Measurements Of Atmospheric Extinction Research Articles

High-resolution spectral atmospheric attenuation measurement for solar power plants

One of the main challenges in solar tower renewable technologies is measurement of solar radiation attenuation at the plants at surface level. This paper describes an improved version of the optical spectrum analysis method for measuring solar radiation attenuation in real time at solar tower plants, as presented in a previous work. The new hardware design and calibration process are explained in detail. This new system has a resolution of 0.25 nm in the VIS range and a precision of 0.5% in real time, improving over the state of the art for the measurement of spectral atmospheric attenuation. These specifications allow the assessment of the contribution of different attenuation factors at surface level, such as absorbance peaks (Na, H, H2O) or scattering and the search for correlation with different weather conditions. They also enable experimental validation of atmospheric extinction simulation methods and their parameters. This work also includes the results of a 6 month-long campaign of solar weighted atmospheric extinction measurements at Atlantica's operating central receiver solar plant PS10 at Sanlúcar la Mayor (Seville, Spain). Fluctuations in the daily averaged transmittance greater than 10% are observed, which shows the importance of monitoring this parameter during the operation of solar plants.

An automated all-sky atmospheric monitoring camera for a next-generation ultrahigh-energy cosmic-ray observatory

We have developed a fully-automated instrument (FASCam—Fast All Sky Camera) designed to capture night all-sky images and analyse cloudiness and coarse atmospheric properties for a proposed next-generation ultrahigh energy cosmic ray observatory, called the Fluorescence detector Array of Single-pixel Telescopes (FAST). FASCam has a 360 × 90̂ (azimuth and elevation) field of view and is equipped with a series of photometric Johnson-Cousins filters (BVR) and an Ultra-violet (UV) filter. The camera is installed on the top of one of the three FAST prototype telescope buildings located in central Utah, U.S.A. . An online analysis of the images provides real time cloud coverage maps and coarse atmospheric extinction measurements and a night sky background estimate in the field of view of the FAST fluorescence telescopes. The result serves as an input parameter for the operation of the telescopes and avoids unnecessary data taking in the case of clouds covering the field of view of the telescopes, high atmospheric extinction, or high night sky background.

Modeling beam attenuation in solar tower plants using common DNI measurements

Solar radiation reflected by concentrating mirrors is attenuated due to atmospheric extinction as it travels to the receiver of a solar tower plant. The lack of information on the magnitude of extinction increases the uncertainties in yield analysis and tower plant design. In-situ measurements of atmospheric extinction as well as measurement correction methods have been recently performed and developed (Hanrieder et al., 2012, 2015), but specific information is unavailable for individual plant projects. It is well known though that the extinction varies significantly with site and time.To overcome this absence of information a model to derive the attenuation loss between heliostat and receiver from common direct normal irradiance (DNI) measurements was developed by Sengupta and Wagner (2011) (SW2011 model). We present an updated version of that model and a comparison between the performance of the models using extinction measurements.In the new approach presented here, different precipitable water vapor (PWV) amounts are considered and the model is adjusted to the elevation of the investigated site.The strongest assumption in this approach is the assumption about the aerosol extinction height profile. Three different height profiles are tested for the Plataforma Solar de Almería (PSA) resulting in three different new transmittance models.The SW2011 as well as the three new models are evaluated with one year of corrected extinction data derived with the ABC (absorption and broadband correction) method of Hanrieder et al. (2015) and a Vaisala FS11 scatterometer at PSA. The new models show a mean difference to the reference data set of 0.01, 0.05 and 0.03 and a root mean square error (RMSE) of 0.052, 0.056 and 0.049 (compared to a mean bias of −0.08 and RMSE of 0.095 for the SW2011 model for transmittances through a 1km slant range). These results indicate the importance of adequate assumptions for the aerosol height profile. Testing the developed TM with the LIVAS height profile (Amiridis et al., 2015) for PSA shows satisfying results and this motivates testing the approach for other sites. By applying an additional correction for the Linke turbidity (TL) derived as in Ineichen and Perez (2002) the mean bias can be further lowered.An uncertainty analysis shows that the absolute uncertainty coincide with the RMSE levels of the evaluation. Performing the additional TL correction promises an improvement of the overall performance of the model.The new models outperform the SW2011 model due to the PWV and elevation adjustments. The approach can be applied for different sites and incorporated in already existing ray-tracing or plant optimization tools. It is expected to be valuable for reducing uncertainty in power tower design and operations.

