The Angular Distribution of Solar Ultraviolet, Visible and Near-Infrared Radiation from Cloudless Skies

Abstract— The amount of solar radiation intercepted by an object depends on the orientation of the object with respect to the sun and the angular distribution of the diffuse component of solar radiation, which is commonly considered to be approximately isotropic. The angular distribution of the diffuse UV, visible and near-infrared insolation was measured at several solar zenith angles between 32° and 68° under cloudless skies at Lauder, New Zealand (45S), and shown to be anisotropic. The diffuse solar UV radiation increases markedly with solar elevation and is a large proportion of the total UV irradiance. The diffuse visible light and infrared radiation are small components of the total irradiance and almost independent of solar elevation. The angular distribution of erythemal UV radiation was tabulated and is available on request.     

Effect of Cloud Cover on UVB Exposure Under Tree Canopies: Will Climate Change Affect UVB Exposure?

The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated with a three-dimensional canopy radiation transport model. The spatial distribution of UVB irradiance at the base of a regular array of spherical tree crowns was modeled under the full range of sky conditions. The spatial mean relative irradiance (I), and erythemal irradiance of the entire below-canopy domain and the spatial mean relative irradiance and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles from 15° to 60°. The erythemal UV irradiance under skies with 50% or less cloud cover was not remarkably different from that under clear skies. In the shade, the actual irradiance was greater under partly cloudy than under clear skies. The mean ultraviolet protection factor for tree canopies under skies with 50% or less cloud cover was nearly equivalent to that for clear sky days. Regression equations of spatially averaged I_r . as a function of cloud cover fraction, solar zenith angle and canopy cover were used to predict the variation in erythemal irradiance in different land uses across Baltimore, MD.     

The ultraviolet radiation environment of high southern latitudes: springtime behavior over a decadal timescale

Four spectroradiometers located at latitudes from 55 degrees to 90 degrees S conducted near-continuous measurements of ground-level solar ultraviolet irradiance from 1990 through 2001. The behavior during months from October through December is of special interest because this period includes the springtime loss in column ozone and the naturally large irradiances of early summer. Monthly integrated irradiances using biological weightings for erythema and damage to DNA show a distortion of the normal annual cycle in irradiance, with enhanced values occurring in October and November. In some cases, these irradiances exceed those near summer solstice in December. Changes in local cloudiness and column ozone both contribute significantly to interannual variability in erythemal irradiance. This is particularly the case at Palmer Station, near 65 degrees S, where the monthly integrated erythemal irradiance in November 1997 was more than double that observed 5 years earlier. In general, at sites on the Antarctic continent, interannual variability in monthly integrated erythemal irradiance is greatest in November, when the observation for any given year can fall 40% above or below the multiyear mean. Near the tip of South America, interannual variability is approximately half that seen in Antarctica.     

Annual and interannual behavior of solar ultraviolet irradiance revealed by broadband measurements

This research examines the behavior of ground-level solar UV radiation as measured by eight broadband meters in the continental United States during the period from late 1994 to late 1998. The goal is to define the variability that occurs in UV irradiance over time scales ranging from one to several years. The monthly integrated irradiances, from latitude 32 degrees N to 47 degrees N, contain large annual cycles and latitudinal gradients which depend on season. Seven of the eight sites show a maximum in July, a behavior related to proximity to the summer solstice, with modifications associated with the annual cycle in column ozone. A large interannual variability in monthly integrated irradiance appears over the 4 year period studied. A comparison of corresponding months during different years shows differences in irradiance of 20% or more in one-third of the cases analyzed. When the solar zenith angle (SZA) is held fixed in the range 60-65 degrees, a substantial annual cycle in UV irradiance remains where the maximum monthly mean irradiance is 1.4-1.9 times the minimum, depending on location. Furthermore, the annual cycle at fixed SZA is not in phase with the normal seasonal cycle. Maximum irradiances at fixed SZA tend to occur in the October to December period, while minima cluster in April through July. The annual cycle in ozone, with maximum column values in spring and minima in autumn, explains the general character of the fixed-SZA data, although changes in cloudiness are significant contributors to interannual variability.     

