Lidar

Lidar techniques have proven very versatile in the study of the space and time variation of atmospheric properties such as wind speeds and concentrations of ozone, pollutants, and aerosols. More recently interest has turned to developing techniques to measure air temperatures and densities. The basic difficulty is in separating the effects of aerosols from those of the air molecules. In our experiments shown schematically in the figure, a pulse (10 nsec) of nearly monochromatic light (~ 0.1 GHz ) is sent into the atmosphere, and the spectrum of the scattered light is shown schematically as the heavy line. The light scattered by the aerosols is slightly Doppler-broadened due to the relatively low speeds of the aerosols (few m/sec) characteristic of the wind. The light scattered by the molecules has a much larger Doppler-broadening ( ~ 2.7 GHz) due to the relatively high speeds of the molecules ( ~ 500 m/s). This Doppler-broadening is roughly proportional to the square root of the air temperature. The method utilizes three signals. The first two signals (shown in the figure) depend on the molecular scattering only and allows the determination of the air temperature and the air density if the atmosphere is assumed to be in hydrostatic equilibrium. This signal is obtained by using a band-stop vapor filter which will block essentially all of the light scattered by the aerosols. A typical transmission function is shown for an atomic vapor filter. Two slightly different vapor filters are used in channels 1 and 2. The third signal depends on both the molecular scattering and the aerosol scattering and is obtained by measurements without the filter. Subtracting the molecular scattering from the third signal allows determination of the aerosol scattering and thus properties such as aerosol density and shape distribution. In addition to providing a useful tool to atmospheric science in general, these air temperature and density profiles and the characterization of the aerosol properties are important in the analysis of other experiments on the processes of ozone depletion in the atmosphere.