Saharan mineral dust outbreaks observed over the North Atlantic island of La Palma in summertime between 1984 and 2012

AbstractWe estimate the frequency of Saharan mineral dust outbreak events observed over the North Atlantic island of La Palma based on in situ nightly atmospheric extinction measurements recorded almost continuously since 1984 by the Carlsberg Meridian Telescope at the Roque de los Muchachos observatory. The outbreak frequency shows a well‐defined seasonal peak in the months of July to September, during which time the occurrence of Saharan dust events (SDEs) is approximately 28 ± 6%. We find considerable year‐to‐year variability in the summertime SDE frequency, observing a steady reduction between 1984 and 1997, followed by a period of relative mean stability from 1999 to 2012. We investigated changes in the atmospheric extinction of the SDEs as an indicator of strength of the episodes and found that this parameter approximately follows the SDE frequency, however, instrumental limitations prevented us from deriving precise conclusions regarding their long‐term changes. A lagged correlation analysis between SDE properties and the El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO) and Sahel rainfall index (SRI) was performed. We found that 55 ± 4% of the year‐to‐year variations in July–September SDE frequency may be reproduced by a lagged relationship to the NAO conditions during the preceding October–December period, and 45 ± 4% may be reproduced by a negative correlation to the SRI during the preceding February–April period. Based on these relationships it may be possible to obtain an approximate indication of the strength of the upcoming summertime dust season over the North Atlantic around half a year in advance.

New techniques for laser beam atmospheric extinction measurements from manned and unmanned aerospace vehicles

Abstract Novel techniques for laser beam atmospheric extinction measurements, suitable for several air and space platform applications, are presented in this paper. Extinction measurements are essential to support the engineering development and the operational employment of a variety of aerospace electro-optical sensor systems, allowing calculation of the range performance attainable with such systems in current and likely future applications. Such applications include ranging, weaponry, Earth remote sensing and possible planetary exploration missions performed by satellites and unmanned flight vehicles. Unlike traditional LIDAR methods, the proposed techniques are based on measurements of the laser energy (intensity and spatial distribution) incident on target surfaces of known geometric and reflective characteristics, by means of infrared detectors and/or infrared cameras calibrated for radiance. Various laser sources can be employed with wavelengths from the visible to the far infrared portions of the spectrum, allowing for data correlation and extended sensitivity. Errors affecting measurements performed using the proposed methods are discussed in the paper and algorithms are proposed that allow a direct determination of the atmospheric transmittance and spatial characteristics of the laser spot. These algorithms take into account a variety of linear and non-linear propagation effects. Finally, results are presented relative to some experimental activities performed to validate the proposed techniques. Particularly, data are presented relative to both ground and flight trials performed with laser systems operating in the near infrared (NIR) at λ= 1064 nm and λ= 1550 nm. This includes ground tests performed with 10 Hz and 20 KHz PRF NIR laser systems in a large variety of atmospheric conditions, and flight trials performed with a 10 Hz airborne NIR laser system installed on a TORNADO aircraft, flying up to altitudes of 22,000 ft.

