Showing papers by "Lucas Alados-Arboledas published in 2008"

Development of a sky imager for cloud cover assessment

Abstract: Based on a CCD camera, we have developed an in-house sky imager system for the purpose of cloud cover estimation and characterization. The system captures a multispectral image every 5 min, and the analysis is done with a method based on an optimized neural network classification procedure and a genetic algorithm. The method discriminates between clear sky and two cloud classes: opaque and thin clouds. It also divides the image into sectors and finds the percentage of clouds in those different regions. We have validated the classification algorithm on two levels: image level, using the cloud observations included in the METAR register performed at the closest meteorological station, and pixel level, determining whether the final classification is correct.

Retrieval of the optical depth using an all-sky CCD camera.

Abstract: A new method is presented for retrieval of the aerosol and cloud optical depth using a CCD camera equipped with a fish-eye lens (all-sky imager system). In a first step, the proposed method retrieves the spectral radiance from sky images acquired by the all-sky imager system using a linear pseudoinverse algorithm. Then, the aerosol or cloud optical depth at 500 nm is obtained as that which minimizes the residuals between the zenith spectral radiance retrieved from the sky images and that estimated by the radiative transfer code. The method is tested under extreme situations including the presence of nonspherical aerosol particles. The comparison of optical depths derived from the all-sky imager with those retrieved with a sunphotometer operated side by side shows differences similar to the nominal error claimed in the aerosol optical depth retrievals from sunphotometer networks.

Aerosol optical properties assessed by an inversion method using the solar principal plane for non-spherical particles

Abstract: Adequate modeling of light scattering by non-spherical particles is one of the major difficulties in remote sensing of atmospheric aerosols, mainly in desert dust outbreaks. In this paper we test a parameterization of the particle shape in size distribution, single-scattering albedo, phase function and asymmetry parameter retrieval from beam and sky-radiance measurements, based on the model Skyrad.pack, taking into account the principal plane measurements configuration. The method is applied under different Saharan dust outbreaks. We compare the results with those obtained by the almucantar measurements configuration. The results obtained by both methodologies agree and make possible to extend the parameter retrieval to smaller zenith angles than that used in the retrieval from almucantar geometries.

Consequences of Uncertainties in CO2 Density for Estimating Net Ecosystem CO2 Exchange by Open-path Eddy Covariance

Abstract: Errors in the estimation of CO2 surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol Soc 106:85–100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65–85, (2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements of absolute CO2 density (ρc). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance system operating in the Sierra de Gador in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to reliably report ρc and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in ρc in a semi-arid climate with large sensible heat fluxes (Hs) and (wet) mineral deposition. When ρc is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO2 uptake.

Altitude effect in UV radiation during the Evaluation of the Effects of Elevation and Aerosols on the Ultraviolet Radiation 2002 (VELETA‐2002) field campaign

Abstract: [1] The Evaluation of the Effects of Elevation and Aerosols on the Ultraviolet Radiation 2002 (VELETA-2002) field campaign was designed to study the influence of aerosols and altitude on solar UV irradiance. The altitude effect (AE) was evaluated for UV irradiance under cloudless conditions by taking spectral and broadband measurements in SE Spain in the summer of 2002 at three nearby sites located at different heights (680 m, 2200 m, and 3398 m). A spectral radiative transfer model (Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART)) was also applied, mainly to evaluate the tropospheric ozone impact on AE. Results are related to the optical properties and air mass origin of the aerosols as determined by back-trajectory analysis. During the 1-week observing period of the campaign, there were two main synoptic situations with different air masses (polar maritime and tropical continental air mass associated with a Saharan dust event). The AE showed a high dependency on wavelength, solar zenith angle, and aerosols, although the growth of the mixing layer during the day also caused substantial AE variability. Saharan dust caused an increase in AE, especially in the UVB region and in the erythemal irradiance. In the UVA (320–400 nm) band the AE ranged 6–8% km−1 at noon, while for the UVB (280–320 nm) band it reached 7–11% km−1. The AE for erythemally weighted irradiance ranged from 11 to 14% km−1 between the lowest and highest stations when it was calculated from spectral measurements.

Application of Sun/star photometry to derive the aerosol optical depth

Abstract: Atmospheric aerosols play a crucial role in the radiative transfer and chemical processes that control the Earth's climate. Aerosol optical depth and other related aerosol characteristics are widely known during daytime through Sun photometers, and so several international networks have been established. However, there are no regular measurements of the spectral optical depth of the atmosphere at night. Such measurements are now possible thanks to the recent CCD camera developments that make feasible the measurements of star radiative fluxes. In this work we present a star photometer based on a CCD, carrying out an exhaustive characterization of the device in order to guarantee the quality of the measurements. We have analysed the applicability of the Langley calibration method in an urban polluted environment. Finally, we present some preliminary results on aerosol optical depth measured by Sun and star photometers under a Saharan dust event.

