Cloud cover and horizontal plane eye damaging solar UV exposures

The spectral UV and the cloud cover were measured at intervals of 5 min with an integrated cloud and spectral UV measurement system at a sub-tropical Southern Hemisphere site for a 6-month period and solar zenith angle (SZA) range of 4.7° to approximately 80°. The solar UV spectra were recorded between 280 nm and 400 nm in 0.5 nm increments and weighted with the action spectra for photokeratitis and cataracts in order to investigate the effect of cloud cover on the horizontal plane biologically damaging UV irradiances for cataracts (UVBE_cat) and photokeratitis (UVBE_pker). Eighty five percent of the recorded spectra produced a measured irradiance to a cloud free irradiance ratio of 0.6 and higher while 76% produced a ratio of 0.8 and higher. Empirical non-linear expressions as a function of SZA have been developed for all sky conditions to allow the evaluation of the biologically damaging UV irradiances for photokeratitis and cataracts from a knowledge of the unweighted UV irradiances.     

Ship-borne measurements of erythemal UV irradiance and ozone content in various climate zones.

Ship-borne measurements of spectral as well as biologically effective UV irradiance have been performed on the German research vessel Polarstern during the Atlantic transect from Bremerhaven, Germany (53.5 degrees N, 8.5 degrees E), to Cape Town, South Africa (33.6 degrees S, 18.3 degrees E), from 13 October to 17 November 2005. Such measurements are required to study UV effects on marine organisms. They are also necessary to validate satellite-derived surface UV irradiance. Cloud free radiative transfer calculations support the investigation of this latitudinal dependence. Input parameters, such as total ozone column and aerosol optical depth have been measured on board as well. Using these measured parameters, the modelled cloudless noontime UVA irradiance (320-400 nm) shows the expected dependence on varying minimum solar zenith angles (SZA) at different latitudes. The modelled cloudless noontime UVB irradiance (290-320 nm) does not show this clear dependence on SZA due to the strong influence of ozone absorption in this spectral range. The maximum daily dose of erythemal irradiance of 5420 J m(-1) was observed on 14 November 2005, when the ship was in the tropical Atlantic south of the equator. The expected UV maximum should have been observed with the sun in the zenith during local noon (11 November). Stratiform clouds reduced the dose to 3835 J m(-1). In comparison, the daily erythemal doses in the mid-latitudinal Bay of Biscay only reached values between 410 and 980 J m(-1) depending on cloud conditions. The deviation in daily erythemal dose derived from different instruments is around 5%. The feasibility to perform ship-borne measurements of spectral UV irradiance is demonstrated.     

Variation of the enhanced biologically damaging solar UV due to clouds.

The variation of the biologically damaging solar UV (UVBE) enhanced by clouds above that of clear sky UVBE has been investigated. This was undertaken for summer through to winter for SZA of 5 to 60 degrees employing an integrated automatic cloud and spectral UV measurement system that recorded the solar UV spectra and the sky images at five minute intervals. The UVBE calculated with action spectra with higher relative effectiveness in the UVA produced the lower percentage of cloud enhanced cases. The DNA UVBE provided the highest percentage of cloud enhanced cases compared to the total number of UV scans with 2.2% cloud enhanced cases. As a comparison, the plant and fish melanoma UVBE provided the lowest percentage of cloud enhanced cases with 0.6 to 0.8% cloud enhanced cases. For the cases of cloud enhanced UVBE, the average ratio of the measured UVBE to calculated cloud free UVBE for the photokeratitis, cataracts, plant, generalized plant damage and fish melanoma action spectra was 1.21 to 1.25. In comparison, the highest value of 1.4 was for the DNA action spectrum.     