Novel atmospheric extinction measurement techniques for aerospace laser system applications

Novel techniques for laser beam atmospheric extinction measurements, suitable for manned and unmanned aerospace vehicle applications, are presented in this paper. Extinction measurements are essential to support the engineering development and the operational employment of a variety of aerospace electro-optical sensor systems, allowing calculation of the range performance attainable with such systems in current and likely future applications. Such applications include ranging, weaponry, Earth remote sensing and possible planetary exploration missions performed by satellites and unmanned flight vehicles. Unlike traditional LIDAR methods, the proposed techniques are based on measurements of the laser energy (intensity and spatial distribution) incident on target surfaces of known geometric and reflective characteristics, by means of infrared detectors and/or infrared cameras calibrated for radiance. Various laser sources can be employed with wavelengths from the visible to the far infrared portions of the spectrum, allowing for data correlation and extended sensitivity. Errors affecting measurements performed using the proposed methods are discussed in the paper and algorithms are proposed that allow a direct determination of the atmospheric transmittance and spatial characteristics of the laser spot. These algorithms take into account a variety of linear and non-linear propagation effects. Finally, results are presented relative to some experimental activities performed to validate the proposed techniques. Particularly, data are presented relative to both ground and flight trials performed with laser systems operating in the near infrared (NIR) at λ=1064nm and λ=1550nm. This includes ground tests performed with 10Hz and 20kHz PRF NIR laser systems in a large variety of atmospheric conditions, and flight trials performed with a 10Hz airborne NIR laser system installed on a TORNADO aircraft, flying up to altitudes of 22,000ft.

SATELLITE-MOUNTED LIGHT SOURCES AS PHOTOMETRIC CALIBRATION STANDARDS FOR GROUND-BASED TELESCOPES

A significant and growing portion of systematic error on a number of fundamental parameters in astrophysics and cosmology is due to uncertainties from absolute photometric and flux standards. A path toward achieving major reduction in such uncertainties may be provided by satellite-mounted light sources, resulting in improvement in the ability to precisely characterize atmospheric extinction, and thus helping to usher in the coming generation of precision results in astronomy. Using a campaign of observations of the 532 nm pulsed laser aboard the CALIPSO satellite, collected using a portable network of cameras and photodiodes, we obtain initial measurements of atmospheric extinction, which can apparently be greatly improved by further data of this type. For a future satellite-mounted precision light source, a high-altitude balloon platform under development (together with colleagues) can provide testing as well as observational data for calibration of atmospheric uncertainties.

Inferring hydroxyl layer peak heights from ground-based measurements of OH(6-2) band integrated emission rate at Longyearbyen (78° N, 16° E)

Abstract. Measurements of hydroxyl nightglow emissions over Longyearbyen (78° N, 16° E) recorded simultaneously by the SABER instrument onboard the TIMED satellite and a ground-based Ebert-Fastie spectrometer have been used to derive an empirical formula for the height of the OH layer as a function of the integrated emission rate (IER). Altitude profiles of the OH volume emission rate (VER) derived from SABER observations over a period of more than six years provided a relation between the height of the OH layer peak and the integrated emission rate following the procedure described by Liu and Shepherd (2006). An extended period of overlap of SABER and ground-based spectrometer measurements of OH(6-2) IER during the 2003–2004 winter season allowed us to express ground-based IER values in terms of their satellite equivalents. The combination of these two formulae provided a method for inferring an altitude of the OH emission layer over Longyearbyen from ground-based measurements alone. Such a method is required when SABER is in a southward looking yaw cycle. In the SABER data for the period 2002–2008, the peak altitude of the OH layer ranged from a minimum near 76 km to a maximum near 90 km. The uncertainty in the inferred altitude of the peak emission, which includes a contribution for atmospheric extinction, was estimated to be ±2.7 km and is comparable with the ±2.6 km value quoted for the nominal altitude (87 km) of the OH layer. Longer periods of overlap of satellite and ground-based measurements together with simultaneous on-site measurements of atmospheric extinction could reduce the uncertainty to approximately 2 km.

Tungsten source integrated cavity output spectroscopy for the determination of ambient atmospheric extinction coefficient

Broadband integrated cavity output spectroscopy (ICOS) utilizing an incoherent tungsten lamp as a spectroscopic source is described. This novel approach has been termed W-ICOS. The technique has been applied to make quantitative measurements of Rayleigh scattering by carbon dioxide between 570 and 590 nm and to make measurements of aerosol and atmospheric extinction. Minimum detectable extinction coefficients (kext) made in a 94 cm optical cavity ranged between 3.4 and 35 Mm(-1) depending on the level of signal averaging employed. The level of sensitivity achieved should allow measurements on static gas samples and regular, quantitative measurements of the atmospheric extinction coefficient.

