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Recent observations made by the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) spacecraft suggest that the Sun’s visible and infrared spectral irradiance increased from 2004 to 2008, even as the total solar irradiance measured simultaneously by SORCE’s Total Irradiance Monitor (TIM) decreased. At the same time, solar ultraviolet (UV) irradiance decreased 3–10 times more than expected from prior observations and model calculations of the known effects of sunspot and facular solar features. Analysis of the SIM spectral irradiance observations during the solar minimum epoch of 2008, when solar activity was essentially invariant, exposes trends in the SIM observations relative to both total solar irradiance and solar activity that are unlikely to be solar in origin. The authors suggest that the SIM’s radically different solar variability characterization is a consequence of undetected instrument sensitivity drifts, not true solar spectrum changes. It is thus doubtful that simulations of climate and atmospheric change using SIM measurements are indicative of real terrestrial behavior.

Spectroradiometric measurements of direct solar irradiance traceable to the SI were performed by three spectroradiometer systems during a 3-week campaign in September 2022 at the Izaña Atmospheric Observatory (IZO) located on the island of Tenerife, Canary Islands, Spain. The spectroradiometers provided direct spectral irradiance measurements in the spectral ranges 300 to 550 nm (QASUME), 550 to 1700 nm (QASUME-IR), 300 to 2150 nm (BiTec Sensor, BTS), and 316 to 1030 nm (Precision Solar Spectroradiometer, PSR), with relative standard uncertainties of 0.7 %, 0.9 %, and 1 % for QASUME/QASUME-IR, the PSR, and the BTS respectively. The calibration of QASUME and QASUME-IR was validated prior to this campaign at Physikalisch-Technische Bundesanstalt (PTB) by measuring the spectral irradiance from two spectral irradiance sources, the high-temperature blackbody BB3200pg as a national primary standard and the tuneable laser facility TULIP. The top-of-atmosphere (ToA) solar irradiance spectra from the spectroradiometers were retrieved from direct solar irradiance measurements using zero-air-mass extrapolation during cloud-free conditions, which were then compared to the TSIS-1 Hybrid Solar Reference Spectrum (HSRS). These ToA solar spectra agreed to within 1 % for spectral ranges longer than 400 nm (for QASUME also at shorter wavelengths) in the spectral regions free of significant trace gas absorption and were well within the combined uncertainties over the full investigated spectral range. Using the results from the comparison with QASUME, the relative standard uncertainty of the TSIS-1 HSRS ToA solar spectrum in the spectral range 308 to 400 nm could be reduced from its nominal 1.3 % to 0.8 %, representing the relative standard uncertainty of the QASUME ToA solar spectrum in this spectral range. The spectral aerosol optical depth (AOD) retrieved from the solar irradiance measurements of these spectroradiometers using the TSIS-1 HSRS as the reference ToA solar spectrum agreed to within 0.01 in optical depth in nearly all common spectral channels of two narrowband filter radiometers belonging to the Global Atmosphere Watch Precision Filter Radiometer (GAW-PFR) network and Aerosol Robotic Network (AERONET). This study shows that it is now possible to retrieve spectral AOD over the extended spectral range from 300 to 1700 nm using solar irradiance measurements traceable to the SI using laboratory-calibrated spectroradiometers with similar quality to that from traditional Langley-based calibrated instruments. The main improvement to previous investigations is the recent availability of the high-spectral-resolution TSIS-1 HSRS with very low uncertainties, which provides the top-of-atmosphere reference for the spectral atmospheric transmission measurements obtained from ground-based solar irradiance measurements.