Risk of Eye Damage from the Wavelength-Dependent Biologically Effective UVB Spectrum Irradiances

A number of previous studies have discussed the risk of eye damage from broadband ultraviolet (UV) radiation. As the biologically damaging effectiveness of UV irradiation on the human body is known to be wavelength-dependent, it is necessary to study the distribution of the UV spectral irradiance. In order to quantify the ocular biologically effective UV (UVBE) irradiance exposure of different wavelengths and assess the risk of eye damage, UV exposure values were measured at Sanya, China (18.4° N, 109.7°E, altitude 18 m), using a manikin and a dual-detector spectrometer to measure simultaneously the ocular exposure and ambient UV spectral irradiance data and solar elevation angle (SEA) range (approximately 7°–85°). The present study uses the ocular UV spectral irradiance exposure weighted with the action spectra for photokeratitis, photoconjunctivitis and cataracts to calculate the ocular UVBE irradiance exposure for photokeratitis (UVBE_pker), photoconjunctivitis (UVBE_pcon) and cataracts (UVBE_cat). We found that the ocular exposure to UV irradiance is strongest in the 30°–60° SEA range when ∼50% of ocular exposure to UV irradiance on a summer’s day is received. In the 7°–30° SEA range, all the biologically highly effective wavelengths of UVBE_pker, UVBE_pcon and UVBE_cat irradiances are at 300 nm. However, in other SEA ranges the biologically highly effective wavelengths of UVBE_pker, UVBE_pcon and UVBE_cat irradiances are different, corresponding to 311 nm, 300 nm and 307 nm, respectively.     

Optimizing solar UV-radiation exposures for vitamin D3: comparing global and diffuse spectral UV radiation

Currently, there is a major gap in the knowledge that is needed to optimize the beneficial effects related to ultraviolet (UV) radiation at wavelengths that induce vitamin D(3) synthesis (UV(D3)) compared to reducing the biologically damaging overexposure to UV radiation. The aim of this study was to investigate the use of diffuse (radiation that is scattered from all directions) UV radiation to optimize exposures to UV(D3) radiation and maximize the reduction of exposure to UVA radiation. Data on global and diffuse solar UV-radiation spectra were collected at 10-min intervals in the Southern Hemisphere in the late spring and summer from 1 November 2006 to 28 February 2007. For a solar zenith angle (SZA) of approximately 5 degrees , the observed maximum UV(D3) irradiances were 0.80 W/m(2) and 0.46 W/m(2) for global and diffuse UV radiation, respectively. The observed maximum UVA irradiances were 79.0 W/m(2) and 36.2 W/m(2) for global and diffuse UV radiation, respectively. For diffuse UV radiation, the maximum ratio of vitamin D(3) to UVA radiation was 1.75% at a SZA of approximately 10 degrees , whereas the maximum ratio for global UV was 1.27% at 10 degrees . For SZAs of 25 degrees and less, more UV radiation is in the wavelength region contributing to vitamin D(3) synthesis (UV(D3)) than in the UVA region for diffuse UV radiation than for global UV radiation.     

Comparison of biologically effective spectra for erythema and pre-vitamin D<Subscript>3</Subscript> synthesis

The short wavelength cut-off (λ_c), the wavelength of the maximum spectral UV (λ_Max) of spectral pre-vitamin D₃ effective solar UV irradiance (UV_D3), and the spectral erythemal UV (UV_Ery) were compared at 5-min intervals over a 6-month period at solar zenith angles (SZA) ranging from 4.7° to 80°. Averaged over the entire period, λ_c for UV_D3 is higher by 1.05 nm than that for UV_Ery. The λ_Max is higher for UV_D3 compared to UV_Ery for SZA < ~50°. For higher SZA (>55°), the ratio of λ_Max for UV_D3 to that for UV_Ery is less than 1. As the erythemal action spectrum extends into the UVA, the ratio of UV_D3 to UV_Ery irradiances decreases with increasing SZA, along with a decrease in the ratio of λ_Max for UV_D3 compared to UV_Ery. The changes in λ_c and λ_Max influence both personal UV_D3 and UV_Ery exposure and, to take this into account, a dual calibration technique for polysulphone dosimeters has been developed to simultaneously provide measurements of both types of exposure.     

