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.     

BIOLOGICAL UV-DOSES AND THE EFFECT OF AN OZONE LAYER DEPLETION

Effective UV-doses were calculated based on the integrated product of the biological action spectrum (the one proposed by IEC, which extends to 400 nm, was adopted) and the spectral irradiance. The calculations include absorption and scattering of UV-radiation in the atmosphere, both for normal ozone conditions as well as for a depleted ozone layer. For Scandinavian latitudes the effective annual UV-dose increases by approximately 4% per degrees of latitude towards the Equator. An ozone depletion of one percent increases the annual UV-dose by approximately 1% at 60 degrees N (increases slightly at lower latitudes). A large depletion of 50% over Scandinavia (60 degrees N) would give these countries an effective UV-dose similar to that obtained, with normal ozone conditions, at a latitude of 40 degrees N (California or the Mediterranean countries). The Antarctic ozone hole increases the annual UV-dose by 20 to 25% which is a similar increase as that attained by moving 5 to 6 degrees of latitude nearer the Equator. The annual UV-dose at higher latitudes is mainly determined by the summer values of ozone. Both the ozone values and the effective UV-doses vary from one year to another (within +/- 4%). No positive or negative trend is observed for Scandinavia from 1978 to 1988.     

POSSIBLE LONG-TERM CHANGES IN BIOLOGICALLY ACTIVE ULTRAVIOLET RADIATION REACHING THE GROUND

Abstract— Three scenarios for long-term changes in atmospheric ozone over the time period 1960 to 2030 lead to different projections for the ultraviolet radiation flux at the earth's surface. Biologically effective fluxes for damage to DNA and generalized damage to plants vary by a factor of 10 or more with latitude and season irrespective of possible changes in ozone. The natural latitudinal gradient in radiation corresponds to spatial changes in biologically effective fluxes which are large compared to temporal changes expected from trends in ozone over the time period analyzed. In an extreme scenario of ozone change, based on an assumed increase in chlorofluorocarbon release rates of 3% per year after 1980, the annually integrated effective flux for damage to DNA increases by 13.5% at latitude 40°N between 1960 and 2030. With chlorofluorocarbon release rates held fixed at their 1980 values, the corresponding radiation increase is only 2.3%. In a scenario where atmospheric chlorine remains fixed at its 1960 value, trends in atmospheric methane and nitrous oxide imply a decrease in biologically effective flux at 40°N of 5.3% between 1960 and 2030.     

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.     

ULTRAVIOLET SOLAR RADIATION IN THE HIGH LATITUDES OF SOUTH AMERICA

Abstract Measurements of the UV solar irradiance are available from Ushuaia, Tierra del Fuego during the spring and summer seasons of 4 consecutive years beginning in 1989. In addition, column ozone amounts derived from satellite-based measurements exist for this location over the entire period from 1980 through 1991. Monthly mean column ozone over Ushuaia shows a general decline over the observing period, and a large day-to-day variability exists within a given month. Ozone amounts for the years 1980 through 1986 combined with a model of radiative transfer provide a climatological baseline against which to interpret the more recent ground-based irradiance data. We focus on monthly mean noontime irradiances integrated over 5 nm wide spectral bands near 305 nm and 340 nm, respectively. Measurements in the 340 nm band show that cloudiness has a large influence on both the absolute monthly mean irradiances and their interannual variability. For example, during December the 340 nm band irradiance varied from approximately 50% of the clear-sky value in 1992 to 65% in 1991. When the influence of cloudiness is removed, most of the months show irradiances in the 305 nm band that are larger than predicted from the climatological ozone amounts. The largest percentage enhancement occurred in October 1991 when the irradiance exceeded the baseline by 56%. The largest absolute irradiances occur in December, where the measurements range from 5.8% below the baseline in 1991 to 31% above in 1990.     

MIDDLE ULTRAVIOLET RADIATION REACHING THE OCEAN SURFACE*

Abstract— Direct measurements of the downwelling spectral irradiance in the middle UV (280–340 nm) have been made for a range of solar zenith angles (20°-70°). These measurements were made for a marine atmosphere at equatorial latitudes. We fit these data to two semi-empirical analytic representations, from which quantitative calculations of spectral irradiance in the middle UV incident at the ocean surface can be made. The formulae accommodate variation in wavelength, solar zenith angle, ozone thickness, aerosol thickness and surface albedo. Our purpose is to provide marine photobiologists and photochemists with a basis for estimating middle UV radiation levels reaching the ocean surface and the approximate changes caused by manmade alterations of the ozone layer.     

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.     

Interannual variability in solar ultraviolet irradiance over decadal time scales at latitude 55 degrees south.

