Assessing Optical Properties and Refractive Index of Combustion Aerosol Particles Through Combined Experimental and Modeling Studies

Abstract

The variability of optical properties of combustion particles generated from a propane diffusion flame under varying fuel-toair (C/O) ratios was studied with a three-wavelength nephelometer, a particle soot absorption photometer, and an integrating sphere photometer. Information on particle size distribution, morphology, and elemental carbon to total carbon (EC/TC) ratios were obtained from scanning mobility particle sizer measurements, transmission electron microscopy analyses, and thermal-optical analyses. Particles generated under a low C/ O ratio (0.22) showed high elemental carbon fraction (EC/TC D 0.77) and low brown carbon to equivalent black carbon (BrC/ EBC) ratio (0.01), and were aggregates composed of small primary particles. Rayleigh-Debye-Gans theory reproduced experimental single-scattering albedo, v, absorption, and scattering A ngstr€ om exponents within 56, 3, and 18%, respectively. In contrast, particles produced under a high C/O ratio (0.60) showed low elemental carbon fraction (EC/TC D 0.09) and high BrC/EBC ratio >100, and were smaller and spherical in shape. Their optical properties were better modeled with Mie theory. By minimizing the difference between calculated and measured v and A ngstr€ om exponents, refractive indices of OC at three visible wavelengths were deduced. Contrary to the widely accepted assumption that refractive index of BC is wavelength independent, BC-rich particles exhibited absorption A ngstr€ om exponent >1.0 which implies some degree of wavelength dependence.

Optical Properties of Biomass Burning Aerosols: Comparison of Experimental Measurements and T-Matrix Calculations

“…These values show very little wavelength dependence, which was similar to the observations. Similar disagreements between the T-matrix calculations and experiments were observed in soot produced by burning ethylene [58] and propane [46]. All of the modeled SSA values were less than the experimental values for lacey soot, fresh soot particles, (~41%), compact soot (~25%) [58], and propane soot (56%).…”

“…Other researchers used a combination of the Rayleigh-Debye Gans (RDG) theory, which treats particles as fractal-like aggregates of small spheres, and Mie theory, to study the optical properties of combustion particles from propane. However, this method produced single scattering albedo (SSA) values that were underestimated by 56% when compared with the experimental results [46]. The imaginary part of the refractive index also showed a wavelength dependence, which decreased with increasing wavelength.…”

“…It has also been shown that the refractive index of brown carbon has some degree of wavelength dependence, contrary to widely held assumptions of wavelength independence [46]. One common method of retrieving the refractive index is based on the work by Raziq et al [47].…”

Recommendations for reporting "black carbon" measurements

“…While BC is characterized by a low value ofå ap between 1.0 and approximately 1.5 (Kirchstetter et al, 2004;Schnaiter et al, 2006;Kim et al, 2012), organic carbon-containing aerosol may show strong light absorption in the blue to ultraviolet spectral range (Kirchstetter et al, 2004;Graber and Rudich, 2006;Adler et al, 2010;Chen and Bond, 2010;Kim et al, 2012) associated withå ap values as high as 7 and beyond for the visible range. However, in a recent paper, Lack and Langridge (2013) investigate the uncertainties of using the method of separating BC and organic non-BC light-absorbing species byå ap values.…”

Response of real-time black carbon mass instruments to mini-CAST soot

“…Mini-CAST soot properties, such as mean agglomerate size, morphology, and EC concentrations, can be varied over a wide range by changing the air-fuel ratio and diluting the fuel with nitrogen (Moore et al 2014). While the burner settings for generating soot with the highest EC content vary for the different models, for any mini-CAST, EC content decreases with decreasing mean agglomerate size (Crayford et al 2013;Maricq 2014;Moore et al 2014;Kim et al 2015). Mini-CAST conditions that produce the largest particles (geometric mean diameter of the agglomerates, d g , typically> 100 nm) have been found to well match the BC content of engine exhaust soot (Maricq 2014).…”