James R. Slusser

On Rayleigh Optical Depth Calculations

Abstract Many different techniques are used for the calculation of Rayleigh optical depth in the atmosphere. In some cases differences among these techniques can be important, especially in the UV region of the spectrum and under clean atmospheric conditions. The authors recommend that the calculation of Rayleigh optical depth be approached by going back to the first principles of Rayleigh scattering theory rather than the variety of curve-fitting techniques currently in use. A survey of the literature was conducted in order to determine the latest values of the physical constants necessary and to review the methods available for the calculation of Rayleigh optical depth. The recommended approach requires the accurate calculation of the refractive index of air based on the latest published measurements. Calculations estimating Rayleigh optical depth should be done as accurately as possible because the inaccuracies that arise can equal or even exceed other quantities being estimated, such as aerosol optica...

Vitamin D Intake Needed to Maintain Target Serum 25-Hydroxyvitamin D Concentrations in Participants with Low Sun Exposure and Dark Skin Pigmentation Is Substantially Higher Than Current Recommendations

Cutaneous cholecalciferol synthesis has not been considered in making recommendations for vitamin D intake. Our objective was to model the effects of sun exposure, vitamin D intake, and skin reflectance (pigmentation) on serum 25-hydroxyvitamin D (25[OH]D) in young adults with a wide range of skin reflectance and sun exposure. Four cohorts of participants (n = 72 total) were studied for 7-8 wk in the fall, winter, spring, and summer in Davis, CA [38.5 degrees N, 121.7 degrees W, Elev. 49 ft (15 m)]. Skin reflectance was measured using a spectrophotometer, vitamin D intake using food records, and sun exposure using polysulfone dosimeter badges. A multiple regression model (R(2) = 0.55; P < 0.0001) was developed and used to predict the serum 25(OH)D concentration for participants with low [median for African ancestry (AA)] and high [median for European ancestry (EA)] skin reflectance and with low [20th percentile, approximately 20 min/d, approximately 18% body surface area (BSA) exposed] and high (80th percentile, approximately 90 min/d, approximately 35% BSA exposed) sun exposure, assuming an intake of 200 iu/d (5 ug/d). Predicted serum 25(OH)D concentrations for AA individuals with low and high sun exposure in the winter were 24 and 42 nmol/L and in the summer were 40 and 60 nmol/L. Corresponding values for EA individuals were 35 and 60 nmol/L in the winter and in the summer were 58 and 85 nmol/L. To achieve 25(OH)D > or =75 nmol/L, we estimate that EA individuals with high sun exposure need 1300 iu/d vitamin D intake in the winter and AA individuals with low sun exposure need 2100-3100 iu/d year-round.

Langley method of calibrating UV filter radiometers

The Langley method of calibrating UV multifilter shadow band radiometers (UV-MFRSR) is explored in this paper. This method has several advantages over the traditional standard lamp calibrations: the Sun is a free, universally available, and very constant source, and nearly continual automated field calibrations can be made. Although 20 or so Langley events are required for an accurate calibration, the radiometer remains in the field during calibration. Difficulties arise as a result of changing ozone optical depth during the Langley event and the breakdown of the Beer-Lambert law over the finite filter band pass since optical depth changes rapidly with wavelength. The Langley calibration of the radiometers depends critically upon the spectral characterization of each channel and on the wavelength and absolute calibration of the extraterrestrial spectrum used. Results of Langley calibrations for two UV-MFRSRs at Mauna Loa, Hawaii were compared to calibrations using two National Institute of Standards and Technology (NIST) traceable lamps. The objectives of this study were to compare Langley calibration factors with those from standard lamps and to compare field-of-view effects. The two radiometers were run simultaneously: one on a Sun tracker and the other in the conventional shadow-band configuration. Both radiometers were calibrated with two secondary 1000 W lamp, and later, the spectral response functions of the channels were measured. The ratio of Langley to lamp calibration factors for the seven channels from 300 nm to 368 nm using the shadow-band configuration ranged from 0.988 to 1.070. The estimated uncertainty in accuracy of the Langley calibrations ranged from ±3.8% at 300 nm to ±2.1% at 368 nm. For all channels calibrated with Central Ultraviolet Calibration Facility (CUCF) lamps the estimated uncertainty was ±2.5% for all channels.

Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model

produce values of w. Its value ranged from 0.65 to 0.91 at 300 nm, 0.71 to 0.96 at 305.5 nm, 0.73 to 0.97 at 311.4 nm, 0.74 to 0.91 at 317.6 nm, 0.76 to 0.96 at 325.4 nm, 0.77 to 0.97 at 332.4 nm, and 0.80 to 0.99 at 368 nm. Error in this procedure decreases with increasing aerosol optical depth (AOD), from ±0.63 at AOD = 0.05 to ±0.04 at AOD = 1.0 averaged over the seven wavelengths. The current values of w have a slightly wider variation than values reported from a previous study at the same site. The lower values of w could indicate that, over the site, preferential absorption of UVradiation by black carbon aerosols could be occurring. More values of w in the UV spectrum will allow for better estimation of this parameter for UV radiative transfer modeling and will lessen error in estimation of surface UV irradiances. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques;

Effects of Supplementary Ultraviolet-B Irradiance on Maize Yield and Qualities: A Field Experiment¶