Using a trichromatic CCD camera for spectral skylight estimation

Abstract: In a previous work [J. Opt. Soc. Am. A 24, 942-956 (2007)] we showed how to design an optimum multispectral system aimed at spectral recovery of skylight. Since high-resolution multispectral images of skylight could be interesting for many scientific disciplines, here we also propose a nonoptimum but much cheaper and faster approach to achieve this goal by using a trichromatic RGB charge-coupled device (CCD) digital camera. The camera is attached to a fish-eye lens, hence permitting us to obtain a spectrum of every point of the skydome corresponding to each pixel of the image. In this work we show how to apply multispectral techniques to the sensors' responses of a common trichromatic camera in order to obtain skylight spectra from them. This spectral information is accurate enough to estimate experimental values of some climate parameters or to be used in algorithms for automatic cloud detection, among many other possible scientific applications.

Characterization of a long range transport pollution episode affecting PM in SW Spain

Abstract: A multidisciplinary study on aerosol characterization was performed at the regional background monitoring station of El Arenosillo, in SW Spain, between 28 June and 5 July 2006. The main aim of the Arenosillo aerosol measurement campaign 2006 was to compare the results of aerosol characterization obtained by different groups by measuring physical and chemical parameters using optical methods and in situ sampling. The campaign coincided with a long-range transport episode from Western Iberia, passing through the Gulf of Cadiz and the Straits of Gibraltar towards the study area. The results of the variability of PM levels and chemical composition of PM10, PM2.5 and PM1 at El Arenosillo and at three nearby regional and urban background sites were interpreted. Mean levels measured during the campaign reached 23, 15 and 12 µg m−3 for PM10, PM2.5 and PM1, respectively, at El Arenosillo. PM during the Arenosillo campaign 2006 was dominated by the secondary inorganic aerosols (SIA, 24, 38 and 39% of PM10, PM2.5 and PM1 mass), carbonaceous aerosols (17, 21 and 23% of the mass), crustal material (13, 9 and 4%), and sea spray (10, 5 and 1%). These values are within the usual range of regional background sites of Southern Spain with the exception of the relatively low crustal load and the high SIA levels. Two major PM episodes were differentiated. The first one was characterized by high levels of Bi, As, Pb, Se, P and Zn, which are the main tracers of the industrial emissions near the town of Huelva. High concentrations of these elements were also recorded at the nearby sites. In the second episode, maximum levels of SO42−, V and coarse Cu as well as the bulk mass of PM1 were determined, tracing the polluted air mass transport from Western Iberia through the Straits of Gibraltar. These results underline the importance of the influence of long-range transport of pollutants on the levels and composition of regional background PM in SW Iberia, where local emissions may also play a role.

Correction factors for a total scatter/backscatter nephelometer

Abstract: Total aerosol scattering and backscattering atmospheric values are typically obtained with an integrating nephelometer. Due to design limitations, measurements do not cover the full (0°–180°) angular range, and correction factors are necessary. The effect of angle cutoff is examined for a number of particle size distributions and refractive indices. Scattering data for sub-micron particles can be corrected by the use of a modified Anderson approximation, while data for larger particle distributions can be approximated by a function of the effective size parameter. Correction factors for the hemispheric backscatter ratio are found to be small if nonsphericity is assumed. Such approximations will help more accurate corrections for angle range, particularly at large size parameter values.

One year of water vapour Raman Lidar measurements at the Andalusian Centre for Environmental Studies (CEAMA)

Abstract: A Raman Lidar system based on a pulsed Nd: YAG laser has been employed for regular monitoring of the columnar water vapour mixing ratio at the Andalusian Centre for Environmental Studies (CEAMA) (37.16° N, 3.58° W, 680 m a.s.l.), at Granada (southeast of the Iberian Peninsula). This methodology is employed continuously for the first time on the Iberian Peninsula. The Raman Lidar water vapour channel has been calibrated by comparison with simultaneous co-located radio-sounding during a field campaign at El Arenosillo, Huelva, Spain (37.11° N, 6.73° W, 0 m a.s.l.) at the end of June 2006. Since September 2005, systematic water vapour Lidar measurements have been performed. Water vapour Raman Lidar measurements taken at night time over 1 year (from September 2005 to August 2006) have been studied and compared in terms of precipitable water with respect to real radio-sounding data from the two closest radio-sounding stations. In addition, the seasonal variability of the water vapour mixing ratio has been analysed.

The European Aerosol Research Lidar Network (EARLINET): An Overview

Abstract: The European Aerosol Research LIdar NETwork (EARLINET) is the first aerosol lidar network on a continental scale with the main goal to provide a comprehensive, quantitative, and statistically significant database for the aerosol distribution over Europe. Next, we present EARLINET along with the main network activities.