An improved estimation of daily clear-sky biologically EER from broadband global solar radiation

To establish a relation between biologically effective erythemal radiation (EER) and global solar radiation, the hourly and daily clear-sky broadband (310–2,800 nm) global solar radiation (G) and spectral ultraviolet radiation incident on a horizontal surface at Esfahan, Iran (32°37′N, 51°40′E) were measured during the period 2001–2005. Good correlations at statistically significant levels between the daily values of EER and the daily G were found. The seasonal variability of EER/G is also discussed and the correction factors are determined for inclusion of vertical column ozone and solar zenith angle (SZA) cycles. The comparison of the estimated daily EER against the independent observed EER revealed that under clear sky conditions the estimations are accurate to 10% or better over SZA of 10–60° and column ozone of 250–350 Dobson. The comparison of the results with the similar works that have used shorter period of experimental data showed more accurate estimates. The deduced relations could be used to a rough estimate of the daily EER from G in arid climate regions, where there is no measured UV radiation or there are instrumental and other difficulties encountered in measuring UV radiation.     

Influence of ground reflectivity and topography on erythemal UV radiation on inclined planes

Erythemal UV irradiance incident on a horizontal surface is not always the best way of estimating the real dose received by humans or animals. For this purpose knowledge of the irradiance incident on inclined planes is required. This study presents a physically accurate model for the calculation of erythemal UV on inclined planes. The influence of ground reflectivity and topography on erythemal UV on inclined planes is investigated as a function of solar zenith and azimuth angle. It is shown that including directional reflectivity does not substantially change the incident dose on inclined planes, the maximum deviation being 10%. The incident erythemal UV may, however, be much more influenced by the surrounding topography and by the direct/diffuse partitioning of the irradiance (which is a function of altitude). Maximum increases in erythemal UV of +57%, compared with the incident erythemal UV on a horizontal plane, were found when the sensor faced the sun with a mountain slope to the left and right of it and for very high altitudes.     

Cloud observations for the statistical evaluation of the UV index at Toowoomba,Australia

The development of a unique statistical model for the estimation of the UV index for all sky conditions with solar zenith angles of 60° or less is reported. The model was developed based on available data from an integrated whole-sky automated sky camera and UV spectral irradiance measurement system that was collected every 5 min when the equipment was operational over a period of 1 year. The final model does not include terms directly associated with solar radiation, but rather employs terms, and interactions between these terms, including the parameters of sky cover, solar obstruction, and cloud brightness. The correlation between the estimations of the model and the measured values was 0.81. The developed model was evaluated on a data set spanning 5 months that had not been employed in the development of the model. The correlation for this new data set was 0.50, which increased to 0.65 for the cases when the clouds were considered to be a contributor to UV enhancement above that of a cloud-free day.     

Vitamin D effective ultraviolet wavelengths due to scattering in shade

Solar UVB radiation (280–320 nm) is an initiator of Vitamin D₃ production in the human skin. While numerous studies have been conducted in relation to the biological impact of UV exposure in full sun, less research has investigated the irradiances in shade. The purpose of this study was to determine the levels of UV radiation in relation to Vitamin D₃ induction with six commonly encountered shade environments for the larger solar zenith angles observed during autumn and winter. Spectral UV irradiance measurements were made under relatively clear sky conditions at a sub-tropical Southern Hemisphere site for six specific shade environments and solar zenith angle between 35° and 60° to investigate the biologically effective UV irradiances for pre-Vitamin D₃ production. Data from this research indicates that pre-Vitamin D₃ effective UV wavelengths in the shade were most significant for tree shade and a shade umbrella. Compared to that in full sun, pre-Vitamin D₃ effective UV wavelengths were at levels of approximately 52 and 55%, respectively, beneath the shade umbrella and in tree shade. UVB irradiance levels in the shade of a northern facing covered veranda and in a car with windows closed were significantly less than those beneath the shade umbrella, with levels of approximately 11 and 0%, respectively, of those in full sun. Shade is important as a UV minimisation strategy; however, it may also play an important role in providing the human body with adequate levels of UVB radiation for pre-Vitamin D₃ production without experiencing the relatively higher levels of UVA irradiances present in full sun.     