Measurements of Atmospheric Extinction at a Ground Level Observatory

In order to determine the atmospheric extinction at Physics Department Astronomical Observatory of the University of Extremadura, located at Badajoz (Spain), several stars were observed during some clear nights of atmospheric stability in the period 1998–2000, at optical wavelengths corresponding to the filters of the Kron-Cousins and Stromgren photometric systems. The determination of the extinction coefficients was made by assuming the Bouguer's law, which was shown to be a good approximation for this study. The results exhibited temporal variations and can be considered to be associated with clean atmospheres at locations of low altitude.

A meteorological and photometric study of the Oukaimeden site

In this paper, we present meteorological and photometric studies of the Oukaimeden astronomical site in Morocco. The meteorological measurements were obtained over 537 days. The photometric data were measured by both FIP and GONG instruments over 1182 days. We have also analysed the weather parameters at the Observatorio del Roque de los Muchachos (ORM) in the Canary Islands. More than ten years of measurements with the Carlsberg Automatic Meridian Circle (CAMC) are included, corresponding to 2931 nights of atmospheric extinction measurements. These different data sets demonstrate photometric sky quality, wind speed and direction, local relative humidity, temperature and other parameters, such as the barometric pressure at the ORM and the precipitation at Oukaimeden. We have separated the day- and night-time data to provide information relevant to solar and stellar observations. Data from the Oukaimeden site have been compared to the ORM site, recognized as one of the best astronomical sites in the world and the results from our data confirm that Oukaimeden constitutes an excellent potential site for astronomical observations.

Aerosol influence on the sea‐viewing wide‐field‐of‐view sensor bands: Extinction measurements in a marine summer atmosphere over the Baltic Sea

Measurements of atmospheric extinction at sea‐viewing wide‐field‐of‐view sensor wavelengths were carried out during the Lagrangian Experiment on the Baltic Sea within the Baltic Sea System Study project, July 2–15, 1997. The empirical orthogonal functions method was applied to the aerosol optical thickness spectra, allowing three air mass types, containing different aerosol types, to be distinguished: continental, continental‐maritime, and maritime, corresponding to different residence times of air masses over the Baltic Sea. The amplitude of the aerosol optical thickness, describing the temporal variability, was found to be strongly affected by the atmospheric stability. The contribution of the aerosol radiance to the atmospheric radiance depends directly on the aerosol type, the wavelength λ, and the solar zenith angle θs. The most significant contribution of more than 30% was detected for the continental and continental‐maritime aerosol types, for θs = 30°, and at λ = 865 nm.

Multiwavelength sun-photometers for accurate measurements of atmospheric extinction in the visible and near-IR spectral range

A new multiwavelength sun-photometer has been developed to obtain accurate measurements of the atmospheric attenuation of direct solar radiation at seven window wavelengths in the visible and near-infrared spectral range. The instrument's optical features and performance are described. The photometer was calibrated by following a careful procedure based on the Langley method. An additional eighth bandpass filter was used to obtain measurements of the precipitable water. Five examples of this instrument have been constructed and calibrated for a network of mountain stations in Southern Italy to monitor Sahara dust transport episodes.

Laser transmissometer --a description

A laser transmissometer is described that is suitable for long path atmospheric extinction measurements. The instrument design incorporates several unique features which greatly enhance the signal to noise ratio of the system. These are apodization of the output beam by conical scan, servo controlled beam aiming and automatic beam width measurement at regular intervals. The system is completely automated and utilizes a micro-computer designed by the DRI for data acquistion and servo control.

