Spectral ultraviolet (UV) and visible irradiance has been monitored at the South Pole with a SUV-100 spectro-radiometer since 1991. A new data version labeled “Version 2” has recently been produced, which has a higher accuracy than the original released “Version 0”. We have established a UV climatology for the South Pole based on the new data set, focusing on the effects of cloudiness, total column ozone, and volcanic aerosols. In addition, total column ozone was calculated from the spectral UV measurements for solar zenith angles (SZA) up to 87° using a recently developed algorithm. This new total ozone data set can complement Dobson ozone observations to validate satellite total ozone data. At the South Pole, NASA/TOMS Version 7 overpass data exceed SUV-100 total ozone measurements by 5-8%. In contrast, TOMS Version 8 data agree on average to within 2% with SUV-100 measurements, and the ratio of the two data sets shows virtually no dependence on SZA and the ozone amount. The good agreement confirms that the well-documented bias of TOMS Version 7 data at high austral latitudes was successfully reduced with the release of Version 8. A similar comparison between SUV-100, Dobson, and TOMS ozone measurements was performed for the NSF/OPP network site at Arrival Heights (78°S), and results are discussed.

The USDA UV-B Monitoring and Research Program maintains a network of Yankee Environmental Systems surface UVB meter instruments at stations throughout the United States. We analyzed the temporal behavior of UV-B at eight stations that were selected because of their early deployment (years beginning in 1995-1997 and ending in 2003). Though calibration methodologies differed before and after 1998, we accounted for variations among instruments and long-term drift in a consistent manner, and estimated a total calibration uncertainty of 7%. We computed mean annual and mean monthly irradiances for each month of the year for each station, and investigated temporal patterns. At Nunn, Colorado, mean annual UV-B irradiance increased by 3.4%, less than the calibration uncertainty. However, UV-B trends in five months exceeded the calibration uncertainty, with the maximum increase occurring in April (28%). Mean annual irradiances increased at all but one of the other seven sites, though only two exceeded the 7% uncertainty.

We report final results of an aerosol UV absorption closure experiment where a UV-shadow-band radiometer (UV-MFRSR, USDA UVB Monitoring and Research Network) and 4 rotating sun-sky radiometers (CIMEL, NASA AERONET network) were run side-by-side continuously for 17 months at NASA/GSFC site in Greenbelt, MD. The aerosol extinction optical thickness τ_ext, was measured by the CIMEL direct-sun technique in the visible and at two UV wavelengths 340 and 380 nm. These results were used for UV-MFRSR daily on-site calibration and 3-min measurements of τ_ext at 325nm, 332nm and 368nm. The τ_ext measurements were used as input to the radiative transfer model along with AERONET retrievals of the column-integrated particle size distribution (PSD)to infer an effective imaginary part of the UV aerosol refractive index, k, by fitting MFRSR measured voltage ratios. Using all cases for cloud-free days, we derive diurnal and seasonal dependence of the aerosol absorption optical thickness, τ_abs with an uncertainty 0.01­-0.02. At our site τ_abs follows pronounced seasonal dependence with maximum values ~0.07 at 368nm (~0.15 at 325nm) occurring in summer hazy conditions and <0.02 in winter-fall seasons, when aerosol loadings are small. Inferred values of k allow calculation of the single scattering albedo, ω, in UVA and comparisons with AERONET almucantar ω₄₄₀ retrievals at 440nm. Overall, ω was slightly lower in UV than in the visible: case average <ω₃₆₈>=0.93 compared to <ω₄₄₀>=0.95. However, the differences (<ω₄₄₀ - ω₃₆₈> ~0.02, rms difference ~0.016) are smaller than uncertainties of both retrievals (δω~0.03). Low <ω₃₆₈> values are consistent with higher values for imaginary refractive index, k: <k₃₆₈> ~0.01 compare to <k₄₄₀> ~0.006. However, mean differences in k (<k₃₆₈-k₄₄₀>~0.004) were only slightly larger than AERONET retrieval uncertainty δk ~0.003²⁷. We also found that ω decreases with decrease in τ_ext, suggesting different aerosol composition in summer and winter months. So far, our results do not allow explaining the causes of apparent larger aerosol absorption in UV. Continuing co-located measurements at GFSC is important to improve the comparison statistics, but conducting aerosol absorption measurements at different sites with varying conditions is also desirable.