LONG-TERM VARIATIONS IN ULTRAVIOLET SUNLIGHT REACHING THE BIOSPHERE: . CALCULATIONS FOR THE PAST THREE DECADES

The long-term data base on atmospheric ozone combined with a set of radiative transfer calculations provides estimates of the variability in ultraviolet (UV) sunlight that should have occurred over the period1957–1988 under clear, pollution-free skies. Results refer to the earth's surface at specific locations in the Northern Hemisphere and to averages over collections of sites located in three latitude bands from 30 to 64°N. For any one year the annually integrated solar irradiance, weighted by the action spectrum for erythema, typically lies within3–4% of the 32-year mean. No statistically significant trends span the entire 32-year time frame. However, over the shorter time period1970–1988 the annually integrated erythemal irradiance shows an upward trend of +2.1 ± 1.2% per decade based on all ozone data at latitudes from 40 to 52°N. No trends exist in lower (30-39°N) and higher (53-64°N) latitude bands. We caution that a trend line provides a very simple index of the variability in UV sunlight, and these results should not necessarily be extrapolated into the future.     

Estimation of pedestrian level UV exposure under trees

Trees influence the amount of solar UV radiation that reaches pedestrians. A three-dimensional model was developed to predict the ultraviolet-B (UV-B) irradiance fields in open-tree canopies where the spacing between trees is equal to or greater than the width of individual tree crowns. The model predicted the relative irradiance (fraction of above-canopy irradiance) under both sunlit and shaded conditions under clear skies with a mean bias error of less than 0.01 and a root mean square error of 0.07. Both model and measurements showed that the locations people typically perceive as shady, low-irradiance locations in the environment can actually have significant UV-B exposure (40-60% of that under direct sunlight). The relationship of tree cover in residential neighborhoods to erythemal UV-B exposure for children and adults was modeled for the 4 h around noon in June and July. Results showed that human exposures (on the horizontal) in cities located at 15 and 30 degrees latitudes are nearly identical. For latitudes between 15 and 60 degrees, ultraviolet protection factors (UPF) were less than 2 for less than 50% tree cover. A UPF of 10 was possible at all latitudes for tree cover of 90%.     

UV index forecasts and measurements of health-effective radiation

While erythemal irradiance as a potentially damaging effect to the skin has been extensively studied and short-term forecasts have been issued to the public to reduce detrimental immediate and long-term effects such as sunburn and skin cancer by overexposure, beneficial effects to human health such as vitamin D(3) production by UV radiation and melatonin suppression by blue visible light have attained more and more attention, though both of them have not become part of forecasting yet. Using 4years of solar radiation data measured at the mid-latitude site Lindenberg (52°N), and forecast daily maximum UV index values, an overall good correspondence has been found. The data base of solar UV radiation and illuminance has also been used to analyze effects of clouds and aerosols on the effective irradiance. Optically thick clouds can strongly modify the ratios between erythemal and vitamin D(3) effective irradiance such that direct radiative transfer modeling of the latter in future UV forecasts should be preferably used. If parameterizations of vitamin D(3) effective irradiance from erythemal irradiance are used instead, the optical cloud depth would have to be taken into account to avoid an overestimation of vitamin D(3) with parameterizations neglecting cloud optical depth. Particular emphasis for the beneficial effects has been laid in our study on low exposure. Daily doses of solar irradiation for both vitamin D(3) and melatonin suppression do not reach minimum threshold doses even with clear sky and unobstructed horizon during the winter months.     

