Atmospheric Extinction Coefficient Research Articles

TWENTY YEARS OF ATMOSPHERIC EXTINCTION AT MCDONALD OBSERVATORY

ABSTRACT Atmospheric extinction coefficients in the U, B, V bands have been measured at McDonald Observatory on 382 nights covering the 20-year period December 1960 to December 1980. Analyses of these data characterize the natural levels and variations in atmospheric extinction during both perturbed (Mount Agung eruption) and unperturbed times. The need for nightly extinction measurements is clearly shown. No significant long-term trends were found. Power spectrum analysis shows only the sinusoidal seasonal variation with minimum extinction in January.

Differences arising in the determination of the atmospheric extinction coefficient by transmission and target reflectance measurements

Measurements of the optical extinction at a wavelength of 1.06 microm have been made in water droplet clouds. The extinction coefficient has been measured in the laboratory using two different methods simultaneously. In the first, measurements of the transmitted signal attenuation over a known path length were used. In the second the extinction coefficient was derived from the two-way attenuation of the signal reflected from a target on the opposite side of the cloud from the laser source and detector. It is found that in general the two values of the coefficient derived differ considerably, and the magnitude of the difference depends on the cloud density, the target size, and the system's optical parameters. The difference is shown to originate in the off-axis forward scattering caused by the cloud droplets, and the implications of the results on the measurement of the atmospheric extinction by reflection (lidar) techniques are discussed.

Statistical Comparisons between Teleradiometer-Derived and Slide-Derived Visibility Parameters

An automated scanning densitometer system developed to estimate visibility-related parameters from photographic slides was tested in comparison to similar teleradiometric measurements. Parameters such as target-sky contrast, visual range, atmospheric light extinction coefficient, and plume and layered contrast can be estimated using this system; however, only target-sky radiance ratios and standard visual ranges were compared. More than 1600 concurrent densitometer and teleradiometer data pairs from eight National Park Service air quality and visibility monitoring locations in the western United States were analyzed using the techniques of correlation, linear regression, average bias and difference calculation, and cumulative frequency distribution generation. Correlation coefficients were good, especially with middle-range camera vistas around 50 km distant. Regression slopes approached unity with intercepts near zero. Average bias introduced into the determination of radiance ratios from slides ranged f...

Optical properties of the atmospheric boundary layer at 1.06 μm: Quantitative remote sensing of aerosols with a Nd: YAG lidar

Quantitative results on atmospheric extinction (α) and backscatter (β) coefficients are reported. From the observations, the lidar ratio ( α β ) was parametrized as a function of relative humidity. Using this parametrization and an integral-inversion technique, the calculated vertical extinction profiles from a lidar are in very good agreement with aircraft data. From a dual-wavelength lidar, a good estimate of the slope of the aerosol size distribution n( r) is obtained [ n( r)(:) r −4.3; r = particle radius] in accordance with the measured particle-size data. Some samples of routine boundary-layer profiles at 1.06 μm will be given. The profiles demonstrate the capability of lidar to monitor boundary-layer dynamics (convective motion, plume mixing). Preliminary experiments in conjunction with ground-level SO 2 measurements were performed, which indicate possible operational applications for IR lidars.

Visibility and telephotometer

This paper discusses the possible error of the calculation of the atmospheric extinction coefficient by the Koschmieder visibility formula. An analysis of the problem shows that an error of 10% can be introduced in the measurement if the target used is a non-blackbody. By using the telephotometer as a tool of measurement, such an error can be markedly reduced.

Solar radiation correlations for the Athens, Greece, area

In the present study, correlations of the solar radiation for the Athens area are provided, based on hourly measurements over the period 1955–1980. These correlations are of two kinds: The first, which provide the average total radiation incident on a horizontal surface in terms of hour, day, and month of the typical year, is suitable for solar energy collection applications. The correlations of the second kind provide the maximum radiation and are suitable for use in the design of air-conditioning installations or other applications requiring knowledge of maximum values. The correlations of this kind are based on the calculation of the solar constants A, B, and C, i.e. the “apparent solar irradiation at zero air mass,” the “atmospheric extinction coefficient,” and the “diffuse radiation factor,” respectively. These solar constants permit the calculation of the maximum direct normal radiation and of the maximum diffuse radiation as well as of the total radiation on surfaces of various orientations and inclinations. The solar heat gain through fenestration of various orientations (needed in air-conditioning applications) is calculated, as an example of the applications of the solar constants A, B, and C.