Ground-based measurements of solar UV spectral irradiance made from Ushuaia, Argentina at latitude 55 degrees S reveal a large degree of variability among corresponding months of different years over the period from September 1990 through April 1998. The magnitude and wavelength dependence of year-to-year changes in monthly spectral UV-B irradiation are consistent with expectations based on the behavior of column ozone and cloudiness. When combined with satellite measurements of column ozone, a regression model fit to the ground-based data set allows estimates of monthly UV-B irradiation over a time frame of two decades, 1978-1998, during several months of the year. Results show a general increase in ground-level irradiation at 305.0 nm from the end of the 1970s to the early 1990s during calendar months from September through December. This is followed by generally smaller irradiances through the middle to late 1990s for all months except November, where the increase continues through the end of the data record. The long-term variability in monthly irradiation over the time period studied is more complicated than can be described by a simple linear trend.     

Monitoring of Solar Irradiance in the High Andes

Solar radiation has been measured in the high Andes near Laguna Lejia (latitude 23° 26′ 23.30" S, longitude 67° 38′ 14.29" W) at an elevation of 4715 m between December 2016 and December 2017. Irradiances were monitored in four wavelength channels: PAR (400–700 nm), UV-A (315–400 nm), UV-B (295–315 nm) and short-wavelength UV-B (295–310 nm) with a new radiometer. In addition, ambient temperatures were recorded. Record values have been found for PAR (exceeding 600 W m⁻²), UV-A (close to 95 W m⁻²), UV-B (3.13 W m⁻²) and short-wavelength UV-B (0.144 W m⁻²) during Austral spring. The winter irradiance values slightly exceeded 50% of these values. Maximal cloud effects due to multiple reflections were 45, 38, 32 and 35% higher than values under cloudless skies for PAR, UV-A, UV-B and short-wavelength UV-B, respectively. Record irradiance for this site shows a UV index reaching and exceeding 20, which is due to low solar zenith angles, the altitude, low water vapor and aerosol concentrations in the atmosphere as well as low total column ozone concentrations.     

TRENDS AND INTERANNUAL VARIATIONS IN ERYTHEMAL SUNLIGHT,1978–1993

Information on column ozone and the earth's reflectivity obtained from satellite-based data allows estimates of the long-term behavior in erythemal irradiance at any location, including the attenuation provided by clouds. Year-to-year changes in cloudiness over specific geographic regions make a major contribution to interannual variability in irradiance at the ground. Although the general decline in column ozone amounts acted to increase erythemal irradiance over the period November 1978 to April 1993, these changes tend to be obscured by the erratic variability associated with cloudiness. The noise introduced into a time series of erythemal irradiance by clouds effectively widens the error bars assigned to derived trends. This behavior could complicate attempts to establish links between any observed changes in the biosphere and measured changes in the ozone layer.     

TEMPORAL CHANGES IN SURFACE ULTRAVIOLET RADIATION: A STUDY OF THE ROBERTSON-BERGER METER AND DOBSON DATA RECORDS

Abstract— Ultraviolet radiation data sets obtained by Robertson-Berger meters located at Bismarck, ND and Tallahassee, FL show variations over the time period 1974–1985 which we interpret in terms of clouds and ozone. Cloudiness is a major source of variance in the irradiance measurements. When this variance is minimized, the monthly mean Robertson-Berger meter record contains trends which are in good agreement with irradiance calculations based on the Dobson ozone measurements in spring, summer and early autumn. Despite the agreement among trends, predictions based on the ozone data explain 40% or less of the variance in the monthly mean radiation values over the 11-year period. The radiation measurements contain negative trends in winter which are contrary to expectations based on the behavior of ozone alone. These trends remain when we minimize the effects of cloudiness. Based on the information available in this study, it is not possible to determine whether the wintertime trends have an instrumental or environmental origin.     

Sensitivity of erythemally effective UV irradiance and daily exposure to temporal variability in total ozone

The provision of information to the public about current levels of the erythemally effective UV radiation is an important issue in health care. The quality of promoted values is therefore of special importance. The atmospheric parameter which affects the erythemally effective UV radiation under clear sky most is the total ozone content of the atmosphere. In this paper we examined the sensitivity of the erythemally effective irradiance and daily radiant exposure to the temporal variability of total ozone on time scales from 1 to 15 days. The results show that the sensitivity is highest for the first 24 h. Larger time scales do not exhibit a similar influence. Total ozone measurements of the previous day may already cause uncertainties higher than 0.5 UV index (UVI) independent of the geolocation. For comparison, a temporal persistence of 15 days may cause uncertainties of 1.2 UVI at 50°N, 1 UVI at 30°S and less than 1 UVI at the equator. The results of this study allow finding the necessary temporal resolution of total ozone values when a certain accuracy for the UVI or for the purpose of sun protection is required. The results are compared with those of two preceding studies where we quantified the influence of measurement uncertainties and spatial total ozone variability to the erythemally effective irradiance at noon and to the daily dose. We conclude that temporal variability of total ozone is the most critical issue, but also measurement uncertainties do have a noticeable influence on the erythemally effective radiation.     

