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

Laser radar (lidar) can be used to estimate atmospheric extinction coeffici ents that are due to aerosols if the ratio between optical extinction and 1 80 degrees backscatter (the Lidar ratio) at the laser wavelength is known o r if Raman or high spectral resolution data are available. Most lidar instr uments, however, do not have Raman or high spectral resolution capability, which makes knowledge of the lidar ratio essential. We have modified an int egrating nephelometer, which measures the scattering component of light ext inction, by addition of a backward-pointing laser light source such that th e 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 c ombined with existing techniques for measuring total scattering and absorpt ion by particles, the new device permits a direct determination of the Lida r ratio. A four-point calibration, run by filling the enclosed sample Volum e with particle-free gases of a known scattering coefficient, indicates a l inear 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 Rayl eigh scattering by air at STP) far a 5-min average and is suitable far grou nd and mobile/airborne surveys, initial field measurements yielded a lidar ratio of similar to 20 for marine aerosols and similar to 60-70 for contine ntal aerosols, with an uncertainty of similar to 20%. (C) 1999 Optical Soci ety of America.