Robert S. Fraser

The Relative Importance of Aerosol Scattering and Absorption in Remote Sensing

Previous attempts to explain the effect of aerosols on satellite measurements of surface properties for the visible and near-infrared spectrum have emphasized the amount of aerosols without consideration of their absorption properties. In order to estimate the importance of absorption, the radiances of the sunlight scattered from models of the Earth-atmosphere system are computed as functions of the aerosol optical thickness and absorption. The absorption effect is small where the surface reflectance is weak, but is important for strong reflectance. These effects on classification of surface features, measuring vegetation index, and measuring surface reflectance are presented.

Size distribution and scattering phase function of aerosol particles retrieved from sky brightness measurements

Ground-based measurements of the solar transmission and sky radiance in a horizontal plane through the Sun are taken in several geographical regions and aerosol types: dust in a desert transition zone in Israel, sulfate particles in Eastern and Western Europe, tropical aerosol in Brazil, and mixed continental/maritime aerosol in California. Stratospheric aerosol was introduced after the eruption of Mount Pinatubo in June 1991. Therefore measurements taken before the eruption are used to analyze the properties of tropospheric aerosol; measurements from 1992 are also used to detect the particle size and concentration of stratospheric aerosol. The measurements are used to retrieve the size distribution and the scattering phase function at large scattering angles of the undisturbed aerosol particles. The retrieved properties represent an average on the entire atmospheric column. A comparison between the retrieved phase function for a scattering angle of 120°, with phase function predicted from the retrieved size distribution, is used to test the assumption of particle homogeneity and sphericity in radiative transfer models (Mie theory). The effect was found to be small (20%±15%). For the stratospheric aerosol (sulfates), as expected, the phase function was very well predicted using the Mie theory. A model with a power law size distribution, based on the spectral dependence of the optical thickness, a, cannot estimate accurately the phase function (up to 50% error for λ = 0.87 μm). Before the Pinatubo eruption the ratio between the volumes of sulfate and coarse particles was very well correlated with α. The Pinatubo stratospheric aerosol destroyed this correlation. The aerosol optical properties are compared with analysis of the size, shape, and composition of the individual particles by electron microscopy of in situ samples. The measured volume size distributions before the injection of stratospheric aerosol consistently show two modes, sulfate particles with rm 0.7 μm. The “window” in the tropospheric aerosol in this radius range was used to observe a stable stratospheric aerosol in 1992, with rm ∼ 0.5 μm. A combination of such optical thickness and sky measurements can be used to assess the direct forcing and the climatic impact of aerosol. Systematic inversion for the key aerosol types (sulfates, smoke, dust, and maritime aerosol) of the size distribution and phase function can give the relationship between the aerosol physical and optical properties that can be used to compute the radiative forcing. This forcing can be validated in dedicated field experiments.

Red and near-infrared sensor response to off-nadiir viewing

Abstract NOAA-6 and 7 AVHRR visible (VIS) and near-infrared (NIR) radiance data from the advanced very high resolution radiometer (AVHRR) on board the National Oceanic and Atmospheric Administrations (NOAA) NOAA-6, 7 and 8 satellites were simulated for a black target, for bare soil and for tow, medium and high levels of green-leaf biomass. Simulations were based on measured reflectance and Daves atmospheric models. Radiance data were generated for viewing and illumination geometry corresponding to the summer solstice, equinox and winter solstice at a latitude of 30°. The results indicate that the radiance responses to atmosphere are less for NOAA-7 geometry than for NOAA-6 and 8. Viewing in the backscatter direction has a more constant radiance response with increasing scan angle than viewing in the forward scatter direction. These results arc discussed with respect to atmospheric composition, surface reflectance and wavelength. Normalized relative differences between NIR and VIS channels, arc also desc...

SATELLITE MEASUREMENTS OF AEROSOL MASS AND TRANSPORT

The aerosol optical thickness over land is derived from satellite measurements of the radiance of scattered sunlight. These data are used to estimate the columnar mass density of particulate sulfur on a day with a large amount of sulfur. The horizontal transport of the particulate sulfur is calculated using wind vectors measured with rawins.

Algorithm for atmospheric corrections of aircraft and satellite imagery

Abstract An algorithm is described for making fast atmospheric corrections. The required radiation parameters are stored in a lookup table. The procedure is to enter the lookup table with the measured radiance, wavelength, view and illumination directions, heights of observation and surface, and the aerosol and gaseous absorption optical thicknesses. The surface radiance, the irradiance incident on a surface, and surface reflectance are computed then. Alternately, the program will compute the upward radiances at specific altitudes for a given surface reflctance, view and illumination directions, and aerosol and gaseous absorption optical thicknesses.

