Mario Blumthaler

INCREASE IN SOLAR UV RADIATION WITH ALTITUDE

Abstract Since 1981, broad-band measurements have been made at Jungfraujoch, Switzerland (3576 m above sea-level (a.s.l.)) and Innsbruck, Austria (577 m a.s.l.), where daily totals of erythemal effective irradiance, UVA irradiance and total irradiance have been compared. Under clear sky conditions, the observed increases in irradiance with altitude (altitude effect) of the daily totals of global irradiance are 8%±2% per 1000 m (total irradiance), 9%±2% per 1000 m (UVA irradiance) and 18%±2% per 1000 m (erythemal effective irradiance) during the summer. The altitude effect of the simultaneously measured erythernal effective irradiance between Innsbruck (577 m a.s.l.) and Hafelekar (2300 m a.s.l.), horizontally separated by 2.5 km, shows a slight dependence on the solar elevation: 15.1%±1.8% per 1000 m at 60° solar elevation and 18.6% ± 2.9% per 1000 m at 20° solar elevation. Simultaneously taken measurements of solar irradiance with high resolution spectrometers at Garmisch-Partenkirchen, Germany (730 m a.s.l.) and Wank (1730 m a.s.l.), horizontally separated by 5 km, show a clear wavelength dependence of the altitude effect of the global irradiance: 9% per 1000 m at 370 nm increasing to 11% per 1000 m at 320 nm and 24% per 1000 m at 300 nm. The altitude effect of direct irradiance is considerably higher than that of global irradiance at all measured wavelenths.

Effect of aerosols on solar UV irradiances during the Photochemical Activity and Solar Ultraviolet Radiation campaign

Surface UV irradiances were measured at two different sites in Greece during June 1996 under noncloudy conditions. The measured UV irradiances are simulated by a radiative transfer model using measured ozone density and aerosol optical depth profiles. The absolute difference between model and measurements ranges between −5% and +5% with little dependence on wavelength. The temporal and solar zenith angle dependence in the difference between model and measurement suggests that part of this difference may be explained by assumptions made about the aerosol single-scattering albedo and phase function. Simulated spectra including aerosols are compared with calculated spectra excluding aerosols. It is found that for otherwise similar atmospheric conditions the UVB irradiance is reduced with respect to aerosol free conditions by 5% to 35% depending on the aerosol optical depth and single-scattering albedo. For the campaign period, changes in the aerosol loading gave larger variations in the surface UV irradiances than the changes seen in the ozone column.

Comparing ground-level spectrally resolved solar UV measurements using various instruments: A technique resolving effects of wavelength shift and slit width

Spectrally resolved UV measurements are important for the study of biologically relevant UV in relation to changes in atmospheric parameters. The inter-comparison of spectral instruments is essential as measurement techniques and calibrations are not standardized. The differences in slit functions cause large spectral variations when comparing the spectral readings directly. The method described, which compares spectral readings using different instruments, corrects for differences of wavelength calibrations and slit functions, and does not require knowledge of additional atmospheric parameters and UV-transfer model calculations. The wavelength alignment has an accuracy of 0.02 nm over the wavelength interval from 300–400 nm, and a reproducibility of 0.01 nm. The robustness of the methods and reproducibility of results are shown in the evaluation of a seven day intercomparison campaign with three different scanning spectroradiometers.

SUSPEN intercomparison of ultraviolet spectroradiometers

Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece July, 1–13 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wave-length registration of the recorded spectra, for most of the spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.

Solar UV-A and UV-B radiation fluxes at two Alpine stations at different altitudes

SummaryDaily totals of UV-A and UV-B radiation fluxes and global radiation were measured since 1981 at Jungfraujoch (3576 m) a.s.l.) and in Innsbruck (577 m a.s.l.) in their seasonal course. The altitude effect of annual totals yields 19%/1000 m (UV-B), 11%/1000 m (UV-A) and 9%/1000 m (global radiation) with reference to Innsbruck station. The ratio of the daily totals of UV-B/global radiation shows a significant seasonal course with the maximum in summer, whereas the ratio of the daily totals of UV-A/global radiation shows no significant seasonal variation. The biological effective doses of erythema reaction, delayed tanning and immediate tanning by UV-A and UV-B radiant exposure are reported in the seasonal course at Jungfraujoch and in Innsbruck.

