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

Figures & data

TABLE 1 Operating conditions of the CAST burner

Condition	Fuel (C3H8) flow rate (L min^−1)	Oxidation air flow rate (L min^−1)	C/O ratio	EC/TC	BrC/EBC	dCMD (nm)	σg
1	0.017	0.564	0.22	0.77	0.01	130	1.66
2	0.017	0.396	0.31	0.62	1.40	125	1.93
3	0.017	0.300	0.40	0.12	71	47	2.22
4	0.025	0.300	0.60	0.09	404	30	1.69
5	0.030	0.240	0.89	—	—	16	1.68

Flow rates for mixing gas (N₂), quenching gas (N₂), and dilution air were set to 0.000, 1.236, and 1.080 L min⁻¹, respectively. Stoichiometric air-fuel ratio and C/O ratio are 15.7 and 0.3, respectively. EC/TC and BrC/EBC ratios, and size distribution (d_CMD: count median diameter and σ_g: geometric standard deviation) of particles generated from different operating conditions are also shown. The BrC/BC ratio for Condition 5 could not be determined because of insufficient filter loading.

FIG. 1. Schematic diagram of the experimental setup. TD is a thermal denuder.

TABLE 2 Real (m_r) and imaginary (m_i) parts of the refractive index of black and organic carbon components of combustion particles

	Black Carbon (BC)		Organic Carbon (OC)
λ/nm	mr	mi	mr	mi
467	1.92	0.67	1.59 ± 0.02	0.11 ± 0.03
530	1.96	0.65	1.47 ± 0.02	0.04 ± 0.02
660	2.0	0.63	1.47 ± 0.02	0 + 0.01

The m_r and m_i values of BC were obtained from a polynomial fit to the data reported by Ackerman and Toon (1981) and those of OC were obtained from this experiment.

FIG. 2. (a) Elemental carbon to total carbon ratio (error bars indicate one standard deviation), (b) brown carbon to black carbon ratio (error bars indicate one standard deviation), and (c) count median mobility diameter (error bars represent the 25th and the 27th percentiles) plotted as a function of C/O ratio. The dotted curve is taken from the data of Schnaiter et al. (2006). The stoichiometric C/O ratio (C/O = 0.3) is also marked. The stoichiometric ratio separates burner conditions into fuel-lean and fuel-rich conditions.

FIG. 3. Mobility size distributions of particles generated from the CAST burner under different combustion conditions. The legend shows measured EC/TC ratios for these particles.

FIG. 4. TEM pictures of combustion particles generated under (a) C/O ratio = 0.22 and (b) C/O ratio = 0.6.

FIG. 5. Contour plots of ∑dev(%) for the 467–530 nm interval in the plane of (a) real and (b) imaginary RI values with the other two RIs fixed to the values shown on each graph. Hollow circles indicate location of the data points.

TABLE 3 Results of the sensitivity analysis for particles containing a high EC fraction

Parameter	ϵxi (%)	Source of ϵxi	Iω	Iσabs	Iσsca
a	20	TEM analysis	1.27	1.03	2.33
D	5	TEM analysis	−0.63	0.00	−0.67
ρBC	5	Bond and Bergstrom (2006)	−0.06	−0.19	−0.23
ρOC	50	Turpin and Lim (2001)	0.06	0.19	0.22
BC mr	5	Bond and Bergstrom (2006)	2.34	−1.06	1.42
BC mi	10	Bond and Bergstrom (2006)	−0.39	0.88	0.48
OC mr	7	Estimated from literatures	1.05	−0.47	0.62
OC mi	50	Estimated from literatures	−0.06	0.08	0.04
EC/TC ratio	10	Thermal-optical analysis	0.15	0.84	0.99

ϵ_xi is the percentage uncertainty associated with each parameter. ‘Source of ϵ_xi’ shows how ϵ_xi for each parameter was chosen. Input parameters used in the calculation of the influence coefficients (I_ω, Iσ_abs, Iσ_sca) were a = 5 nm, D = 1.74, ρ_BC = 1.8 g cm⁻³, ρ_OC = 1.0 g cm⁻³, BC m_r = 1.92, BC m_i = 0.67, OC m_r = 1.59, OC m_i = 0.11, and EC/TC ratio = 0.773.

FIG. 6. Single scattering albedo (ω) vs. elemental carbon to total carbon ratio for λ = 530 nm. Filled squares: experimental data (with error bars of one standard deviation). Hollow triangles: Mie theory. Hollow circles: RDG theory.

FIG. 7. (a) Absorption (α_abs) and (b) scattering (α_sca) Ångström exponents plotted as a function of elemental carbon to total carbon ratio for the 467–660 nm interval. Filled squares: experimental data (with error bars of one standard deviation). Hollow triangles: Mie theory. Hollow circles: RDG theory.

FIG. 8. Imaginary component of the refractive index (m_i) for organic carbon as a function of wavelength. Filled circles: Kirchstetter (2004). Filled triangles: organic soluble matter, Adler et al. (2010). Hollow triangles: water soluble matter, Adler (2010). Filled squares: Sato (2003). Filled stars: from this study. The solid line is a polynomial fit through the Kirchstetter data with the equation m_i = 1.4304–6.49 × 10⁻³λ + 1.0126 × 10⁻⁵λ²–5.3939 × 10⁻⁹λ³.

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