Abstract Stratospheric ozone depletion has caused an increase in the amount of ultraviolet-B (UV-B) radiation reaching the earths surface. Numerous investigations have demonstrated that the effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, susceptibility to disease and changes in plant structure and pigmentation. Many experiments examining UV-B radiation effects on plants have been conducted in growth chambers or greenhouses. It has been questioned whether the effect of UV-B radiation on plants can be extrapolated to field responses from indoor studies because of the unnaturally high ratios of UV-B/ultraviolet-A radiation (320–400 nm) and UV-B/photosynthetically active radiation (PAR) in many indoor studies. Field studies on UV-B radiation effect on plants have been recommended to use the UV and PAR irradiance provided by natural light. This study reports the growth and yield responses of a maize crop exposed to enhanced UV-B radiation and the UV-B effects on maize seed qualities under field conditions. Enhanced UV-B radiation caused a significant reduction in the dry matter accumulation and the maize yield in turn was affected. With increased UV-B radiation the flavonoid accumulation in maize leaves increased and the contents of chlorophyll a, b and (a + b) of maize leaves were reduced. The levels of protein, sugar and starch of maize seed decreased with enhanced UV-B radiation, whereas the level of lysine increased with enhanced UV-B radiation.

Methodology for Deriving Clear-Sky Erythemal Calibration Factors for UV Broadband Radiometers of the U.S. Central UV Calibration Facility

Abstract In the United States, there are several federal agencies interested in the effects of UV radiation, which has resulted in the establishment of UV monitoring programs each with their own instrumentation and sites designed to address their specific needs. In 1993, participating agencies of the U.S. Global Change Research Program organized a UV Panel for coordinating the different agencies’ programs in order to ensure that UV data are intercalibrated, have common quality assurance and control procedures, and that the efforts among agencies are not duplicated. In order to achieve these goals, in 1994 the UV Panel recommended formation of the U.S. Central UV Calibration Facility (CUCF), which is operated by the Surface Radiation and Research Branch of the Air Resources Laboratory of National and Oceanic Atmospheric Administration. The CUCF is responsible for characterizing and calibrating UV measuring instruments from several U.S. federal agencies. Part of this effort is to calibrate UVB broadband rad...

Annual and Interannual Behavior of Solar Ultraviolet Irradiance Revealed by Broadband Measurements

Abstract 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°N to 47°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°, 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.

Establishing the stability of multifilter UV rotating shadow-band radiometers

A modified Langley technique is used in a preliminary evaluation of the stability of ion-assisted deposition interference filters. The technique uses lamp- calibrated direct-normal irradiance measurements to return estimates of the extraterrestrial solar irradiance over the passband of the filter. Results from prototype and production UV-multifilter rotating shadow-band radiometers deployed in the U.S. Department of Agriculture UV Radiation Monitoring program indicate a 1%/yr drift in filter measurements. Measurements are reported to be within 8% of the SUSIM Atlas-3 extraterrestrial solar irradiance.

Diurnal changes in epidermal UV transmittance of plants in naturally high UV environments.

Studies were conducted on three herbaceous plant species growing in naturally high solar UV environments in the subalpine of Mauna Kea, Hawaii, USA, to determine if diurnal changes in epidermal UV transmittance (T(UV)) occur in these species, and to test whether manipulation of the solar radiation regime could alter these diurnal patterns. Additional field studies were conducted at Logan, Utah, USA, to determine if solar UV was causing diurnal T(UV) changes and to evaluate the relationship between diurnal changes in T(UV) and UV-absorbing pigments. Under clear skies, T(UV), as measured with a UV-A-pulse amplitude modulation fluorometer for leaves of Verbascum thapsus and Oenothera stricta growing in native soils and Vicia faba growing in pots, was highest at predawn and sunset and lowest at midday. These patterns in T(UV) closely tracked diurnal changes in solar radiation and were the result of correlated changes in fluorescence induced by UV-A and blue radiation but not photochemical efficiency (F(v)/F(m)) or initial fluorescence yield (F(o)). The magnitude of the midday reduction in T(UV) was greater for young leaves than for older leaves of Verbascum. Imposition of artificial shade eliminated the diurnal changes in T(UV) in Verbascum, but reduction in solar UV had no effect on diurnal T(UV) changes in Vicia. In Vicia, the diurnal changes in T(UV) occurred without detectable changes in the concentration of whole-leaf UV-absorbing compounds. Results suggest that plants actively control diurnal changes in UV shielding, and these changes occur in response to signals other than solar UV; however, the underlying mechanisms responsible for rapid changes in T(UV) remain unclear.

Coupling short-term changes in ambient UV-B levels with induction of UV-screening compounds.

A substantial number of studies have been conducted over the last several decades to assess the potential impacts of long‐term increases in ultraviolet‐B radiation (UV‐B between 280 and 320 nm) that will result from continued depletion of stratospheric ozone. However, seasonal changes, tropospheric chemistry and cloudiness are the dominant factors controlling ambient UV‐B levels on a short‐term or daily basis. The effects of short‐term changes in UV‐B on plant growth, phytochemistry and physiological processes have received relatively little attention. The USDA UV‐B Monitoring and Research Program provides an excellent network of stations that provide an opportunity to monitor long‐term changes in solar UV‐B radiation and evaluate the responses of plants to short‐term variation in UV‐B levels on a near‐real‐time basis. In this study barley (Hordeum vulgare L.) and soybean (Glycine max [L] Merr.) were used as model systems. Emerging seedlings of these species were grown under either near‐ambient levels of UV‐B or under reduced levels (ca 90% reduction) in the field. Periodic measurements of foliar UV‐screening compounds were made on separate groups of seedlings planted at intervals over the growing season during contrasting periods of ambient levels of UV radiation. The levels of UV‐screening compounds correlated with UV‐B levels in both species and with UV‐A in soybean but the sensitivity of the response differed between the two species and among the soybean cultivars. Response differences among species may be related to unique secondary chemistry of each species, so one response estimate or action spectrum may not be appropriate for all species.