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The results of two methods retrieving actinic flux and ozone photolysis frequencies from measurements of irradiance with a Brewer MKIII spectroradiometer are investigated in this paper. The first method uses actinic flux retrieved from irradiance measurements by the use of known formulas while the second is an empirical method converting irradiance to ozone photolysis frequencies through polynomials extracted from a study of synchronous actinic flux and irradiance measurements. When examining the actinic fluxes derived from the first method to those measured by an actinic flux spectrometer, data agree within ±10% for solar zenith angles lower than 75° for the UV-B and the UV-A wavelength band. Also, the actinic to global irradiance ratio derived, deviates within ±6% for solar zenith angles lower than 70° compared with cloudless sky calculations of the TUV model. For both cases the deviations are in the order of the magnitude of the measurements or model uncertainties. The ozone photolysis frequencies calculated by the second method show a mean ratio of 0.98±0.12 (1σ and 0.97±0.06 for all data and for cloudless skies, respectively, when compared with ozone photolysis frequencies derived by a Bentham actinic flux spectroradiometer. Finally, the agreement of the two methods is within ±5% comparing two years' data of ozone photolysis frequencies retrieved from irradiance measurements at Thessaloniki, Greece. The use of such methods on extensive data sets of global irradiance can provide JO1D values with acceptable uncertainty, a parameter of particular importance for chemical process studies.

A large set of synchronous actinic flux and global irradiance data recorded in Thessaloniki (Greece) is used to determine the characteristics of the ratio of actinic flux to global irradiance as a function of global irradiance. The shape of this function is dominated by the values for clear and overcast sky conditions. Hence, model calculations carried out with the radiative transfer model UVSPEC for these two types of atmospheric conditions serve as boundary conditions. Then the ratio of actinic flux to global irradiance as a function of global irradiance is reproduced by an analytic function. This function depends on the boundary conditions and on one wavelength-dependent parameter. With the analytic function, the boundary conditions and the empirically determined parameter it is possible to convert any global irradiance data into actinic fluxes within reasonable uncertainties. This is demonstrated with data recorded in Thessaloniki (Greece), at the high mountain station Jungfraujoch (Switzerland), in Innsbruck (Austria) and in Weybourne (Great Britain). In the UVA and UVB for solar zenith angles up to about 60° the uncertainty in calculated actinic fluxes is 6% (twofold standard deviation, k=2) for the data taken from all four measurement sites. For larger solar zenith angles the uncertainties are on average 8% (k=2) in the UVB and about 12% (k=2) in the UVA. Afterwards photolysis rates for ozone and nitrogen dioxide derived from measured and calculated actinic fluxes are compared. The uncertainty in photolysis rates obtained from calculated actinic fluxes for ozone is about 7% (k=2) and for nitrogen dioxide about 12% (k=2).

A method is presented that combines individual ground based ultraviolet (UV) measurements from the Austrian UVB monitoring network and area-wide data of the distribution of clouds derived from satellite images to generate a UV-Index map all over the region. The Austrian UVB Monitoring network provides near real time ground based measurements of surface UV irradiance from fifteen selected locations throughout and in the vicinity of Austria. The amount of ultraviolet radiation passing through the atmosphere as measured by the UVB detectors is indicated in units of the UV-Index, the internationally agreed unit for erythemally weighted solar UV irradiance. Together with clear sky model calculations the measured UV-Index is used to determine the cloud modification factor (CMF), a scaling factor giving the reduction of radiation due to the presence of clouds. Moreover satellite images from MSG (Meteosat Second Generation) with a time resolution of 15 min and a spatial resolution of 0.05° are received. From the satellite images the CMFs for the area of Austria are obtained using an algorithm provided by Jean Verdebout. Then both independent data sets of cloud modification factors are checked for consistency by comparing satellite derived and ground based values at the positions of the monitoring stations. If necessary the satellite derived cloud modification factors are corrected by about ±20% according to the results of the ground based measurements. Afterwards realistic UV-Index maps of the whole area are generated by scaling model derived UV-Indexes with the corresponding cloud modification factors. Since all the data is available in almost real time, the calculated UV-Index maps are available in the web at http://www.uv-index.at/ with a time delay of about 30 min., JRC.H.5-Land Resources Management

Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral actinic flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of actinic flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O1D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total flux profiles lies in the range of ±15%. © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License.

