The influence of ångström parameters on calculated direct solar spectral irradiances at high turbidity

This work analyzes the influence of ångström parameters on modeled spectral solar irradiance data. The ångström parameters are obtained in the same way as in the Volz method but using continuous record of spectral direct solar irradiance measures in the 400–1000 nm spectral range. Different pairs of α-β values are obtained depending on the two selected wavelengths. The comparison between the calculated irradiance data with these different α-β pairs and experimental data show important differences that may be taken into account in irradiance models.     

Comparison of aerosol optical thickness retrieval from spectroradiometer measurements and from two radiative transfer models

The spectral values of the aerosol optical thickness τ_aλ in the 400–670 nm band have been determined from 500 solar direct irradiance spectra at normal incidence registered at Valencia (Spain) in the period from July 1993 to March 1997. The τ_aλ values obtained from experimental measurements have been compared with the boundary layer aerosol models implemented in the radiative transfer codes ZD-LOA and LOWTRAN 7. For the ZD-LOA code, the continental and maritime models have been considered and for the LOWTRAN 7 code the rural, maritime, urban and tropospheric models have been used. The obtained results show that the aerosol model that best represents the average turbidity of the boundary layer for the urban area of Valencia (Spain) is the continental model when the ZD-LOA code is used and the urban model when the LOWTRAN 7 code is used.     

The parameterisation of the atmospheric aerosol optical depth using the Ångström power law

We have analysed the ability of the Ångström power law to model the spectral aerosol optical depth, τ_aλ, for the 400–670 nm band, obtained from spectral direct irradiance measurements at normal incidence. The spectra were registered at ground level in Valencia, Spain, using a Li-cor 1800 spectroradiometer. The results obtained showed that the fitting method that introduces lower errors in the determination of the Ångström power law coefficients is to adjust directly the spectral experimental data. In this way the errors obtained for the turbidity coefficient, β, were about 0.004 and for the wavelength exponent, α, 0.07. The correlation coefficient was always greater than 0.95. These values of the correlation coefficient could be improved by parameterisation of τ_aλ using an alternative function of the wavelength to the Ångström power law. But this may not be justified for the turbidity values attained in Valencia.     

A preliminary assessment of a detailed two stream short-wave narrow-band model using spectral radiation measurements

A data bank of measurements of global, direct and diffuse solar spectral irradiances at ground level for clear skies has been compiled for Valencia (Spain) dating back to December 1992. The measurements were made with a commercial Li-cor 1800 spectroradiometer with a range of 300–1100 nm and a spectral resolution of 6 nm. A preliminary comparative assessment has been carried out between the experimental data and model data. The chosen model was a detailed narrow-band model (208 spectral intervals from 0.2 to 4 μm) developed at the “Laboratoire d'Optique Atmosphèrique (LOA)” of the University of Lille (France). This plane-parallel multilayer model uses a two-flux method to solve the radiative transfer equation and an exponential sum-fitting procedure to solve the absorption–scattering problem for finite spectral intervals. For this first comparative assessment we focused our attention on the capability of the LOA model to predict irradiance data (direct, global and diffuse) using four values of visibility (40, 23, 14 and 5 km) for two aerosol models (maritime and continental) in the boundary layer. These first results show the low sensitivity of global irradiance to different turbidity conditions. Conversely, the spectral direct and diffuse irradiances were highly influenced by the chosen aerosol model taking into account the visibility values. The spectral distribution of predicted global and direct irradiances are in relatively good agreement with the observed values. The diffuse data show larger discrepancies, which are in part due to the nature of the measurement process itself. However, the observed differences can be partially explained by taking into account the associated errors of the measured data, the elapsed time between the measured spectra and the prediction power of the LOA model.     

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.     

Error source in AOD retrieval from filter radiometer data in the UV due to filter band function

The filter band function of filter radiometers is frequently used in AOD retrieval to improve the accuracy of the Rayleigh and gaseous absorption contributions to the total optical depth. These contributions to the total optical thickness are overestimated when the band-pass filter curve used in the computation exceeds the lower limit of the detector response range (around 320 nm). It can be the case for some typical band-pass filters used in the ultraviolet region (e.g. 340 or 380 nm).This error can involve a strong impact on the aerosol optical depth accuracy, underestimating its value. Errors as large as 0.047 in the evaluation of ozone optical depth at 340 nm, and 0.009 in the Rayleigh optical depth were found, leading to final errors of 50–100% in the AOD for remote locations, like Polar regions or high mountains.To avoid this significant error, the detector spectral response must be taken into account in the computations. Further, it is recommended to discard the filter band-pass function when the transmittance falls below 1% of its maximum value at the central wavelength.