Radiative models for the evaluation of the UV radiation at the ground.

The variety of radiative models for solar UV radiation is discussed. For the evaluation of measured UV radiation at the ground the basic problem is the availability of actual values of the atmospheric parameters that influence the UV radiation. The largest uncertainties are due to clouds and aerosol, which are highly variable. In the case of tilted receivers, like the human skin for most orientations, and for conditions like a street canyon or tree shadow, besides the classical radiative transfer in the atmosphere additional modelling is necessary.

Visualization of UV exposure of the human body based on data from a scanning UV-measuring system.

In general, measurements of UV radition are related to horizontal surfaces, as in the case of the internationally standardized and applied UV index, for example. In order to obtain more relevant information on UV exposure of humans the new measuring system ASCARATIS (Angle SCAnning RAdiometer for determination of erythemally weighted irradiance on TIlted Surfaces) was developed and built. Three systems of ASCARATIS have been in operation at different locations in Bavaria for 3 years, providing erythemally weighted UV irradiation data for 27 differently inclined surfaces every 2 min. On the basis of these data virtual three-dimensional models of the human body surface consisting of about 20,000 triangles could be created and each of these triangles coloured according to its UV irradiation. This allowed the UV exposure of the human body to be visualized for any kind of body posture and spatial orientation on the basis of real measuring data. The results of the UV measurements on inclined surfaces have shown that measuring UV radiation on horizontal surfaces, as done routinely worldwide, often underestimates the UV exposure of the human skin. Especially at times of the day or year with low solar elevations the UV exposure of parts of the human skin can be many times higher than that of the horizontal surface. Examples of three-dimensional modelling of the human UV irradiation are shown for different times of the day and year, altitudes above sea level, body postures and genders. In these examples the UV "hotspots" can be detected and, among other things, used to inform and educate the public about UV radiation.

Tobacco smoke is phototoxic.

BACKGROUND: Both cigarette smoke and ultraviolet (UV) radiation are known to cause changes of the skin which can be regarded as premature ageing. OBJECTIVES: To assess the theory that the effects of these two exposures could be linked by a phototoxic action of cigarette smoke. METHODS: A photohaemolysis test was used, in which human erythrocytes were incubated with cigarette smoke condensate, followed by UV irradiation and measurement of exposure-dependent haemolysis. RESULTS: Cigarette smoke condensate was clearly phototoxic. Photohaemolysis depended on the concentration of the condensate and UV dose and was more pronounced after exposure to UVA-rich than UVB-rich radiation. CONCLUSIONS: Phototoxicity may be a mechanism by which cigarette smoking causes premature skin ageing. An enhancing effect on photocarcinogenesis has also to be considered.

Routine measurement of erythemally effective UV irradiance on inclined surfaces.

Measurements of erythemally weighted UV radiation are usually related to a horizontal surface. The radiation is weighted with the sensitivity of the human skin, but the surface of the human body has only few horizontal surfaces. Therefore the UV radiation on inclined surfaces has to be quantified to investigate UV effects on humans. To fulfill this task three fully automatic measuring systems were built to measure the erythemally weighted UV radiation in 27 directions within 2 min. This system measures routinely during the whole day and has now been in operation for nearly three years (in total 2000 measurement days) under any kind of meteorological conditions. The measurements provide the informations needed for further investigations concerning the UV effects on humans. The calibration of the erythemally weighting radiometers was performed in a way to provide reliable UV index measurements for all directions. The results of four exemplary measurement days in summer and winter for clear sky and overcast conditions are presented.

Simplified calibration for broadband solar ultraviolet radiation measurements.

Aspects of different calibration procedures for erythemally weighing broadband radiometers are presented in this study. These instruments are common in projects dealing with ultraviolet radiation effects on humans. Many erythemally weighing broadband radiometers are still operated using a single calibration factor (cf) that is provided with the instrument. The individual characteristics of every instrument are strongly dependent on the total ozone amount and the solar elevation. Therefore, a calibration procedure also has to take into account the ozone concentrations and the solar elevation to compensate for the effects of the individual characteristics and to provide comparable measurements. Given the variation of the ozone concentrations and the solar elevation, an individual cf has to be calculated for every measurement. Using a simplified version of the calibration procedure, which is presented in this study, can lessen this effort. Taking into account the relevant meteorological conditions for a measuring site, a single cf is calculated to compensate the individual characteristics of the instruments and therefore deliver comparable measurements with less effort.

Future UV radiation in Central Europe modelled from ozone scenarios.

