Ángel M. de Frutos

Modified calibration procedures for a Yankee Environmental System UVB-1 biometer based on spectral measurements with a brewer spectrophotometer.

Abstract The calibration of the erythemal irradiance measured by a Yankee Environmental System (YES) UVB-1 biometer is presented using two methods of calibration with a wide range of experimental solar zenith angles (SZAs) and ozone values. The calibration is performed through simultaneous spectral measurements by a calibrated double-monochromator Brewer MK-III spectrophotometer at “El Arenosillo” station, located in southwestern Spain. Because the range of spectral measurements of the Brewer spectrophotometer is 290–363 nm, a previously validated radiative transfer model was used to account for the erythemal contribution between 363 and 400 nm. Both methods are recommended by the World Meteorological Organization and we present and discuss here a wide range of results and features given by modified procedures applied to these two general methods. As is well established, the calibration factor for this type of radiometric system is dependent on atmospheric conditions, the most important of which are the ozone content and the SZA. Although the first method is insensitive to these two factors, we analyze this behavior in terms of the range used for the SZA and the use of two different mathematical approaches for its determination. The second method shows the dependence on SZA and ozone content and, thus, a polynomial as a function of SZA or a matrix including SZA and ozone content were determined as general calibration factors for the UV radiometric system. We must note that the angular responses of the YES radiometer and Brewer spectroradiometer have not been considered, because of the difficulty in correcting them. The results show in detail the advantages and drawbacks (and the corresponding associated error) given by the different approaches used for the determination of these calibration coefficients.

Columnar physical and radiative properties of atmospheric aerosols in north central Spain

Direct solar irradiance spectra under clear skies have been measured in a rural station in the region of Castilla y Leon in north central Spain from March to November 1995 to determine the physical and radiative characteristics of atmospheric aerosols. About 300 spectra were measured with a LI-COR Model LI-1800 spectroradiometer, with a spectral resolution (full width at half maximum) of 6 nm from 300 nm to 1100 nm. This moderately high spectral resolution permitted the retrieval of the spectral aerosol optical depth (AOD) using a method based on spectral windows in nonabsorbing regions and modelling by the Angstrom formula in the spectral range 370-1000 nm. These modelled Angstrom AOD spectra were used to determine the columnar particle size distribution of atmospheric aerosols using Mie theory and a pseudoinversion method. We assumed a monomodal lognormal function with a fixed standard deviation, σ = 2.5, and two particle refractive indices (absorbing and nonabsorbing aerosols) based on climatological characteristics of the continental area of study. Physical parameters, such as the effective radius, the total vertical volume, and mass loading, have been derived. The effective radius ranged from 0.015 to 1 μm, and the values of the vertical aerosol volume, from 0.01 to 0.3 μm. Aerosol mass loading varied from 0.03 to 0.53 g m -2 . Radiative properties represented by the asymmetry parameter g, the single-scattering albedo ω o , and the phase function P were also evaluated for each of the retrieved particle size distributions as a function of wavelength. Because the radiative parameters show low wavelength dependence from 300 to 1000nm, we present their behavior at 500 nm. The parameter g varies from 0.45 to 0.75 (the average value is 0.6), and ω o varies from 0.75 to 0.94. Relationships of the effective radius and the asymmetry parameter with the Angstrom turbidity parameter α have been found, which is of interest for the parametrization of aerosol properties. Finally, the phase function at 500nm, 120° (where it has a minimum) has also been evaluated and correlated with the α parameter, thus providing information about the size of aerosol particles. The uncertainties of all of these parameters are also estimated and discussed.

UV Index Experimental Values During the Years 2000 and 2001 from the Spanish Broadband UV-B Radiometric Network¶

Abstract An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson–Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes.

Determination of the atmospheric-water-vapor content in the 940-nm absorption band by use of moderate spectral-resolution measurements of direct solar irradiance

We have analyzed three methods that can be used to determine the integrated water vapor of the atmosphere in the 940-nm band by means of modeled and measured direct solar spectral irradiance. The experimental irradiance data were obtained with a commercial LI-COR 1800 spectroradiometer, based on a monochromator system, of high to moderate spectral resolution (6 nm) in the 300-1100-nm range. The modeled data are based on monochromatic approaches to determine atmospheric transmittance constituents; for those of water vapor we used the lowtran7 model. The first method is a curve-fitting procedure that makes use of the entire shape band absorption information to retrieve a unique water-vapor value. The second method makes use of the monochromatic approach of the absorption transmittance formula to determine the amount of water vapor at each wavelength of the absorption band, and the third method is the classic differential absorption technique suitably applied to our data. Spectral analysis showed the advantages and disadvantages of each method, such as problems linked to the various spectral resolutions of the experimental and the modeled data, the width of the spectral range used to define the water-vapor absorption band, and the dependence of the retrieval on the choice of the two selected wavelengths in the last-named technique. All these problems were considered so they could be avoided or minimized and the associated errors estimated. We used the methods to determine water-vapor values for the period from March to November 1995 at a rural station in Vallodolid, Spain, allowing for the evaluation of the differences in real monitoring conditions. Finally, the contribution of continuum absorption was also evaluated, yielding lower water-vapor values between 13 and 30%. These differences were considerably greater than those that were due to the problems that we have just enumerated.