Who, what, where and when-influences on cutaneous vitamin D synthesis

The synthesis of vitamin D in skin is a two-stage process that begins with the production of previtamin D after irradiation of 7-dehydrocholesterol by ultraviolet (UV) radiation. A number of personal and environmental factors control the probability of a suitable UV photon reaching a molecule of 7-dehydrocholesterol in the skin. These are astronomical factors that govern the solar zenith angle (SZA), and the local state of the atmosphere, determining the available solar UV radiation; skin pigmentation and age, determining competing absorbers of UV radiation and available 7-dehydrocholesterol; individual behaviour in the local surroundings, determining exposure of unprotected skin to available UV radiation. The only one of these influences that can be determined unequivocally for any situation is the SZA. The other influences must be considered either as individual case studies, or be represented by "typical" and "idealised" situations for the weather, skin and behaviour. At large SZAs there is insufficient solar UV radiation to initiate significant vitamin D synthesis. At smaller SZAs assessment of solar exposure necessary for vitamin D synthesis can only be indicative and application of any such assessment necessarily requires awareness of both self- and the local environment.     

Wavelength-dependent induction of UV-absorbing mycosporine-like amino acids in the red alga Chondrus crispus under natural solar radiation

Polychromatic response spectra for the induction of UV absorbing mycosporine-like amino acids (MAAs) were calculated after exposing small thalli of the red alga Chondrus crispus under various cut-off filters to natural solar radiation on the North Sea island Helgoland, Germany. The laboratory-grown specimens typically contain only traces of palythine and synthesise five different MAAs rapidly and in high concentrations after being transplanted into shallow water. The resulting qualitative and quantitative patterns of MAA induction differed markedly with respect to spectral distribution. Furthermore, the wavebands effective for MAA induction vary within the MAA. UV-B radiation had a negative effect on the accumulation of the major MAAs shinorine (λ_max=334 nm) and palythine (λ_max=320 nm), while short wavelength UV-A exhibits the highest quantum efficiency on their synthesis. In contrast, the synthesis of asterina-330 (λ_max=330 nm), palythinol (λ_max=332 nm) and palythene (λ_max=360 nm) was mainly induced by UV-B radiation. Whether the synthesis of shinorine and palythine is induced by a photoreceptor with an absorption maximum in the short wavelength UV-A and whether a second photoreceptor absorbing UV-B radiation is responsible for the induction of asterina-330, palythinol and palythene remains to be studied.Our results show that C. crispus has a high capacity to adapt flexibly the qualitative and quantitative MAA concentration to the prevailing spectral distribution of irradiance. On one hand, this is regarded as an important aspect with respect to the acclimation of algae to increasing UV-B irradiance in the context of ongoing depletion of stratospheric ozone. On the other hand, the experiment demonstrates that UV-A irradiance is more important for the induction of the major MAAs shinorine and palythine than UV-B.     

Insect perception of ambient ultraviolet-B radiation

Solar ultraviolet‐B radiation (UV‐B, 290–315 nm) has a strong influence on the interactions between plants and animal consumers. Field studies in various ecosystems have shown that the intensity of insect herbivory increases when the UV‐B spectral band of solar radiation is experimentally attenuated using filters. This effect of UV‐B on insect herbivory has been attributed to UV‐B‐induced changes in the characteristics of plant tissues, and to direct damaging effects of UV‐B photons on the animals. We tested for effects of UV‐B radiation on insect behaviour using field experiments with the thrips Caliothrips phaseoli. When placed in a ‘choice’ tunnel under natural daylight, these insects showed a clear preference for low‐UV‐B environments, and this preference could not be accounted for by differences between environments in total irradiance. These results provide the first evidence of ambient UV‐B photoperception in an insect, challenging the idea that animals are unable to detect variations in the narrow UV‐B component of solar radiation.     

Estimating ultraviolet radiation at the earth's surface

Surface ultraviolet (UV) irradiance depends not only on stratospheric ozone amounts, but also varies with time and date, latitude, cloud amount and aerosol load. Any assessment of the effect of stratospheric ozone depletion on surface UV irradiance must take into consideration all of the above parameters. Measurements in the UV-B region may be accomplished using filter and detector combinations which mimic a biological response curve. However there are uncertainties such as in determining the exact filter response and in the cosine error of the detector. The UV-A region lacks a strong ozone absorption band and approaches which relate measured UV-A irradiance to measured global irradiance show promise. Theoretical models have been derived which calculate spectral UV irradiance in cloudless and cloudy conditions. Results show that cloud transmissivities increase as wavelength increases; however, there is a strong dependence on cloud type. In the absence of surface observations of clouds, satellite data may be used to map UV-A and UV-B irradiance in a region, and this approach is illustrated using two specific examples.     