Effects of cloudiness on global and diffuse UV irradiance in a high-mountain area

SummaryAt the high-mountain station Jungfraujoch (3576 m a.s.l., Switzerland), measurements of the radiation fluxes were made during 16 periods of six to eight weeks by means of a Robertson—Berger sunburn meter (UVB data), an Eppley UVA radiometer and an Eppley pyranometer. Cloudiness, opacity and altitude of clouds were recorded at 30-minute intervals. A second set of instruments was employed for separate measurement of the diffuse radiation fluxes using shadow bands. The global and diffuse UVA- and UVB radiation fluxes change less with cloudiness than the corresponding total radiation fluxes. When the sun is covered by clouds, the global UVA- and UVB radiation fluxes are also affected less than the global total radiation flux. The roughly equal influence of cloudiness on the UVA- and UVB radiation fluxes suggests that the reduction is influenced more by scattering than by ozone. Also, the share of diffuse irradiance in global irradiance is considerably higher for UVA- and UVB irradiance than for total irradiance. At 50° solar elevation and 0/10 cloudiness, the share is 39% for UVB irradiance, 34% for UVA irradiance and 11% for total irradiance. The increased aerosol turbidity after the eruptions of El Chichon and Pinatubo has caused a significant increase in diffuse total irradiance but has not produced any significant changes in diffuse UVA- and UVB irradiances.

Correcting global solar ultraviolet spectra recorded by a brewer spectroradiometer for its angular response error.

We present a methodology for correcting the global UV spectral measurements of a Brewer MKIII spectroradiometer for the error introduced by the deviation of the angular response of the instrument from the ideal response. This methodology is applicable also to other Brewer spectroradiometers that are currently in operation. The various stages of the methodology are described in detail, together with the uncertainties involved in each stage. Finally global spectral UV measurements with and without the application of the correction are compared with collocated measurements of another spectroradiometer and with model calculations, demonstrating the efficiency of the method. Depending on wavelength and on the aerosol loading, the cosine correction factors range from 2% to 7%. The uncertainties involved in the calculation of these correction factors were found to be relatively small, ranging from ~0.2% to ~2%.

Erythemally weighted radiometers in solar UV monitoring: Results from the WMO/STUK intercomparison

The first international intercomparison of erythemally weighted (EW) broadband radiometers was arranged in 1995 to improve the accuracy and comparability of the measurements carried out by solar UV monitoring networks. The intercomparison was arranged at the Radiation and Nuclear Safety Authority in Helsinki, Finland, in cooperation with the University of Innsbruck and with support from the World Meteorological Organization. Altogether 20 EW meters of six different types from 16 countries were (1) tested in the laboratory by measuring the spectral and angular responsivities and (2) calibrated in solar radiation against two reference spectroradiometers. Calibration factors (CFs) for the EW meters were determined by using simultaneously measured EW solar UV spectra as a calibration reference. The CFs averaged over solar elevations higher than 35° varied from 0.87 to 1.75, with the estimated uncertainty being ±10%. As a result of this intercomparison, for the first time the calibrations of more than 100 EW radiometers around the world are possible to trace to the same origin. The present experience indicates that the accuracy of temperature‐controlled EW radiometers is not significantly lower than the accuracy of spectroradiometers provided that strict quality assurance/quality control procedures are followed.

Simultaneous spectroradiometry: a study of solar UV irradiance at two altitudes

Spectrally resolved measurements of solar ultraviolet (UV) radiation were made at adjacent mountain top and valley sites, vertically separated by 1 km. Data was collected over a period of 4 days in August 1993 providing simultaneously measured spectra for a range of zenith angles with both sites in direct sunlight and less than 2 octas of cloud cover. As there was no snow cover the local albedo was similar at both sites. Irradiances at the mountain site were 9% higher than the valley at 370 nm, increasing gradually to 11% at 320 nm. Across the UVB waveband the altitude effect increased dramatically to 24% at 300 nm. The magnitude of the altitude effect was independent of zenith angle, within the measurement accuracy, for the range z = 30°–70°. These measured differences between the mountain and valley station can be interpreted by model calculations using the corresponding differences in air pressure, column ozone and aerosol content.

From model intercomparison toward benchmark UV spectra for six real atmospheric cases

The validity of a radiative transfer model can be checked either by comparing its results with measurements or with solutions for artificial cases. Unfortunately, neither type of comparison can guarantee that the spectral UV surface irradiance is accurately calculated for real atmospheric cases. There is a need therefore for benchmarks, i.e., standard results that can be used as a validation tool for UV radiation models. In this paper we give such benchmarks for six cloud-free situations. The chosen cases are characterized by different values of solar zenith angle, ozone column, aerosol loading, and surface albedo. Observations are also available for these cases to allow a further comparison between model results and measurements. An intercomparison of 12 numerical models is used to construct the benchmarks. Each model is supplied with identical input data, and a distinction is made between models that assume a planeparallel geometry and those that use a pseudospherical approximation. Differences remain between the model results, because of different treatments of the input data set. Calculations of direct and global transmission and direct and global irradiance are within 3% for wavelengths longer than 320 nm. For the low-Sun cases the calculations are within 10% for wavelengths longer than 300 nm. On the basis of these calculations, six benchmark UV spectra (295–400 nm) are established with a standard deviation of 2%. Relative standard deviations are higher for the lowest absolute intensities at low Sun (5% at 300 nm). The variation between models is typically less than the variation seen between model and measurement. Differences between the benchmarks and the observed spectra are mainly due to the uncertainty in the input parameters. In four of the six cases the benchmarks agree with the observed spectra within 13% over the whole UV spectral region.