The solar spectrum at the top of the atmosphere contains crucial data for solar physics, astronomy, and geophysics. Accurately determining high-resolution solar reference spectra, whether they are disk-integrated, disk-center, or intermediate cases, represents a new challenge and is of primary importance for all applications where spectral solar radiation needs to be evaluated. These spectra are also essential for interpreting remote sensing measurements that rely on sunlight, such as those obtained by Earth observation satellites or spacecraft exploring other planets. This paper lays a foundation for the implementation of multiple new solar irradiance reference spectra that have high resolution and are representative of solar minimum conditions. We developed the SOLAR high-resolution extraterrestrial reference spectra (SOLAR-HRS disk-integrated spectra) by normalizing high-spectral-resolution solar line data to the absolute irradiance scale of the SOLAR-ISS reference spectrum. The resulting one-of-a-kind SOLAR-HRS disk-integrated spectrum has a spectral resolution varying between 0.001 and 1 nm in the 0.5–4400 nm wavelength range. We also implemented a new high-resolution solar spectrum at the disk-center, covering a range of 650–4400 nm with a spectral resolution of 0.001 to 0.02 nm. We further expanded our analysis by producing several solar spectra for ten different solar view angles ranging from μ = 0.9 to μ = 0.05 (SOLAR-HRS intermediate cases). Finally, we developed new Merged Parallelised Simplified ATLAS spectra (MPS-ATLAS) based on solar modeling with Kurucz and Vald3 solar linelists for both the disk-integrated and disk-center spectra. One of the objectives of implementing all these new solar spectra is to fulfill the requirements of the MicroCarb space mission, which focuses on measuring greenhouse gas emissions. The solar data of this study are openly available.

The accurate measurement of the solar spectrum at the top of the atmosphere and its variability are fundamental inputs for solar physics (Sun modeling), terrestrial atmospheric photochemistry, and Earth’s climate (climate’s modeling). These inputs were the prime objective set in 1996 for the SOLAR International Space Station (ISS). The SOLAR package represents a set of three solar instruments measuring the total and spectral absolute irradiance from 16 nm to 3088 nm. SOLAR was launched with the European Columbus space laboratory in February 2008 aboard the NASA Space Shuttle Atlantis . SOLAR on the ISS tracked the Sun until it was decommissioned in February 2017. The SOLar SPECtrum (SOLSPEC) instrument of the SOLAR payload allowed the measurement of solar spectra in the 165 – 3000 nm wavelength range for almost a decade. Until the end of its mission, SOLAR/SOLSPEC was pushed to its limits to test how it was affected by space environmental effects (external thermal factors) and to better calibrate the space-based spectrometer. To that end, a new solar reference spectrum (SOLAR-ISS – V1.1) representative of the 2008 solar minimum was obtained from the measurements made by the SOLAR/SOLSPEC instrument and its calibrations. The main purpose of this article is to improve the SOLAR-ISS reference spectrum (between 165 and 180 nm in the far ultraviolet, between 216.9 and 226.8 nm in the middle ultraviolet, and between 2400 and 3000 nm in the near-infrared). SOLAR-ISS has a resolution better than 0.1 nm between 165 and 1000 nm, and 1 nm in the 1000 – 3000 nm wavelength range. Finally, a first comparison is made between the new SOLAR-ISS spectrum (V2.0) and the Total and Spectral solar Irradiance Sensor (TSIS-1) spectrum obtained from its first observations from the ISS. Indeed, the launch of TSIS in December 2017 provides a new light on the absolute determination of the solar spectrum and especially in the infrared region of the spectrum.

Carbon dots (CDs) featuring low‐cost, non‐toxic, and appealing optical properties demonstrate promising applications in energy, e.g. solar energy capture and conversion. However, it remains a significant challenge to expand the absorption bands of CDs from visible to near‐infrared (NIR) spectral regions to harness the entire spectrum of sunlight for efficient solar energy utilization. Herein, hierarchical assemblies of CDs (HA‐CDs) are constructed by stepwise assembling monodispersed ultraviolet‐absorbing CDs to water‐soluble visible‐NIR absorbing supra‐CDs (PA‐CDs), and then complexing PA‐CDs with Fe3+ ions to form 3D porous architectures (HA‐CDs) with full solar spectrum absorption and good water resistance. Notably, the HA‐CDs exhibit good hydrophilicity and superior photothermal conversion efficiency of 84% under simulated solar irradiation. The facile Fe3+ ion cross‐linking assembly property enables the in situ preparation of HA‐CDs on various fabric substrates, resulting in low‐cost, high‐performance photothermal conversion products. High‐performance 2D solar‐driven interfacial water evaporation, electricity generation, and water‐electricity cogeneration have been demonstrated in the HA‐CDs in situ coated fabric (HA‐CDs‐fabric). This study provides a novel and effective design approach for the development of high‐performance CD‐based photothermal materials for solar energy applications. Hierarchical assemblies of carbon dots are constructed, achieving full‐solar‐spectrum absorption, superior photothermal properties, and good hydrophilicity. These assemblies can be in situ prepared on a fabric, which exhibits good performance for applications in solar‐driven interfacial water evaporation and electricity generation. This paper provides a novel and practical design strategy for developing high‐performance carbon‐based photothermal materials for solar energy applications.

