Some computations concerning the actual light scattering coefficient as measured in haze and fog

The aerosol light scattering coefficient is a parameter of paramount interest in problems related to diffuse transmission, visibility, remote sensing, and radiative transfer. Its measurement is actually carried out with sources of angular divergence θd ≠ 0 and detection systems of half-angle acceptance θ0 ≠ 0; the scattering coefficient so measured is called actual (or apparent) scattering coefficient and is denoted as β(θd, θ0). The total (or true) scattering coefficient is defined as the limit case for θd = θ0 = 0 and usually denoted as β; this is the quantity which characterizes the attenuation of a monochromatic radiation propagating through an aerosol and has to be recovered from the measured β(θd, θ0). Experimental works, carried out for different values of θd, θ0 and with different aerosol particles attenuating the light beam, have shown that, even in the case where θd and θ0 can be reduced to few mrad, the actual value of the scattering coefficient β(θd, θ0) is less than the true value β. In an earlier article we reported computations of the actual Mie scattering efficiency factor K(m,x; θ0), θd = 0, for the following values of the refractive index (m), size parameter (x), and θ0: m = 1.33, x = 0.1 (0.1) 200 and m = 1.55, x = 0.1 (0.1) 100; θ0 = 0.1° (0.1°) 1.0°. Tables and card decks have been arranged and are available from the author. The results show that for aerosol radii r < 3 µm, at visible wave lengths, the ratio of the actual to the total (K) efficiency factor is practically 1 for both the refractive indices; for r > 3 µm the ratio decreases, and at r ~= 20 µm and θ0 = 1.0° it is 0.58. In this Letter we compute β and β (θ0), θd = 0, on the basis of 89 size spectra of condensation nuclei (CN), haze, and fog, with the aim of determining the limits between which the ratio R(θ0) = β (θ0)/ β can range in different situations.