Solar UV Exposure of Seafarers along Subtropical and Tropical Shipping Routes

Seafarers working on decks of vessels at low latitudes are exposed to extremely high solar UV radiation. Their risk of developing skin cancer may be enhanced. Solar erythemal UV irradiance and exposure were measured for the first time on merchant vessels going along typical international routes at low latitudes. The measurements taken at horizontal incidence on the observation deck, and on different parts of the seaman (head, shoulder, chest and back) doing typical outdoor work show the highest portion (40–80% of horizontal exposure) incident on the head. 2 years of measurements of solar UV and VIS/NIR irradiance taken on the mast top of the Research Vessel METEOR were added to the data base. Radiative transfer model calculations were performed along all the routes with satellite‐based input data of ozone and aerosol for clear sky health‐effective radiation including vitamin D3 (VD3). Measured data show extremely high noontime UV index values up to 19 with clear sky, and up to 22 due to cloud scattering. Eight hours erythemal exposure values are more than double of typical midlatitude summer values. Based on the results, an algorithm is presented to derive a seafarer's personal erythemal exposure according to his/her personal record of sea service.     

Erythemal UV Irradiances at Lauder, New Zealand: Relationship between Horizontal and Normal Incidence

Abstract— Measurements from sensors designed to measure erythemal UV irradiance were used to relate the UV incident on a horizontal surface to that incident on a surface maintained normal to the sun throughout the day at Lauder, New Zealand. These UV measurements were also related to variations in global radiation, total column ozone and atmospheric pressure at the surface. Strong correlations were found between these variables over the 37 day observation period in the summer of 1995/1996. Results from these cross-calibrated UV sensors show that the irradiance incident on a surface normal to the sun can be significantly different from that on a horizontal surface. On clear days, the normal-incidence signal can be 30-40% greater for solar zenith angles in the range 60-70A_o. Consequently, the risk of UV damage can be greater than reported by measurements or models that assume horizontal incidence (e.g. UV index). On cloudy days the normal-incidence UV can be less than 50% of the horizontal-incidence UV. Averaged over a day, any enhancements in normal-incidence UV over horizontal-incidence UV are smaller. The effects were strongly dependent on cloud conditions. Under clear skies the enhancements are generally less than 10%, and the integrated excess over horizontal-incidence UV is usually less than 5%. However, under cloudy skies the reductions can still be large.     

SOLAR ULTRAVIOLET RADIATION IN SOUTHEAST ENGLAND: THE CASE FOR SPECTRAL MEASUREMENTS

Spectral measurements of the solar ultraviolet spectrum have been made at Reading, England, since July 1989. The data presented show the daily and annual changes in the ultraviolet-B (UV-B) part of the spectrum, and illustrate the dominance of the longer wavelengths in grouping the data into a single broadband measurement. The temporal changes (diurnal and annual) cover 2 orders of magnitude at 300 nm, and a factor of 5 at 320 nm. In a single statement of UV-B levels the trend at longer wavelengths predominates, hiding the larger differences at the more biologically important wavelengths. However, the data also show that at mid-high latitudes the UV irradiance at noon in winter is less than that received at any time during the middle 12 h of daylight in summer, and this should be acknowledged when assessing the consequences of ozone depletion. Atmospheric scattering of short wavelength radiation is compared to that of the entire solar spectrum from measurements of diffuse radiation: on a clear day70–100% of UV-B was diffuse in Reading, with a slight wavelength dependency increasing diffuse radiation at short wavelengths. Under the same conditions scattering of total solar radiation was 21%. The effect of cloud cover is briefly discussed for two specific cases of complete, uniform cloud cover, when attenuation by clouds was approx. 40 and 68%, with little wavelength dependence in either case.     

Seasonal erythemal UV doses in Belo Horizonte, Brazil

The ultraviolet radiation (UVR) emitted by the Sun causes many effects on the biosphere. On human beings they vary from the benefit of vitamin D synthesis to the harm of skin cancer induction. The biological dose depends on the effect, the exposure time to the Sun and the amount of UVR received. In this work we show that the measured incidence of erythemal dose (ED) in Belo Horizonte (19.92 degrees S, 43.94 degrees W, 858 m a.s.l., Brazil) for a cloudless day can vary from 7503 to 2926 J m(-2) in the summer-winter seasonal variation. In addition, supposing a linear relationship between the ED and the geophysical parameters of solar zenith angle cosine (cos(SZA)), column ozone and reflectivity from the ozone monitoring instrument overpass measurements, a model for the forecast of UVR incidence on a monthly-based period is developed. From this an annual ED of 1,451,099 J m(-2) is obtained.     