Light Extinction by Aerosols during Summer Air Pollution

Abstract In order to utilize satellite measurements of optical thickness over land for estimating aerosol properties during air pollution episodes the optical thickness was measured from the surface and investigated. Aerosol optical thicknesses have been derived from solar transmission measurements in eight spectral bands within the band λ440–870 nm during the summers of 1980 and 1981 near Washington, DC. The optical thicknesses for the eight bands are strongly correlated. It was found that first eigenvalue of the covariance matrix of all observations accounts for 99% of the trace of the matrix. Since the measured aerosol optical thickness was closely proportional to the wavelength raised to a power, the aerosol size distribution derived from it is proportional to the diameter (d) raised to a power for the range of diameters between 0.1 to 1.0 μm. This power is insensitive to the total optical thickness. Changes in the aerosol optical thickness depend an several aerosol parameters, but it is difficult to ...

Temporal and spatial variability of aerosol optical depth in the Sahel region in relation to vegetation remote sensing

Abstract A network of Sun photometers was established in the Sahel region of Senegal, Mali and Niger in order to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data. The aerosol optical thickness τa computed from the spectral Sun photometer measurements exhibited very high day-to-day variability ranging from approximately 01 to greater than 20 at 875 nm for both the wet and dry seasons. A gradient of decreasing τa from north-to-south latitudes in the Sahel for the wet season, July-September, was observed, which may be caused jointly by increased washout owing to the gradient of increasing precipitation to the south and the location of source regions for dust in the north. The Angstrom wavelength exponent a was found to vary with the magnitude of the aerosol optical thickness, with values as high as 0-75 for very low Ta, and values of 0-25 to 0-0 for high τa conditions. Analysis of τa data from this observation network suggests that there is a high spatial variabil...

An Iterative Radiative Transfer Code For Ocean-Atmosphere Systems

Abstract We describe the details of an iterative radiative transfer code for computing the intensity and degree of polarization of diffuse radiation in models of the ocean-atmosphere system. The present code neglects the upwelling radiation from below the ocean surface and as such is applicable to the part of the spectrum where the absorption by water is strong. TO establish the reliability of our numerical scheme as well as the computer code, we compare our results with those of Fraser and Walker (1968), Dave (1972) and Mullamaa (1964) and find them to be in excellent agreement. We also obtained reasonably good qualitative agreement with Plass et al. (1975) who utilize the Monte Carlo technique to solve the radiative transfer equation for the ocean-atmosphere system. Our computations also show that both the intensity as well as the degree of polarization of the upwelling diffuse radiation at the top of the atmosphere vary significantly, when the rough ocean at the base of the atmosphere is replaced by a ...

Satellite measurement of mass of Sahara dust in the atmosphere

The mass of particulates in a vertical column of dust outflow from northwestern Africa is derived with the aid of satellite measurements of nadir radiance. The measurements were made from the Landsat-1 satellite. Measurement accuracies are discussed. A radiative transfer model with a known mass of dust is developed to account for the measured radiance values. The model is constructed with knowledge of a few values of optical parameters measured from a surface ship. The accuracy of the model is discussed. The resulting mass of particulates smaller than 10-microm radius in a vertical column is 1.6 g m(-2).

Adjacency effects on imaging by surface reflection and atmospheric scattering: cross radiance to zenith

An analytical solution is discussed for the nadir radiance as measured from a satellite, based on a simplified single-scattering approximation in which the scattered radiation is not subject to extinction. In the solution, terms can be identified as due to a reflection from the vicinity of the object pixel, and, respectively, (1) upward scattering to zenith above the object pixel, and (2) downward scattering from the entire atmosphere to the object pixel. The first term is referred to as the cross radiance, the second as the cross irradiance. The cross radiance is proportional to the forward scattering optical thickness, as defined, and the cross irradiance to the backscattering optical thickness. The cross radiance usually constitutes the predominant effect. The effect, even at low atmospheric turbidity, can be large enough to constitute a significant fraction of the radiance registered at the satellite, thus hampering determination of spectral signature of the object pixel or identification of pixels with inherently the same spectral signature. Explicit expressions and computer solutions for the cross radiance from annular or from rectangular reflecting areas are presented. The effect depends on the height distribution and on the sharpness of the forward peak of the scattering particles.