Measurement of the Lidar Ratio for Atmospheric Aerosols Using a 180 Degree-Backscatter Nephelometer.

摘要

Laser radar (lidar) can be used to estimate atmospheric extinction coefficients due to aerosols if the ratio between optical extinction and 180DG backscatter (the 'lidar ratio') at the laser wavelength is known or if Raman or high spectral resolution (HSR) data is available. Most lidar instruments do not have Raman or HSR capability, however, making knowledge of the lidar ratio essential. We have modified an integrating nephelometer, which measures the scattering component of light extinction, by addition of a backward pointing laser light source such that the detected light corresponds to integrated scattering over 176DG-178DG at a common lidar wavelength of 532 nm. Mie calculations indicate that the detected quantity is an excellent proxy for 180DG backscatter. When combined with existing techniques for measuring total scattering and absorption by particles, the new device permits a direct determination of the lidar ratio. A fourpoint calibration, run by filling the enclosed sample volume with particle-free gases of known scattering coefficient, indicates a linear response and calibration reproducibility to within 4%. The instrument has a detection limit of 1.5x10-7 m-1 sr-1 (10% of Rayleigh scattering by air at STP) for a 5 minute average, and is suitable for ground and mobile/airborne surveys. Initial field measurements yielded a lidar ratio of 20 for marine aerosols and 60-70 for continental aerosols, with an uncertainty of 20%.