Integration method and ratio method for retrieving extinction coefficient from lidar signals

New solution techniques to improve the accuracy of quantitatively determining the atmospheric extinction coefficient and the backscattering-to-extinction ratio from lidar signals are developed. The integration method is proposed to analytically retrive the extinction coefficient at ground level, which has the advantage of eliminating the effect of backscattering fluctuations on the inversion results. The ratio method, on the other hand, deals with the inversion of the vertical distribution of the extinction coefficient. The main idea of this method is to begin with a calculation of the transmittance by eliminating the backscattering through ratioing lidar signals at two elevations, and subsequently derive the extinction coefficient from the transmittance, thus avert from ambiguous results caused by inappropriate assumptions on the backscattering-to-extinction ratio. Observational investigations have demonstrated that the integration method is superior to the slope method in terms of accuracy and stability, and the ratio method is reasonable and feasible as well.

Horizon line scanning telephotometry

The horizon line softening method of determining the atmospheric spectral extinction coefficient σ(λ) is analyzed in detail indicating an inherent lower limit σ0(λ) on the range of σ(λ) which can be measured by this technique. As the observation height h above the earth surface increases, σ0(λ) decreases. A comparative experimental study of horizon line scanning telephotometry and contrast telephotometry is described, the results of which show the former technique to be useful under certain observing conditions.

Evaluation of the atmospheric content of particulate mass from visibility observations

Accurate estimates of the average value of the atmospheric extinction coefficient in the visible spectral range can be obtained from observations of visibility if they are carefully made according to Horvath's criteria. Since the perception function depends on the spectral features of atmospheric attenuation and illumination, the average coefficients due to Rayleigh scattering and to absorption by water vapour, ozone and nitrogen dioxide are evaluated as a function of the wave-length of maximum perception. Thus, the average extinction coefficient by particulate matter can be obtained as the difference between the average value of the atmospheric extinction coefficient and the sum of the various coefficients given by Rayleigh scattering and absorption by minor gases. From this evaluation, the mass content of particulate matter along the horizontal path at ground level can be estimated by making use of a large set of mass extinction coefficients obtained for different models of both airborne particles and water droplets and calculated at the wave-length of maximum perception for various turbidity conditions.

Measurements of the spatial distribution of particulate pollution with a telephotometer

Recently an instrument has been developed which measures the atmospheric extinction coefficient by measuring the contrast of distant targets. The extinction coefficient thus measured is the average value between the target and the location of the telephotometer. When two objects located behind each other at different distances are used as targets, average extinction coefficients between the instrument and the first target and between the first and the second target, are obtained. If this is done for several targets and in different directions, the average value of the extinction coefficient in the corresponding annular sectors can be determined. Since it is possible to correlate mass concentration and extinction coefficient, the mass concentration in different sections can be determined with one instrument from one point. Measurements in Vienna have shown that almost always an inhomogeneous distribution of particulates occurs, although it is not visible by eye.

