David Krueger
Professor
B.S., Montana State University, 1961;
Ph.D., University of Washington, 1967.
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.
