Measurement of the lidar ratio for atmospheric aerosols with a 180° backscatter nephelometer

Laser radar (lidar) can be used to estimate atmospheric extinction coefficients that are due to aerosols if the ratio between optical extinction and 180 degrees backscatter (the Lidar ratio) at the laser wavelength is known or if Raman or high spectral resolution data are available. Most lidar instruments, however, do not have Raman or high spectral resolution capability, which makes 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 176-178 degrees at a common lidar wavelength of 532 nm. Mie calculations indicate that the detected quantity is an excellent proxy for 180 degrees 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 four-point calibration, run by filling the enclosed sample Volume with particle-free gases of a known scattering coefficient, indicates a linear response and calibration reproducibility to within 4%. The instrument has a detection limit of 1.5 x 10(-7) m(-1) sr(-1) (similar to 10% of Rayleigh scattering by air at STP) far a 5-min average and is suitable far ground and mobile/airborne surveys, initial field measurements yielded a lidar ratio of similar to 20 for marine aerosols and similar to 60-70 for continental aerosols, with an uncertainty of similar to 20%. (C) 1999 Optical Society of America.