Retrieval of atmospheric aerosol characteristics from visible extinction data at valladolid (spain)

Abstract Ground-based visible spectral extinction data and modelling of atmospheric aerosols has been carried out to retrieve the columnar size distribution function of aerosol particles. By a minimization method the Junge aerosol size distribution function is obtained and compared with that retrieved by an inversion procedure. The Angstrom α and β turbidity coefficients have also been determined. A relationship between the two turbidity parameters has been found for the different atmospheric conditions. As the β empirical parameter is related to the concentration of aerosol particles, correlations between it and the derived mass loading M of aerosol particles, which depend on the aerosol size distribution, have been established. Results at different atmospheric turbidity conditions are presented which recover the measured aerosol optical thickness with adequate accuracy.

Design of aberration-balanced high-efficiency focusing holographic gratings

A volume and phase focusing holographic grating to be used as the principal element of a high-resolution spectrometer has been designed with the aid of a computer. A two-step method was used; first designs were obtained with an aberration-balancing technique, and, in the second step, these early designs are optimized by a ray tracing procedure to obtain aberration-balanced high-efficiency holograms that can be used in highresolution spectroscopy.

Comparison between measurements and model simulations of solar radiation at a high altitude site: Case studies for the Izana BSRN station

In this work we have carried out a comparative study of shortwave downward radiation (SDR) measurements and simulations obtained with the radiative transfer model (RTM) LibRadtran in order to be used as a model quality control at Izana BSRN (Baseline Surface Radiation Network). We selected cases corresponding to the most common atmospheric conditions at the Izana station (IZA), we analyzed: clear-sky and African dust intrusions. The input parameters for the model, such as a total ozone column, surface albedo, water vapour column and aerosols parameters, were measured at IZA. The results of the two clear-sky case studies (16 May and 26 October 2010) analyzed here show an underestimation of the simulations for the SDR global and direct for solar zenith angles (SZA) lower than 70°. The maximum relative differences for SDR global, direct and diffuse are 6%, 2% and 6%, respectively. For dust intrusion case studies (22 July 2009 and 10 July 2010) and for SZA lower than 70°, the simulations of the direct normal ...

Aerosol scattering optical properties by nephelometer measurements at the El Arenosillo site (SW coastal area of Spain)

Aerosol light scattering coefficients, the hemispheric σsp and the back-scattering coefficient σbsc, have been measured using a 3-wavelengths integrating nephelometer over two years (January 2006 to May 2008) at the monitoring station ESAT-El Arenosillo. This station is located in the coastal area of the province of Huelva, in the southwest of the Iberian, Peninsula. The Angstrom exponent α, has been also derived from the spectral dependence of σsp. All these parameters have been carefully analyzed to investigate their general characteristics and features, and diurnal variability. A general statistic gives mean values and std of σsp = 48.5 ±38.1 Mm−1 with a large range of variation showing moderate values of this rural and coastal site with marine prevalence but with significant influence of local sources of pollution. The daily cycle of σsp and α presents different behaviour depending on the season and is modulated by sea-land breeze regime.

Observation and characterization of aerosols above ALOMAR (69 degrees N) by tropospheric lidar, sun-photometer, and VHF radar

At ALOMAR (Arctic Lidar Observatory of Middle Atmosphere Research, 69°N, 16°E) an exemplary co-location of tropospheric Lidar, sun-photometer and VHF Radar is used for aerosol investigations. Recently the University of Oslo, the Norwegian Institute for Air Research and the Andøya Rocket Range started to operate a new troposphere Lidar system. The system uses two elastic backscatter channels (1064nm, 355 nm), two polarization channels (532p nm and 532s nm) and a Raman channel (387nm). The co-located sun photometer is of Cimel type and the VHF Radar is operating at 53 MHz. The data from the Cimel instrument are collected in cooperation with a group from Valladolid (Spain) and the Radar is operated by the Institute for Atmospheric Physics from Kühlungsborn. The location of ALOMAR, north of the Arctic Circle and on an island, a few hundred meters from seashore and about 30 km off the continent, makes it ideal for investigations related to Sub-Arctic aerosols. The present paper presents the first results from simultaneous and collocated tropospheric measurements. We compare aerosol stratification derived from Lidar data with simultaneous measurements of total aerosol content, derived from Cimel data in dependence of simultaneous winds, stratified layers and echo power from radar data. Diurnal cycles for both summertime and wintertime are shown.

Research areas of the Optics Department at University of Valladolid

In this article we present the different groups of investigation which work at the Department of Optics and Applied Physics of the University of Valladolid (Spain). The areas covered by these groups are the following: (1) optical diagnostic techniques in plasmas, (2) calculation of spectral line shapes, (3) atmospheric optics, (4) radiometry and photometry, and (5) history of science and techniques.