Scattering Channel Truncation in CAPS PMssa Monitor

Brian Heffernan, Stephen Jones, Fred Bacon, Andrew Freedman, TIMOTHY ONASCH, Aerodyne Research, Inc.

     Abstract Number: 492
     Working Group: Instrumentation and Methods

Abstract
The CAPS PMssa monitor measures both total particle optical extinction and the fraction of that extinction which is scattering – the single scattering albedo. Extinction is measured using an optical cavity defined by two parallel, nearly confocal, high reflective mirrors. Modulated, incoherent LED light travels more than 1 km through the sample before leaking out of the cavity. Particle leaden sample air enters the extinction cell, travels down the cell axis into and out of an integrating sphere, and exits the extinction cell. A transparent glass tube in the middle of an integrating sphere keeps the particle leaden flow separate from the integrating sphere’s inner scattering surface. The integrating sphere acts as an inverse nephelometer, collecting light scattered by particles within the sample cell while the LED is off. Light scattered by particles within the integrating sphere toward the entrance/exit ports is lost, causing truncation of the collected scattering angles in the forward/backward directions. Light scattered by particles in the sample cell, but outside the integrating sphere, can enter into the integrating sphere, partially offsetting light lost through this truncation.

Several research groups have recently investigated the scattering angle truncation in the CAPS PMssa monitor as a function of particle size with varying approaches to model the results, including the effects of the glass tube. We have undertaken a modelling effort based on complete ray-tracing efforts using Zemax Optics Studio to recreate the complete monitor geometry and scattering effects. The ray-tracing model results will be presented, providing insight into the factors controlling the truncation in the CAPS PMssa monitors and enabling sensitivity analyses.