The University of Vienna telephotometer

The measurement of the contrast reduction of distant targets is a useful tool in the determination of atmospheric visibility and atmospheric extinction coefficients. For this purpose an astronomic reflective telescope has been converted to a telephotometer. At the location of the real image of the target an adjustable stop with normal size of 1 mm is inserted, thus selecting a small portion of the target to be measured. The light emerging from the stop passes interchangeable interference filters and reaches a photodiode. The photocurrent is proportional to the luminance of the target. A plane parallel plate, which is inserted in the path of light, permits the measurement of the brightness of target and horizon repeatedly without realignment of the telescope. The extinction coefficient of the atmospheric aerosol can be calculated from the measured luminance of horizon, distant and close target and the distance of the target, using Middletons contrast reduction formula. Normally the intrinsic luminance of the target is determined from a similar target at short distance using calibration factors. The accuracy of the measured extinction coefficients depends on errors arising from the determination of the luminance of the distant target (apparent luminance), the inherent luminance, the distance of the target and from light scattered in the instrument. Model calculations have shown that the optimum distance of the target is 1 4 to 3 4 of the visibility. The light scattered in the instrument is less than 2 % at all wavelengths and thus can be neglected. Under conditions of inhomogeneous illumination (clouds) the extinction coefficient determined with the telephotometer can differ from the true value up to 30% with very specific cloud distributions in the sky. It can be shown that certain cloud distributions will give no error at all in the determination of the extinction coefficient, whereas for all possible cloud coverages the average of several measurements will have deviations from the true value of a few percent. Multiwavelength telephotometry also permits the calculation of the mass of the suspended particulates. Examples of measurements are presented.

The influence of the color of visibility targets on the visibility

Reduced visibilities have been simulated in the laboratory by means of a hydrosol. It was illuminated homogeneously by iodine quartz lamps producing an illuminance similar to a bright day. In the hydrosol several visibility targets were suspended at different distances and the visibility observation was performed by counting the number of visible objects. By this method a very accurate and fast determination of the visibility was possible. The extinction coefficient of the hydrosol was determined with a long path photometer: the intrinsic brightness of the objects in the illuminated hydrosol was determined for several wavelengths by means of a fiber optic. The correlation between the measured visibility and the extinction coefficient shows that the frequently used visual range underestimates the visibility, since the highest perception for an atmospheric aerosol is not at 550 nm but at 580 nm wavelength. Therefore it is suggested to use the atmospheric extinction coefficient at 580 nm for the calculation of the visual range. All colored objects could be seen less far, since they had a smaller contrast at the wavelength of maximum perception than the black object. Dark colored objects can be seen further than bright ones. Especially objects that have a high reflection in the yellow and organe have a small visibility. If the intrinsic brightness of the objects and the extinction coefficient (both as a function of wavelength) are known, the wavelength of maximum perception can be calculated and from this the visibility. The visibility thus calculated agrees with the observed visibility. If visibility observations in the atmosphere have to be performed with non-black objects, methods for corrections to the visibilty of a black object are given.

Visual range monitors

The determination of the atmospheric extinction coefficient is necessary to allow visual range to be calculated. The limitations of nephelometers (devices which measure light scattered in the atmosphere) are discussed. These limitations lead to the conclusion that optical transmissometers are the only valid devices for assessing extinction coefficients. New techniques for maintaining the accuracy of transmissometers over many months of use are described. The high accuracies achieved allow a wide dynamic range of visibility to be measured with a single base line instrument.

Determination of the atmospheric extinction coefficient by measurement of distant contrasts

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text H. Horvath and G. Presle, "Determination of the atmospheric extinction coefficient by measurement of distant contrasts," Appl. Opt. 17, 1303-1304 (1978) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Visibility in turbid media with colored illumination

Reduced visibilities were simulated in a hydrosol, which was illuminated with light from iodine quartz lamps. Visibility observations were performed through different color niters, thus having the same effect as if the hydrosol were illuminated by the colored light obtained by passing the white light through the filter. Two types of hydrosols were used : One had a wavelength-dependent extinction coefficient similar to the atmosphere with higher extinction in the blue than in the red while the other consisted of monodisperse latex spheres of 2.02 μm diameter having a higher extinction in the red. For observations in white light, the visual range using the extinction coefficient at 550 nm underestimates the visibility for the hydrosol similar to the atmosphere and overestimates the visibility for the latex hydrosol. In colored lights, the visual range has no meaning; calculation of the visibility from the extinction coefficient at either the dominant wavelength of the filters or at their wavelength of maximum transmission gives wrong results, too. However, if the wavelength of maximum perception is used instead, the experimentally determined visibilities agree in all cases with the visibilities calculated from the extinction coefficient. Since the wavelength of maximum perception is calculated from the wavelength dependent contrast of the object seen through the turbid medium, the energy distribution of the illumination and the spectral sensitivity of the eye, it depends both on the aerosol and the illumination. For daylight conditions and the normal atmospheric aerosol the wavelength is 580 nm and it is suggested that the extinction coefficient at this wavelength be used, rather than that at 550 nm for the calculation of the visual range. When color filters are used for visibility observations, the atmospheric extinction coefficient can easily be obtained by means of the visibility formula at different wavelengths. For several conditions likely to occur in the atmosphere, the wavelength of maximum perception has been calculated and is listed in a table.

