A laser doppler method for the determination of particle deposition velocities

The measuring technique is based on Laser Doppler Anemometry (LDA). When two laser beams intersect, an interference pattern is formed. Particles crossing this interference pattern emit a pulse of scattered light which is modulated by the spatially varying fringe intensity. In order to allow for directional discrimination, one laser beam is frequency-shifted in a Bragg cell with a moving fringe pattern resulting. The use of two optical systems is mandatory in order to cover a wide particle size range while maintaining a reasonable data rate. Large particle need a large scattering volume because they are comparatively rare in the atmosphere. This is achieved by a cylindrical lens together with a beam calibrated using the advective flux in a small wind tunnel which can be tilted. Deposition velocity as well as flux of lycopodium spores of different sizes was successfully measured. Size calibration with the present equipment is hampered by the fact that in the undisturbed flow particle cro ss the scattering volume at arbitrary places. A method is described to circumvene these difficulties.