a water vapour band. Two water vapour bands are investigated: the 0.9- and l.4-μm H z0 bands. Since the method is based on the establishment of ratios of optical signals at given channels, there is no need for absolute calibration. Using an extensive set of rawinsonde temperature and water vapour mixing ratio profiles along with LOWTRAN 7 code, empirical curves between precipitable water and ratios of solar direct irradiance at differentially absorbing channels have been derived. The interrelationship with atmospheric aerosol has been particularly analysed. The method which has been proposed is based on observing the extinction of solar beam irradiance at four adjacent wavelengths rather than at two as it is habit. The result is a good independence of the method on the aerosol atmospheric load.

Middle Ultraviolet Solar Spectral Irradiance Measurements, 1985-1992, from the SBUV/2 and SSBUV Instruments

Solar spectral irradiance measurements from the NOAA-9 and NOAA-11 SBUV/2 instruments and SSBUV are presented and compared. Inter-instrument solar spectral irradiance differences on the order of 5-10% are observed. Internal comparisons of the SBUV/2 solar data suggest instrument sensitivity changes ranging from less than 1% per year near 400 nm to approximately 5% per year near 200 nm.The mission-to-mission accuracy of the SSBUV solar spectral irradiance measurements is better than ±2%. Initial results of our investigation into the feasibility of using SSBUV data to maintain the long-term calibration of the SBUV/2 instruments for solar spectral irradiance monitoring are reviewed.