Analyses of atmospheric extinction data obtained by astronomers—II. Seasonal variations in astronomical extinction

We have examined atmospheric extinction coefficients from 15 observatories obtained during the time period 1956–1972. We find definite seasonal variations in the northern hemisphere with maxima occurring during the local summer (June–August). On the other hand, the southern hemisphere observatories exhibit very little or no seasonal effect, with the exception of two South African sites, where a definite late winter (August–October) maximum is observed. These latter exceptions may be due to local effects.

ATMOSPHERIC EXTINCTION AT MCDONALD OBSERVATORY, 1960-68

ABSTRACT Atmospheric extinction coefficients measured in the U,B,V bands at McDonald Observatory on 280 nights between December 1960 and October 1968 have been analyzed to derive the effects of seasonal variations, volcanic eruptions, and possibly secular trends. The spectral dependence of the various effects are interpreted in terms of a variable tropospheric dust content with a A 1 extinction law, and a fluctuating amount of roughly neutral volcanic smoke averaging 0.03 mag since March 1963. There is no clear indication yet for a long term increase of atmospheric extinction over West Texas, but the pre-eruption minimum seasonal values are about 0.02 magnitude greater than the calculated theoretical minimum for molecular scattering, corrected for ozone and water vapor absorption. Key words: atmospheric extinction - seasonal variations

The Wavelength Dependence of Atmospheric Extinction

Atmospheric extinction coefficients determined at Flower and Cook Observatory for the Johnson B and V spectral responses are compared with the predictions of two models based on theoretical and observational parameters. Some of the observed deviations from the models can be explained as telluric line absorption in the wavelength region of the V response. The remaining differences arise from the use of different observational data in calculating the Rayleigh attenuation coefficients and the ozone attenuation coefficients, and local differences in the characteristics of the atmosphere. High and variable extinction coefficients seem to show an individual wavelength dependence and a periodicity with respect to time.

レーザー・レーダーによる大気の後方散乱関数および減衰係数の決定について

レーザー・レーダーによる大気の後方散乱関数および減衰係数を求める一方法を提出した。従来大気の後方散乱関数をレーザー・エコーより求めるのに,光の通過路程の透過関数を仮定して求めているが,これはかなりの誤差を伴う。一方後方散乱関数が分っていても,減衰係数はこれから一義的に求め難い事が論じられている。この論文では大気の後方散乱係数をレーザー・エコーより求めるのに通過路程の透過関数についてなにも仮定せずに大気が水平に成層していることのみを仮定して二方向のエコーの測定より大気の後方散乱関数を求め,又この散乱関数を用いてレーザー・レーダー方程式より導かれる微分方程式を数値的に解くことにより減衰係数が一義的に求められることを示した。

Equations for solar heat gain through windows

An analysis of solar-radiation records obtained at Scarborough, Ontario, indicates that the insolation in Canada can be significantly greater than the values given by Moon's 1 standard solar-radiation curves. It is possible to represent the solar data by a single analytical expression involving an atmospheric extinction coefficient and an apparent value of the solar constant. This expression allows the calculation of insolation on any surface and the determination of the time and date when the maximum insolation can occur for any surface. Simple expressions relate the time, date, latitude, building orientation, and the type of window glass and shading with the solar heat gain through windows. These can be used to program any digital computer to compute design values of the instantaneous heat gain through the windows of a building.