TOMS UV algorithm is capable of taking into account the scattering aerosols via its scene reflectivity. It also accounts for absorbing aerosols in free troposphere (dust and smoke plumes) via aerosol index correction. On the other hand, the effects of aerosol absorption in the boundary layer are not properly taken into account, because they do not appear as absorbing aerosols in the TOMS AI data (positive AI). This additional error has been claimed to be the reason for the observed positive bias between TOMS derived UV and ground-based measurements. We compared TOMS overpass irradiances against the Brewer measurements in NASA/GSFC site in USA and Thessaloniki, Greece with the main objective of evaluating the effect of absorbing aerosols with the measurements of aerosol optical properties. We found that the bias between TOMS UV and ground-based data depends on the aerosol absorption. In other words, the bias was increasing with the increasing aerosol absorption, τ_abs. A simple correction to account for this effect is proposed, assuming that the climatology of τ_abs is known.

UV radiation measurements from many sites throughout the US are employed on a routine basis to assess biologically significant dosages to plant, animal and humans and to retrieve aerosol optical depths and ozone column concentrations. In various recent studies the impact of clouds on the derived quantities has become a topic of interest. In particular, when clouds are present during measurements used for aerosol retrievals, their impact may depend on the cloud’s height in the atmosphere, the cloud thickness and, for clouds of limited horizontal extent, on the horizontal displacement from the instrument site and the relative geometry between the cloud, the instrument and the sun. Results from a Monte Carlo radiative transfer model, which has been adapted to include the effects of ozone absorption in the UV, will be presented. The impact of clouds with high aspect ratio (cloud height / cloud width) on biologically significant dosages will also be addressed.

This is the second continuation of work begun by Dave Bigelow and James Slusser in their study of the same name published in 2000 in J. Geophys. Res., 105, 4833-4840, which studied only a few instruments over a limited in-service time span. Part 1 expanded the Langley stability analysis by using 42 instruments over 5 years of field service. This part 2 expands stability as expressed with repeated laboratory lamp calibrations of the instruments, and compares these to the prior Langley analysis. 115 cases representing 44 instruments covering seven years of deployment are studied. Complicating this analysis are the four versions of the UV-MFRSR instrument that span the analysis time frame, and the results are presented as such. These results show the mean annual drift in sensitivity for the seven nominal wavelengths of the UV-MFRSR instrument are: pre-Rev.M: 300nm -8.8%, 305nm -8.1%, 311nm -7.4%, 317nm -8.3%, 325nm -7.3%, 332nm -7.6%, 368nm -7.2%; Rev.M: 300nm -7.5%, 305nm -7.1%, 311nm -6.5%, 317nm -5.6%, 325nm -5.8%, 332nm -5.3%, 368nm -5.1%; Rev.N and P; 300nm -10.1%, 305nm -7.2%, 311nm -8.3%, 317nm -4.3%, 325nm -3.6%, 332nm -3.7%, 368nm -3.5%; and Rev.Q: 300nm -5.6%, 305nm -5.8%, 311nm -3.8%, 317nm -4.4%, 325nm -4.8%, 332nm -4.6%, 368nm -3.5%.

Since 1990, global solar UV-B irradiance has been measured by a narrow-band solar UV-B radiometer at Hiratsuka, Japan. To detect long-term trend of global solar UV-B irradiance measured by the narrow-band solar UV-B radiometer, evaluation of a calibration constant of the radiometer for accurate measurement is essential. Thus, for estimating trend of global solar UV-B irradiance from 2000 to 2003, the calibration constant of the narrow-band solar UV-B radiometer was evaluated by applying three methods. The first method is based on a spectral irradiance standard lamp, which has been applied when evaluating long-term trend of global solar UV-B irradiance from October 1990 to September 2000. The second one is based on a reference narrow-band solar UV-B radiometer. Global solar UV-B irradiance measured by our narrow-band solar UV-B radiometer was compared with that of the reference narrow-band solar UV-B radiometer under natural sunlight. The third one is based on the total ozone amount. The results with three methods showed good agreement within 5%, leading to the conclusion that three methods are comparable in evaluating the calibration constant of the narrow-band solar UV-B radiometer.

Many experiments examining plant responses to enhanced UV-B radiation simply compare an enhanced UV-B radiation treatment with ambient UV-B (or no UV-B in most greenhouse and controlled environment studies). However, some experiments utilize multiple doses of UV-B radiation. A number of different techniques have been used to adjust the UV dose, each with advantages and disadvantages. One common technique is to place racks of fluorescent UV-B lamps at different heights above the plant canopy. A generally ignored consequence of this technique is that the pattern of shade which plants receive from the lamps is distributed differently over the course of the day at different lamp heights. To determine the effects of using lamps at different heights above the canopy, we grew three species (canola, sunflower, and maize) in the greenhouse under racks of unenergized lamps that were placed at two different heights above the plant canopy. Many plant growth characteristics differed between plants grown under the two lamp heights. These differences can potentially enhance or obscure true UV-B effects. Even more troubling is that changes in leaf mass per foliage area, which were observed in this experiment, could contribute to differences in plant UV-B sensitivity. We recommend the use of other techniques for achieving multiple doses of UV-B radiation. These range from simple and inexpensive approaches (wrapping individual fluorescent tubes in different layers of a neutral density filter such as cheese cloth) to more technical and expensive alternatives (electronically modulated lamp control systems). These choices should be determined by the goals of the particular experiment.

