A tutorial review is provided of UV radiation transport in the atmosphere-ocean system. Emphasis is placed on the basic physical principles involved rather than on mathematical/numerical aspects. To illustrate the application of the theory, the effects of an ozone depletion on UV irradiance at the surface are discussed. A comparison of measured and predicted UV penetration into the ocean under the Antarctic ozone hole is also provided.

Spectral measurements of solar global and direct UV irradiance are routinely conducted at Thessaloniki, Greece with a double monochromator Brewer spectroradiometer. From absolutely calibrated direct and global irradiance spectra, aerosol optical depth and diffuse irradiance spectra between 300 nm and 365 nm are derived. The modification of the global and diffuse irradiance and of the relationship between direct and diffuse irradiance by the aerosol single scattering albedo (SSA) is investigated with respect to aerosol optical depth and solar zenith angle using radiative transfer model calculations. Model calculations were used also to investigate the effect of SSA on the radiance distribution in the ultraviolet. The model-derived relations are compared with measurements, with the aim to establish an indirect method of deriving an effective single scattering albedo from spectral measurements of the direct and global irradiance. The uncertainties introduced by different sources into the SSA estimates are discussed. Finally the effective SSA is determined for two days with different aerosol amounts and composition. The overall accuracy in determining indirectly the SSA depends strongly on the amount of aerosols varying between 0.1 and 0.2 units of SSA.

At present the USDA Reference Spectroradiometric Network consists of 3 sites: Table Mt. CO, Lamont OK (the ARM program SGP site), and Beltsville MD. At each site we deploy and continuously operate a 1-meter cascaded additive-double Czerny-Turner scanning monochromator with a bi-alkali Photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80 degrees. The instruments use photon counting and make measurements not affected by stray light at 290 nm under typical conditions. The basic performance specifications of the instrument were demonstrated by a prototype at the 1997 North American UV Spectroradiometer Intercomparison. Data shown here demonstrate that these are met in routine operation.

We show comparisons between ground-based measurements of UV irradiance less 380 nm and satellite TOMS UV retrievals within the whole period of TOMS measurements (1979-2000) over Moscow. We analyze the scale of temporal averaging of ground-based UV data taken with 1 minute resolution which should be used while comparing with TOMS data measured once per day within a relatively large footprint area (50-100 km²). Another objective is to study interannual variability of UV irradiance obtained by ground-based UV measurements and TOMS UV retrievals for the whole period of observation (1979-2000) over Moscow area. The analysis of interannual variations in satellite UV retrievals and ground-based UV irradiance is given together with examination of different atmospheric parameters, which are available from ground and satellite observations. A special attention is given to the comparisons of UV radiation obtained from ground and satellite measurements in spring season when the maximum ozone loss is observed. This is done together with the analysis of interannual variations in snow characteristics (snow albedo, snow depth, etc) and in cloudiness. We revealed the uncertainties in TOMS UV retrievals at specified atmospheric conditions by using ancillary information. The comparisons between TOMS and ground-based UV radiation in cloudless atmosphere with different aerosol optical properties are of particular concern.

Comparison of measured UV irradiance with estimates from satellite observation is potentially effective for the validation of the data from the two sources. Data from 10 Canadian Brewer sites were compared in this study with noon UV irradiance estimated from TOMS measurements. In general, TOMS estimates can successfully reproduce long-term and major short-term UV variations, although there are some systematic differences between the measurements at the ground and satellite-retrieved UV irradiance. Up to 9% of the Brewer-TOMS difference can be attributed to the Brewer cosine response error. This error depends on the solar zenith angle and cloud conditions and is different from instrument to instrument. When the cosine response of the Brewer instrument is considered and applied, the Brewer data are still lower than TOMS-estimated UV irradiance at most of the sites by 10% on average. The bias for clear-sky condition is smaller, about 4%, than for overcast conditions (about 20% on average) with some wavelength dependence. The bias was close to 0 at one station (Saturna Island), possibly due to its much cleaner air.

Comparisons of UV irradiances measured by the USDA UVB Monitoring and Research Network at 305 and 368 nm with retrievals from the NASA TOMS and a multiple scattering radiative transfer code were made for an 18-month period from January 1, 2000 through May 31, 2001 for Las Cruces, New Mexico, USA (32.6 degree(s)N, 106.7 degree(s)W, 1317 m elevation) and Billings, Oklahoma, USA (36.6 degree(s)N, 97.5 degree(s)W, 317 m elevation). Agreement is generally within +/- 12% for all sky conditions and 8% for clear skies. The effects of aerosols is mostly less than 5%, consistent with the measured aerosol optical depths at 368 nm within the range of 0.05 and 0.25.

A network of 21 Brewer spectroradiometers, owned by the U.S. Environmental Protection Agency and operated by the University of Georgia, is measuring UV spectral irradiances throughout the United States. Corrections to the raw data have now been implemented. These corrections include (1) stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments and (4) the temporal variation in the instrument response due to optical changes in the characteristics of the instruments. While for many sites the total corrections amount to less than 10%, for certain sites they are much larger, in some cases amounting to more than 25%. Application of these corrections brings the errors of the absolute irradiance values to approximately +/- 5 to 7% for all sources of error. Comparisons of corrected daily integrated erythemal UVR data (DUV) to model and TOMS- inferred values are performed for sites at Acadia National Park, Bigbend National Park, Everglades National Park and the Virgin Islands. All sites show very good agreement with the TUVSPEC model but comparison with TOMS-inferred DUV values indicate a 10-20% overestimate by TOMS for the four sites.

