AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Study of the Truncation Issue in the Cavity Attenuation Phase Shift PMSSA Monitor using Two Novel Approaches
FENGSHAN LIU, David Snelling, Kevin Thomson, Gregory Smallwood, National Research Council Canada
Abstract Number: 181 Working Group: Instrumentation and Methods
Abstract To monitor black carbon (BC) mass concentrations from emission sources and in the atmosphere, it is desirable to have the real-time measurement capability. The recently developed cavity attenuated phase shift particulate matter single scattering albedo (CAPS PMSSA) monitor has shown good performance for real-time aerosol optical properties and black carbon mass concentration measurements. The scattered light by particles in the sampling glass tube of CAPS PMSSA is measured by an integrating sphere. However, similar to an integrating nephelometer, the scattering component of the measurement also suffers the truncation error due to the loss of scattered light in the forward and backward directions. Such loss can be significant if the particles scatter radiation predominately in the forward directions, which is the case for fairly large BC mass-fractal aggregates. It is therefore important to develop correction algorithms to quantify the truncation error for the purpose of improving the accuracy of CAPS PMSSA.
The theories developed so far in the aerosol science community to deal with the truncation issues have been based on simple considerations of the nephelometer geometry and the scattering phase function of spherical particles from the Mie theory. Such simplified models are unable to account for non-spherical shape of aerosol particles, such as BC particles, the interactions among aerosol particles in the sampling tube, as well as to the effect of the boundary condition at the inner surface of the sampling tube. To overcome the difficulties of these simplified models and to more accurately estimate the truncation error of the CAPS PMSSA monitor, this study proposed two novel approaches: one is based on solving the radiative transfer equation (RTE) and the other is based on a ray-tracing (RT) algorithm with detailed treatment of boundary conditions at the glass tube.
Numerical calculations of the scattering loss were conducted for different particle sizes characterized by the asymmetry parameter of the scattering phase function using the geometrical dimensions of the typical Aerodyne CAPS PMSSA at 660 nm. The two approaches developed in this study predicted larger scattering loss than the simplified theoretical estimate in the literature. Results of the two approaches were compared and their advantages and disadvantages were also discussed.