Optical, chemical and physical properties of aerosols over the antarctic ice sheet

Multiwavelength optical measurements of atmospheric extinction and sky brightness in the solar aureole were made at Ross Island and at South Pole Station, Antarctica, to derive the column-integrated size distribution and mass loading of suspended aerosol. In addition, the composition and particle size distribution near the surface at South Pole, the Ross Ice Shelf and from the summit of Mt Erebus volcano were derived by electron microscopic analysis and X-ray spectrometry. The aerosol over the ice sheet has a bimodal size distribution with modal radii in dn/dr at < 0.05 μm and at 0.14 ± 0.02 μm. The number concentration and mass loading for the large mode are 40cm −3 and 300 ng m −3, respectively, while the number concentration of particles in the smaller mode is ~ 10 3 cm −3. The particles over the polar ice sheet are dominated by ammonium sulfate and hydrated droplets of sulfuric acid (60–80% by mass) although trace amounts of oceanic and crustal-derived material have also been detected. Aerosols on the Ross Ice Shelf are relatively large and consist mainly of crustal material, presumably from the exposed land within 20 km distance. Submicron particles collected at the fuming vent of Mt Erebus volcano contained 34% Cl, 22% K and only 18%S. The sulfur aerosol residing over the ice cap shows evidence of being produced by nucleation of trace sulfurbearing gases; the most likely source is organic compounds, possibly CS 2 and COS, released by the oceans. These compounds have long lifetimes (3 and 20, respectively), hence they can be transported to the central polar ice cap and converted to sulfates. It is estimated that downwelling air from the stratosphere is probably not an important source for sulfate particles. The rate of removal of particles to the snow and ice surface of the continentis −300 × 10 −16g cm −2s −1; ninety percent of the removal is by nucleation of snow flakes and ice crystals.

Atmospheric extinction measurements of a He–Ne laser

The extinction of a He-Ne laser in a 300-m long urban path has been measured and correlated to temperature and relative humidity variations. The results are contrasted with the scattering coefficient theoretically calculated for a model of aerosols. The agreement seems to indicate that the values of the extinction are strongly correlated with relative humidity in inhomogeneous urban paths.

Measurements of Atmospheric Extinction in the Spectral Region 360–800 nm by Spectroradiometry

Abstract Measurements of atmospheric extinction in the spectral region 330–803 nm have been made by measuring the apparent radiance of a hole in a black box and relating this to the radiance of the horizon sky. The spectroradiometer used for this was bat at our institute. Good correlation was achieved between the extinction coefficient at 500 nm measured in this way and the scattering coefficient wavelength measured with a nephelometer. The wavelength dependence of the extinction was in most cases in agreement with Angstrom's power law with the extinction coefficient bext ∝ λ−α. Typical values of α were in the interval 0.75 ≤ α ≤ 1.5. The influence of the relative humidity on α and on the extinction has been studied.

Atmospheric extinction measurements by photoelectric star photometry

Extinction coefficients observed in three colours during 102 nights on Rockdale Mountain (Republic of South Africa) are discussed. Instrumental errors are statistically separated from other variations. In 85 per cent of the nights dust extinction was smaller than 0 m·03 in yellow light. Seasonal variations are small (0 m·01 in yellow). The observed extinction coefficients agree reasonably well with extinction values computed for a Rayleigh atmosphere, to which 2 mm of ozone and small amounts of dust are added. The mean extinction differences found between red and blue stars are reproduced by calculations using observed spectral energy distributions of stars. The dependence of extinction on the colours of the stars and on zenith distance in the ultra-violet should be taken into account in astronomical broad-band photometry.

Observations of Solar and Lunar Radiation at 15 Millimeters

Observations of the radiation from the sun and the moon in a wide wavelength band centered approximately at 1.5 mm are reported. These waves were detected with a Golay detector using a searchlight mirror for a telescope. Measurements of atmospheric extinction are reported and compared with calculations of water vapor absorption which have been made by J. H. Van Vleck. It is found that the measured extinction is greater than the calculated.Observations of the radiation emitted by the moon at different phases are reported.The transmittances of some materials to the 1.5-mm waves from the sun are included.