We evaluate the effects of possible enhancements of the current (version 1) TOMS surface UV irradiance algorithm. The major enhancements include more detailed treatment of tropospheric aerosols, effects of diurnal variation of cloudiness and improved treatment of snow/ice. The emphasis is on the comparison between the results of the version 1 TOMS UV algorithm and each of the changes proposed. TOMS UV algorithm does not discriminate between nonabsorbing aerosols and clouds. Absorbing aerosols are corrected by using the TOMS aerosol index data. The treatment of aerosol attenuation might have been improved by using newly derived TOMS products: optical depths and the single-scattering albedo for dust, smoke, and sulfate aerosols. We evaluate different approaches for improved treatment of pixel average cloud attenuation, with and without snow/ice on the ground. In addition to treating clouds based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate attenuation of the UV irradiance throughout the day. The improved (version 2) algorithm will be applied to reprocess the existing TOMS UV data record (since 1978) and to the future satellite sensors (e.g., Quik/TOMS, GOME, OMI on EOS/Aura and Triana/EPIC).

Side by side comparisons of Langley type Dobson AD double wavelength pair direct sun observations between SBUV/2, SSBUV flight models and the NOAA world primary standard Dobson spectrophotometer 83 and 61 show that the SBUV/2 type instruments yield column ozone amounts that are 2% higher than the NOAA Dobson spectrometers. Similar results have been obtained with a radiometrically stable multi-filter spectroradiometer (MFS) under less than ideal conditions. A new approach based on a modeled table look-up method for using zenith sky radiances to derive total column ozone from zenith clear sky conditions has been tested using SBUV/2, SSBUV flight models and the MFS equipped with narrow band interference filters at the Dobson AD wavelength pairs. These clear zenith sky observations yield total column ozone that is in good agreement with ozone from the NOAA Dobson spectrophotometer direct sun observations. New model calculations on the relationship between satellite nadir radiances and surface based zenith clear sky radiances suggest that the combination of a very radiometrically stable MFS combined with a compact high performance double monochromator could be used to derive a common radiometric scale among satellite borne ozone monitoring instruments using the solar backscatter ultraviolet technique, and to be able to determine the drift in the radiometric calibration of ozone remote sensing instruments in space.

The vast majority of radiation measurements, including UV, refer to the radiation incident on a flat horizontal plate. However, this may not be the most appropriate way to specify radiation for bodies affected by UV, since they are rarely flat or horizontal. In particular the target molecules involved in atmospheric chemistry are approximately spherical and the actinic flux would be a better measure of the incident radiation. The ADMIRA project is addressing the issue of converting spectral UV irradiances to spectral actinic fluxes that can then be weighted with any required cross-section or action spectrum to give photolysis rates or biologically effective radiation incident on a sphere. The success with which this conversion can be made will depend on the prevailing atmospheric conditions and the knowledge of such at the time the irradiance measurements were made. Several different approaches to the conversion are being assessed, together with their associated uncertainties. These range from the simple empirical method to more complex radiative-transfer model based algorithms. Here we report on a coordinated campaign of simultaneous irradiance and actinic flux measurements supported by a wide range of ancillary measurements and their application to a simple empirical approach to converting irradiances to actinic fluxes.

Global solar UV measurements performed with high-resolution SUV-100 spectroradiometers in Antarctica and Alaska are compared with results of the radiative transfer model UVSPEC/libRadtran. The instruments are part of the National Science Foundation's Office of Polar Programs (NSF/OPP) UV monitoring network, and are located at the South Pole (90 degree(s)S), McMurdo (78 degree(s)S), Palmer Station (65 degree(s)S), and Barrow, Alaska (71 degree(s)N). A new algorithm to retrieve total column ozone from the ratio of measured and modeled UV spectra is presented, which is then used to uncover biases in column ozone data from different sources (Earth Probe TOMS Version 7, Dobson, GOME, TOVS) at the previously mentioned high-latitude locations. The analyses suggest that EP/TOMS overestimates total column ozone at all Antarctic sites by 4-10%, which is consistent with recent findings reported elsewhere. SUV-100 and Dobson total column ozone measurements at the South Pole, Barrow and McMurdo agree to within +/- 1.5%, +/- 2%, and +/- 1%, respectively. GOME measurements at Palmer and McMurdo Station are 2% and 6% lower than the SUV-100 data. TOVS ozone values show in general a larger deviation. The data further reveal that ozone and temperature profiles used in the model have an important influence, particularly at low sun elevations. This is quantified by comparing the UV measurements with model calculations using either standard profiles or actual profiles measured by balloon sondes. When using Dobson ozone measurements and actual ozone profiles, and correcting SUV- 100 UV measurements for the cosine error of the entrance optics, spectral clear-sky measurements typically agree with model results to within +/- 5% for solar elevations greater than 5 degrees.

Previous studies elucidated the relationship between total ultraviolet and total solar radiant exposures at four meteorologically different sites: desert, subtropical, urban smog and high altitude. These studies, which have only been briefly summarized elsewhere, demonstrated that at-latitude daily total ultraviolet plotted against daily at-latitude total solar radiant exposure follows well-defined, site- specific relationships; namely, data for month-pairs that are symmetrical about the two solstices plot on the regression line with high correlation coefficients. It was found that the algebraic behavior of the family of regression expressions (DEC, JAN-NOV, FEB-OCT, MAR-SEP, APR- AUG, MAY-JUL, JUN) depend on whether either pollution- related absorption or humidity-related scattering mechanisms predominate one over the other at any given site. More recent studies, which are on-going, have determined the extent to which daily UV-B radiant exposure exhibits the same dependence on latitude, altitude, and local climate atmospheric conditions that was observed for daily UV-A (e.g., total) radiant exposures. It was found that the UV-B histograms for both Arizona and Florida show a much greater degree of scatter than do the UV-A histograms. Solar UV-B radiation is much more sensitive to a combination of atmospheric conditions and ozone than is UV-A, and the differences between the Florida and Arizona UV-B histograms are ascribed to these effects.

Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface. Global mapping of the underwater UV irradiance creates further challenges for the models. The uncertainties in physical input parameters become more serious because of the presence of absorbing and scattering quantities affected by biological processes within the oceans. In this presentation we summarize the problems encountered in the assessment of the underwater UV irradiance from space-based measurements, and propose approaches to resolve the problems. We have developed a radiative transfer scheme for computation of the UV irradiance in the atmosphere-ocean system. The scheme makes use of input parameters derived from satellite instruments such as TOMS and SeaWiFS. The major problem in assessment of the surface UV irradiance is to accurately quantify the effects of clouds. Unlike the standard TOMS UV algorithm, we use the cloud fraction products available from SeaWiFS and MODIS to calculate instantaneous surface flux at the ocean surface. Daily UV doses can be calculated by assuming a model of constant daily cloudiness. Both SeaWiFS and MODIS provide some estimates of seawater optical properties in the visible.

Albedo inversion techniques are investigated in this work. Several methods are applied to spectral irradiance data from a measurement campaign held in the German Alps during the spring of 1999. One first method is based on the comparison of measurements of absolute levels of UV irradiance with model calculations. The second method takes advantage of changes in the spectral slope of spectral UV irradiance, which is a function of the surface albedo. In the third method, the surrounding area is partitioned into snow- covered and snow-free regions, and the effective albedo estimated by applying a higher or lower reflectivity to each facet before integrating over the surroundings. We present sensitivity analysis, the differences and the correlations between the various methods as well as the results for the different locations.

A major limitation in predicting the ultraviolet-B (UVB) irradiance on humans, plant leaves and flowers and aquatic organisms is the difficulty in determining the UVB under partly cloudy sky conditions. This study analyzes the UV diffuse fractions under partly cloudy and clear conditions for nine locations in the USA over a period of 1997 through 1999. Radiation measurements, made as part of the United States Department of Agriculture UVB Monitoring Program using multi-filter rotating shadow band radiometers, were paired with cloud cover and other atmospheric measurements made with National Weather Service Automated Surface Observation Systems within 30 km of the radiation measurement location to evaluate the accuracy of using a relatively simple model to describe the diffuse fraction of UV radiation under partly-cloudy skies. The diffuse fraction was modeled as the summation of clear and overcast sky diffuse fractions, weighted by the probability of the sun's direct beam being obstructed or not for a given cloud cover fraction. For the nine locations evaluated, the model had a mean bias error (MBE) of 0.0037 and a root mean squared error (RMSE) of 0.0361. Simplifying the model by assuming a diffuse fraction of 1 for the overcast sky resulted in a slightly higher error (MBE error of -0.0045 and an RMSE of 0.0387). Model errors were greatest for low solar zenith angles and high cloud fractions. The greatest error, associated with overcast sky conditions, appeared to be a result of scattering off the clouds during the period of time where the sun's beam was unobstructed. Error analysis also showed that the diffuse fraction of partly cloudy skies when the sun's beam was not obstructed is well approximated by the clear sky condition to within approximately 0.1, supporting the use of aerosol optical thickness estimates by Langley plot under partly cloudy skies.

An efficient method for calculating and mapping surface UV radiation by use of satellite data and based on the independent pixel approximation has been developed. The method principally allows to take into account the total ozone column amount, the cloud optical depth, the surface elevation and albedo (snow, no snow) and the solar zenith angle. It is currently used for creating a UV climatology to study the effects of UV daily doses on marine ecosystems around the Norwegian Lofoten Islands. This study focuses on the temporal sampling of the cloud field and its effects on the calculation of UV daily doses. Three-dimensional radiative transfer calculations have been performed to estimate how many satellite overpasses or slots are needed to derive daily UV doses within certain limits of error. Shown are results for a synthetic cloud field having a pronounced asymmetry in cloud cover around noon time. For this case the daily dose can be derived with an error of 10% if the cloud field is at minimum probed every 60 minutes within the time interval from 10:00 to 14:00 assuming a constant cloud cover from sunrise to 10:00 and from 14:00 to sunset. Such a time sampling may be obtained from Meteosat data or from a combination of Meteosat and NOAA/AVHRR data. By using only NOAA/AVHRR data with a temporal resolution of typically 100 minutes at least 3 overpasses are needed to obtain the 10% relative difference between the derived daily dose and the exact reference value.

Direct-sun UVB, total column ozone and water vapor, and aerosol optical thickness have been measured since 1990 at Geronimo Creek Observatory in South-Central Texas (29.6N 97.9W). Full sky and diffuse solar irradiance at UVB, visible and near-infrared wavelengths have been measured since 1994. Comparisons of total ozone measurements made from the surface by TOPS and MICROTOPS and from space by the Total Ozone Mapping Spectrometer (TOMS) show excellent agreement. Comparisons of UVB at solar noon measured at the surface by inexpensive UVB radiometers and inferred from TOMS ozone measurements give excellent agreement on days free of clouds. Scattering from the sides of clouds on summer days can cause UVB at the surface to exceed by 10-20% the UVB inferred from TOMS ozone measurements. Public service announcements of predicted peak UVB should include a correction factor for summer days when clouds are expected to be near the sun. Ultraviolet measurements at Geronimo Creek Observatory support various educational and biological studies, including development of inexpensive instruments, avoidance of UV by mosquito larvae (Culex pipiens), preferential selection by female C. pipiens of nursery pools with low levels of ambient UV, suppression of non-pigmented bacteria suspended in air and E. coli in water, and studies of sub-canopy and open-field growth of sapling trees.

Within the frame of the Austrian UV Monitoring Network, repeated recalibrations of Solar Light Sunburn Meters between December 1997 and March 2000 have shown significant temporal changes in the instruments' relative spectral response function as well as in their absolute calibration. Therefore, laboratory investigations of the effects of ambient temperature and internal relative humidity on the behavior of two Sunburn Meters have been performed. Despite internal temperature stabilization, both instruments show significant dependence of their spectral response function on ambient temperature. When the outside temperature of the detector's housing varies between 13 degree(s)C and 44 degree(s)C, spectral sensitivity changes by up to 10% in the UVB range and by up to a factor of 2 in the UVA range, depending on internal relative humidity. As a consequence, output voltage variations of 10% are observed when the detector is mounted in front of a 1000 W halogen lamp and its internal relative humidity is changed while its ambient temperature is kept constant. Whereas temperature effects take place within several hours, instabilities due to variations in internal relative humidity show typical time constants in the order of several days.

