An Inter-Comparison of Two Black Carbon Aerosol Instruments and a Semi-Continuous Elemental Carbon Instrument in the Urban Environment

Figures & data

TABLE 1 Published values for the specific attenuation cross-section of ambient aerosols

σ ATN (m^2g^− 1)	Optical method	λ (nm)	Thermal method	Authors
9.8 to 25.4	Aethalometer/ PSAP	550 ^a	Cachier	(Liousse et al. 1993)
5 to 25	PSAP	550 ^a	NIOSH	(Quinn et al. 2004)
5.6 to 10.8	PSAP	550 ^a	EGA	(Mayol-Bracero et al. 2002)
6.6 to 7.0	Aethalometer	880	Cachier	(Kuhlbusch 1995)
3 to 9	PSAP	550 ^a	NIOSH	(Huebert et al. 2003)
6.4 to 20.1	Aethalometer/ PSAP	550 ^a	Various	(Sharma et al. 2002)
9.3	Aethalometer	880	Cachier	(Lavanchy et al. 1999)
5.9 to 54.8	Aethalometer	880	Sunset	(Jeong et al. 2004)

FIG. 1 Comparisons of semi-continuous thermal EC (Sunset Labs instrument) with integrated filter measurements of EC (Figure 1a) and with absorption coefficients (b_ap) as determined by two co-located PSAPs (Figures 1b and 1c). PSAP data is segregated by transmittance values and by instrument in Figures 1b and 1c, respectively. The resulting slope of these plots represents the specific attenuation cross-section (σ_ATN). Two PSAPs were used sequentially during the study. Regression statistics for each subset are available in Table S1 in the supplemental materials accompanying this article.

FIG. 2 Comparison of absorption coefficients (b_ap) determined by two co-located Aethalometers. Aethalometer #1 utilized the standard “high-sensitivity” collection spot while Aethalometer #2 was fitted with the optional “extended-range” collection head. Six of the seven available wavelengths are shown. All values are in Mm⁻¹.

FIG. 3 Comparison of absorption coefficients determined by PSAP and Aethalometer. PSAP data is segregated by transmittance in Figure 3a and by instrument in Figure 3b. Note that while the correlation between the two instruments is strong, the slope demonstrates a significant deviation from a 1:1 relationship. Regression statistics for each subset are available in Table S1 in the supplemental materials accompanying this article.

FIG. 4 Comparison of hourly thermal EC (Sunset Labs RT instrument) and absorption coefficients as determined by Aethalometer #1. Although significant zero-offsets are observed, a strong correlation between attenuation and thermal EC across a wide range of wavelengths is demonstrated by these plots. As in Figure 1b and Figure 1c, the slopes represent the specific attenuation cross section (σ_ATN). Six of the seven available wavelengths are shown.

FIG. 5 The wavelength dependence of the slopes (σ _ATN) and intercepts shown in Figure 4.

FIG. 6 Comparison of the performance of PSAP and Aethalometer filters. Figures 6a and 6b show a side-by-side comparison of absorption coefficients and transmittance values, respectively, from two co-located PSAPs both using the tissue-glass filters. Figures 6c and 6d show side-by-side comparisons of b_ap values and transmittance values from the same two instruments when one of the tissue-glass filters is replace with the tissue-quartz filter used in the Aethalometer (in effect, changing both reference and sample filters). Dashed lines in the transmittance plots represent the 1:1 line.

TABLE 2 Specific attenuation cross-section (σ_ATN) measured in this study

Wavelength (nm)	Slope (σ ATN) (m^2g^− 1)	Intercept (Mm^− 1)	R^2
370	34.50 ± 0.61	10.6 ± 10.83	0.83
470	30.16 ± 0.50	8.3 ± 10.68	0.85
520	27.50 ± 0.45	7.36 ± 0.61	0.88
565 (PSAP)	18.26 ± 0.46	4.35 ± 0.59	0.83
590	24.93 ± 0.40	6.52 ± 0.54	0.86
660	23.73 ± 0.37	6.11 ± 0.51	0.86
880	18.54 ± 0.38	4.62 ± 0.38	0.87
950	17.04 ± 0.35	4.15 ± 0.35	0.88
PSAP 550^a	17.92 ± 0.45	4.24 ± 0.58	0.83
Aethalometer 550 ^a , ^c	17.77 ± 0.34	4.77 ± 0.48	0.86

