AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Variation of Mass Absorption Coefficient and Internal Structure of Laboratory Generated Soot Particles with Mass
RAMIN DASTANPOUR, Steven Rogak, Ali Momenimovahed, Kevin Thomson, Jason S. Olfert, University of British Columbia
Abstract Number: 86 Working Group: Combustion
Abstract Light absorption and scattering properties of combustion generated soot particles are important for accurate estimation of emission rates and are used as the inputs for climate models. For decades, mass absorption coefficient (MAC) of soot particles has been assumed to be independent of particle size and mass. MAC numbers approximately in the range of 6 to 8 (m$^2g$^(-1) at a wavelength of 660 nm) have been reported in literature. Here we consider the possibility that particles produced by a single source can have varying optical properties.
Soot particles were produced by combustion of CH$_4 and CH$_4+N$_2 in air, within a laminar inverted diffusion flame. Particles were diluted and passed through a catalytic stripper, charged by a unipolar diffusion charger, and mass-classified by a centrifugal particle mass analyser (CPMA). Extinction and scattering cross sections were measured by Cavity Attenuated Phase Shift Spectrometer (CAPS-PMssa). Mobility size distributions of mass classified particles were used in combination with effective density measurements for the calculation of arithmetic mean mass for each mass to charge ratio. Primary particle sizing and morphology characterisation were performed by transmission electron microscopy (TEM). Variations of the internal structures of size classified particles were also investigated by a combination of high resolution TEM (HRTEM), and Raman spectroscopy.
Here, for the first time we have shown that MAC changes with the particle mass. Ensemble average MAC numbers of 6.2 and 7.5 (m$^2g$^(-1) at 660 nm) were calculated for the two operating conditions (methane with and without N$_2 dilution, respectively), but MAC varies with size for both sources, and the size-resolved MAC values lie on the same curve for both sources. MAC number increases from 4.5 to 8.5 (m$^2g$^(-1)) with the particle mass increasing from 0.05 to 8 (fg) (roughly an order of magnitude increase in mobility diameter).