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To retrieve columnar intensive aerosol properties from sun–sky photometers, both irradiance and radiance calibration factors are needed. For the irradiance the solar calibration constant, V0 , which denotes the instrument counts for a direct normal solar flux extrapolated to the top of the atmosphere, must be determined. The solid view angle, SVA, is a measure of the field of view of the instrument, and it is important for obtaining the radiance from sky diffuse irradiance measurements. Each of the three sun-photometer networks considered in the present study (SKYNET, AERONET, WMO GAW) adopts different protocols of calibration, and we evaluate the performance of the on-site calibration procedures, applicable to every kind of sun–sky photometer but tested in this analysis only on SKYNET Prede POM01 instruments, during intercomparison campaigns and laboratory calibrations held in the framework of the Metrology for Aerosol Optical Properties (MAPP) European Metrology Programme for Innovation and Research (EMPIR) project. The on-site calibration, performed as frequently as possible (ideally monthly) to monitor changes in the device conditions, allows operators to track and evaluate the calibration status on a continuous basis, considerably reducing the data gaps incurred by the periodic shipments for performing centralized calibrations. The performance of the on-site calibration procedures for V0 was very good at sites with low turbidity, showing agreement with a reference calibration between 0.5 % and 1.5 % depending on wavelengths. In the urban area, the agreement decreases between 1.7 % and 2.5 %. For the SVA the difference varied from a minimum of 0.03 % to a maximum of 3.46 %. [ABSTRACT FROM AUTHOR]

To retrieve columnar aerosol properties from sun-photometers both irradiance and radiance calibration factors are needed. For the irradiance the solar calibration constant, V0, that is the instrument counts for a direct normal solar flux extrapolated to the top of the atmosphere, must be determined. The solid view angle, SVA, is the measure of the field of view of the instrument, and it is important for obtaining the Radiance from sky diffuse irradiance measurements. Each of the three sun-photometers networks considered in the present study (SKYNET, AERONET, WMO-GAW) adopts different protocols of calibration and we evaluated the performance of the on-site calibration procedures, applied to SKYNET PREDE-POM instruments, during intercomparison campaigns and laboratory calibrations held in the framework of the Metrology for Aerosol Optical Properties (MAPP) EMPIR project. The on-site calibration, performed as frequently as possible (rather monthly) to monitor change of the machine condition, allow operators to track and evaluate the calibration status on a continuous basis considerably reducing the data gaps incurred by the periodical shipments for performing centralized calibrations. The performance of the on-site calibration procedures for V0 was very good in sites with low turbidity, showing an agreement with a reference calibration between 0.5% and 1.5% depending on wavelengths. In the urban area the agreement decreases between 1.7% and 2.5%. For the SVA the difference varied from a minimum of 0.03 % to a maximum of 3.46%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The optical parameters of a Si[O.sub.2] thin-film coating determined from the spectral reflectance and transmittance measurements at various incidence angles, including the normal incidence and the Brewster's angle, are compared in this paper. The high-accuracy measurements were carried out through visible-near-infrared spectral regions by using our purpose-built instruments. The optical parameters obtained from the reflectance and the transmittance data are consistent over the angles of incidence and agree within 0.2%. The effect of important systematic factors in the oblique-incidence spectrophotometric measurements is also discussed. OCIS codes: 310.1620, 310.3840, 260.5430, 120.5700, 120.7000, 120.6200.

Gonioreflectometric determination of reflectance factors that involves hemispherical collection of reflected flux, which is an alternative to integrating sphere-based methods, is discussed. A detailed description of a gonioreflectometer built at the Helsinki University of Technology is presented. The instrument is used to establish an absolute scale of total diffuse reflectance factors throughout the spectral range 360-830 nm. The hemispherical reflectance factors are obtained through integration of the gonioreflectometric measurement results. The reflectance factors of white high-quality artifacts can be determined with a combined standard uncertainty of 0.20%. Results of test measurements were found to be in agreement with values traceable to other absolute scales based on integrating-sphere methods. OCIS codes: 120.1840, 120.3930, 120.3940, 120.4800, 120.5700, 120.6200.

Three reference Dobsons (regional standards Dobsons No. 064 Hohenpeissenberg - Germany and No. 074 Hradec Kralove - Czech Republic and primary = world standard Dobson No. 083 Boulder - USA) were optically characterized at PTB (Physikalisch-Technische Bundesanstalt in Braunschweig) in 2015 and at CMI (Czech Metrology Institute in Prague) in 2016 within the EMRP ENV 059 project Traceability for the total column ozone. Bandpass functions and the related parameters of the instruments were measured and compared with G. M. B. Dobson's specification in his handbook. A predominantly good match of the bandpass functions and the peak (centroid) wavelengths of D083, D064 and D074 with the nominal values could be observed. Slightly larger deviations from the nominal Dobson data can be seen in the longer wavelengths, especially in the D-wavelength. As consequence of these findings the differences of the derived Effective Absorptions Coefficients (EACs) for ozone to Dobson's nominal ones are not too large in both old Bass-Paur (BP) and new IUP-ozone absorption cross sections. Their consideration in the calculation of the total ozone column (TOC) leads to improvements of significantly less than ±1 % in the AD- and between -1 % and -2 % in the CD-wavelengths pairs. Besides this positive effect of the achievement of data with higher quality needed for trend analyses and satellite validation, it will be possible to explain uncommon behaviours of field Dobsons during calibration services. [ABSTRACT FROM AUTHOR]

The instrumental temperature correction to be applied to the ozone measurements by the Brewer spectrophotometers is derived from measurements of the irradiance from an internal halogen lamp in the instrument. These characterisations of the Brewer instruments can be carried out within a thermal chamber, varying the temperature from −5 °C to +45°C, or during field measurements, making use of the natural change in ambient temperature. However, the internal light source used to determine the thermal sensitivity of the instrument could be affected in both methods by the temperature variations as well, which may affect the determination of the temperature coefficients. In order to validate the standard procedures for determining Brewer's temperature coefficients, two independent experiments using both external light sources and the internal lamps have been performed within the ATMOZ Project. The results clearly show that the traditional methodology based on the internal lamps is not affected by possible temperature effects on the internal lamps. The three methodologies yielded equivalents results, with differences in total ozone column below 0.08 % for a mean diurnal temperature variation of 10 °C. [ABSTRACT FROM AUTHOR]

A compact array spectroradiometer technology that enables precise and robust measurements of UV spectral irradiance is presented. The spectroradiometer design allows various applications such as measurement of high-power UV lamps, risk assessment of sources and measurement of solar irradiance. We show that this technology can perform precise total ozone column (TOC) retrieval. The internal stray light, which is often the limiting factor for measurements in the UV spectral range and increases the uncertainty for TOC analysis significantly, is physically reduced so that no other stray light 15 reduction methods, such as mathematical corrections, are necessary. The instrument and its elaborate components have been extensively characterised at the Physikalisch-Technische Bundesanstalt (PTB) in Germany. During an international total ozone measurement intercomparison at the Izaña observatory in Tenerife, the high-quality applicability of the instrument was verified with measurements of the direct solar irradiance and subsequent TOC evaluations based on the spectral data between 12 and 30 September 2016. The results showed deviations of less than 1.5 % to most other instruments in most 20 situations not exceeding 3 % compared to established TOC measurement systems such as Dobson or Brewer. [ABSTRACT FROM AUTHOR]