Development and operation of the Hemispherical Blackbody (HSBB) for the calibration of infrared radiometers with a hemispherical acceptance angle.

Longwave downward radiation measurements are fundamental for investigating the surface and atmospheric energy budget of the Earth. Such measurements are performed using broadband infrared radiometers that have a hemispherical acceptance angle. A new hemispherical reference blackbody has been developed to provide a new traceability path of longwave downward radiation measurements to the SI. The Hemispherical Blackbody (HSBB) was specifically designed to meet the large opening angle of radiometers used for measuring longwave downward radiation. The first step was to find a suitable design candidate for the HSBB by simulating the effective emissivity using the Monte Carlo ray-tracing software STEEP. The most promising candidate was then realized in two versions with similar coatings and both versions were taken into operation.

UV Index monitoring in Europe.

The UV Index was established more than 20 years ago as a tool for sun protection and health care. Shortly after its introduction, UV Index monitoring started in several countries either by newly acquired instruments or by converting measurements from existing instruments into the UV Index. The number of stations and networks has increased over the years. Currently, 160 stations in 25 European countries deliver online values to the public via the Internet. In this paper an overview of these UV Index monitoring sites in Europe is given. The overview includes instruments as well as quality assurance and quality control procedures. Furthermore, some examples are given about how UV Index values are presented to the public. Through these efforts, 57% of the European population is supplied with high quality information, enabling them to adapt behaviour. Although health care, including skin cancer prevention, is cost-effective, a proportion of the European population still doesn't have access to UV Index information.

Traceability of solar UV measurements using the Qasume reference spectroradiometer.

One major objective of the European Joint Research Project "Traceability for surface spectral solar ultraviolet (UV) radiation" was to reduce the uncertainty of spectral UV measurements. The measurement instrument used for this work was the portable UV European reference spectroradiometer Qasume. The calibration uncertainty of this instrument was decreased and validated by a comparison of direct calibrations against a primary standard for spectral irradiance, a high temperature blackbody radiator, and against a reference detector using a spectrally tunable laser as a monochromatic source. The spectral irradiance responsivity of the reference detector is traceable to the primary standard of optical power, realized through a cryogenic radiometer, and to the SI unit of meter. The measuring technique was improved by the construction of a new reference spectroradiometer, QasumeII. An improved input optics removes the dependences of the measured solar irradiance on the angle of incident for solar zenith angle smaller than 75 deg. Moreover, a hybrid photon detection system enables continuous tracking of the instrument's responsivity changes. For both spectroradiometer systems an uncertainty budget was calculated. The improvements have reduced the measurement uncertainties of solar spectral UV irradiance measurements from 4.8% in 2005 to 2.0% (k=2) in the spectral region above 310 nm. The largest sources of uncertainty were the absolute spectral irradiance responsivity calibration, the angular response uncertainty, and the instrument stability using the hybrid detector, which were reduced from 3.6% to 1.1%, from 1.2% to 0.6%, and from 0.65% to 0.4%, with respect to the situation prior to the project. The new instrument was validated during a four month intercomparison relative to the Qasume reference. The mean ratio of the solar irradiance scans between the two reference spectroradiometers has an offset of +0.7% and a standard deviation of ±1.5% for a wavelength greater than 305 nm, which is well within the combined uncertainty of 3.7% calculated from the uncertainties of the two systems.

Quantifying UV exposure, vitamin D status and their relationship in a group of high school students in an alpine environment.

The relationship between personal UV exposure and vitamin D status was studied among 7 high school students from Davos, Switzerland from March to August 2013. The personal UV exposure was monitored using electronic dosimeters, while blood samples were taken at monthly intervals to determine the serum concentration of 25-hydroxyvitamin D3 (25(OH)D3). During school days students were exposed to 1.7% of the ambient UV irradiance, while 85% of the cumulative UV dose was obtained on weekends and holidays. Insufficient vitamin D levels in March (9 ng ml(-1) 25(OH)D3) rose to 25(OH)D3 concentrations of over 40 ng ml(-1), meeting sufficient levels in August. The increase in vitamin D levels among 5 high school students correlated well (r = 0.89) with their measured personal UV exposure, yielding a mean increase in serum 25(OH)D3 concentration of 0.38 ± 0.22 ng ml(-1) per 100 J m(-2) of vitamin D-weighted UV exposure, a value consistent with other studies. During certain periods of the study, increases in vitamin D status and UV doses differed from the average of the whole study, implying that other factors must influence vitamin D metabolism.

