UV Spectroradiometer Research Articles

Annual solar spectral energy distributions in North America

Predicting outdoor service life of polymers and coatings requires knowledge of the spectral power distribution (SPD) of the sunlight and the amount of radiant energy they receive. Standard solar SPDs such as ASTM G173 and ASTM G177 are snapshots of the SPD at a single moment under model conditions. A better measure would be average annual spectral energy distributions (SED) for specific locations, especially benchmark outdoor weathering sites. The annual SED is the energy received at each wavelength over the course of a year. A Smithsonian/NIST collaboration placed SR-18 UV spectroradiometers in Miami, FL, Phoenix, AZ, and Madison, WI during the period 1997-2012, collecting data at ca. 2 nm intervals between 290 and 324 nm. Similar data have been collected at Edgewater, MD for many years. The data are not complete for any year, but gaps could be patched using data from other years. These data sets were compared to data available on-line from ongoing U.S Department of Agriculture (USDA)/Colorado State University outdoor spectroradiometer measurements, after similar patching. The USDA data have fewer points in the UV but extend into the visible portion of the spectrum. The data sets give consistent results and show that the SEDs can be fit by SPDs calculated using the SMARTS 2.9.5 model and satellite ozone data. The annual standard deviations are < 5% for wavelengths > 310 nm. SEDs for average annual radiant energy received by horizontal and 45° south exposures are reported for benchmark sites near Miami and Phoenix. The paper provides a general approach for determining an SED for a location based on SMARTS modeling of the autumnal equinox and subsequent scaling based on reported annual total solar radiant energy.

Unraveling the Seasonality of UV Exposure in Reef Waters of a Rapidly Warming (Sub-)tropical Sea

Ultraviolet radiation (UV) is a crucial abiotic stressor that can have severe impacts on biota residing in the upper euphotic zone, especially if UV stress coincides with other stressors such as extreme sea surface temperatures (SSTs). Exposure-dependent effects of UV exposure have been described for a broad range of marine taxa and ecosystems such as coral reefs, yet little is known about the magnitude and seasonality of UV exposure in natural waters. In the present study, we determined how daily exposure of UV-B and UV-A varies seasonally along the water column of a reef system in the central Red Sea, and identified periods when damaging UV levels are likely to coincide with episodes of extreme SST, both presently and in the future. Between July 2016 and September 2018, UV spectroradiometer profiles were recorded fortnightly at a pelagic site adjacent to a mid-shore reef off the Saudi Arabian Red Sea coast, while atmospheric UV-B and UV-A irradiances were measured in 10-minute intervals. Additionally, we quantified the concentration of chlorophyll-a (Chl-a) and absorption by chromophoric dissolved organic matter (aCDOM) in the reef as well as the surrounding waters. Biologically effective optical depths (Z10%) ranged from 6.3–12.9 m (UV-B) and 14.4–27.3 m (UV-A), with the highest UV transparency being observed in late summer when photodegradation of dissolved organic matter (DOM) was most intense and the concentration and molecular weight of CDOM were at their lowest. Incident UV peaked a few weeks prior to this later summer maximum in UV transparency. Consequently, organisms living close to the water surface experienced their most intense UV exposure in May/June, while the timing of maximum UV exposure for biota below ~ 2–4 m coincided with the annual peak in water transparency and water temperature, i.e., in July/August. However, SSTs in the Red Sea are increasing at a rapid rate due to climate change, with the consequence that extreme temperatures are occurring earlier in the year and may eventually coincide with extreme UV radiation in shallower areas of the reef. This development could have potentially detrimental effects on highly sensitive, immotile reef biota such as reef-building corals.