Photobiologically and photochemically relevant UV radiation for the time around the years 2015 and 2050 is estimated by radiative transfer calculations using variable ozone content based on model simulations. The future cloud conditions are assumed unchanged. Assuming various emission scenarios of chlorfluorohydrocarbons (CFCs) and other trace gases, and taking into account future temperature development and changing atmospheric dynamic conditions, ozone values are simulated. On the basis of these data, three different scenarios of the future total ozone content over Central Europe are analysed, which represent from current knowledge, probable as well as optimistic (high ozone and low UV irradiance) and pessimistic (low ozone and high UV irradiance) conditions. According to these scenarios the future development of the UV radiation is expected not to follow the increasing trend of UV irradiation observed during the last three decades. The predicted changes are highly variable with season. During late winter and spring, the enhanced recent UV values will persist for the next decades. Till 2015 a further slight increase is predicted for springtime. In contrast, during summer and fall, the UV level is assumed to remain on the recent level. For 2050 a decrease to values close to that of an anthropogeneous nearly undisturbed ozone chemistry, as it was found around 1970, is predicted. In addition to average long-time variations of the UV irradiance, short-time increase may occur due to ozone minihole events or due to a large volcanic eruption. The latter can produce a marked increase in UV radiation for several months. During ozone minihole events, with maximum occurrence in spring, UV irradiance is typically increased for a few days. Such episodes must be taken into account additionally to the average UV development. They will occur also in the future and result in UV radiation increases against undisturbed conditions, which are similar to present minihole events. These differences are much larger than the average changes predicted for future ozone development.

Spectral radiative-transfer modeling with minimized computation time by use of a neural-network technique.

A new approach based on a neural-network technique for reduction in the computation time of radiative-transfer models is presented. This approach gives high spectral resolution without significant loss of accuracy. A rigorous radiative-transfer model is used to calculate radiation values at a few selected wavelengths, and a neural-network algorithm replenishes them to a complete spectrum with radiation values at a high spectral resolution. This method is used for the UV and visible spectral ranges. The results document the ability of a neural network to learn this specific task. More than 20,000 UV-index values for all kinds of atmosphere are calculated by both the rigorous radiative-transfer model alone and the model in combination with the neural-network algorithm. The agreement between both approaches is generally of the order of ?1%; the computation time is reduced by a factor of more than 20. The new algorithm can be used for all kinds of high-quality radiative-transfer model to speed up computation time.

Method to determine snow albedo values in the ultraviolet for radiative transfer modeling.

For many cases modeled and measured UV global irradiances agree to within +/-5% for cloudless conditions, provided that all relevant parameters for describing the atmosphere and the surface are well known. However, for conditions with snow-covered surfaces this agreement is usually not achievable, because on the one hand the regional albedo, which has to be used in a model, is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method is given to determine the regional albedo values for conditions with snow cover by use of a parameterization on the basis of snow depth and snow age, routinely monitored by the weather services. An algorithm is evolved by multiple linear regression between the snow data and snow-albedo values in the UV, which are determined from a best fit of modeled and measured UV irradiances for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18-0.5 range. Since the constants of the regression depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with different snow conditions improves the accuracy of modeled UV irradiances considerably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average deviations between measured and modeled UV global irradiances.

Determination of erythema-effective solar radiation in Japan and Germany with a spore monolayer film optimized for the detection of UVB and UVA--results of a field campaign.

The available physical and biological broad-band radiometers designed to determine erythema-effective radiation do not show any response or over/underestimate the biologically effective radiation to a high extent in the ultraviolet (UV)A spectral region. The data presented in this paper demonstrate that the biological system used in this study is the first one to make possible measurements of erythema-effective radiation in the sun in the UVA and UVB spectral region. These measurements were performed with a spore-film filter system as well as with spectroradiometers. It was demonstrated that this biotechnological method could be used to determine exact values expressed as minimal erythemal dose (MED). The spore-film system was tested in various field campaigns performed in Germany and in Japan. The seasonal daily variation of UV radiation in Germany determined in the period November 1995 to December 1996 using the spore-film filter system in sunny conditions tallied well with model calculations. The daily dose in Germany measured with the spore-film system close to the summer solstice, in sunny conditions (20.45 MED), was approximately 20 times higher than the lowest value measured close to the winter solstice (0.82 MED), a result which was in accordance with model calculations. The data determined with the spore-film filter system in Sapporo and Naha, Japan, fitted to the erythema-weighted data calculated from spectroradiometric measurements (Brewer), even at low solar radiation angles in a solar spectrum with less UVB but significant UVA. The spore-film dosimeter values were about 103 +/- 8% of the integrated dose of the Brewer instrument. The standard deviation of the spore-film measurements obtained in Japan was 12.8%. The responsivity of the spore-film system towards longer wavelengths within the UVA spectrum was tested with the Okasaki Large Spectrograph with monochromatic radiation. At a wavelength of 365 nm--in a spectral region which is dominant in many tanning lamps and with minor importance for solar radiation in summer conditions--the tested spore-film system gave results that were close (112% compared to the calibration dose) to the calibration dose which was used for irradiation.

Comparison of models used for UV index calculations.

