CONSIDERATIONS IN THE ACCURACY OF A LONG-PATH TRANSMISSOMETER

A recently developed long-path (almost-equal-to 15 km) transmissometer (LPT) was configured with a design criteria of measuring transmission independent of atmospheric turbulence accurately enough to determine ambient aerosol extinction coefficients with better than 10% error. To assess the ability of the LPT to measure atmospheric extinction accurately and to assess its response to atmospheric turbulence, a series of field programs was initiated to compare it to other measures of extinction as well as to itself using different path lengths. Relative accuracy of the transmissometer-derived extinction is calculated by comparing it to other measurements of extinction when it is expected that those measurements would be unperturbed by meteorological conditions and/or relative humidity. For instance, integrating nephelometers can accurately measure near-Rayleigh extinction, and teleradiometer contrast measurements of artificial black targets under standard meterological conditions eliminate the errors associated with the uncertain inherent contrast and nonuniform illumination. Turbulence effects were independently investigated by simultaneously operating two transmissometers over varying path lengths at Grand Canyon National Park from June 1986 to September 1986.