A prototype UVA dosimeter that is responsive to the UVA wavelengths only has been developed for measurement of personal exposures. The chemical phenothiazine, cast in thin film form and which is responsive to both the UVA (320-400 nm) and UVB (280-320 nm) part of the spectrum was used and filtered with mylar. This combined system responded to the UVA wavelengths only and underwent a change in optical absorbance as a result of UVA exposure. The wavelength of 370 nm was employed for quantifying the change in optical absorbance of the combined mylar/phenothiazine dosimeter and a calibration curve determined for measuring the UVA exposures. UVA exposures to approximately 50 J cm^-2 may be measured prior to saturation of the response.

The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated using a three-dimensional canopy radiation transport model. The UVB irradiance across a horizontal domain at the base of a regular array of spherical tree crowns of varying radius was modeled under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast. Differences in crown radius created differences in crown cover (m) with resulting differences in portions of the domain in direct beam shade. The spatial mean relative irradiance and erythemal irradiance of the domain and the spatial mean relative irradiance (I_r) and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles of 15°, 30°, 45°, and 60°. The mean I_r and erythemal UV irradiance under skies with 4 octas or less was not remarkably different from that under clear skies. Broken cloud cover reduces the spatial mean irradiance by approximately 20% to 30% across the 15o to 60o solar zenith range. In the shade, the irradiance was greater under partly cloudy than under clear skies. Partial cloud cover did not greatly influence the irradiance in the shade of the canopies. Significant changes in erythemal irradiance in the shade did not occur except with cloud cover of 8 octas (overcast) with solar zenith angles less than 45°. Consequently the mean ultraviolet protection factor for vegetation canopies under partly cloudy skies (50% or less cloud cover) is nearly equivalent to that for clear sky days. Regression equations were developed to estimate the areally averaged relative irradiances across the entire domain and only the shaded regions of the domain for each cloud cover fraction as functions of the solar zenith angle and canopy cover. These equations were then used to predict the variation in erythemal irradiance received across a region of suburban Baltimore, Maryland.

A limited evaluation of Ultraviolet Multifilter Rotating Shadowband Radiometer (UVMFRSR) based monitoring was conducted for instruments operated at the Mauna Loa Observatory, Hawaii 2/1/04 - 5/31/04. Two UV radiometers were installed, an UVMFRSR (shadowband) and a tracker mounted UVMFR (no shadowband). Measurements from the two UVMFR's, collocated Brewer and Dobson UV spectrophotometers, and the TOMS satellite, allowed a number of intercomparisons. The Langley-Forgan calibration technique proved more effective than lamp or Langley-SLR (simple linear regression) techniques when applied to an UVMFR that exhibited substantial change in response. Direct irradiance from an UVMFRSR compared very well with direct irradiance measured by a tracker mounted UVMFR for full-sun days (r²'s = 1). Mean ratios of the UV-MFRSR column ozone retrievals to the collocated Dobson and Brewer were 1.015 and 1.011 respectively. The mean ratio of UVMFRSR ozone to TOMS satellite ozone was 0.907. The UVMFRSR advantages of relatively low cost, unattended operation, automated calibration stability checks using Langley plots, and minimal maintenance make it a unique instrument for column ozone measurement.

The USDA UV radiation network currently consists of four high resolution spectroradiometers located at Table Mountain, Colorado (deployed 11/1998); the Atmospheric Radiation Measurement testbed site at Southern Great Plains, Oklahoma (deployed 10/1999); Beltsville, Maryland (deployed 11/1999); and Fort Collins, Colorado (deployed 10/2002). These spectroradiometers contain Jobin Yvon’s one meter asymmetric Czerny-Turner double additive spectrometer. The instruments measure total horizontal radiation in the 290nm to 360nm range, once every 30 minutes, with a nominal full-width at half-maximum (FWHM) of 0.1nm. We describe data quality control techniques as well as the data processing required to convert the raw data into calibrated irradiances. The radiometric calibration strategies using NIST FEL lamps, portable field calibrators, and vicarious calibrations using UVMFRSR data are discussed and a statistical summary of network performance is presented. All results are presented in the context of data processing and analysis tools including software and database systems.