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.

Quality assurance of solar UV irradiance in the Arctic.

The first Arctic intercomparison of three solar ultraviolet (UV) spectroradiometers and two multifilter radiometers was held in May and June 2009 at Ny-Alesund, Svalbard, Norway. The transportable reference spectroradiometer QASUME acted as reference instrument for this intercomparison. The measurement period extended over eleven days, comprising clear sky and overcast weather conditions. Due to the high latitude, measurements could be performed throughout the day during this period. The intercomparison demonstrated that the solar UV measurements from all instruments agreed to within +/-15% during the whole measurement period, while the spectroradiometer from the Alfred-Wegener Institute agreed to better than +/-5%. This intercomparison has demonstrated that solar UV measurements can be performed reliably in the high-latitude Arctic environment with uncertainties comparable to mid-latitude sites.

Europe's darker atmosphere in the UV-B.

Irradiation in the ultraviolet wavelength range is found to be up to 50% lower in the European summer compared to sites with comparable latitudes in New Zealand. We have developed a method to quantitatively attribute the causes for such differences between sites by analysis of spectra. We conclude that these large differences are caused mainly by differences in total ozone, cloudiness, aerosol loading and Sun-Earth separation. The relative contribution of clouds varies from year to year and it is site dependent. Averaged over several years we find a strong latitudinal gradient of the cloud impact within Europe, with much less cloud attenuation in southern Europe. Due to the differences in total ozone and aerosol loading, the UV-B levels are generally lower in Europe compared to New Zealand. It is likely that inter-hemispheric differences will change in coming decades due to a combination of changes in ozone concentrations, air pollution and cloudiness as a result of climate change. However, since the future evolution of these major parameters is highly uncertain, the magnitude and even the sign of such changes are not known yet.

Operation and investigation of a tilted bottom cavity for pyrgeometer characterizations.

A new cavity for pyrgeometer characterizations has been built at the Infrared Radiometry Section of the World Radiation Center (WRC-IRS) at PMOD/WRC. The calculated effective emissivity of 0.99993+/-0.00033 was obtained from Monte Carlo simulations taking into account the geometry and the measured temperature distributions of the cavity. The cavity is operated in a temperature range of -30 degrees C to +30 degrees C and is initially flushed with nitrogen to reduce the relative humidity in the cavity. The estimated uncertainties of retrieved pyrgeometer parameters k1, k2, and k3 are +/-0.024, +/-0.0008, and +/-0.03, respectively. The relative uncertainty of pyrgeometer sensitivity C is 0.8%. The comparison with the cavity used since 1995 at PMOD/WRC gave average differences of 0.005, 0.00026, and 0.08 for k1, k2, and k3, respectively. The pyrgeometer sensitivity retrieved with the new cavity is on average 1.0% higher than with the original cavity.

Variability of UV irradiance in Europe.

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used one-dimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m(-2) at 70 degrees N and 5.2 kJ m(-2) at 35 degrees N, whereas these values are reduced to 1.5-4.5 kJ m(-2) if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

Laboratory calibration of pyrgeometers with known spectral responsivities.

A methodology is presented to calibrate pyrgeometers measuring atmospheric long-wave radiation, if their spectral dome transmission is known. The new calibration procedure is based on a black-body cavity to retrieve the sensitivity of the pyrgeometer, combined with calculated atmospheric long-wave spectra to determine a correction function in dependence of the integrated atmospheric water vapor to convert Planck radiation spectra to atmospheric long-wave spectra. The methodology was validated with two custom CG4 pyrgeometers with known dome transmissions by a comparison to the World Infrared Standard Group of Pyrgeometers at the World Radiation Center-Infrared Radiometry Section. The responses retrieved using the new laboratory calibration agree to within 1% with the responses determined by a comparison to the WISG, which is well within the uncertainties of both methodologies.

Characterization and calibration of ultraviolet broadband radiometers measuring erythemally weighted irradiance.

