Measurements Of Solar Ultraviolet Irradiance Research Articles

Measurements of total ozone amount over Badajoz (Southwestern Spain) by means of a GUV multiband radiometer

This paper presents the first measurements of total ozone amount performed at the radiometric station of Badajoz (Spain) by means of a multichannel radiometer GUV-2511. The retrieval of total ozone amount from GUV measurements of solar ultraviolet irradiance is performed with the support of look-up tables calculated using the SBDART radiation transfer code. This methodology is based on the different absorption by ozone suffered by the radiation at two certain near wavelengths. In this case the pair 305, 340 nm was chosen because of its better performance than the pair 305, 320 nm. The look-up table contains the ratio between irradiances at the mentioned wavelengths calculated under different theoretical conditions of solar zenith angle and total ozone amount. The estimations of actual total ozone amount are calculated applying the look-up table to the experimental irradiance ratios as measured by the GUV radiometer. In order to validate these estimations, they are compared with the values provided by the OMI (Ozone Monitoring Instrument) aboard NASA EOS/Aura satellite during April 2010. Results indicate a general good agreement between both datasets, showing a notable dependence with the cloudiness conditions.

Solar forcing of winter climate variability in the Northern Hemisphere

An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested. Simulations with a climate model driven by satellite measurements of solar ultraviolet irradiance show an atmospheric response to the solar minimum that resembles the negative phase of the North Atlantic Oscillation.

Improving solar ultraviolet irradiance measurements by applying a temperature correction method for Teflon diffusers

To establish trends in surface ultraviolet radiation levels, accurate and stable long-term measurements are required. The accuracy level of today's measurements has become high enough to notice even smaller effects that influence instrument sensitivity. Laboratory measurements of the sensitivity of the entrance optics have shown a decrease of as much as 0.07-0.1%/deg temperature increase. Since the entrance optics can heat to greater than 45 degrees C in Dutch summers, corrections are necessary. A method is developed to estimate the entrance optics temperatures from pyranometer measurements and meteorological data. The method enables us to correct historic data records for which temperature information is not available. The temperature retrieval method has an uncertainty of less than 2.5 degrees C, resulting in a 0.3% uncertainty in the correction to be performed. The temperature correction improves the agreement between modeled and measured doses and instrument intercomparison as performed within the Quality Assurance of Spectral Ultraviolet Measurements in Europe project. The retrieval method is easily transferable to other instruments.

Quality assurance of spectral solar UV measurements: results from 25 UV monitoring sites in Europe, 2002 to 2004

With the transportable reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) routine quality assurance of spectrally resolved solar ultraviolet irradiance measurements were successfully performed at 25 UV monitoring sites in Europe. The absolute scale carried by the QASUME reference spectroradiometer is traceable to the primary irradiance standard of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, and has proved to represent the average scale in use at 25 independent European laboratories; it can thus be taken as a European irradiance reference. Out of the 27 instruments 13 showed deviations relative to the QASUME reference spectroradiometer of less than 4% in the UVB (15 instruments in the UVA) for solar zenith angles below 75°. The results so far have shown the unique possibilities offered by this transportable reference spectroradiometer for providing on-site quality assurance of solar ultraviolet irradiance measurements.

Ultraviolet radiation and clouds: Couplings to tropospheric air quality

Measurements of solar ultraviolet irradiance obtained by a Brewer spectrophotometer located in Chicago, Illinois, reveal a wavelength‐dependent excess attenuation associated with cloudy skies beyond that expected from the column ozone amount. When each observation is expressed as a fraction of the irradiance that would have existed under clear skies, a quantity called the transmission ratio, values for a wavelength band near 305 nm are generally smaller than those for a band near 345 nm under dense cloud cover. Transmission ratios computed for wavelength bands near 310 nm and 315 nm lie in between those for the above wavelengths. The observations are consistent with absorption of radiation in the interstitial air of a cloud, while the magnitude and wavelength dependence are like those expected from ozone. A radiative transfer calculation shows that ozone amounts typical of the troposphere are accompanied by enhanced absorption when placed in a medium that is optically thick in scattering. This mechanism constitutes a coupling between tropospheric air quality and the attenuation of sunlight provided by cloudy skies in the chemically active ultraviolet portion of the spectrum.

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.

Measurements of solar ultraviolet irradiance on inclined surfaces

We present pyranometer and biometer results of total solar UV irradiance (295?385?nm) and erythemal irradiance measured on inclined surfaces in Rosario, Argentina (32?57' S, 60?38' W, 25?m a.s.l.), on clear-sky days near the southern hemisphere summer and winter solstices and spring equinox. The pyranometer and a biometer were placed on a rotation device in order to modify the detector inclination in the solar plane. We determined in detail a systematic difference between the detector inclination at the highest intensity and the detector inclination in the Sun direction. The maximum intensity was always registered at a detector inclination greater than the solar elevation. This can be explained by the varying relation between the diffuse and direct contributions to the irradiance at different inclinations. For total UV irradiance, the absolute difference between the detector inclination at maximum intensity and the solar elevation is less than 12? for solar elevations greater than 30? and for erythemal irradiance less than 20?. The relative difference of the intensities is less than 2% for total UV irradiance and less than 5% for erythemal irradiance. For smaller solar elevations both differences grow rapidly.

Analysis of direct solar ultraviolet irradiance measurements in the French Alps. Retrieval of turbidity and ozone column amount.