Ozone and UV radiation over southern South America: climatology and anomalies

Ozone and UV radiation were analyzed at eight stations from tropical to sub-Antarctic regions in South America. Ground UV irradiances were measured by multichannel radiometers as part of the Inter American Institute for Global Change Radiation network. The irradiance channels used for this study were centered at 305 nm (for UV-B measurements) and 340 nm (for UV-A measurements). Results were presented as daily maximum irradiances, as monthly averaged, daily integrated irradiances and as the ratio of 305 nm to 340 nm. These findings are the first to be based on a long time series of semispectral data from the southern region of South America. As expected, the UV-B channel and total column ozone varied with latitude. The pattern of the UV-A channel was more complex because of local atmospheric conditions. Total column ozone levels of < 220 Dobson Units were observed at all sites. Analysis of autocorrelations showed a larger persistence of total column ozone level than irradiance. A decreasing cross-correlation coefficient between 305 and 340 nm and an increasing cross-correlation coefficient between 305 nm and ozone were observed at higher latitudes, indicating that factors such as cloud cover tend to dominate at northern sites and that ozone levels tend to dominate at southern sites. These results highlight the value of long-term monitoring of radiation with multichannel radiometers to determine climatological data and evaluate the combination of factors affecting ground UV radiation.     

Adjusting timing of weathering test to account for seasonal variations in UV exposure

Satellite-derived UV climatology has been used to design a timetable for the outdoor UV exposure of polymeric material specimens. By pre-exposure computation, fixed time increments are transformed into a schedule with a predicted average accumulation of UV dose. The method was applied to produce a timetable for an ongoing exposure programme in a network of seven European test sites over the latitude range 28.5-67.4°N. The effect of the average seasonal variability of solar UV irradiance on the predicted accumulated UV exposure is shown to be notably diminished by the method. Relative underestimations of 5-82% of cumulative UV stress are estimated to be avoided by extensions of 6-167 days exposure durations as suggested by the adjustment procedure. Hence, more reproducible degradation data are expected to be obtainable for exposures that are not multiples of the full annual cycle of UV, especially for those with a duration <1 year.     

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.     

Solar Spectral Irradiance and Summary Outputs Using Excel

The development of an Excel spreadsheet is described that calculates solar spectral irradiance between 290–3000 nm on an unshaded, horizontal surface under a cloudless sky at sea level, together with summary outputs such as global UV index, illuminance and percentage of energy in different wavebands. A deliberate goal of the project was to adopt the principle of Ockham's razor and to develop a model that is as simple as it can be commensurate with delivering results of adequate accuracy. Consequently, just four inputs are required—geographical latitude, month, day of month and time of day—resulting in a spreadsheet that is easily usable by anyone with an interest in sunlight and solar power irrespective of their background. The accuracy of the calculated data is sufficient for many applications where knowledge of the ultraviolet, visible and infrared levels in sunlight is of interest.     

UV exposure of elementary school children in five Japanese cities

A 1 week UV‐exposure measurement and outdoor‐activity pattern survey was conducted for elementary school children for four seasons at five sites in Japan, i.e. Sapporo (43°05′N, altitude 40 m), Tsukuba (36°05′N, 20 m), Tokyo (35°40′N, 45 m), Miyazaki (31°60′N, 40 m) and Naha (26°10′N, 5 m), and UV exposure was measured directly and estimated using outdoor‐activity records. The study site with largest UV exposure was Miyazaki, a southern rural area. Comparing the results for boys and girls, UV exposure was larger in boys. UV exposure was large in spring and summer and small in winter. The total amount of UV exposure in spring and summer contributed 57.7–73.4% of total exposure for the year. As a whole, 8.1% and 1.8% of the schoolchildren were exposed to more than 1 minimum erythemal dose (MED) and 2 MED of solar UV in a day, respectively. The estimated yearly UV exposure ranged from 49 207 J/m² in Miyazaki to 31 520 J/m² in Tsukuba. The actual UV exposure correlated to potential UV exposure, estimated using outdoor‐activity records and ambient UV irradiance, but the ratio differed by season and site. The yearly average of percent UV exposure to ambient UV on a horizontal plane ranged from 9.9% in Tokyo to 4.0% in Naha. In the questionnaire survey on outdoor‐activity pattern, a short question “How long did you spend time outdoors between 0900 and 1500 h?” gives the best estimates of UV exposure.     

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.     

Variability of Cloud-free Ultraviolet Dose Rates on Global Scale Due to Modeled Scenarios of Future Ozone Recovery

Simulations of the total ozone content and vertical ozone and temperature profiles during the period 1980–2080 from three chemistry climate models (CCMs) were used and the future variability of five UV dose rate types in global scale was simulated. For each CCM, radiative transfer calculations for cloud-free skies and constant values of aerosol optical properties and surface reflectivity were performed and the percentage difference, relative to the mean over the period 1996–2005, was calculated. The potential biological consequences of ozone recovery are quantified due to the different influence of ozone-absorbing wavelengths on the selected UV action spectra: average percentage differences between a few and 60% are revealed during the 2070s, depending on the latitude zone and the season. Although the research into the prediction of UV radiation levels is ongoing, due to the possible future changes in cloudiness, aerosols or surface reflectivity, the long-term changes in ozone, as projected by the CCMs in a similar way, will affect strongly some of the selected UV dose rates in the future.