Assessment of aerosol extinction to backscatter ratio measurements made at 694.3 nm in tucson, arizona

The aerosol extinction-to-backscatter ratio, S_a, is a key parameter in interpreting scattering measurements made with lidar. Whereas solution techniques for solving the lidar equation generally assume some constraining relation for S_a (i.e., such as S_a is constant with range), few measurements of S_a have been made to establish the statistics and properties of this parameter. Measurements of S_a, for a wavelength 694.3 mm, obtained from slant-path lidar observations made in Tucson, Arizona, from May 1979 to June 1982 yielded values of S_a between ∼ 5 and 100 with the majority concentrated between ∼ 10 and 45. The weighted mean (weighted by inverse variances of S_a) of all S_a values and the arithmetic mean of the main grouping of S_a values (10 < S_a < 45) both equaled approximately 25. Interpretation of the S_a measurements in terms of various size distributions and refractive index values representative of atmospheric aerosols indicated the following: 1) S_a values in the 10 to 20 range are indicative of size distributions with significant coarse mode particle concentrations, larger real refractive index values (greater than ∼ 1.50), and zero or nearly zero imaginary refractive index values; 2) S_a values in the 20 to 45 range appear to correspond to size distributions with less significant but still noticeable coarse mode particle concentrations, somewhat lower real refractive index values, and still fairly low imaginary refractive index values (less than ∼ 0.005); and 3) S_a values greater than about 45 indicate even lower real refractive index values (values of ∼ 1.45 andlower) and even larger imaginary refractive index values (values in the 0.005 to 0.01 range).