Enhanced UV-B radiation due to stratospheric ozone depletion may have impacts on the productivity of agricultural crops. Which crop will be more sensitive to increased UV-B has received little attention. This paper presents a comparative study of the effects of supplemental UV-B on plant height, leaf area, biomass and yield among soybean, cotton, corn and wheat which were cultivated in fields in Nanjing, China. The experimental results showed that the four crops response to enhanced UV-B irradiation was shortened plant height, decreased leaf area and reduced biomass and yield of crops. Using the same criteria, the response of soybean and cotton to elevated UV-B is bigger than that of wheat and corn. RI (response index) is an integrated index which is the accumulation of relative change in plant height, leaf area, biomass and yield, reflecting general impact of increased UV-B on crops. The results suggested that the RI for the four crops was minus, demonstrating a negative impact of enhanced UV-B on the crops. According to the RI, the soybean and cotton belong to the sensitive plants category, wheat is a moderately sensitive plant and corn is a tolerant plant.

Ultraviolet radiation-B (UV-B) would increase due to the Ozone depletion. Global climatic factors, such as temperature, precipitation, evapotranspiration, soil moisture and CO₂ content, are changing because of the increase of greenhouse emission and the destruction of ecosystems. Climate stress factors including enhanced UV-B irradiance have effects on crop production. Many studies have assessed the effects of enhanced UV-B on crops and impacts of global climatic change on crops separately. However, when UV-B effects were discussed, other environmental stress factors were generally neglected. It is well-known that crops in nature are seldom affected by only a single stress factor, such as UV-B radiation. The impacts of enhanced UV-B radiation can be greatly increased or decreased by other environmental stress factors. In this paper, through field and plant growth chambers experiments, combined effects of enhanced UV-B radiation with other environmental stress factors including solar visible light, temperature and soil moisture content on winter wheat were investigated. The experimental results showed that enhanced UV-B irradiance can restrain growth and development of winter wheat, which leads to reduction of plant height, leaf area, and slowing physiological activity and decreasing biomass and yield of winter wheat. The response of winter wheat to enhanced UV-B varied under different UV-B intensity and its combination with other environmental stress factors.

Herbivores represent the interface between primary production and higher trophic levels. The effects of enhanced UV-B radiation on microbes, invertebrate herbivores, and detritivores has received limited study in both terrestrial and aquatic ecosystems. However, although direct effects (e.g. melanoma, cataracts) on mammals have been documented, indirect effects (e.g., resulting from changes in plant chemistry) of enhanced UV-B on mammalian herbivores have not been evaluated. Although the diet of mammalian herbivores has little effect on nutritional quality for their associated predators, to the extent changes in plant chemistry affect aspects of population dynamics (e.g., growth, fecundity, densities), higher trophic levels can be affected. In this study, different forage species of varying inherent levels of key secondary metabolites are being grown in the field under either ambient or ambient plus supplemental UV-B radiation simulating a 15% stratospheric ozone depletion for Pullman, Washington. At various time intervals, foliage is being sampled and analyzed for changes in secondary metabolites and other attributes. Using controlled feeding trials, changes in plant secondary metabolites are being related to preference and digestibility in specialist and generalist mammalian hindgut herbivores, digestion in ruminants and non-ruminants, and to selected aspects of population dynamics in mammalian herbivores. Results suggest how UV-B-induced changes in plant secondary chemistry affect animal nutrition, and thus animal productivity in a range of mammalian herbivores. Reductions in palatability and digestibility of plant material along with reductions in fecundity and other aspects of population dynamics could have significant economic ramifications for farmers, ranchers and wildlife biologists.

The polarization sensitivity of a Brewer MKIII double spectrophotometer was measured in the laboratory. We found two major sources of polarization sensitivity. 1) The flat quartz plate as the first optical element alters the polarization state of the transmitted light by Fresnel reflection at oblique incident angles. 2) The internal grating produces almost 100% polarization of the incident light perpendicular to the direction of the ruled grating. The combination of both effects results in a zenith angle (ZA) dependence of the instrument’s sensitivity for unpolarized input such as from Direct Sun measurements. The Brewer is 2% more sensitive at ZA=0° and 10% less sensitive at ZA=80° with respect to normal incidence (ZA=35°). Since the ZA-dependence is independent of wavelength this effect cancels out when calculating wavelength-ratios as used for total ozone retrieval. However the ZA-dependence causes errors when absolute signals at single wavelengths are needed as for aerosol optical depth (AOD) retrievals. Based on our laboratory measurements an overestimation of the Langley extrapolation between 3% and 5% is estimated even at best atmospheric conditions. The ZA-dependence causes 0.025-0.045 overestimation of AOD and an underestimation of the Angstrom exponent. We believe that this effect has not been detected from Brewer AOD-measurements since it is masked by larger uncertainty sources of other nature and AOD-comparisons to other instruments in the short UV-region are rare. Knowing the ZA-dependence it is possible to correct for the ZA-effect. We modified our Brewer by incorporating a depolarizer in its optical path and replacing the flat quartz window by a curved one, so that the input is always at normal incidence, which reduces the ZA-effect.