A high-resolution extraterrestrial solar spectrum has been determined from ground-based measurements of direct solar spectral irradiance (SSI) over the wavelength range from 300 to 500 nm using the Langley-plot technique. The measurements were obtained at the Izaña Atmospheric Research Centre from the Agencia Estatal de Meteorología, Tenerife, Spain, during the period 12 to 24 September 2016. This solar spectrum (QASUMEFTS) was combined from medium-resolution (bandpass of 0.86 nm) measurements of the QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) spectroradiometer in the wavelength range from 300 to 500 nm and high-resolution measurements (0.025 nm) from a Fourier transform spectroradiometer (FTS) over the wavelength range from 305 to 380 nm. The Kitt Peak solar flux atlas was used to extend this high-resolution solar spectrum to 500 nm. The expanded uncertainties of this solar spectrum are 2 % between 310 and 500 nm and 4 % at 300 nm. The comparison of this solar spectrum with solar spectra measured in space (top of the atmosphere) gave very good agreements in some cases, while in some other cases discrepancies of up to 5 % were observed. The QASUMEFTS solar spectrum represents a benchmark dataset with uncertainties lower than anything previously published. The metrological traceability of the measurements to the International System of Units (SI) is assured by an unbroken chain of calibrations leading to the primary spectral irradiance standard of the Physikalisch-Technische Bundesanstalt in Germany.

Uvsq-Sat NG is a French 6U CubeSat (10 × 20 × 30 cm) of the International Satellite Program in Research and Education (INSPIRE) designed primarily for observing greenhouse gases (GHG) such as CO2 and CH4, measuring the Earth’s radiation budget (ERB), and monitoring solar spectral irradiance (SSI) at the top-of-atmosphere (TOA). It epitomizes an advancement in CubeSat technology, showcasing its enhanced capabilities for comprehensive Earth observation. Scheduled for launch in 2025, the satellite carries a compact and miniaturized near-infrared (NIR) spectrometer capable of performing observations in both nadir and solar directions within the wavelength range of 1100 to 2000 nm, with a spectral resolution of 7 nm and a 0.15° field of view. This study outlines the preflight calibration process of the Uvsq-Sat NG NIR spectrometer (UNIS), with a focus on the spectral response function and the absolute calibration of the instrument. The absolute scale of the UNIS spectrometer was accurately calibrated with a quartz-halogen lamp featuring a coiled-coil tungsten filament, certified by the National Institute of Standards and Technology (NIST) as a standard of spectral irradiance. Furthermore, this study details the ground-based measurements of direct SSI through atmospheric NIR windows conducted with the UNIS spectrometer. The measurements were obtained at the Pommier site (45.54°N, 0.83°W) in Charentes–Maritimes (France) on 9 May 2024. The objective of these measurements was to verify the absolute calibration of the UNIS spectrometer conducted in the laboratory and to provide an extraterrestrial solar spectrum using the Langley-plot technique. By extrapolating the data to AirMass Zero (AM0), we obtained high-precision results that show excellent agreement with SOLAR-HRS and TSIS-1 HSRS solar spectra. At 1.6 μm, the SSI was determined to be 238.59 ± 3.39 mW.m−2.nm−1 (k = 2). These results demonstrate the accuracy and reliability of the UNIS spectrometer for both SSI observations and GHG measurements, providing a solid foundation for future orbital data collection and analysis.

