Obtaining calibrated marine aerosol extinction measurements using horizontal lidar measurements, differential lidar-target measurements, and a polar nephelometer

Abstract

Lidars are ideal for mapping the spatial distribution ofaerosol concentrations, however efforts to convertthe lidar measurements into estimates of the aerosol extinction or scattering coefficient are usually complicated. The difficulties arise from the uncertainty in the aerosol backscatter-to-extinction ratio and the lidar calibration. In marine conditions with little absorption, the aerosol backscatter-to-extinction ratio is identical to the aerosol phase function/4? at 1 80 degrees (backscatter) . Uncertainty in the lidar calibration is another source ofuncertainty, which can change with time depending on the state ofthe optics (clean or dirty). Here we investigate several techniques to obtain calibrated aerosol extinction coefficient values. The first approach uses horizontal lidar measurements over the open ocean where the atmosphere is horizontally homogeneous. The lidar calibration or aerosol phase function is adjusted until the derived aerosol extinction coefficients are flat with distance. Modeling shows that this provides correct aerosol extinction values. A second approach uses a target to reflect the lidar beam at different distances. The aerosol extinction is derived from the differential transmission measurements. As an independent measurement, the aerosol phase function and scattering coefficients can be measured with a polar nephelometer.