Eighteen radiative transfer models in use for calculation of UV index are compared with respect to their results for more that 100 cloud-free atmospheres, which describe present, possible future and extreme conditions. The comparison includes six multiple-scattering spectral models, eight fast spectral models and four empirical models. Averages of the results of the six participating multiple-scattering spectral models are taken as a basis for assessment. The agreement among the multiple-scattering models is within +/- 0.5 UV index values for more than 80% of chosen atmospheric parameters. The fast spectral models have very different agreement, between +/- 1 and up to 12 UV index values. The results of the empirical models agree reasonably well with the reference models but only for the atmospheres for which they have been developed. The data to describe the atmospheric conditions, which are used for the comparison, together with the individual results of all participating models and model descriptions are available on the Internet: http://www.meteo.physik.uni-muenchen.de/++ +strahlung/cost/.

Accounting for the multiple-scattering effect in radiation intensities at the top of the atmosphere.

For obtaining aerosol optical depths over the ocean by using satellite measured radiances computations of backscattered solar radiation fields are necessary. Since a detailed multiple-scattering algorithm is time consuming a simple approach that allows parametrization of multiple-scattering processes is desired. Here the differences between multiple-scattered radiances, which are calculated by the successive order of scattering method, and single-scattered radiances, which use a simple single-scattering equation, and are obtained for different aerosol models defined as a correction term. It is shown that all correction terms could be linearly fitted with aerosol optical depths if they are scaled by the single-scattering albedo and 1 minus the asymmetry factor of the aerosol models considered. An analytic expression for the correction factor is obtained. Thus it is shown that multiple-scattered radiances at the top of the atmosphere can be estimated directly from the single-scattered radiances and with the co rection term for all Sun-satellite geometries that are usually used for remote sensing of aerosol turbidities over ocean surfaces with an uncertainty of less than 10% for aerosol optical depths up to 0.5 in the visible region.

Scattering functions of tropospheric aerosols: the effects of nonspherical particles.

Scattering functions, i.e., the scattered intensity as a function of angle, are modeled for tropospheric aerosol types with respect to the effect of the nonspherical shape of the particles. Scattering functions of nonspherical particles compared with those of equivalent spheres show differences increasing with particle size. Thus, for aerosol types with a relatively low amount of large particles, such as continental and urban aerosols, the effect due to uncertainty about particle shape can be ignored, compared to effects due to uncertain particle size and refractive index. In desert aerosol the nonspherical particles systematically increase side scatter with a maximum around a scattering angle of 120 degrees , while around 160 degrees the difference between scattering functions of spheres and nonspheres is small. With increasing wavelength the influence of nonspherical particles decreases.

Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat.

The method of vicarious calibration by means of calculated radiances allows absolute calibration of satellite radiometers in orbit. It works by comparing counts from the radiometer to be calibrated with corresponding absolute radiances, calculated from actual values of the relevant optically acting parameters of the atmosphere and the earth's surface. The method is applied to the VIS-channel (it measures in the visible and near IR) of the European geostationary satellite Meteosat-1. To minimize uncertainties, the procedure is carried out over different surfaces, at different atmospheric conditions, and at different sun and satellite angles. The ratio between the effective radiances (the radiances at the satellite weighted with its spectral response) and the measured 6-bit counts of the Meteosat-1-VIS-channel is the calibration constant c(sat) = 2.66 W . m(-2) . sr(-1)/count. The accuracy of the calibration is +/-6%. The inaccuracy is mainly due to the broad digitization steps of the channel. Conversion factors are presented which allow one to calculate from the effective radiance the radiance at the satellite (the radiance leaving the atmosphere).

Influence of measured reflection properties of vegetated surfaces on atmospheric radiance and its polarization.

Based on measured values of the spectral bidirectional reflection functions of four vegetated surfaces, the influence of their angular anisotropy on the upward and downward emerging radiance and its polarization is calculated. By means of a realistic model of the atmosphere and with the assumption of completely depolarizing reflection properties of the surfaces, results are obtained in dependence of wavelength and solar zenith angle. The angular anisotropy influences considerably the upward emerging radiance. On the degree of polarization and on the downward emerging radiance the anisotropy has negligible to small influence. Due to the angular anisotropy of the reflection properties the spectral albedo depends strongly on the solar zenith angle. This influences upward and downward radiance as well as its degree of polarization. Therefore, for the interpretation of radiation measurements, those spectral albedo values should be used which correspond to the respective solar zenith angle. This is essential especially at longer wavelengths where vegetated surfaces have high spectral albedos.

Water vapor: spectral transmission at wavelengths between 0.7 microm and 1 microm.

Spectral water vapor transmission of the bands between 0.7 microm and 1 microm is determined from ground-based measurements of direct solar radiation with a high resolution grating spectrometer. These transmission values are compared with those published by Moskalenko, which are the only spectral water vapor data below 1 microm applicable to real atmospheres. Our values of the spectral transmission and those reported by Moskalenko differ considerably. New beta(nu) values to calculate spectral transmission values by means of Moskalenko's transmission function are given. The resulting absorptions of the total bands agree better with absorptions obtained from Yamamoto than with those from Moskalenko.