Total column ozone (TCO) is commonly measured by Brewer and Dobson spectroradiometers. Both types of instruments use solar irradiance measurements at four wavelengths in the ultraviolet radiation range to derive TCO. For the calibration and quality assurance of the measured TCO both instrument types require periodic field comparisons with a reference instrument. This study presents traceable TCO retrievals from direct solar spectral irradiance measurements with the portable UV reference instrument QASUME. TCO is retrieved by a spectral fitting technique derived by a minimal least square fit algorithm using spectral measurements in the wavelength range between 305 and 345 nm. The retrieval is based on an atmospheric model accounting for different atmospheric parameters such as effective ozone temperature, aerosol optical depth, Rayleigh scattering, SO2, ground air pressure, ozone absorption cross sections and top-of-the-atmosphere solar spectrum. Traceability is achieved by fully characterizing and calibrating the QASUME spectroradiometer in the laboratory to SI standards (International System of Units). The TCO retrieval method from this instrument is independent from any reference instrument and does not require periodic in situ field calibration. The results show that TCO from QASUME can be retrieved with a relative standard uncertainty of less than 0.8 % when accounting for uncertainties from the measurements and the retrieval model, such as different ozone absorption cross sections, different reference top-of-the-atmosphere solar spectra, uncertainties from effective ozone temperature or other atmospheric parameters. The long-term comparison of QASUME TCO with TCO derived from a Brewer and a Dobson in Davos, Switzerland, reveals that all three instruments are consistent within 1 % when using the ozone absorption cross section from the University of Bremen. From the results and method presented here, other absolute SI calibrated cost-effective solar spectroradiometers, such as array spectroradiometers, may be applied for traceable TCO monitoring.

Spectral beam split is attracting more attention thanks to the efficient use of whole spectrum solar energy and the cogenerative supply for electricity and heat. Nanofluids can selectively absorb and deliver specific solar spectra, making various nanofluids ideal for potential use in hybrid photovoltaic/thermal (PV/T) systems for solar spectrum separation. Clarifying the effects of design parameters is extremely beneficial for optimal frequency divider design and system performance enhancement. The water-based SiO 2 nanofluid with excellent thermal and absorption properties was proposed as the spectral beam splitter in the present study, to improve the efficiency of a hybrid PV/T system. Moreover, a dual optical path method was applied to get its spectral transimissivity and analyze the impact of its concentration and optical path on its optical properties. Furthermore, a PV and photothermal model of the presented system was built to investigate the system performance. The result indicates that the transimissivity of the nanofluids to solar radiation gradually decreases with increasing SiO 2 nanofluid concentration and optical path. The higher nanofluid concentration leads to a lower electrical conversion efficiency, a higher thermal conversion efficiency, and an overall system efficiency. Considering the overall efficiency and economic cost, the optimal SiO 2 nanofluid concentration is 0.10 wt.% (wt.%, mass fraction). Increasing the optical path (from 0 to 30 mm) results in a 60.43% reduction in electrical conversion efficiency and a 50.84% increase in overall system efficiency. However, the overall system efficiency rises sharply as the optical path increases in the 0–10 mm range, and then slowly at the optical path of 10–30 mm. Additionally, the overall system efficiency increases first and then drops upon increasing the focusing ratio. The maximum efficiency is 51.93% at the focusing ratio of 3.

Passive differential optical absorption spectroscopy (DOAS) is widely used to monitor the three-dimensional distribution of atmospheric pollutants. However, the observational and retrieval accuracy of this technique is significantly influenced by the precise wavelength calibration of solar spectra. Current calibration methods face challenges in automation when dealing with complex remote-sensing conditions. We introduce a novel automatic wavelength calibration algorithm for passive DOAS based on sequence-matching technology to estimate the spectral parameters of the spectrometer channels, integrating advanced processing measures such as feature structure enhancement and sub-pixel interpolation. These measures significantly reduce the dependency on reference spectrum resolution and accurately correct even minor spectral shifts. We perform sensitivity experiments using synthetic spectra to determine optimal retrieval configurations, followed by field tests at four cities on the Yangtze River Delta, China, to calibrate and compare passive DOAS instruments of various resolutions. Comparative verification in these field studies demonstrated that our algorithm was suitable for rapid spectral calibration within a wider resolution range of 0.03 nm to 0.1 nm with a wavelength inversion error < 0.01 nm. This highlights the applicability and calibration precision of our algorithm.