Parameters influencing the accuracy and practical applicability of UV indicator cards.

In order to evaluate the potential hazard for the skin and the eye presented by solar ultraviolet radiation, and to take appropriate precautions, it is necessary to characterise the biological effective irradiance or dose. Recently, inexpensive UV indicator cards became available that in principle would provide an attractive means to roughly indicate the local level of erythemal solar UV irradiance. We have characterised the properties of a number of different types of UV indicator cards. Several parameters which may influence the colour of the cards were examined with both outdoor trials under solar UV as well as indoor trials using a filtered xenon arc lamp. Our findings show that the tested cards do not give an appropriate estimation of the effective irradiance due to their spectral sensitivity and their temperature dependence. The application area of the tested UV indicator cards is therefore limited to certain temperature ranges and to seasons where a certain ratio between solar UV-A and solar UV-B occurs.     

Seasonal and diurnal variations in ultraviolet-B and ultraviolet-A irradiances at and below the sea surface at Helgoland (North Sea) over a 6-year period

Ultraviolet (UV) radiation at four wavelengths (305, 320, 340 and 380 nm) and photosynthetically active radiation (PAR) were measured from May 1994 to October 1999 using Biospherical UV radiometers. A surface reference sensor located on the roof of the Marine Station at Helgoland recorded values every 5 min, and an equivalent profiling underwater sensor was used for measurements in the sea at approximately monthly intervals. The ratio of 305-nm radiation to PAR varied seasonally, with a 14-fold increase from winter to summer. A much weaker seasonal trend (ca. 1.5-fold) was apparent in the ratio of 320-nm radiation to PAR, but there was no seasonal trend in the ratios of 340- or 380-nm radiation to PAR. The year-to-year variations in 305-nm radiation were also much greater relative to PAR than for the other UV wavelengths, but there was no evidence of a change in the 305 nm:PAR ratio over the study period. The ratios of both 305- and 320-nm radiation to PAR increased from dawn to midday, but those of 340- and 380-nm radiation were almost constant through the day, except shortly before sunrise and after sunset when the proportions of 340- and 380-nm radiation increased. Underwater measurements of PAR and UV suggest that the 1% depth for 305-nm radiation was little more than 1 m, but this estimate is valid only for summer and autumn because, in other seasons, few reliable readings for 305-nm radiation could be obtained underwater, and no attenuation coefficient could be calculated. The 1% depths recorded for the other UV wavelengths in the middle 6 months of the year were 2.0 m for 320 nm, 2.6 m for 340 nm and 4.6 m for 380 nm, compared with 12 m for PAR, but the attenuation of all wavebands increased sharply in October and remained higher until March. An analysis of the influence of sun angle, total column ozone concentration, the proportion of skylight, and cloud cover on the ratio of UV wavelengths to PAR in surface irradiance demonstrated that solar angle has a greater influence than ozone concentration on the irradiance at 305 nm, and that the typical occurrence of ozone "holes" in spring may not result in higher UV-B irradiances than occur under higher ozone concentrations in summer. The implications of the data for attempts to model the biological effects of natural UV radiation on marine organisms are considered. Received in revised form: 3 July 2000 Electronic Publication     

Three dimensional visualisation of human facial exposure to solar ultraviolet.

A three dimensional computer model of the human face has been developed to represent solar ultraviolet exposures recorded by dosimeter measurements on a manikin headform under low cloud conditions and various solar zenith angles. Additionally, polysulfone dosimeters have been successfully miniaturised to provide the detailed measurements required across the face. The headform used in this research was scanned at 709 individual locations to make a wireframe mesh consisting of 18 vertical contours and 49 horizontal contours covering half the manikin's frontal facial topography. Additionally, the back of the headform and neck have also been scanned at 576 locations. Each scanned location has been used as a viable dosimeter position on the headform and represents a grid intersection point on the developed computer wireframe. A series of exposures recorded by dosimeters have been translated into three dimensional exposure ratio maps, representing ambient solar ultraviolet exposure. High dosimeter density has allowed for the development of individual topographic contour models which take into account complex variation in the face and improve upon previously employed techniques which utilise fewer dosimeters to interpolate exposure across facial contours. Exposure ratios for solar zenith angle ranges of 0 degrees -30 degrees, 30 degrees -50 degrees, and 50 degrees -80 degrees have been developed.     