After the discovery of the ozone hole the use of spectral and multiband instruments in UV radiation measurements became more common, but time series for these instruments are still relatively short to determine trends. Nevertheless, systematic measurements of ozone column have been performed since late fifties at several stations and with worldwide coverage since late seventies, and long time series of broadband instruments (Pyranometers, UV and erythemally weighted) are also available at stations all over the world. In this paper we introduce a multi- regressive model that allows inference of spectral or narrowband irradiances from ozone total column and broadband irradiances, in places where a relatively short time series of spectral irradiances is available. To test the model, measurements under all weather, solar zenith angle and ground conditions performed at three of the stations of the NSAF UV Radiation Monitoring Network were used. The model generated very good results over a wide variety of situations. Broadband pyranometer data from the NOAA/CMDL surface radiation budget database for the South Pole Station were used in the model to estimate the daily-integrated narrowband irradiances. A time series of monthly means for the narrowband 303.030-307.692 nm were then computed, dating back to 1978.

Ground-based global solar ultraviolet-B (UVB) irradiance has been measured by a narrowband-spectral UVB radiometer at Tokai University, Hiratsuka (35 degree(s)21' N, 139 degree(s)16' E) Japan for a 10-year period from October 1990 to September 2000. A precise calibration of the UVB radiometer was periodically performed and the yearly decay sensitivity was found to be -3.7% of the initial sensitivity. Using this decay rate, the measured UVB irradiance was corrected, and the long-term trends of the UVB irradiance were estimated. An increase in the UVB irradiance was demonstrated to be 1.57% per year with a seasonal variation correction, and 1.22% per year with global total solar irradiance and the seasonal variation corrections in addition to quasi-biennial oscillation (QBO) correction. A significant, negative correlation was confirmed between the UVB irradiance normalized by the total solar irradiance and the effective ozone amount ((total ozone amount) x sec (theta) , where (theta) is the solar zenith angle) at Tsukuba, the closest ozone amount measuring site. These findings can be supportive evidence for a direct relationship between solar UVB irradiance and stratospheric ozone amount.

This study presents measurements of Aerosol Optical Depth (AOD) spectral dependence in the Ultraviolet (UV) (317-368 nm) from a USDA UV-Multi-Filter Rotating Shadow band Radiometer (UV-MFRSR) and an EPA Brewer spectrophotometer collocated at Boulder, Colorado. The square of the correlation coefficients between the two instruments are: 0.93 for the 317-320 nm wavelength pair, 0.86 for the 332- 340 nm wavelength pair, and 0.94 for the 368-360 nm wavelength pair (first wavelength represents the UV-MFRSR, while the second represents the Brewer). The ratio of the UV-MFRSR to Brewer AOD for all wavelengths is 1.01 +/- 0.35. The daily-averaged AOD for wavelengths 317/320, 332/340, and 368/360 nm range from 0.112 to 0.373, 0.136 to 0.328, and 0.103 to 0.289, respectively, based on averages from both instruments. The hourly mean angstrom exponents were calculated to be 0.8 +/- 1.5 for the UV-MFRSR and 1.3 +/- 2.4 for the Brewer (which are not statistically different than zero at the 95% confidence level). The wavelength dependence of the AOD is explained based on reasonable assumptions for the size distribution and optical properties of the aerosol particles using Mie theory.

A sensitivity analysis is performed for the expected changes to the photodissociation rates of various key species that are relative to tropospheric chemistry, due to changes in the radiative transfer, caused by contrails and clouds generated from the use of hydrogen as fuel. The results are compared to those which correspond to contrails from conventional aircraft. For the determination of cirrus clouds as a model input the cirrus clouds climatology developed in the frame of the ISCPP has been used. The optical parameters of the contrails generated by conventional fuel and hydrogen fuel are adopted from the recent literature. The mean total ozone field, which corresponds to present status and has been used as an input is based on both TOMS and ground-based data, while for the future projections the IPCC 1999 scenario has been used. The Tropospheric Ultraviolet and Visible (TUV) radiative transfer model has been used, to calculate the actinic flux in the UV and visible part of the spectrum, at 3-km height steps in the troposphere. For this purpose, the model has been initially tested against spectral measurements, and an agreement of +/- 10% has been achieved when the appropriate input parameters are well documented. The different optical properties of the aerosols and clouds have been considered in the model calculations. For the calculation of the photodissociation rates the latest available chemicokinetical data have been incorporated into the model. It was found that the additional perturbations induced from contrails are larger for the kerosene contrails compared to the ones induced by LH₂ contrails, since they are expected to have smaller optical depths.

The Alps are one of the regions where the highest UV levels are measured in Europe, which is a consequence of low aerosol levels, high altitude and snow-covered surfaces. Solar UV radiation increases with altitude mainly due to decreasing amounts of air molecules, ozone, aerosols and clouds in the atmosphere. The altitude effect plays a key role in the understanding of the UV radiation field in mountainous terrain. Therefore simultaneous measurements of erythemal UV radiation (broadband) on three different heights were performed in the Swiss Alps during more than four years. Under clear-sky conditions, the altitude effect of daily noon-time yearly mean values of direct, diffuse and global erythemal UV radiation results in 17.4%/1000 m (direct), 8.5%/1000 m (diffuse) and 10.7%/1000 m (global). Seasonal variations of the altitude effect are mainly influenced by changes of solar elevation, albedo values and turbidity levels during the year. In addition, measured altitude effects are shown and compared to calculated altitude effects obtained by application of the MODTRAN radiative transmission model.

