Feasibility of Shipboard Laser-Attenuation Measurements With a Portable Transmissometer.

A 2-meter-long cell has been devised in which aerosol extinction and scattering coefficients can be measured simultaneously. Extinction is obtained by measuring the change in the intensity of a HeNe laser beam which is stepped through the cell for a total path length of 20 meters. A reciprocal nephelometer consisting of a sensor with a precise Lambertian response is mounted on the side of the cell to give a direct measure of the scattering coefficient. Filling the cell with ambient aerosol that has been concentrated centrifugally increases the effective path length of the beam in the cell to 300 meters. The cell was taken on the 1977 EOMET cruise of the USNS Hayes in order to determine its capabilities when exposed to the rigors of a ship's environment. The variance of data collected during the cruise was analyzed. The mean resolution limit of the transmittance measurements made during the cruise was + or - 0.16%. This limit was primarily influenced by mechanical instabilities in the nephelometer operated precisely. Vibrations from the ship's machinery was damped out successfully. An improved cell could increase the resolution by a factor of 10 or more. Absorption coefficients of the aersol particles in the eastern North Atlantic and in the Mediterranean were plotted. This technique has potential in the infrared. (Author)