Polarimetric remote sensing of aerosols over land surfaces

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Writing from the S.S. Narkunda, near Aden, [1921] that using a Nicol prism ‘serves to cut off a great deal of the blue atmospheric “haze” which usually envelops a distant view, and mostly consists of polarized light. ’ Although the reason for the color and polarization of the sky had been explained some time before by [1871], later Lord Rayleigh, and the neutral points, where the polarization of the sky becomes zero, had already been named after their discoverers Arago [Barral, 1858], [1840] and [1842], this simple observation of Raman’s was still considered noteworthy, because of the difference between the behavior of the object being observed and the haze. The reason for this difference is that light scattered by molecules and small aerosol is strongly polarized in a plane perpendicular to the scattering plane (the plane defined by the sun, the object being viewed and the observer) while light scattered by surfaces is only weakly polarized. Thus, when Raman oriented the polarizer to transmit light in the plane parallel to the scattering plane the contributions from light scattered by aerosols and molecules were suppressed while the lighthouse was made more visible (had more contrast). This difference between the polarizing properties of aerosols and molecules as compared to surfaces is used by modern polarimetric remote sensing instruments to determine the amount, size and type of aerosols that are present above the surface.