FIG. 7 Diurnal variations in the light absorbing properties of ambient aerosols during SOAR along with selected bulk chemical measurements. Similar patterns in weekday σ _ATN are observed in data from the Aethalometer and PSAP (Figure 7a). These patterns appear related to the OC/EC ratio (Figure 7b) and changes in Å_ap (Figure 7e) but unrelated to average OC (Figure 7c) and EC (Figure 7d) concentrations. Error bars shown represent standard error. n = 504 hours.

FIG. S1 Specific attenuation cross section (σ_ATN) values for Fridays demonstrate two distinct populations. The morning values (open circles) demonstrate a low attenuation cross section, presumably due to an infusion of fresh, externally mixed LAC from morning rush-hour traffic. During the afternoon and evening (closed circles), σ_ATN is distinctly lower, representing the dominance of externally mixed LAC. The result of these distinct populations of σ_ATN is a significant intercept in the linear relationship between ATN and EC mass concentration. The slope of the b_ap/EC line during the morning (dashed line) is significantly lower than that of the afternoon/evening line (upper line). The best fit line though all points results in an intercept due to the influence of morning data and this may be a primary cause of the intercepts seen in the relationship between ATN reported by both the PSAP and the Aethalometer and EC mass concentration.

TABLE S1 Regression statistics for subsets of data shown in Figure 1 and Figure 3. All statistics were calculated using SPSS for Windows v.13.0

Figure	Y-axis (Mm^− 1)	X-axis	Slope ± Unc	Intercept ± Unc	R^2	n
1B	bap (PSAP) All Transmittances	Thermal EC (μ g m^− 3)	18.24 ± 0.46	4.35 ± 0.59	0.83	328
	bap (PSAP) Transmittance (0.30 to 0.49)	Thermal EC (μ g m^− 3)	15.73 ± 0.96	5.08 ± 1.08	0.74	94
	bap (PSAP) Transmittance (0.50 to 0.69)	Thermal EC (μ g m^− 3)	16.76 ± 0.93	6.58 ± 1.09	0.73	120
	bap (PSAP) Transmittance (0.70 to 1.00)	Thermal EC (μ g m^− 3)	17.47 ± 0.65	5.87 ± 1.04	0.86	114
1C	bap PSAP #1 & #2	Thermal EC (μ g m^− 3)	18.24 ± 0.46	4.35 ± 0.59	0.83	328
	bap PSAP #1	Thermal EC (μ g m^− 3)	16.98 ± 0.47	5.54 ± 0.62	0.81	303
	bap PSAP #2	Thermal EC (μ g m^− 3)	20.08 ± 1.27	0.00*	0.91	25
3A	bap (PSAP) all Transmittances	bap Aethalometer (590 nm) (Mm^− 1)	0.71 ± 0.02	0.00*	0.86	331
	bap (PSAP) Transmittance (0.30 to 0.49)	bap Aethalometer (590 nm) (Mm^− 1)	0.60 ± 0.24	0.00*	0.87	94
	bap (PSAP) Transmittance (0.50 to 0.69)	bap Aethalometer (590 nm) (Mm^− 1)	0.65 ± 0.02	0.00*	0.86	121
	bap (PSAP) Transmittance (0.70 to 1.00)	bap Aethalometer (590 nm) (Mm^− 1)	0.80 ± 0.02	0.00*	0.92	116
3B	bap PSAP #1 & #2	bap Aethalometer (590 nm) (Mm^− 1)	0.71 ± 0.02	0.00*	0.86	331
	bap PSAP #1	bap Aethalometer (590 nm) (Mm^− 1)	0.72 ± 0.02	0.00*	0.87	306
	bap PSAP #2	bap Aethalometer (590 nm) (Mm^− 1)	0.85 ± 0.04	0.00*	0.95	25

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