The method combines the use of a standard radiative transfer code (UVspec) and various sources of information on the influencing parameters to map surface UV radiation. GOME, TOMS or TOVS data are used for the total column ozone. The cloud optical thickness is estimated using METEOSAT/MVIRI data. Other influencing factors taken into account include tropospheric aerosols, snow cover and surface elevation. The resulting products are maps of surface dose rates and daily doses, covering Europe with a spatial resolution of 0.05 deg. Because the method is using METEOSAT the cloud optical thickness can be estimated every half-hour. This allows quantifying how well doses can be reconstructed from one or a few cloud images per day. The method is now being used to reconstruct the surface UV conditions during the last 10 to 15 years, in particular to support a study on the effects of UV radiation on marine biology in the Northeast Atlantic. Initial results for the month of March, from 1990 to 1999 are presented. Their validity is discussed with respect to the consistency of the input data during this period. A scheme to deal with drifts in METEOSAT sensitivity is presented.

The UV rotating shadowband spectroradiometer (UV-RSS) is capable of measuring direct, diffuse and total horizontal irradiances simultaneously with spectral resolution of 0.25- 0.45 nm in the 290-380 nm range. It is based on a two-prism spectrograph that has very high out-of-band rejection of 2*10-6) as defined by the 325 nm HeCd laser line. Without moving parts, the radiometric stability is limited by the stability of the diffuser throughput and the stability of the cooled CCD. The wavelength stability is maintained by temperature control of the fused silica prisms and air pressure in the spectrograph. The current signal-to- noise ratio allows optical depth retrievals in the 305-360 nm range at mid latitudes in summer for typical ozone loading of 300 DU. This signal-to-noise can be increased by a factor of 5 within a one-minute shadowbanding cycle by means of multiple exposures. The UV-RSS permits ozone retrieval from diffuse irradiance using the DOAS method or from direct irradiance via Langley regression. Either method is robust as the UV-RSS provides 205 pixels of data within 310-330 nm range.

Since October 1995 a global daily forecast of the UV index and the daily dose, as the irradiance of the biologically effective ultraviolet radiation, for clear sky is calculated. The Austrian model as well as the input parameters are described. By connecting the daily dose with the sensitivity of the photobiological skin types, a recommendation is given to select an appropriate sun protection factor of a sunscreen to avoid overexposure of the skin. The validation of the Austrian forecast model is done by long-term measurements of the biologically effective ultraviolet radiation. Measurements were taken from 6 different sites at 4 continents (Antarctica, Australia, America and Europe) covering the latitudinal range from 67 degree(s)N to 60 degree(s)S. By using the underestimation as criteria in the sense of radiation protection, the Austrian model shows less than 12% underestimation over the whole period for the UV index and less than 10% for the daily dose. The evaluation shows further that the forecast of the daily dose is much more influenced by the attenuation due to clouds than the UV index.

Measurements of aerosol UV optical depths are described as part of an ongoing study of surface ultraviolet irradiances over the southwestern United States. Global UV irradiances are continuously monitored using a moderate-bandwidth radiometer (Biospherical, GUV-511), which has been in operation since June 1997. Irradiances at 305 and 320 nm are used to derive column ozone; erythemal doses are determined with the additional consideration of 340 and 380-nm irradiances. The clear sky relationship between ozone and UV dose is well characterized by a power law with an exponent that decreases with increasing solar zenith angle, from 1.12 to 0.99 between solar zenith angles of 20 and 60 degrees. Most recently, aerosol optical depths at 340 nm have been estimated using standard direct sun techniques applied to model corrected global measurements. These are compared with direct sun measurements (Solar Light, Microtops II) over a five-month period. Mean values agree well, but daily observations show differences in aerosol optical depths of up to 0.1, with direct sun measurements indicating larger variability. Aerosol optical depths inferred from global irradiances vary between a minimum of about 0.03 in winter and a maximum of 0.10 in summer.

An alternative method using the spectrum retrieval technique for accuracy increase of ultraviolet UV measurements is presented. The irradiances measured by the UV filter radiometer with narrow filters in investigated range for retrieval spectral function are used. Irradiances in narrow intervals are substituted to average values, which do for an effective wavelength. An approximated function is employed to extract the source spectrum from number of irradiances average values. The extracted spectrum is useful in the correction of UVA, UVB, UVC and total UV irradiances, and in calculating the biologically effective ultraviolet radiation for studies of the effect of increased UV radiation. The criterion of reached level of accuracy is relation of measured irradiance value to calculated one. If the reached level of accuracy was not enough, the procedure repeated. The mathematical model of measurement process to test the technique and installation of its optimal parameters was employed. Some UV spectrum sources were used in modeling, for example, solar, tungsten lamp, deuterium lamp and other. They were calculated or measured before. The results show that using the new technique ensures the reasonable level of uncertainty not more than 10%. It quite satisfies the needs of natural monitoring and medicine. The new technique also has appeared useful to some problems of pirometry and photometry.

Many of the effects of ultraviolet radiation (UVR) on people and their environment--damage to various materials, survival of insects and microbial pathogens, growth of vegetation, and adverse or beneficial effects on human health--are modified by the presence of trees. Human epidemiological investigations generally consider exposure as given by indices of UVR irradiance on horizontal surfaces in the open. Though many people are exposed to UVR while reclining at a beach or swimming pool, thus experiencing irradiance on essentially horizontal surfaces in the open, exposure to UVR during daily routines in urban areas may also be important in affecting human health. Tree influences on UVR irradiance, particularly in the UVB, can differ substantially from influences on the visible portion of the solar spectrum. Trees greatly reduce UVB irradiance in their shade when they obscure both the sun and sky. Where trees obscure the sun but leave much of the sky in view, UVB irradiance will be greater than suggested by the visible shadow. In small sunny areas near trees that block much of the sky from view, UVB irradiance is reduced substantially, whereas visible irradiance may be nearly as great or slightly greater than in the open.

The design and optical characteristics of a polychromatic ultraviolet radiation exposure system are described. The system uses a Xe lamp and long-pass cutoff filters to provide exposures to plankton suspensions in quartz cuvettes. Up to 80 simultaneous exposures can be made for eight spectral regimes with ten irradiance levels for each regime. Treatment spectra are presented for a system configured for the study of UV responses of biological and chemical processes in the Chesapeake Bay. Experimental exposures bracket full solar spectral irradiance for the site.

