Ricardo Vergaz

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.

Intercomparison of Spectroradiometers for Global and Direct Solar Irradiance in the Visible Range

Abstract This paper presents the results of the analysis of the spectral, global, and direct solar irradiance measurements in the visible range (400–700 nm) that were made in the framework of the first Iberian UV–visible (VIS) instruments intercomparison. The instruments used in this spectral range were four spectroradiometers: three Licor 1800s equipped with different receiver optics and one Optronic 754. For the direct solar irradiance measurements the spectroradiometers were equipped with collimators with different fields of view. Parallel studies have been carried out with the data given by the spectroradiometers with their original calibration file and with the same data that is corrected, following in situ calibration of the instruments using a laboratory reference lamp. To compare the series of spectral data the relative values of mean absolute deviation (MAD) and root-mean-square deviation (rmsd) have been used. The results obtained from the measurements of global irradiance show that the Licor 18...

The PHOTONS - AERONET Network Stations in Spain

Abstract The Atmospheric Optics Group of the University of Valladolid, Spain (GOA-UVA) in collaboration with the Instituto Nacional de Tecnica Aeroespacial (INTA) has been measuring direct solar irradiance and sky radiance with a Cimel sun-photometer belonging to PHOTONS-AERONET since February 2000 up to date. The site for measurements “El Arenosillo” is located in the southwest coast of Spain (37.1° N, 6.7° W). Since January 2003 another Cimel sunphotometer is operating in Palencia (41.9°N, 4.5°W), located in a continental area of north-central Spain. The characteristics and protocols for measurements, data transmission and processing of the sunphotometers according to AERONET standards are described. Problems due to calibration were detected and solved by development of a new correction method (KCICLO). The method provides in situ correction for Aerosol Optical Depth (AOD) measurements as well as it explains the AOD fictitious diurnal cycle detected in the data series.

Aerosol Arctic Campaign at ALOMAR (69N, 16E, Norway) in June-July 2002

The GOA Aerosol Arctic Campaign 2002 was carried out during the months of June and July at ALOMAR (Arctic Lidar Observatory for Middle Atmosphere Research). Aerosol measurements at this latitude are scarce, so the aim of this campaign is the characterization of the Arctic aerosols. Two instruments, the Cimel photometer at the AERONET network and the EPP-3000 spectrometer, are used for the measurements. This work is focuses on the Cimel data. These data show a low aerosol optical thickness (AOT), with an average value of 0.135 at 440 nm. The AOT decreases strongly with wavelength, so the /spl Aring/ngstro/spl uml/m parameter /spl alpha/(440,670,870) presents high values, with an average of 1.61 for a log-log fit at these three wavelengths.

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.

Analysis of the atmospheric water vapor content determination in the 940-nm band using moderate spectral resolution measurements of direct solar irradiance

We have determined the vertical integrated water vapor of the atmosphere based on the absorption features of the 940- nm band by means of ground-based measurements of direct solar spectral irradiances and modeled ones. The experimental irradiance data were performed under clear skies with a LIcor 1800 spectroradiometer, based on a monochromator system, of high to moderate spectral resolution in the 300-1100 nm range. The modeled data are based on the monochromatic approaches for atmospheric transmittance constituents, where for water vapor we used the band-model transmittance of LOWTRAN7 code. The method here used is a curve fitting procedure making use of the whole shape band absorption information and the contribution of molecular and aerosol constituents retrieving a unique water vapor value. The method were used to determine water vapor values for the period from March to November of 1995 at a rural station in Valladolid under different atmospheric conditions. The contribution of continuum absorption was also evaluated in the retrieval, obtaining lower values from 13 to 30 percent. This contribution appears as considerable greater than those expected.

Atmospheric correction for hyperspectral DAIS imagery of a forested zone

Airborne imaging spectrometers, like DAIS (Digital Airborne Imaging Spectrometer) sensor, are powerful tools to monitor surface biophysical parameters, but need atmospheric correction to be compared to reference spectra. The aim of this work is the atmospheric correction of the DAIS images acquired by DLR during a flight campaign in the area of Les Gavarres (Girona, NE Spain). Experimental atmospheric measurements of direct and global irradiances together with downward radiance allow us to monitor the atmospheric radiative properties in order to carry out the atmospheric correction. An operational algorithm has been developed to process the DAIS images at low-time cost in the visible and near infrared; range (from 500 to 1100 nm).