An ultraviolet calibration center has been established in Davos, Switzerland. It provides a laboratory for characterizing the spectral and angular response of broadband radiometers. The absolute calibration of these instruments is performed through the comparison to the reference spectroradiometer QASUME. We present what we believe to be a novel calibration methodology that explicitly includes the information of the angular and spectral response functions. From the results of the latest broadband intercomparison campaign, the typical uncertainties of these instruments could be obtained. Most radiometers have an expanded uncertainty of approximately 7%. The angular response introduces an uncertainty of 0.9%-7.2%, depending on the cosine error of the radiometer.

Experimental determination of the reference plane of shaped diffusers by solar ultraviolet measurements.

The optical reference plane of a J1002 shaped dome diffuser from CMS-Schreder was determined using direct normal spectral solar UV irradiance measurements relative to a flat Teflon diffuser. The spectroradiometers were calibrated relative to the same irradiance standard. The optical reference plane of the shaped J1002 diffuser is 5.3 mm behind the top of the dome with an uncertainty of 1.0 mm. Solar UV irradiance measurements based on a lamp calibration using the top of the dome as the reference will overestimate the global solar irradiance by 2.1% for the usual calibration distance of 500 mm.

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.

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements.

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.

Intercomparison of monochromatic source facilities for the determination of the relative spectral response of erythemal broadband filter radiometers.

The relative spectral responses of erythemally weighted broadband radiometers determined at three different laboratories are compared, and the systems are described. The results of measurements of four different broadband radiometers are discussed. Although the common dynamic range of the measured relative spectral responses is approximately 10(4), the differences in the relative spectral response functions are lower than 20%. These differences are related mostly to measurement uncertainties and differences in the spectral response facilities.

Improved entrance optic for global irradiance measurements with a Brewer spectrophotometer.

A new entrance optic for a Brewer spectrophotometer has been designed and tested both in the laboratory and during solar measurements. The integrated cosine response deviates by 2.4% from the ideal, with an uncertainty of +/- 1%. The systematic uncertainties of global solar irradiance measurements with this new entrance optic are considerably reduced compared with measurements with the traditional design. Simultaneous solar irradiance measurements between the Brewer spectrophotometer and a spectroradiometer equipped with a state-of-the-art shaped diffuser agreed to within +/- 2% during a five-day measurement period.

Quality assurance of reference standards from nine European solar-ultraviolet monitoring laboratories.

A program for quality assurance of reference standards has been initiated among nine solar-UV monitoring laboratories. By means of a traveling lamp package that comprises several 1000-W ANSI code DXW-type quartz-halogen lamps, a 0.1-ohm shunt, and a 6-1/2 digit voltmeter, the irradiance scales used by the nine laboratories were compared with one another; a relative uncertainty of 1.2% was found. The comparison of 15 reference standards yielded differences of as much as 9%; the average difference was less than 3%.

Retrieval of the ultraviolet aerosol optical depth during a spring campaign in the Bavarian Alps.

A measurement campaign was organized in March 1999 in the Bavarian Alps as part of the European project, Characteristics of the UV Radiation Field in the Alps (CUVRA), to analyze the effect of altitude, aerosols, and snow cover on ground-level UV spectral irradiance. We present the results of simultaneous measurements of aerosol optical depth (AOD) made at various sites on two cloudless days in March 1999. The two days exhibited different aerosol conditions. Results derived from spectral measurements of UV irradiance are compared with data from filter radiometer measurements made at discrete wavelengths extending from the UV to the near IR. The different methods generated values for the AOD that were in good agreement. This result confirms that one can use either method to retrieve the AOD with an uncertainty of approximately 0.03-0.05. On 18 March, high turbidity was observed at low altitude (400-nm AOD approximately 0.5 at 700 m above sea level), and the AOD decreased regularly with altitude; on 24 March, the turbidity was much less (0.11 at 700 m above sea level). On both days very low AODs (0.05-0.09) were measured at 3000 m above sea level. The spectral dependence of the AOD is often parameterized by the angstrom relationship; the alpha parameter is generally difficult or impossible to retrieve from spectral measurements because of the relatively narrow wavelength range (320-400 nm), and only one of the spectro-radiometers used during the campaign permits this retrieval. In most cases, during this field campaign, alpha was found by filter sunphotometers to be 1.1-1.5.