Phylloplane microorganism survival is presumably affected by ultraviolet radiation (UV) penetrating into plant canopies, but little field data exist relating microorganism population dynamics to canopy UV level. Recent advances in field measurements involving the use of biological dosimeters and miniature radiometers make possible data sets for use in assessing the impact of UV on phylloplane microbe survival. The objective of this study was to compare field survival of a bacterial species, applied to turfgrass as a biological disease control agent, with predicted survival based on the prevailing UV-B environment under natural and attenuated UV conditions. Derived survival curves and radiation penetration equations (based on radiometer and biodosimeter field measurements of UV-B transmittance) were applied to predict bacterial survival within a turfgrass canopy at different leaf area indices. Due to the range in UV levels within a canopy, as indicated by the maximum (sunfleck areas) and minimum (shaded areas) transmitted irradiance values, bacterial survival can vary; predicted bacterial survival based only on average light penetration tended to underestimate survival. Further study should address contributions due to microenvironmental effects (e.g., canopy temperature, leaf wetness, and canopy structure), the spatial distribution of bacterium leaf microsites and bacterium survival on leaf surfaces.

Variations of solar ultraviolet (UV) radiation by clouds and aerosols that have a comparable effect on UVB (280-315 nm) caused by variations in stratospheric ozone hinder accurate detecting mid-latitude UVB trends. In this connection it is desirable to use a UVB dosimeter that has at least two independent parameters, namely, a parameter responding to the integral intensity of UVB radiation and an additional one exclusively sensitive to the short wavelength variations in solar UV spectrum related to ozone depletion. The desired spectral selectivity is intrinsic in D-dosimeter that was recently introduced for an in situ monitoring of vitamin D synthetic capacity of solar UVB radiation. D-dosimeter is based on an in vitro model of vitamin D synthesis. The photoreaction rate (decay of provitamin D and formation of previtamin D) depends upon the integral UV intensity whereas maximum achievable concentration of previtamin D is solely dictated by the spectral position of the short-wave edge of solar spectrum. This makes it possible to reveal ozone depletion under conditions of opaque atmosphere when clouds and aerosols attenuate solar UV flux like a gray filter.

Background: Recent advances confirming the role of vitamin D in prevention of cancer have created new scientific interest. The main source of vitamin D is exposure to ultraviolet B (UVB). Factors that reduce atmospheric penetration of UVB play a role in increasing risk of cancers of the colon, breast, and other sites. Objective: To systematically review available epidemiological and laboratory studies concerning effects of UVB or vitamin D on colon, breast, prostate and ovarian cancer. Methods: All published research articles that identified the role of ultraviolet B, vitamin D, and its metabolites in conjunction with colon and breast cancer were ascertained and abstracts or articles were reviewed. Results: The preponderance of epidemiological and laboratory studies support the hypothesis that moderate exposures to ultraviolet B and vitamin D provide protection against colon and breast cancer, among others. The effect is present throughout life for colon cancer, but is exerted mostly during the first two decades for breast cancer. Conclusion: Latitude, climate, sulfate air pollution, stratospheric ozone, and behavioral factors combine to reduce the dermal synthesis of vitamin D to virtually zero during winter months. Populations at 37+ degrees of latitude are at markedly elevated risk of vitamin D deficiency, and, consequently, of colon, breast and prostate cancer incidence and mortality.

Based on geographical distributions of various cancers in the U.S. and elsewhere, ecologic studies comparing these distributions to indices of solar radiation during the past 20 years have led to the understanding that solar UVB radiation is a risk reduction factor for breast, colon, ovarian, prostate cancer and non-Hodgkin's lymphoma (NHL). Photo-initiated production of vitamin D is the mechanism that enables solar UVB to play this role. The work presented here explores the use of the USDA UVB Radiation Monitoring Program ground station values to confirm and extend the prior results, and compares these results to those obtained using UVB data from the Total Ozone Mapping Spectrometer (TOMS). Inverse correlations between solar UVB radiation have been found for a total of 14 cancers using mortality data from the U.S. with these data sets, and are supported by additional studies using cancer mortality and dietary supply data from Europe. While vitamin D has been shown to be a risk reduction factor for several of these cancers, it is hoped that additional studies will be conducted to confirm or disprove the protective role of UVB and/or vitamin D for the remaining cancers.

The stratospheric ozone decrease has heightened concern over the ecological implications of increasing solar UVB radiation on agricultural production and natural plant ecosystems. UVB is absorbed and can damage many important plant species through a variety of interacting mechanisms. The effects of enhanced UVB exposure on yield and yield formation of winter wheat associated with photosynthetic activity and total biomass development were investigated in this study. The overall experimental design was three UVB treatments (two supplemental UVB treatments and an ambient level) and three replicates of each treatment. The results suggested that the supplemental UVB can cause the decrease of yield of winter wheat up to 24% with 11.4% increased UVB. The key growing stage which cause the yield decrease is during the jointing-spike formation. This study also investigated the effects of supplemental UVB on production of dry matter, allocation, LAI, total biomass, and photosynthetic activity of winter wheat. The combined UVB effect with other environmental stress factors on wheat was discussed in this study.

Since our major concern on the stratospheric ozone depletion is possible adverse effects on the biosphere, it is important to establish the way to determine biologically effective doses of solar UV radiation. The spore dosimetry system measuring the lethality of dry bacterial spores on membrane filters has been developed to meet this purpose. The methodology to evaluate experimental correlation with spectral measurements based on the effectiveness calculation has been applied in several field comparisons carried out at Nea Michaniona (Greece), Brussels (Belgium), and Sao Martinho (Brazil). When plotted against UVB irradiance (total energy below 320 nm), the calculated values of MED (minimal erythema dose), SID (spore inactivation dose) and DND (DNA damage dose) exhibited increasing exponents in power regressions, while the exponents from spore dosimetry exceeded those of the calculated values. The results of calculated versus observed values of SID indicate a general convergence at low to modest dose rates, but at high dose rates the calculated ones tended to yield lower values than those obtained from direct biological measurements.