Success of Montreal Protocol Demonstrated by Comparing High-Quality UV Measurements with \u201cWorld Avoided\u201d Calculations from Two Chemistry-Climate Models

The Montreal Protocol on Substances that Deplete the Ozone Layer has been hailed as the most successful environmental treaty ever (https://www.unenvironment.org/news-and-stories/story/montreal-protocol-triumph-treaty). Yet, although our main concern about ozone depletion is the subsequent increase in harmful solar UV radiation at the Earth’s surface, no studies to date have demonstrated its effectiveness in that regard. Here we use long-term UV Index (UVI) data derived from high-quality UV spectroradiometer measurements to demonstrate its success in curbing increases in UV radiation. Without this landmark agreement, UVI values would have increased at mid-latitude locations by approximately 20% between the early 1990s and today and would approximately quadruple at mid-latitudes by 2100. In contrast, an analysis of UVI data from multiple clean-air sites shows that maximum daily UVI values have remained essentially constant over the last ~20 years in all seasons, and may even have decreased slightly in the southern hemisphere, especially in Antarctica, where effects of ozone depletion were larger. Reconstructions of the UVI from total ozone data show evidence of increasing UVI levels in the 1980s, but unfortunately, there are no high-quality UV measurements available prior to the early 1990s to confirm these increases with direct observations.

Validation of Ozone Monitoring Instrument UV Satellite Data Using Spectral and Broadband Surface Based Measurements at a Queensland Site.

This research reconstructed and validated the broadband UVA irradiances derived from discrete spectral irradiance data retrieved from the Ozone Monitoring Instrument (OMI) satellite from 1 January to 31 December 2009. OMI data at solar noon were compared to ground-based spectral irradiances at Toowoomba (27°36' S 151°55' E), Australia, at 310, 324 and 380 nm for both cloud-free and all sky conditions. There was a strong relationship between the ground-based UV spectroradiometer data and satellite-based measurements with an R2 of 0.89 or better in each waveband for cloud-free days. The data show an overestimate of the satellite-derived spectral irradiances compared to the ground-based data. The models developed for the subtropical site data account for this overestimation and are essential for any data correlation between satellite- and ground-based measurements. Additionally, this research has compared solar noon broadband UVA irradiances evaluated with a model and the discrete satellite spectral irradiances for the solar noon values of cloud-free days to those measured with a ground-based UVA radiometer. An R2 of 0.86 was obtained confirming that for cloud-free days the broadband UVA can be evaluated from the OMI satellite spectral irradiances.

Very short-term reactive forecasting of the solar ultraviolet index using an extreme learning machine integrated with the solar zenith angle

Exposure to erythemally-effective solar ultraviolet radiation (UVR) that contributes to malignant keratinocyte cancers and associated health-risk is best mitigated through innovative decision-support systems, with global solar UV index (UVI) forecast necessary to inform real-time sun-protection behaviour recommendations. It follows that the UVI forecasting models are useful tools for such decision-making. In this study, a model for computationally-efficient data-driven forecasting of diffuse and global very short-term reactive (VSTR) (10-min lead-time) UVI, enhanced by drawing on the solar zenith angle (θs) data, was developed using an extreme learning machine (ELM) algorithm. An ELM algorithm typically serves to address complex and ill-defined forecasting problems. UV spectroradiometer situated in Toowoomba, Australia measured daily cycles (0500–1700h) of UVI over the austral summer period. After trialling activations functions based on sine, hard limit, logarithmic and tangent sigmoid and triangular and radial basis networks for best results, an optimal ELM architecture utilising logarithmic sigmoid equation in hidden layer, with lagged combinations of θs as the predictor data was developed. ELM’s performance was evaluated using statistical metrics: correlation coefficient (r), Willmott’s Index (WI), Nash-Sutcliffe efficiency coefficient (ENS), root mean square error (RMSE), and mean absolute error (MAE) between observed and forecasted UVI. Using these metrics, the ELM model’s performance was compared to that of existing methods: multivariate adaptive regression spline (MARS), M5 Model Tree, and a semi-empirical (Pro6UV) clear sky model. Based on RMSE and MAE values, the ELM model (0.255, 0.346, respectively) outperformed the MARS (0.310, 0.438) and M5 Model Tree (0.346, 0.466) models. Concurring with these metrics, the Willmott’s Index for the ELM, MARS and M5 Model Tree models were 0.966, 0.942 and 0.934, respectively. About 57% of the ELM model’s absolute errors were small in magnitude (±0.25), whereas the MARS and M5 Model Tree models generated 53% and 48% of such errors, respectively, indicating the latter models’ errors to be distributed in larger magnitude error range. In terms of peak global UVI forecasting, with half the level of error, the ELM model outperformed MARS and M5 Model Tree. A comparison of the magnitude of hourly-cumulated errors of 10-min lead time forecasts for diffuse and global UVI highlighted ELM model’s greater accuracy compared to MARS, M5 Model Tree or Pro6UV models. This confirmed the versatility of an ELM model drawing on θsdata for VSTR forecasting of UVI at near real-time horizon. When applied to the goal of enhancing expert systems, ELM-based accurate forecasts capable of reacting quickly to measured conditions can enhance real-time exposure advice for the public, mitigating the potential for solar UV-exposure-related disease.