Direct ultraviolet spectral solar irradiance is regularly obtained by the difference between global and diffuse irradiances at the French Alpine station of Briançon; the data of years 2001 and 2002 are analyzed in this paper. Comparison with modeled values is used for cloud screening, and an average UV-A aerosol optical depth is used as an index of turbidity; it is found to be around 0.05 for the clear winter days and around 0.2 in summer. Langley plots are used to verify the instrument calibration; they confirm the expected uncertainty smaller than 5%. The ozone total column amount is estimated with an uncertainty between -3 and Dobson units; comparisons with TOMS (Total Ozone Mapping Spectrometer) overpass values shows agreement within the expected uncertainties of both instruments.

Lamps as means to homogenize solar ultraviolet irradiance measurements performed with different spectroradiometers

Portable lamp irradiance calibration units of different size were used to transfer a relative calibration in a group of spectroradiometers operating at the same site. Every unit was used as normally in the field or at a home laboratory, i.e., including any errors related to the method itself. The calibration obtained for each instrument was used to reprocess their synchronous solar measurements. Solar measurements deviating by typically ±10% got a standard deviation as low as 3–4% when corrections derived from a common lamp measurement were applied. Because instrumental features remained the same in the original and in the reprocessed solar data, the improvement of the agreement in the solar measurements was considered as an indication of a more homogeneous irradiance scale. The remaining disagreement must be a sum of the uncertainty related to the use of each calibration unit and any instrumental peculiarities. Each of the lamp units could bring the scales of the specific instrument group closer. This was especially true for distinct outliers in the group. Even the smallest units performed almost as well as more sophisticated equipment, and an agreement of about 5% in the sky measurements could be reached, which is better than that seen in most recent intercomparisons.

The role of the cloud optical thickness in the attenuation of the solar ultraviolet radiation reaching the ground: Implications to the human health impacts

Cloud optical depth, as it is derived from the King's method, is compared with that deduced from the simple two stream approximation for the greater Athens, Greece area. The results show that King's method overestimates the cloud optical thickness, in comparison to the two stream approximation. These estimations were also compared with satellite observations provided by the International Satellite Cloud Climatology Project. This comparison showed that there are large deviations between ground‐based and satellite observations. These deviations are probably due to the uncertainty introduced by the integration procedure of the satellite spatial analyzing resolution (distinctness). Furthermore, values of the simple scattering albedo were calculated by employing a recently proposed theoretical method and using the solar ultraviolet irradiance measurements performed at the Athens University on a routine basis. These results are of great importance for the densely populated areas like Athens, in a sense of health impacts, from enhanced incident solar ultraviolet radiation.

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].

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS)

The state of solar ultraviolet irradiance measurements in 1978, when NASA requested proposals for a new generation of solar ultraviolet monitors to be flown on the Upper Atmosphere Research Satellite (UARS), is described. To overcome the radiometric uncertainties that plagued the measurements at this time, the solar ultraviolet spectral irradiance monitor (SUSIM) instrument design included in‐flight calibration light sources and multichannel photometers. Both are aimed at achieving a maximum precision of the SUSIM measurements over a long period of time, e.g., one solar cycle. The design of the SUSIM‐UARS instrument is compared with the original design specifications for the UARS instruments. Details including optical train, filters, detectors, and contamination precautions are described. Also discussed are the SUSIM‐UARS preflight calibration and characterization, as well as the results of the inflight performance of the instrument during the first 3 months of operation. Finally, flight operations, observation strategy, and data reduction schemes are outlined.

Time variations of solar UV irradiance as measured by the Solstice (UARS) instrument

An analysis is presented of solar ultraviolet irradiance measurements made by the SOLSTICE spectrometers on the Upper Atmosphere Research Satellite (UARS). Reported observations cover the wavelength interval 119–420 nm, and the analysis discussed here is for the time period 26 Nov 1991 to 31 Dec 1992, during which time solar activity decreased in intensity. At the time of peak activity, the average 27‐day variation had a relative amplitude of ∼8% at Ly‐α, tailing off to about 0.6% at 260 nm. It is shown that over the spectral interval 119–260 nm, the relative 27‐day harmonic was about a factor of two larger during the strongly disturbed as compared with the moderately disturbed period.

Solar irradiance between 120 and 400nm and its variations

The solar ultraviolet irradiance measurements in the 120–400 nm wavelength range are reviewed and compared showing still important discrepancies between the irradiance values deduced from the most recent observations.

Solar UV Irradiance Measurements at Four Sites in Tibet

We present solar ultraviolet (UV) irradiance measurements at four sites in Tibet (Lhasa, Linzhi, Nagqu, and Tingri) for the period between July 2008 and September 2010. Erythemal UV dose rates and irradiances of solar UVA, UVB for all four sites are presented. Days with clear sky cloud transmittance (CLT) larger than 100% are particularly selected and analyzed for the four sites. Comparisons between the four sites are also carried out, and the clear-sky data analyses for the summer time show that Tingri (near Mt. Everest) has the highest UV irradiance, Lhasa the second highest, while Nagchu (Northen Tibet) and Linzhi (Eastern Tibet) have a bit lower values. Tingri has the strongest UV level among the four sites around the year, mainly due to increased albedo caused by snow-covered surroundings. UV data for Lhasa for the period from 2004 to 2010 are also analyzed, showing that the UV instantaneous irradiance trend for the recent six-year period is stable. However, the number of clear-sky days in Lhasa has increased over the recent six years, causing an increase in the yearly-integrated total UV irradiance. DOI: http://dx.doi.org/10.3126/jie.v8i3.5934 JIE 2011; 8(3): 75-86