Quantitative changes in foliar chemistry in response to UVB radiation are frequently reported but less is known about the qualitative changes in putative UV-screening compounds. It has also not been conclusively shown whether qualitative differences in screening compounds or differences in localization patterns influences the sensitivity of plants to damage from UVB radiation. In this study we evaluated the chemical composition and deposition patterns of UV-absorbing compounds in three tree species and assayed these species for possible effects on gas exchange and photosynthetic carbon assimilation. Branches of mature trees of sweetgum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera) and red maple (Acer rubrum) were exposed to supplemental levels of UVB radiation over three growing seasons. Controls for UVA were also measured by exposing branches to supplemental UVA only, and additional branches not irradiated were also used for controls. These species demonstrated contrasting chemical composition and deposition patterns with poplar being the most responsive in terms of epidermal accumulation of phenolics including flavonols and chlorogenic acid and hydroxycinnamates. Sweetgum and red maple showed increases primarily in hydroxycinnamates, particularly in the mesophyll in red maple. Leaf area was marginally influenced by UV exposure level. Assimilation was generally not reduced by UVB radiation in these species and was enhanced in red maple by both UVB and UVA and by UVA in sweetgum. These finding are consistent with a hypothesis that epidermal attenuation of UVB would only be reduced in poplar, which accumulated the additional epidermal screening compounds. It is possible that photosynthetic efficiency was enhanced in red maple by the increased absorption of blue light within the mesophyll. Stomatal conductance was generally reduced, and this led to an increase in water use efficiency in red maple and poplar.

Biological systems are extremely sensitive to changes in ultraviolet radiation reaching the Earth's surface. Atmospheric ozone absorbs considerable part of the UV radiation coming from the sun and harmful for biosphere so only a very small part of it can reach the Earth's surface; thus, organisms adapted to that intensity. The light-trap success of European corn borer (Ostrinia nubilalis Hbn.) was examined at those nights when during the previous day the UVB radiation had different intensity. UVB data used for examination come from measurements in the Keszthely observatory of the Hungarian Meteorological Service by Robertson-Berger UV-Biometers. The light-trap catch data of European corn borer originated from the national light-trap network between 1994-1998. Relative catch (RC) values were calculated from the daily data of UVB radiation relating to the summer half-year. The daily data were divided with the weighted average values of previous, actual and following ten days. We calculated RC values from daily light-trap results of European corn borer for all observing stations and swarming times. Our results prove light-trap catch is low if the values of UVB radiation are significantly lower considering the average.

Decreasing stratospheric ozone concentrations have led to concern for an enhanced UVB radiation (wavelength range of 280-315 nm) reaching the Earth's surface. An understanding of the mechanism of UVB radiation tolerance and sensitivity may improve our ability to assess the potential effects of changes in solar UVB radiation on plant species. The purpose of this research was to conduct an assessment of tolerance of southern trees to future enhanced UVB radiation. This paper describes the systematic approach developed for such an assessment and the preliminary results associated with thirty-five broadleaf tree species in the South studied during two growing seasons. Results indicated that the diverse trees in the South possessed various biological characteristics in defense of UVB damage. It is suggested that the approach to comprehensive evaluation of UVB tolerance in diverse trees should focus on leaf optical properties, mainly depth of light penetration into leaves, leaf anatomical and morphological changes, and concentration of leaf UVB absorbing compounds during leaf development.

The stratospheric ozone depletion and enhanced solar ultraviolet-B (UVB) irradiance may have adverse impacts on the productivity of agricultural crops. The effect of UVB enhancements on agricultural crops includes reduction in yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in structure and pigmentation. Many studies have examined the influence of supplementing UVB irradiance on different crops, but the effect of UVB irradiance on cotton crops has received little attention. The cotton crop is one of the most versatile of all the crops. It is a major fiber crop of the world and a major source of trade and economy in many countries. In this study we provide quantitative examination of the effects of elevated UVB irradiance on cotton. The tested cotton crop was grown under natural and supplemental levels of UVB irradiance in the cotton field. The results in this study showed that the supplemental UVB irradiance has negative impacts on cotton growth, development, yield, and fiber quality. A brief discussion is included on how the impacts on cotton contrast with impacts that have been observed in other studies on other plants, including trees.

A decrease in stratospheric ozone may result in a serious threat to plants, since biologically active short-wavelength ultraviolet-B (UVB 280-320 nm) radiation will increase even with a relatively small decrease in ozone. Experimental work has shown that various cultivars and species respond to UVB in different ways. To determine the physiological effects on plants of any increases in UVB radiation, the irradiances at the potential sensitive plant surface need to be known. Numerical models are needed to calculate UVB irradiance. This paper compares spatially and temporally averaged measurements of UVB canopy transmittance (T_canopy, irradiance below canopy/irradiance above canopy) to that predicted by three models. Maize was selected as the canopy for the study because direct measurements of leaf area and leaf angle distribution are manageable. The models can be applied to other plants including urban trees, though other methods of characterizing leaf area and angle distributions generally would be used. Using measurements of canopy parameters as inputs to the numerical scheme, the models attempt to simulate the UVB T_canopy that the UVB sensors measure. The purpose of this paper is: (1) to describe the models developed for calculating UVB irradiances (as measured by UVB T_canopy) at given positions in maize canopies; and (2) to report the results of experimental tests of the models.

The United States Department of Agriculture (USDA) UVB Radiation Monitoring and Research Program began installing the UV Multi-Filter Rotating Shadow-band Radiometer for long-term measurements of UV radiation in 1995, and the program now has 28 sites across the U.S., as well as 2 sites in Canada. The UV-MFRSR uses 7 independent interference filter photodiode detector combinations to make total horizontal solar irradiance measurements at 300, 305.5, 311.4, 317.6, 325.4, 332.4 and 368 nm (nominal 2 nm FWHM bandwidth) through a single Lambertian detector. UV effects researchers want to apply their particular action spectrum to the measured spectra to estimate damage due to UV. The UV synthetic spectra retrieval model is used to estimate the continuous spectral distribution based on the seven UV radiometer channel measurements. In this study, we made comparisons of these synthetic spectra with the spectra measured from co-located USDA Reference UV and NIWA UV spectroradiometers at Table Mountain near Boulder, Colorado, U.S. A preliminary comparison of modeled erythemal-weighted dose with measurements performed by the two spectroradiometers is presented.