Stray light correction of array spectroradiometers for solar UV measurements.

An approach is presented to characterize and correct stray light in spectra measured with array spectroradiometers and caused by out-of-spectral range radiation. A prerequisite for out-of-range stray light correction is knowledge of the spectral irradiance not measured by the instrument itself. A way of solving this problem for solar UV measurements is shown. The effect of out-of-range stray light is especially important for solar UV spectroradiometers typically having a spectral range narrower than that of the silicon detectors in use. Two different types of instruments used for solar UV measurements were characterized and corrected for out-of-range and in-range stray light. As a hardware solution to the out-of-range stray light problem, a bandpass filter was fitted in one array spectroradiometer. Results of test measurements using this modified instrument are also shown.

Assessment of the UVB Transmittance Measurement Uncertainty in Conformity with ISO/IEC 17025

The main purpose of this research is to construct a laboratory for testing standard. It refers to the spirit of ISO/IEC 17025 Standard, and follows Guide to the Expression of Uncertainty in Measurement (ISO GUM) made by ISO organization. The content includes UV test system construction, UVB transmittance measurement uncertainty, and the plan of measurement quality assurance that assures the stable operation of the laboratory system. Construct the equations of measurement uncertainty by the system methods that apply to the UVB transmittance measurement of car insulation film, and evaluate the variance with transmittance measurement measured by UV spectroradiometer. As for the results of transmittance measurement, the transmittance is between 0.87214 and 1.02146(%) with single wavelength of 294.5nm. There is little difference between the manufacturer label value and the measured one. The verification proofs the effectiveness of the system, and allows the tests of commercial UV-cut products fit the international and world standards.

Estimation of the optical receiving plane positions of solar spectroradiometers with spherical diffusers on the basis of spatial responsivity data

A straightforward method for estimating the position of the optical receiving plane of a spherical, dome-shaped diffuser from its spatial responsivity data is presented. The method is tested with two diffusers, types J1002 and J1015 from CMS Schreder, commonly used in solar UV spectroradiometers. The shift of the receiving plane from its nominal position determines a potential measurement error that occurs when measurements and calibrations are carried out with sources at different distances from the diffuser. Such information is particularly valuable for voluminous solar UV monitoring spectroradiometers that cannot easily be transported to laboratory calibrations. The results suggest that systematic measurement errors are at least of the order of 1%, if the position of the receiving plane is not properly taken into account, thus indicating a need to study the effect more carefully. This method can also be used to minimize measurement errors when designing diffusers.

Calibration, data processing, and maintenance of the United States Department of Agriculture high-resolution ultraviolet spectroradiometers

The USDA ultraviolet radiation network currently includes four high-resolution spectroradiometers, located at Table Mountain, Colorado (deployed November 1998); the Atmospheric Radiation Measurement Climate Research Facility in Oklahoma (October 1999); Beltsville, Maryland (November 1999); and Fort Collins, Colorado (October 2002). These spectroradiometers contain Jobin Yvon's 1-m Czerny-Turner double additive spectrometers. The instruments measure total horizontal radiation in the 290- to 371-nm range, once every 30 min, with a nominal FWHM of 0.1 nm. We describe data quality control techniques as well as the data processing required to convert the raw data into calibrated irradiances. The radiometric calibration strategies using Central UV Calibration Facility FEL lamps that are directly NIST-traceable, portable field calibrators, and vicarious calibrations using data from UV multifilter rotating shadowband radiometers (MFRSRs) are discussed. Using direct-to-diffuse ratios from UV MFRSRs, we derive direct and diffuse high-resolution horizontal spectra from the collocated UV spectroradiometers of the USDA network. The direct-beam spectra can be used in a Langley regression that leads to spectroradiometric in situ calibration and to ozone column and aerosol optical depth retrievals. The high-resolution direct spectra are used to obtain the ozone column and aerosol optical depth in the 290- to 360-nm range at 0.1-nm resolution. A statistical summary of network performance is presented.

