Facile Method for Determining the Aspect Ratios of Mineral Dust Aerosol by Electron Microscopy

Figures & data

TABLE 1 Average aspect ratios and standard deviations for the minerals used in this study for both the top-down and side-on orientations

Species	Aspect ratio top	Aspect ratio side
Calcite	1.29 ± 0.20	1.27 ± 0.20
Quartz	1.38 ± 0.28	1.64 ± 0.49
NX-Illite	1.44 ± 0.32	4.80 ± 2.23
Kaolinite KGa-1b	1.35 ± 0.22	7.98 ± 3.94
Kaolinite KGa-2	1.39 ± 0.27	8.89 ± 3.72
Montmorillonite STx-1b	1.38 ± 0.25	7.49 ± 4.20
Montmorillonite SWy-2	1.44 ± 0.29	9.14 ± 4.67

FIG. 1. (a) A representative scanning electron microscopy (SEM) image of the calcite particles in the top-down orientation. (b) A representative SEM image of the calcite particles in the side-on orientation, imaged at an angle of 5˚ from normal. (c) The distribution of calcite aspect ratios obtained from SEM images. The fraction of particles by number is plotted vs. aspect ratio. (d) SEM images of quartz particles in the top-down orientation and the (e) side-on orientation. (f) The distribution of quartz aspect ratios obtained from the SEM images. The tick marks label the bin to the left of the number, making the aspect ratio of the first bin 1.0.

FIG. 2. SEM images of KGa-1b (kaolinite) in the (a) top-down and (b) side-on orientations. (c) The distribution of aspect ratios determined for KGa-1b in the two orientations. (d) SEM images of KGa-2 (kaolinite) in the top-down and (e) side-on orientations. (f) The distribution of aspect ratios for KGa-2. The tick marks in (c) and (f) denote the bin to the left of the number.

FIG. 3. SEM images of NX-illite in the (a) top-down and (b) side-on orientations. (c) The distribution of aspect ratios determined for NX-illite from the SEM images. (d) SEM images of STx-1b (montmorillonite) in the top-down and (e) side-on orientations. (f) The measured distribution of aspect ratios for STx-1b. (g) SEM images of SWy-2 in the top-down and (h) side-on orientations. (i) The distribution of aspect ratios for SWy-2. For (c), (f), and (i), the tick marks denote the bin to the left of the number.

FIG. 4. Comparison of the aspect ratios for each species studied. For each type of mineral, the aspect ratios observed for the top-down and side-on orientations are shown. In the box and whiskers plots, the center line is the average of the data, the bottom of the box is the 25th percentile, the top of the box is the 75th percentile, and the whiskers correspond to the 5th and 95th percentiles.

TABLE 2 Comparison of measured average aspect ratios to literature values for the clay minerals. The NX-illite is compared to measurements of illite

Species	Top	Side	Lit. top	Ref.	Lit. side	Ref.
NX-Illite	1.44	4.80	min = 2.10 max = 5.33	Inoue and Kitagawa (1994)Nadeau (1985)	min = 11.3 max = 120	Becket et al. (1997) Nadeau (1985)
Kaolinite	1.35–1.39	7.98–8.89	min = 1.29 max = 1.47	Robertson et al. (1954) Robertson et al. (1954)	min = 2.8 max = 17.67	Becket et al. (1997) Nadeau (1985)
Montmorillonite	1.39–1.44	7.49–9.14	1.68	Nadeau (1985)	min = 3.10 max = 448	Becket et al. (1997) Nadeau (1985)

TABLE 3 Specific surface area (SSA) measurements using SEM, BET, and AFM. The SSA calculations from the SEM measurements are performed assuming the particles are hexagonal prisms, rectangular prisms, and spheroids. NX-Illite is compared to BET measurements of illite

Species	SSA hex. prism (m^2/g)	SSA rect. prism (m^2/g)	SSA spheriod (m^2/g)	BET	Ref.	AFM	Ref.
Calcite	8.8	12.0	12.2	—		—
Quartz	9.1	11.9	12.2	—		—
NX-illite	33.5	38.3	45.6	min = 94 max = 104.2	Steudel et al. (2009) Broadley et al. (2012)	—
KGa-1b	19.6	21.5	27.4	min = 10.7 max = 16.1	Schuttlefield et al. (2007) Sanders et al. (2010)	10.1	Bickmore et al. (2002)
KGa-2	28.7	31.1	40.4	min = 22.4 max = 24	Bereznitski et al. (1998) Madsen (1977)	24.1	Bickmore et al. (2002)
STx-1b	45.1	49.6	62.9	min = 82.9 max = 83.8	Sanders et al. (2010) van Olphen and Fripiat (1979)	—
SWy-2	54.5	58.9	77.1	min = 31.82 max = 34.0	van Olphen and Fripiat (1979) Sanders et al. (2010)	—

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