The presence of clouds is responsible for an important variation in the UV and visible radiation at the Earth's surface. Although for practical purposes cloud transmittance is often considered plane in the UV and visible, a wavelength dependence is observed. In this paper we performed a statistical study of cloud transmittance at Ushuaia for wavelengths between 295 and 600 nm, following different procedures. A decrease of the transmittance for increasing wavelengths in the UVA and the visible was observed (0 to 50% decrease at 600 nm regarding to 340 nm) in good agreement with the observations made by other authors. Nevertheless, for wavelengths below 320 nm our results show discrepancy with other papers. Since Ushuaia is a small town n a fairly unpolluted area, a possible reason for this differences is that, as a consequence of low ozone amounts in the troposphere, Rayleigh scatter is more important than ozone absorption, even in this part of the spectrum.

One aim of the project CUVRA was to improve the modeling of effects of inhomogeneous albedo fields on ground UV irradiance. For the determination of the effect of heterogeneous illumination (shading effects, inclination and orientation of the facet) on the reflectivity of a facet in a mountainous region more complex approaches (than for the calculation of the reflectivity of flat surfaces) have to be chosen. We determined the 'effective' albedo for the surroundings of the Observatory Sonnblick (3106 m) by using three different methods. The first method consisted of a combination of a 3D albedo model calculation and 1D radiative transfer calculation. By using this method and a digital elevation map, the reflectivity of the region surrounding Sonnblick Observatory was calculated. The second method was an inversion method using a 1D radiative transfer model. The routine spectral UV measurements performed at Sonnblick were used to calculate the average effective albedo. The third method was entirely experimental. An albedo measuring system was used to perform reflectivity measurements of the surrounding of Sonnblick. Overall, the results showed that the average albedo of a topographically structured surface is lower than the average albedo of a corresponding (surface with same ground characteristics) flat surface.

Beside the sun elevation angle, the total ozone content (TOC) of the atmosphere is a main input parameter of models to predict the biologically-effective UV radiation. Therefor TOC was analysed for the domain of Central Europe. The data domain contains the area from 7 degree(s)E to 17 degree(s)E in longitude and from 45 degree(s) to 50 degree(s) in latitude. Ground based as well as satellite based measurements from EPTOMS were involved. The temporal and spatial variability of TOC was analysed using auto-correlation and fitting auto- correlation functions. It becomes evident that temporal correlation decrease rapidly within the first 9 days down to the level of 1/e. From spatial correlation functions de- correlation distances (1/e) are estimated with 180 km E-W and 145 km for S-N direction. The analysis of TOC variability per 100 km for the domain of Austria was also examined. The results show that the highest values can reach up to 18%/100 km. Day-to-day variability for a certain location may exceed this level by a few percent (25%/day). These results show that for the calculation of the biologically effective UV radiation a high spatial resolution of TOC data is necessary to ensure an appropriate quality.

To evaluate regional differences of global ultraviolet-B (UVB: 290-315 nm) irradiance, eight band-spectral UVB radiometers (MS-210W, EKO), which were used in the UV monitoring network organized by five universities, were compared at the Shonan Campus of Tokai University, Japan from August 2-10, 1997. Reference irradiances were measured by a spectroradiometer (OL752, Optronic Lab.) and the reference band-spectral UVB radiometer (MS-210W, EKO). UVB radiometers participated were classified into three groups by the characteristics of the spectral response depending on the UVB irradiance. It suggests that characteristics of out of band is different from the reference radiometer. Furthermore, to access regional difference of UVB, it is necessary to use the calibration factor that is not constant but a function depending on output.

Routine standard spectral UVB measurements with Brewer spectrophotometers in the Canadian network began in 1989. The time interval of UV measurements militates against reliable detection of long-term changes in UV. To estimate the UV irradiance (at individual wavelengths and spectrally integrated) values prior to 1989 a statistical model has been developed to derive UVB from other variables, which have been measured since the 1960s. These variables include global solar radiation, total ozone, dew point temperature, and snow cover. The model results are demonstrated to be in good agreement with the measurements since 1989. For example, the standard deviations of the difference between monthly values of measured and derived erythemaly weighted UV irradiation is 3.3% for summer months. The major source of error in the model estimates is likely linked to rare occurrences of absorbing aerosols in the atmosphere. Long records of reliable measurements of total ozone, global solar radiation and other parameters made it possible to derive UVB values at three Canadian stations from the mid 1960s. Trends in derived erythemaly weighted UV at two stations (Toronto and Edmonton) are similar to those expected from total ozone trends although the estimated error of the UV trends is more than 2 times larger. However the increase in annual UV at Churchill (59 N) in 1979-1997 was found to be more than twice that expected from the ozone decline. This is a result of long-term changes in snow cover and clouds.

Changes in ultraviolet (UV) radiation in Nanjing region were investigated in this study by using the simplified atmospheric radiation transfer computation model, the discrete-ordinate-method (DISORT) of radiation transfer, the climatic empirical model and observational method. The computational results of UV radiation were compared against observations showing that the computational values of UV radiation from the simplified model, and the DISORT and climatic methods were reasonable in agreement with observations.

An episode of low ozone values over Central Europe on November 29 2000 was observed at some Italian Brewer stations. Middle latitudes ozone variability, at time scales of 1-3 days, depends mainly on the atmospheric motions transport. This paper reports an investigation on synoptic scale ozone components, based on the O₃ tendency equations for determining the dynamic cause of the episode. Ultraviolet (UV) irradiance variations are also analyzed in order to find out how the weather patterns may lead to enhanced levels of UV radiation within few days.