The penetration of ultraviolet radiation to the ground

The evolution of analytic formulas for characterizing the ultraviolet spectral ir-radiance penetrating to the ground is briefly described. Analytic spectral functions for the extraterrestrial solar spectral irradiance, the ozone absorption coefficient, Rayleigh scattering coefficients and aerosol scattering and absorption coefficients, which are used as basic inputs, are given. With Beer's law, these give immediately the direct solar spectral irradiance. A ratio technique described in quantitative detail gives a procedure for calculating the skylight component of the UV radiation reaching the ground. The influence of ground reflectivity, clouds and a possible connection between photobiology and radiological physics are discussed. Finally the advantages of multiwavelength monitoring are described, using monochromators similar to those used in satellite ozone sounding to serve the needs of the photobiology and the atmospheric science communities.     

A biological spectral weighting function for ozone depletion research with higher plants

Biological spectral weighting functions (BSWF) play a key role in assessing implications of stratospheric ozone reduction. They are used to calculate the increase in biologically effective solar UV radiation due to ozone reduction (radiation amplification factor, RAF), assess current latitudinal gradients of solar UV radiation, and compare solar UV radiation with that from lamps and filters used in experiments. As a basis for a BSWF, we developed an action spectrum for growth responses of light-grown oat ( Avena sativa L. cv. Otana) seedlings exposed to narrowband UV radiation from a large double water-prism monochromator. Five UV wavelength peaks were used (275, 297, 302, 313 and 366 nm) in the absence of any visible radiation. Growth responses were measured from 1 to 10 days after the treatments. At all these wavelengths, the UV radiation inhibited height growth, including the height at which the first leaf separated from the stem. Radiation at all wavelengths used, except the one UV-A wavelength, promoted the length of the second leaf. The resulting action spectrum closely resembles the commonly used generalized plant response function except that it indicates continued sensitivity into the UV-A region. When used as a BSWF for the ozone depletion problem, this new function for plant growth would suggest substantially less impact of ozone depletion because it results in only a modest increment of biologically effective UV for a given level of ozone depletion (a lower RAF). Yet this new BSWF also suggests that experimental treatments based on previous BSWF with less emphasis on the UV-A may have resulted in simulations of less pronounced ozone depletion than intended. The validity of this new BSWF with UV-A sensitivity, designated the UV plant growth weighting function, was verified in field experiments as described in the companion paper.     

Reconstruction of daily solar UV irradiation from 1893 to 2002 in Potsdam,Germany

Long-term records of solar UV radiation reaching the Earth’s surface are scarce. Radiative transfer calculations and statistical models are two options used to reconstruct decadal changes in solar UV radiation from long-term records of measured atmospheric parameters that contain information on the effect of clouds, atmospheric aerosols and ground albedo on UV radiation. Based on earlier studies, where the long-term variation of daily solar UV irradiation was derived from measured global and diffuse irradiation as well as atmospheric ozone by a non-linear regression method [Feister et al. (2002) Photochem Photobiol 76:281–293], we present another approach for the reconstruction of time series of solar UV radiation. An artificial neural network (ANN) was trained with measurements of solar UV irradiation taken at the Meteorological Observatory in Potsdam, Germany, as well as measured parameters with long-term records such as global and diffuse radiation, sunshine duration, horizontal visibility and column ozone. This study is focussed on the reconstruction of daily broad-band UV-B (280–315 nm), UV-A (315–400 nm) and erythemal UV irradiation (ER). Due to the rapid changes in cloudiness at mid-latitude sites, solar UV irradiance exhibits appreciable short-term variability. One of the main advantages of the statistical method is that it uses doses of highly variable input parameters calculated from individual spot measurements taken at short time intervals, which thus do represent the short-term variability of solar irradiance.