3-WAVELENGTH NEPHELOMETER SUITABLE FOR AIRCRAFT MEASUREMENT OF BACKGROUND AEROSOL SCATTERING COEFFICIENT

A new nephelometer suitable for aircraft measurements of aerosol scattering extinction coefficient (sigma-sp) has been constructed and operated under field conditions. This instrument is vacuum tight for operation in a pressurized aircraft cabin and is capable of measuring background tropospheric sigma-sp at an averaging time of 1 min. For example, in a typical atmospheric profile the instrument can measure values of about 10(-4) m-1 with a time resolution of 2 s in a polluted region, and about 10(-7) m-1 with a time resolution of 1 min in a clean region. This sensitivity is made possible by: (1) subtracting in real time the air Rayleigh scattering from the total scattering signal by continuously measuring pressure and temperature in the sampling volume of the instrument; (2) correcting for the dark count and sensitivity of the photomultipliers using a rotating shutter; and (3) using a beam splitter arrangement to allow simultaneous detection by three photomultipliers. A laboratory measurement of instrument noise suggests a 550-nm noise level of about 5 x 10(-8) m-1 at an averaging time of 1 min.