Aerosol radiative forcing efficiency in the UV region over southeastern Mediterranean: VELETA2002 campaign

Atmospheric aerosol effects on spectral global UV irradiance were evaluated during the VELETA2002 field campaign between 8 and 19 July 2002 in southeast Spain. In the first stage, seven UV spectroradiometer and six CIMEL Sun photometer measurements were carried out simultaneously, allowing them to be calibrated and intercompared. The mean ratio obtained for the global irradiance between the spectroradiometers, with regards to a reference instrument, ranges from 0.98 up to 1.04 with standard deviations that oscillate between ±0.01 and ±0.17. In particular, the two spectroradiometers used to obtain the aerosol forcing efficiencies have a ratio of 1.000 ± 0.001. The aerosol optical depth (AOD) obtained with the CIMEL Sun photometers has a standard deviation of lower than ±0.01 for all the channels. Under clear sky conditions, the diurnal aerosol forcing efficiency (ΔDFe) and fractional diurnal forcing efficiency (ΔFDFe) was calculated for two Mediterranean stations: Armilla (691 m.a.s.l.) within the boundary layer and Sabinas (2200 m.a.s.l) on the lower limit of the free troposphere and 25 km away from the first station. The ΔDFe values obtained at Armilla range between −2.72 ± 0.45 W m−2/τ380 and −2.88 ± 0.45 W m−2/τ440 and between −3.22 ± 0.61 W m−2/τ380 and −3.40 ± 0.62 W m−2/τ440 at Sabinas station; the ΔFDFe values range from −8.0 ± 1.4%/τ380 to −8.6 ± 1.3%/τ440 and −12.0 ± 2.3%/τ380 to −12.6 ± 2.3%/τ440 at the two stations, respectively. Also, an experimental aerosol transmittance factor, CT, used to obtain UV satellite derived products was found as a result of the dependence of the global irradiance with the AOD, under cloudless conditions. The average aerosol attenuation factor, η, obtained from the CT, is 6 ± 2% under weakly absorbing aerosols, with a negligible spectral dependence.

Spectral dependency of cloud enhanced UV irradiance

This paper addresses two questions of primary importance to the solar UV community: 1) “Are cloud induced UV enhancements always wavelength dependent?” and 2) “Are the enhancements greatest in the UVA or UVB wavebands?” The answer to the first question is a definite no, with the conclusion to the second question that most of the enhancements found at this southern hemisphere measurement site are in the UVB waveband. This research is based on the results from a scanning UV spectroradiometer and a colour, all-sky camera over a 19-month period. In both the UVB and UVA wavebands there were cases that showed increasing, decreasing and no spectral dependence towards the shorter and longer wavelengths respectively. This research has found that cases of spectral dependence that decreased with wavelength tended to correspond to cloud fraction distributed in the outer field of view of the sky camera images for relatively low solar zenith angles. It is speculated that this is most likely due to an increase of scattered UV, compared to cases of increasing trends with wavelength, which would be accounted for by an increase in reflected UV from cloud surfaces in closer proximity to the sun. It also appears that wavelength dependency trends are related to the overall cloud fraction.

Portable device for characterizing the angular response of UV spectroradiometers

This paper introduces a device that was developed to measure the angular response of UV spectroradiometers in the field. This device is designed to be used at the operating position of spectroradiometers; thus the derived angular response also includes any effects from imperfect leveling of the diffuser and corresponds to the actual operational angular response. The design and characterization of the device and the results from its application on 11 different spectroradiometers that operate at different European UV stations are presented. Various sources of uncertainties that were identified result in a combined uncertainty in determining the angular response, which ranges between approximately 1.5% and 10%, depending on the incidence angle and the characteristics of the diffuser. For the 11 instruments, the error in reporting the diffuse irradiance ranges between 2% and - 13%, assuming isotropic distribution of the downwelling radiances.

