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 curvefitting 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 optical depth, particularly in the UV region of the spectrum. All of the calculations are simple enough to be done easily in a spreadsheet.

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.

[1] A new dynamic-statistical parameterization of snow-free land surface albedo is developed using the Moderate Resolution Imaging Spectroradiometer (MODIS) products of broadband black-sky and white-sky reflectance and vegetation and the North American and Global Land Data Assimilation System (LDAS) outputs of soil moisture during [2000][2001][2002][2003]. The dynamic component represents the predictable albedo dependences on solar zenith angle, surface soil moisture, fractional vegetation cover, leaf plus stem area index, and greenness, while the statistical part represents the correction for static effects that are specific to local surface characteristics. All parameters of the dynamic and statistical components are determined by solving nonlinear constrained optimization problems of a physically based conceptual model for the minimization of the bulk variances between simulations and observations. They all depend on direct beam or diffuse radiation and visible or near-infrared band. The dynamic parameters are also functions of land cover category, while the statistical factors are specific to geographic location. The new parameterization realistically represents surface albedo variations, including the mean, shape, and distribution, around each dependent parameter. For composites of all temporal and spatial samples of the same land cover category over North America, correlation coefficients between the dynamic component of the new parameterization and the MODIS data range from 0.39 to 0.88, while relative errors vary within 8-42%. The gross (i.e., integrated over all categories) correlations and errors are 0.57-0.71 and 17-26%, changing with direct beam or diffuse radiation and visible or near-infrared band. The static local correction results in a further reduction in relative errors, producing gross values of 11-21%. The new parameterization is a marked improvement over the existing albedo scheme of the state-of-the-art Common Land Model (CLM), which has correlation coefficients from À0.57 to 0.71 and relative errors of 18-140% for individual land cover categories, and gross values of 0.03-0.32 and 37-71%, respectively.

Abstract. Surface measurements of direct and diffuse voltages at UV wavelengths were made at the T1 site during the MILAGRO (Megacity Initiative: Local and Global Research Observations) field campaign in March 2006, using a multifilter rotating shadowband radiometer (UV-MFRSR). We used the MFRSR data, together with measurements from a co-located CIMEL Sun photometer at the site operating as part of the AERONET network, to deduce aerosol single scattering albedo (ω) at 368 and 332 nm for four cloudfree days during the study. Our retrievals suggest that T1 aerosols with aerosol extinction optical depth τ 368 >0.1 that are influenced by Mexico City emissions, blowing dust, and biomass burning, are characterized by low ω 368 =0.73-0.85 and ω 332 =0.70-0.86, with small or no spectral variation of ω between 368 and 332 nm. Our findings are consistent with other published estimates of ω for Mexico City aerosols, including those that suggest that the absorption attributable to these aerosols is enhanced at UV wavelengths relative to visible wavelengths. We also demonstrate, via sensitivity tests, the importance of accurate τ and surface albedo measurements in ω retrievals at UV wavelengths.

The U.S. Department of Agriculture's Ultraviolet (UV) Radiation Monitoring Program has been measuring UV radiation since 1994. The initial network of 12 stations employed broadband meters to measure UVB irradiance and included ancillary measurements of temperature, humidity, and irradiance at seven wavelengths in the visible produced by a Multi-Filter Rotating Shadowband Radiometer (MFRSR). Since that beginning the network has expanded to more than 20 stations and the broadband meters have been supplemented with a seven-wavelength Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR). The network has been designed to include 30 stations, each with a full complement of instrumentation. Annual characterizations of the network's filter radiometers indicate that gradual shifts in instrument response are manageable but must be accounted for to achieve accurate and precise measurements of UV irradiance. The characterization and calibration of the filter instruments is discussed along with filter stability and instrument precision. Broadband instruments are shown to be quite stable and collocated instruments are shown to agree to within 2.3% for zenith angles less than 80° under all sky conditions. Preliminary investigations into the accuracy of the UV-MFRSR calibrated with the Langley method are presented and successful column ozone retrievals are demonstrated with the UV-MFRSR under clear skies.