Daily Maximum Erythemal Dose Rates in the Tropics

The maximum erythemal dose rate (EDR_max) at the Earth's surface tends to occur at local noon. However, clouds can make the timing of EDR_max significantly away from local noon. In fact, EDR_max and its time of occurrence depend mainly on the solar zenith angle at noon (SZA_n), site's altitude, the total ozone column (TOC), cloud cover, cloud genera and aerosols. This work depicts the daily incidence of EDR_max for Belo Horizonte (19.92°S, 43.94°W, 858 m a.s.l.) in the Southern Hemisphere tropics for a period of five years (2005–2010). Daily values of EDR_max ranged from 0.063 W m^?2 (1σ > 6.9%, Moderate UV‐Index of 3, winter) to 0.486 W m^?2 (Extreme UVI of 19, summer). Indicative values of EDR_max for cloudless days were 0.336 W m^?2 (summer, TOC = 258 Dobson Units), 0.311 W m^?2 (fall, 260 DU), 0.253 W m^?2 (spring, 274 DU) and 0.143 W m^?2 (winter, 246 DU). Radiation enhancement events by clouds made EDR_max up to 45% higher than the reference EDR_max for cloudless summer skies at a time resolution of a few minutes. The main cloud genera to be associated with such events are Cumulus, Altocumulus, Altostratus and Stratocumulus. The EDR can also be significantly affected by aerosols, which attenuated on average 0.031 W m^?2 (22%) of the erythemal UV in a case study at the site.     

Parameterization of daily solar global ultraviolet irradiation

Daily values of solar global ultraviolet (UV) B and UVA irradiation as well as erythemal irradiation have been parameterized to be estimated from pyranometer measurements of daily global and diffuse irradiation as well as from atmospheric column ozone. Data recorded at the Meteorological Observatory Potsdam (52 degrees N, 107 m asl) in Germany over the time period 1997-2000 have been used to derive sets of regression coefficients. The validation of the method against independent data sets of measured UV irradiation shows that the parameterization provides a gain of information for UVB, UVA and erythemal irradiation referring to their averages. A comparison between parameterized daily UV irradiation and independent values of UV irradiation measured at a mountain station in southern Germany (Meteorological Observatory Hohenpeissenberg at 48 degrees N, 977 m asl) indicates that the parameterization also holds even under completely different climatic conditions. On a long-term average (1953-2000), parameterized annual UV irradiation values are 15% and 21% higher for UVA and UVB, respectively, at Hohenpeissenberg than they are at Potsdam. Daily global and diffuse irradiation measured at 28 weather stations of the Deutscher Wetterdienst German Radiation Network and grid values of column ozone from the EPTOMS satellite experiment served as inputs to calculate the estimates of the spatial distribution of daily and annual values of UV irradiation across Germany. Using daily values of global and diffuse irradiation recorded at Potsdam since 1937 as well as atmospheric column ozone measured since 1964 at the same site, estimates of daily and annual UV irradiation have been derived for this site over the period from 1937 through 2000, which include the effects of changes in cloudiness, in aerosols and, at least for the period of ozone measurements from 1964 to 2000, in atmospheric ozone. It is shown that the extremely low ozone values observed mainly after the eruption of Mt. Pinatubo in 1991 have substantially enhanced UVB irradiation in the first half of the 1990s. According to the measurements and calculations, the nonlinear long-term changes observed between 1968 and 2000 amount to +4%, ..., +5% for annual global irradiation and UVA irradiation mainly because of changing cloudiness and + 14%, ..., +15% for UVB and erythemal irradiation because of both changing cloudiness and decreasing column ozone. At the mountain site, Hohenpeissenberg, measured global irradiation and parameterized UVA irradiation decreased during the same time period by -3%, ..., -4%, probably because of the enhanced occurrence and increasing optical thickness of clouds, whereas UVB and erythemal irradiation derived by the parameterization have increased by +3%, ..., +4% because of the combined effect of clouds and decreasing ozone. The parameterizations described here should be applicable to other regions with similar atmospheric and geographic conditions, whereas for regions with significantly different climatic conditions, such as high mountainous areas and arctic or tropical regions, the representativeness of the regression coefficients would have to be approved. It is emphasized here that parameterizations, as the one described in this article, cannot replace measurements of solar UV radiation, but they can use existing measurements of solar global and diffuse radiation as well as data on atmospheric ozone to provide estimates of UV irradiation in regions and over time periods for which UV measurements are not available.     