Bias in Dobson total ozone measurements at high latitudes due to approximations in calculations of ozone absorption coefficients and air mass

The Dobson spectrophotometer is the primary standard instrument for ground‐based measurements of total column ozone. The accuracy of its data depends on the knowledge of ozone absorption coefficients used for data reduction. We document an error in the calculations that led to the set of absorption coefficients currently recommended by the World Meteorological Organisation (WMO). This error has little effect because an empirical adjustment was applied to the original calculations before the coefficients were adopted by WMO. We provide evidence that this adjustment was physically sound. The coefficients recommended by WMO are applied in the Dobson network without correction for the temperature dependence of the ozone absorption cross sections. On the basis of data measured by Dobson numbers 80 and 82, which were operated by the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory at the South Pole, we find that omission of temperature corrections may lead to systematic errors in Dobson ozone data of up to 4%. The standard Dobson ozone retrieval method further assumes that the ozone layer is located at a fixed height. This approximation leads to errors in air mass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles (SZA). At the South Pole, systematic errors caused by this approximation may exceed 2% for SZAs larger than 80°. The bias is largest when the vertical ozone distribution is distorted by the “ozone hole” and may lead to underestimation of total ozone by 4% at SZA = 85° (air mass 9). Dobson measurements at the South Pole were compared with ozone data from a collocated SUV‐100 UV spectroradiometer and Version 8 overpass data from NASA's Total Ozone Mapping Spectrometer (TOMS). Uncorrected Dobson ozone values tend to be lower than data from the two other instruments when total ozone is below 170 Dobson units or SZAs are larger than 80°. When Dobson measurements are corrected for the temperature dependence of the ozone absorption cross section and accurate air mass calculations are implemented, data from the three instruments agree with each other to within ±2% on average and show no significant dependence on SZA or total ozone.

Fast measurements of solar spectral UV irradiance—first performance results of two novel spectroradiometers

Short spectral scan times of preferably less than 1 min in the UV region are an important prerequisite for modern spectroradiometers to reflect short-term solar irradiance radiation variations that can occur as a result of fast changes in cloud cover and/or cloud optical depth. Two different types of fast measuring spectroradiometers for solar UV irradiance are compared in a first field campaign: (1) the UV spectroradiometer on filter model basis (UV-SPRAFIMO) and (2) the modified version of the spectroradiometer SPECTRO 320D by Instrument Systems. The UV-SPRAFIMO instrument combines a filter radiometer with five narrowband (FWHM2.0 to 2.7 nm) filters centered at fixed wavelengths in the UV-B and UV-A regions with an advanced neural network-based model. Up to 5 measurements/s can be taken concurrently in the five wavelength channels. After averaging the measurements over preselectable time intervals, the measured irradiances are converted by the neural network model into a full spectrum from 280 to 450 nm at arbitrary wavelength steps (0.05 nm). The SPECTRO 320D spectroradiometer consists of a grating double monochromator with a cooled photomultiplier tube (PMT) receiver. The instrument version run by Deutscher Wetterdienst (DWD) is thermostatted and equipped with a Schreder type cosine diffuser as the entrance optics. A spectral scan from 290 to 450 nm with a selected 0.2-nm wavelength step takes less than 30 s. The two spectroradiometers are used in a field campaign at Izana (Tenerife Island) at a height of 2440 m above sea level (ASL) to compare measured spectral and integral values of solar irradiance. Results of that comparison and the instruments' characteristics are discussed. This first field comparison shows that due to the fast measurements regime, cloud effects on the measured spectra can be appreciably reduced. The campaign shows an acceptable agreement between the spectra measured by both instruments. It also reveals some issues for further improvements of the instrument design.

Focus on Spectroscopy

Focus on Spectroscopy

The use of diode array spectroradiometers for dosimetry in phototherapy

An evaluation of two diode array radiometers, an UV spectroradiometer, Type SC-MP-A, from 4D Controls (Redruth, UK) and an USB2000-UV-VIS spectrometer from Ocean Optics (Duiven, NL), was carried out at the Photobiology Unit, University of Dundee. Three parameters of the instruments' performance were investigated, having been identified as the most likely sources of error in phototherapy dosimetry: (1) calibration, (2) stray light rejection, (3) angular response. An assessment was then made of the reliability of this type of instrument for dosimetry in clinical practice by measurement of a selection of phototherapy sources, in direct comparison with calibrated radiometers. Both instruments were found to have significant stray light levels (SC: 13% and USB: 39%). The use of stray light compensation and a high output calibration source improves accuracy to within acceptable limits. Angular responses were satisfactory: f2 values (±60°) of 5.9% and 7.8% for SC and USB, respectively. The SC spectroradiometer is supplied as a calibrated instrument. Using the supplied calibration resulted in errors in measuring phototherapy sources of up to 44% in UVA. Alternative calibration reduced the error in measuring UVA and UVB sources to within 12%. The USB spectrometer was found to have insufficient responsivity in both UVB and UVA to provide reproducible measurements of most phototherapy sources.