On 29 March 2006, a total eclipse of the Sun was visible on the Greek island Kastelorizo (36.150°N, 29.596°E). An extended set of instruments was installed in order to measure the variability of different components of the radiation field during the eclipse. Seven spectroradiometers (two scanning double monochromators measuring especially in the UV range, 4 photo diode array instruments and one CCD-spectrograph for the UV and visible wavelength range) performed measurements during 28 and 29 March. A narrow band multi-filter radiometer and two broadband erythemal and UVA radiometers were operated with about 1 sec temporal resolution. Two sun-photometers were used to measure ozone column and aerosol optical depth. The weather conditions on March 28 were almost perfect, whereas on 29 March thin cirrus clouds were occasionally present in front of the sun. Details about the observed changes in the radiation field on the eclipse day are presented and compared with model calculations of the change in extraterrestrial solar irradiance. The results show an underestimation of the model calculations compared with measurements, with respect to the effect of the limb darkening on the spectral behavior of the solar irradiance during the eclipse. The absolute changes in the global and direct irradiance and their wavelength dependencies are discussed. Finally, the decrease in total ozone retrieved during the course of the eclipse from direct irradiance measurements is investigated with respect to the effect of the limb darkening and the influence of the diffuse radiation entering the field of view of the Brewer spectroradiometer.

[1] The actinic flux describes the number of photons incident at a point, while the irradiance describes the radiant energy crossing a surface. An algorithm for conversion of irradiance to downwelling actinic flux has been developed and tested. The algorithm uses a simple method to distinguish between a cloudy and a cloudless sky. It is necessary to separate cloudy and cloudless situations, as the irradiance to actinic flux conversion depends on the radiance field which is rather different for cloudy and cloudless skies. Surface UV irradiance and downwelling actinic flux spectra were measured at five European locations which were representative of different climates. A total of 9015 synchronized actinic flux and irradiance spectra were available to test the proposed algorithm. The measured irradiance spectra were used to estimate downwelling actinic flux spectra. The estimated actinic flux spectra were compared with the measured actinic flux spectra for all cloud and aerosol situations encountered, a wide range of solar zenith angles, and surface conditions. The average ratio of the reproduced to measured downwelling actinic flux is 1.021 ± 0.085 in the UV-B and 1.015 ± 0.105 in the UV-A. In general, the performance of the algorithm is better at smaller solar zenith angles.

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within 2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth 300365 nm derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to 0.3 at 340 nm in winter and 0.7 in summer. © 2005 Optical Society of America OCIS codes: 120.6200, 010.1110, 010.1290.

In September 2002, the first INSPECTRO campaign to study the influence of clouds on the spectral actinic flux in the lower troposphere was carried out in East Anglia, England. Measurements of the actinic flux, the irradiance and aerosol and cloud properties were made from four ground stations and by aircraft. The radiation measurements were modelled using the uvspec model and ancillary data. For cloudless conditions, the measurements of the actinic flux were reproduced by 1-D radiative transfer modelling within the measurement and model uncertainties of about ±10%. For overcast days, the ground-based and aircraft radiation measurements and the cloud microphysical property measurements are consistent within the framework of 1-D radiative transfer and within experimental uncertainties. Furthermore, the actinic flux is increased by between 60-100% above the cloud when compared to a cloudless sky, with the largest increase for the optically thickest cloud. Correspondingly, the below cloud actinic flux is decreased by about 55-65%. Just below the cloud top, the downwelling actinic flux has a maximum that is seen in both the measurements and the model results. For broken clouds the traditional cloud fraction approximation is not able to simultaneously reproduce the measured above-cloud enhancement and below-cloud reduction in the actinic flux. © Author(s) 2005. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.