Scattered UV beneath public shade structures during winters

Broadband field measurements were conducted beneath three different-sized public shade structures, small, medium and large, during winter in the Southern Hemisphere. These measurements were compared with the diffuse UV to quantify the relationship of the UV under and around the shade structures to the diffuse UV. For the shade structures, a relationship between the diffuse UV and the UV in the shade has been provided for clear skies and solar zenith angles (SZA) of 49-76 degrees. This allows the prediction of the UV in the shade of these structures if the diffuse UV is known. The ultraviolet protection factors for the three shade structures ranged from 1.5 to 5.4 for decreasing SZA. For the greater SZA of 70-76 degrees, the erythemal UV in the shade was 65%, 59% and 51% of that in full sun for the small, medium and large structures, respectively. For the smaller SZA of 50-53 degrees the erythemal UV in the shade was 35%, 41% and 18% for the small, medium and large shade structures, respectively. From this research it can be concluded that the UV radiation levels in the shade in winter could cause erythema and other sun-related disorders.     

Albedo-enhanced maximum UV irradiance--measured on surfaces oriented normal to the sun

UV radiation measured on normal-to-the-sun-oriented surfaces can show significantly higher global UV irradiance values compared to measurements on horizontal receivers. The direct component is amplified by the inverse cosine of the zenith angle, but over surfaces with high local albedo this accounts for only about half of the signal rise of global irradiance. The signal rise of the diffuse component, however, is strongly related to local albedo and solar elevation, which is demonstrated by 2 years of measurements of direct, diffuse, global, reflected and global normal-to-the-sun erythemal effective UV irradiance (UVery). Global UVery signal rises, on normal-to-the-sun-oriented versus horizontal receivers, of up to 65% were measured on fresh snow and solar elevation angles below 30 degrees. An empirical expression has been deduced from the measurements relating the ratio of normal-to-the-sun versus horizontal measurements of global UVery to surface albedo and solar elevation. This allows one to calculate the maximum global UVery irradiance levels which are to be expected on normal-to-the-sun-oriented surfaces with respect to horizontal measurements or model calculations.     

UV index experimental values during the years 2000 and 2001 from the Spanish broadband UV-B radiometric network

An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson-Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes.     

BIOLOGICALLY EFFECTIVE DOSES OF SUNLIGHT FOR IMMUNE SUPPRESSION AT VARIOUS LATITUDES AND THEIR RELATIONSHIP TO CHANGES IN STRATOSPHERIC OZONE