United States Department of Agriculture reference ultraviolet spectroradiometer: current performance and operational experience at Table Mountain, Colorado

At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of three sites: Table Mountain, Colorado, Lamont, Oklahoma (the ARM program SGP site), and Beltsville, Maryland. At each site we deploy and continuously operate a 1-m cascaded additive-double Czerny-Turner scanning monochromator with a bialkali photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80°. The instruments use photon counting and make measurements at 290 nm not affected by stray light under typical conditions. The basic performance specifications of the instrument were demonstrated by a prototype at the 1997 North-American UV Spectroradiometer Intercomparison. Data shown here demonstrate that these are met in routine operation. The fundamental instrument performance specifications are: Optical resolution: 0.1 nm FWHM, triangular slit-function. Wavelength reproducibility: ′0.0025-nm 2<T with 296-nm Hg retrace-scan corrections applied, ′0.007 nm 2σ over typical diurnal variability, without correction. Wavelength accuracy: Limited by calibration systematic errors. Believed to be 0.005-nm worst case. Stray light: < 10 - 7 at 4 FWHM, 10 - 1 0 at 20 nm, slit-scattering function versus 325 nm HeCd. Angular response: less than 1% error from cosine over the range of zenith angles from 0 to 80°. Signal linearity: The instrument uses a photomultiplier with 2-ns rise-time and photon counting detection. The dual-threshold discriminator has a 700-Mhz synchronous signal counting limit. The maximum counting rates seen at the longest wavelengths are less than 10 MHz; less than 1/5 of the frequency where nonlinearity can be detected, as tested for the 1997 Intercomparison. 2000 was the first full year of operation of our instrument at the NOAA Table Mountain site (140.177 °N 105.276 °W, 1900 m asl) for which the operational and calibration frequencies justify making the data accessible to outside users for scientific application. We show performance in routine operation and issues of calibration over the period April 2000 to 31 December 2001.

Measuring Spectral Actinic Flux and Irradiance: Experimental Results from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) Project

Abstract Results are presented from the Actinic Flux Determination from Measurements of Irradiance (ADMIRA) campaign to measure spectral global UV irradiance and actinic flux at the ground, beneath an atmosphere well defined by supporting measurements. Actinic flux is required to calculate photolysis rates for atmospheric chemistry, yet most spectral UV measurements are of irradiance. This work represents the first part of a project to provide algorithms for converting irradiances to actinic fluxes with specified uncertainties. The campaign took place in northern Greece in August 2000 and provided an intercomparison of UV spectroradiometers measuring different radiation parameters, as well as a comprehensive radiation and atmospheric dataset. The independently calibrated spectroradiometers measuring irradiance and actinic flux agreed to within 5%, while measurements of spectral direct irradiance differed by 9%. Relative agreement for all parameters proved to be very stable during the campaign. A polarizat...

Retrieval of ozone column from global irradiance measurements and comparison with TOMS data. A year of data in the Alps

Spectral UV global irradiance has been measured regularly during the year 2000, in the French Alps, using two UV spectroradiometers. The total ozone column amount is retrieved from the ratio of irradiances around 305 and 340 nm, using look up tables based on modeling data. During cloudless days, ozone values from direct sun observations agree to within 5 DU. These ground based values compare reasonably well (to within about 4%, or 12 DU) with TOMS ozone data; this is consistent with the uncertainty of both methods which is estimated to 5 DU for TOMS, and 5–7 DU for clear sky ground based measurements. Generally the ground based ozone values seem to be slightly higher than the TOMS ozone values. For a few cases, the difference between TOMS and ground based values, can reach 10 to 15%. Different possible explanations of the discrepancies are considered.

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.