RESUMEN
In the course of a lengthy series of observations since 1975, a large, continuous decrease of the brightness of the solar aureole has been found west of Boston, at Hanscom Air Force Base and at Lexington, Mass. This points to the virtual disappearance from the lower atmosphere of giant particles larger than ~ 10 microm. in size while total suspended particulates in Boston and other U.S. cities have barely decreased. Results of calculations to better understand the relation between forward scattering and aerosol mass distribution [coarse fraction (CF)] are presented. In addition, a method to modify steep scattering functions calculated for a plane-wave source (Sun treated as a star) to those of the actual (and limb-darkened) Sun is presented. The calculated wavelength dependence of extinction, which is lower than that observed, is found to be little affected by the CF, but seems, like forward scattering, to be sensitive to mass distribution of sizes of < 0.4 and ~ 0.6 microm because of the anomal scattering behavior of spheres.
RESUMEN
A simple method is described that yields, in reflective optics as well as in Fourier-transformin frared spectrometers,a beam of bright visible light coincident with the invisible infrared beam so that foci and beam locations can be checked easily.
RESUMEN
A small, handheld skylight photometer, incorporated into a sun photometer and capable of measuring sky radiation to within 2 degrees of the sun at lambda 0.50 and 0.93 microm is described. Calibration procedures are discussed; solar measurements in the wideband 0.93-microm channel show the expected influence of water vapor. Formulas to obtain the aerosol scattering function are presented. Samples of measured and theoretical aerosol scattering functions are discussed.
RESUMEN
Spectra of absorption indices and data on gross chemical composition for two size classes of water soluble and dust fractions of high-volume aerosol samples are presented for several locations (mainly Bedford, Massachusetts; Meppen in Germany; Riverside and San Nicolas Island in California and some maritime locations). The absorption data were obtained by the KBr-pressed disk method. It has been found that re-grinding of the pellets increases the absorption by salt samples considerably.
RESUMEN
The excellent data of zenith polarization and color ratio (CR) during twilight obtained by Coulson at the Mauna Loa Observatory, Hawaii are subjected to a reinterpretation, especially with regard to the frequent deviations from the clear average. It is shown that a moderate lift of the earth's shadow by distant clouds (or by low level haze) will reduce the red/green CR, while greater lift shifts the CR peak to a smaller solar depression. The effect on zenith polarization at wavelengths >/=0.6 microm is seen to correspond to a slight reduction of overall polarization for a moderate lift, and for a large lift to a shift of the polarization minimum (depolarization by stratospheric aerosol) from 4 degrees solar depression angle to 2 degrees . A generally high level of polarization, but with a barely noticeable 4 degrees minimum, had also been observed earlier in Germany by Steinhorst when the stratospheric aerosol load was very small. The difference is explained by a higher and stronger aerosol layer in the tropics combined with a higher and cleaner troposphere.
RESUMEN
Striated twilight glows have been observed since 26 November 1974 in New England, indicating the spread of stratospheric dust earlier observed over Arizona. Similar photometric results were obtained from New Mexico and Florida, and twilights in Puerto Rico showed features not hitherto measured. Letters and verbal reports indicate the source to be eruptions of Fuego Volcano in Guatemala between 13 and 23 October 1974.
RESUMEN
Spectra of the real and imaginary parts of the refractive index m = n ' - in from 2.5 microm to 40 microm of ammonium sulfate, Sahara dust (as collected at Barbados), volcanic dust (pumice), and dust from a coalfired power plant are presented. With a peak absorption index n = 1.0 at 10 microm Sahara dust turns out to be the strongest absorber in the atmospheric window. The dispersion of n' is also very pronounced near 9 microm and 20 microm. Volcanic and coal-fire dust also absorb in the window region stronger than most of our earlier-investigated more normal aerosol fractions. The data for ammonium sulfate are compared with Remsberg's measurements on an aqueous solution.
RESUMEN
The optical constants in the ir from lambda2.5 microm to 40 microm (4000-250 cm(-1)) of dry natural aerosol substances and of sea salt are presented. The aerosol substances were obtained from rain and snow water: dust and soot by sedimentation, and water soluble salts by evaporation. The spectra of the absorption index n' were derived from our published transmittance measurements of potassium bromide disks. The real part n of the refractive index was calculated from the specular reflectance at near normal incidence of disks of pure aerosol substance. The observed spectral features are being related to chemical constituents, notably sulfates and alcohol soluble organics. Optical constants of composite and wet aerosol are discussed. A simple model confirms the measured transmission of a coarse dry powder of water solubles and shows that the extinction by natural aerosol should have a minimum near 8 microm and a strong maximum near 9 microm.
RESUMEN
Sky radiation near zenith and solar radiation in the rhosigmatau band region were recorded by means of a rotating interference filter (lambda0.98-0.88 microm) and a silicon detector. Although the spectral resolution of the simple spectrometer was not high, the water vapor content of the cloud free atmosphere was obtained with reasonable accuracy. The band depth of the radiation from thin, bright clouds was only slightly greater than that of the cloud free atmosphere, but dense and dark clouds showed deep bands mainly caused by increased path length as a result of multiple scattering. Considerable distortion of the band due to absorption by liquid water is observed in the radiation from very dark and dense clouds, and sometimes during snowfall. Some laboratory measurements are also discussed.
RESUMEN
Many absolute measurements of twilight sky radiance were made in six spectral ranges from lambda377 nm to lambda970 nm at 20 degrees elevation in the solar vertical, and mostly from sunset to 6 degrees -10 degrees sun depression (sd) in southern Germany, from fall 1962 to summer 1967, and at Bedford, Massachusetts, during winter 1967-1968. A strong change of twilight radiance, especially a large increase in the amplitude of the red/green color ratio between 4.5 degrees (main purple light) and 1 degrees sd, was observed in January 1964 following the explosive eruption of the Agung volcano in March 1963, but twilights were still abnormal and variable in early 1968. Dust mixing profiles of some typical twilight groups were derived, and the volcanic twilights show, as do other optical and direct soundings of the same period, a strong maximum near 20 km. A strong secondary red/green maximum (visually observed as secondary purple light) developed at sun depressions greater than 6 degrees of the volcanic twilights. This seems to have been caused by multiple scattering and not by mesospheric dust. A few twilight observations in 1964 from Australia indicate considerably more volcanic dust in accordance with transmission data, while no stratospheric dust is indicated in Arctic twilights of spring 1966. In order to verify the persistence of abnormal twilight conditions for more than five years, the dependence on red wavelength of the red/green color ratio is investigated, and some earlier twilight measurements are discussed. This persistence may have to be related to even higher amounts of dust in the tropical stratosphere.