AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Limits to the Absolute Accuracy of the Optical Closure Technique
JAMES RADNEY, Christopher Zangmeister, National Institute of Standards and Technology
Abstract Number: 285 Working Group: Instrumentation and Methods
Abstract The optical closure technique represents a general methodology that has received wide usage in aerosol science. Historically, the optical closure technique was only used in the forward direction where integrated scattering coefficients were calculated from measured size distributions of a spherical non-absorbing aerosol population (i.e. ammonium sulfate or dioctyl sebacate) of well-known refractive index. This technique was most commonly used for nephelometer calibrations and typically resulted in scattering coefficient uncertainties on the order of 10 %.
In recent years, the optical closure technique has seen increasing use in the reverse direction to calculate a refractive index (using Mie theory) for an arbitrarily-shaped aerosol from parallel measurements of scattering coefficients, absorption coefficients and size distributions. Here, we compare the accuracy of refractive indices calculated using reverse optical closure to those calculated from size- and mass-selected particles; test cases consisted of pure ammonium sulfate, pure nigrosin, and 50:50 (m/m) and 75:25 (m/m) mixtures of ammonium sulfate and nigrosin. We find, in agreement with previous studies, that the optical closure method can suitably capture the refractive index of the pure, non-absorbing ammonium sulfate. For the absorbing particles, average absolute differences in refractive index are on the order of 3 % and 14 % in the real and imaginary components, respectively, without applying appropriate statistical methods for data analysis. Notably, these uncertainties were obtained for spherical, homogeneously mixed particles using a calibrated scanning mobility particle sizer with an accuracy better than a couple of percent (roughly half the width of a typical size bin at 64 bins/decade resolution). Lower resolution or accuracy significantly increases the magnitude of these errors. While these uncertainties in refractive index are modest for these idealized particles, propagating these values forward into calculated absorption and scattering coefficients for a theoretical distribution results in absolute errors of up to 50 % that are strongly size dependent. These sensitivities imply that, for absorbing particles, the reverse optical closure technique should only be used obtain relative, not absolute, refractive indices unless large errors in forward calculated absorption and scattering coefficients (and albedos) are acceptable.