Amplification of Light Absorption of Black Carbon by Organic Coating

Figures & data

TABLE 1 Measured and calculated absorption cross sections (σ_abs) and mass absorption cross sections (C _abs) of graphite particles. The units of σ_abs are 10^–12 m², C _abs are m² g^–1

D c (nm)	r ^2	σabs,mea	σabs,cal (m graphite)	σabs,cal (m graphite-air)	C abs,mea	C abs,cal (m graphite)	C abs,cal (m graphite-air)
185	0.99	0.046	0.047	0.045	6.09	6.22	5.95
234	0.91	0.075	0.067	0.071	4.90	4.38	4.64
281	0.92	0.103	0.098	0.102	3.89	3.69	3.85

FIG. 1 Absorption coefficient (b _abs) of uncoated graphite particles with D _c of 185, 234, and 281 nm. Model calculations by Mie theory are also shown.

FIG. 2 Size distribution of graphite particles (D _c = 185 nm) coated by oleic acid measured with the laser of the photo-acoustic absorption spectrometer (PASS) on or off.

TABLE 2 Terminology of effective diameters used in this study

Terminology	Method	Description
D c	SP2	BC mass-equivalent core diameter
D p	SP2	Optical shell diameter
D p/D c	SP2	Shell/core diameter ratio determined by SP2
Core mobility diameter	DMA	Core diameter selected by DMA1
Shell mobility diameter	DMA	Shell diameter selected by DMA2

FIG. 3 Schematic of the laboratory experiment for measuring absorption of organics-coated graphite particles. The system uses differential mobility analyzers (DMA1 and DMA2), a single particle soot photometer (SP2), and a photo-acoustic absorption spectrometer (PASS).

FIG. 4 (a) Distribution of graphite mass-equivalent core diameter (D _c) and (b) distribution of shell/core diameter ratio (D _p/D _c) of oleic acid-coated graphite particles with D _c of 185 nm and the shell mobility diameters of 200, 300, 400, and 500 nm selected by DMA2.

TABLE 3 Uncertainties of calculated and observed absorption coefficient (b _abs), mass absorption cross section (C _abs), and absorption amplification factor (γ)

	b abs		C abs		γ
	Observed	Calculated	Observed	Calculated	Observed	Calculated
Uncertainty	7%	15%	19%	4%	25%	15%

FIG. 5 Observed absorption amplification factor (γ) for organics-coated graphite particles with D _c of (a) 185, (b) 234, and (c) 281 nm. The bottom axis shows the median of the shell/core diameter ratio (D _p/D _c) and the top axis shows the coating thickness (nm). Shell/core model calculations are also shown for m _graphite and m _graphite-air.

FIG. 6 Comparison of the b _abs (Mm^–1) calculated using Mie theory and that observed by PASS for (a) uncoated and (b) coated graphite particles. For coated graphite, b _abs was calculated assuming the shell/core model and no absorption enhancement. The dotted 1:1 line is a visual guide.