Evaluation of a Method for Measurement of the Concentration and Size Distribution of Black Carbon Particles Suspended in Rainwater

Figures & data

FIG. 1 Schematic diagram of the experimental setup for measuring BC particles in rainwater. (Color figure available online.)

FIG. 2 Expanded diagram of the ultrasonic nebulizer (U-5000AT). (Color figure available online.)

FIG. 3 Mass size distributions of (a) Aqua Black 001 and (b) Aqua Black 162. Approximately 36% and 17% of the BC particle mass were not detected by the SP2 (BC < 70 nm) for AB-001 and AB-162, respectively. Bars indicate 1 σ values.

FIG. 4 Efficiencies of the nebulizer in extracting BC measured as a function of BC mass concentration in water. Dashed lines show the average values. Bars indicate 1 σ values of each data.

FIG. 5 Efficiencies of the nebulizer for Aqua Black 001 and Aqua Black 162 as a function of the liquid flow rate of the peristaltic pump. Open and closed circles stand for the average values. Dashed and solid bars indicate the range of the data and 1 σ values, respectively.

FIG. 6 Measured BC concentration in rainwater versus dilution ratio. The solid line indicates linear fitting.

FIG. 7 BC concentrations in airflow measured by the SP2 versus SRFA concentration added to the rainwater sample.

FIG. 8 Changes in the BC mass concentration measured by the SP2 for about 100 min. The solid line and dashed lines stand for the average and 1 σ values, respectively.

FIG. 9 Measured BC concentrations in rainwater during storage of a few months. The solid line and dashed lines stand for the average and 1 σ values, respectively.

TABLE 1 Sample water used in this study. Rainwater samples were collected at the University of Tokyo campus in December 2009 and in October and December 2010 and at Cape Hedo in Okinawa in June 2010. BC mass concentrations in rainwater samples were determined by the efficiency shown in Section 3

Sample name	Sample number	BC mass concentration in sample water (μg L^−1)	PSL or BC number concentration after nebulization (STP cm^−3)	BC mass concentration after nebulization (STP μg cm^−3)
AB-001	8	3.5–35		4.1 × 10^−8–4.9 × 10^−7a
AB-162	8	3.4–34		6.8 × 10^−8–6.9 × 10^−7a
PSL	3		PSL: 4.5 × 10^2–6.6 × 10^2
Rainwater	9	9–208	BC: 6.1 × 10–7.9 × 10^2 ^a	1.6 × 10^−7–3.0 × 10^−6

FIG. 10 Expanded view of the process of evaporation of droplets containing (a) single BC particle and (b) multiple particles.

TABLE 2 Ratios in number and mass concentrations of doublets or triplets of PSL particles to the total PSL particles measured by the SP2

Diameter (nm)	Number ratio (%)	Mass ratio (%)
202	11	18
294	10	19
402	1	2

TABLE 3 Slopes and correlation coefficients (R ²) of the fitted lines to MMD and σ _gm shown in Figures 11, 12, 15, and 16

Sample name	Slope for MMD [nm (μg L^−1)^−1]	R ^2 for MMD	Slope for σ gm [(μg L^−1)^−1]	R ^2 for σ gm
AB-001	5.1 × 10^−2	0.40	−2.5 × 10^−4	0.09
AB-162	7.9 × 10^−2	0.94	−6.0 × 10^−4	0.49
BC in rainwater	7.7 × 10^−2	0.76	−6.8 × 10^−4	0.92
BC in rainwater	1.7 × 10^−1 (for CMD)	0.86 (for CMD)	−4.6 × 10^−4 (for σgc)	0.32 (for σgc)

FIG. 11 Mass median diameter (MMD) and geometric standard deviation (σ _gm) of Aqua Black 001 and Aqua Black 162 versus the BC concentration in water. The solid and dashed lines indicate linear fitting to the MMD and σ _gm, respectively.

FIG. 12 (a) Normalized number and (b) mass size distributions of BC in original rainwater (208 μg L⁻¹) and in diluted rainwater (35 μg L⁻¹). Error bars show 1 σ values. Some error bars were removed for clarity.

FIG. 13 (a) CMD and MMD and (b) σ _gc and σ _gm of BC in diluted rainwater versus the BC concentrations in rainwater. Solid lines indicate linear fitting.

TABLE 4 BC size distribution in rainwater collected in Tokyo in December 2010 and average BC size distribution in ambient air in Tokyo in August and September 2009

Sample name	CMD (nm)	σ gn	MMD (nm)	σ gm
BC in rainwater	98	1.60	191	1.59
BC in ambient air	64	1.66	146	1.82

TABLE 5 BC concentrations and size distributions in rainwater along with rain rate on December 11, 2009, at the University of Tokyo campus

December 11, 2009 (LT)	Rain duration (h)	Rain amount (mm)	Rain rate (mm h^−1)	BC concentration (μg L^−1)
No. 1 (0945–1325)	3.67	6.0	1.63	123
No. 2 (1350–1550)	2.00	3.5	1.75	51
No. 3 (1550–1750)	2.00	4.5	2.25	16
No. 4 (1750–1950)	2.00	8.0	4.00	7
No. 5 (1950–2150)	2.00	5.5	2.75	8
No. 6 (2150–2250)	1.00	1.0	1.00	15
Total	12.67	28.5	2.25	39
Note: LT, local time.