Using information on solar irradiance at different latitudes derived from a radiative transfer model and a detailed in vivo action spectrum for immune suppression in a murine system, we report here calculations of the "biologically effective" irradiance of sunlight for immune suppression. From 40 degrees N to 40 degrees S in summer, under normal stratospheric ozone concentrations this value ranged from 0.27 W/m2 (40 degrees N or S) to a peak of 0.33 W/m2 (20 degrees N or S) predicting that 50% immune suppression in the Balb/c mouse would occur after 21-26 min of sunlight exposure within this latitude range. We also found that the most effective wavelengths for immune suppression shift from a peak of 270 nm in the laboratory to near 315 nm in sunlight. Furthermore, using ozone depletion scenarios of 5 to 20%, at latitudes 20 degrees S and 40 degrees N, a 0.6% increase in biologically effective irradiance levels of solar UVB for immune suppression was predicted for each 1% decrease of ozone. This value rose to a nearly 1% increase for each 1% decrease in ozone at 60 degrees N latitude in wintertime. These data indicate that activation of immune suppression, in a murine model, requires relatively low levels of sunlight and that these levels are easily obtainable over most of the populated regions of the world. Since a UVB-activated photoreceptor, urocanic acid, regulates immune suppression in mice and since this same compound exists on other mammalian skin, including human skin, suppression of the mammalian immune system is predicted to increase if substantial stratospheric ozone depletion takes place.     

Diurnal variation of ocular exposure to solar ultraviolet radiation based on data from a manikin head

Measurements were conducted at San Ya, China (18.4°N, 109.7°E, altitude 18 m) to investigate the diurnal variation of ocular exposure to ultraviolet (UV) radiation. The experimental apparatus was composed of a manikin and a dual-detector spectrometer to simultaneously measure ocular and ambient UV data. The experimental apparatus was rotated clockwise to simulate three different types of exposure. When the manikin was facing into the sun, the ocular exposure to UV radiation on a summer day was bimodally distributed. The maximum ocular UV irradiance occurred at solar elevations of around 40° and 50° for UVA and UVB respectively. The spectral irradiances were measured at specific wavelength to obtain the ocular biologically effective UV (UV(BE) ) irradiances for photokeratitis, photoconjunctivitis and cataract, and the UV index (UVI) was calculated at the same time point for comparison. When the manikin faced the sun, the maximal ocular UV(BE) irradiance values were obtained at the solar elevation where the UVI value was 8. The results of this study showed that protection against ocular overexposure during outdoor activities should be taken not only at noon but also at other times.     

Ultraviolet Radiation at Sites on the Antarctic Coast

Ground-based measurements of solar UV irradiance combined with calculations using satellite-based ozone data are able to define the variability in UV sunlight at Palmer Station and McMurdo Station, Antarctica over time scales of years. Special attention focuses on the spring and summer seasons. Satellite data show that the annual ozone loss that occurs during October was greater in1991–1992 than in1979–1980. This led to average noontime UVB irradiances computed for clear skies in the latter period that exceeded those in the earlier time by50–65%. However, a biologically weighted irradiance for suppression of photosynthesis in phytoplankton indigenous to the area near McMurdo Station increased by at most 5% over this period in response to the change in ozone owing to an important contribution from the UVA. At Palmer Station the behavior of ozone and cloudiness can mesh so as to produce the largest noontime UVB irradiances of the year in October as opposed to near summer solstice in December and January. Interannual variability in UVB irradiance during October, the month of the major ozone loss, is larger at Palmer than at McMurdo during the time spanned by ground-based irradiance measurements, being1990–1994. However, interannual variations in cloudiness were more important than changes in ozone in causing the observed year-to-year variability at Palmer Station. The opposite situation prevailed at McMurdo during October, where interannual variations in ozone were responsible for most of the year-to-year differences in UVB received at the ground.     

An estimation of biological hazards due to solar radiation

A spectrum evaluator based on four different dosimeter materials has been employed to estimate the spectral irradiances of solar radiation for exposed humans. The result is used to calculate the biologically effective irradiance using the erythemal action spectrum and a fish melanoma action spectrum. Measurements are made in winter at a sub-tropical site on the chest and shoulder of subjects during normal daily activities. Up to 95% of the total UV exposure received is in the UV-A waveband (320-400 nm). The UV-A waveband is found to contribute approximately 14% of the erythemal UV and 93% of the biologically effective UV for fish melanoma. Extrapolation to humans suggests that exposure to the UV-A band will contribute to photodamage in human skin during exposure to solar radiation.