Slant-Lidar Aerosol Extinction Measurements and their Relation to Measured and Calculated Albedo Changes

Abstract

The slant-path (or zenith-angle scanning) lidar technique is used to measure boundary-layer optical depths for ten different times during a single day. For the range of optical depths encountered (0.03?0.14 at 0.69 ?m wavelength), 1σ measurement uncertainties are about 0.03. The major source of uncertainty is daytime skylight background, effects of which are mitigated in this experiment by the low boundary-layer top height (<1.4 km above the lidar). Resulting particle column backscatter-to-extinction ratios vary between 0.010 and 0.053 sr?1. By assuming that these ratios are independent of height, vertical extinction profiles and the optical depth of a layer below a tower-mounted radiometer pair are derived. This radiometer pair is used to measure the change in surface plus layer albedo caused by the aerosol layer. The consistency of albedo and optical thickness measurements is tested by comparing measured albedo changes to those calculated from the optical thickness measurements. When reasonable values are assumed for particle refractive index and relative size distribution (not measured in this experiment), results agree to within the uncertainty arising from the 0.03 optical thickness measurement uncertainty.