Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement

Figures & data

Table 1 A summary of the instrumental analysis during the measurement campaign

Instruments	Contents^a	Resolution	Particle type	Instrument principle
Single Particle Soot Photometer	Refractory black carbon (rBC)	15 min	PM2.5 cyclone	Intracavity Nd:YAG laser light at 1064 nm to quantify the rBC mass in individual rBC-particles.
Photoacoustic Extinctiometer	Absorption/scattering coefficient	15 min	PM2.5 cyclone	A laser beam in the acoustic chamber of the instrument heats absorbing aerosol particles, and this heating produces pressure waves that are detected with a microphone.
Aerosol Chemical Speciation Monitor	Organics, SO4^2−, NO3^−, NH4^+, and Cl^−	1 h	PM2.5 cyclone	Individual particles (∼40–1000 nm) into a beam, and non-refractory components are vaporized, ionized, and finally analyzed with a quadrupole mass spectrometer.
Environmental Beta Attenuation Monitor	PM2.5	1 h	PM2.5 impactor	Attenuation of beta ray energy as aerosol mass accumulates.
Automatic Weather Station	Visibility, humidity, wind, temperature	1 min		Visibility sensor (Model PWD22), RH/temperature probe (Model QMH101), and wind sensor (Model QMW101-M2).

^aThe concentrations of PM_2.5, rBC, organics, SO₄²⁻, NO₃⁻, NH₄⁺, and Cl⁻ as well as light absorption, were corrected to standard temperature and pressure (STP: 273.15 K, 1013.25 hPa).

Table 2 Summary of rBC concentrations and number fraction of thickly coated rBC particles (f_BC) as well as related chemical concentrations of particulate matter

	Clean periods		Polluted periods		All samples
Parameter	Mean	SD	Mean	SD	Mean	SD
rBC (μg m^−3)	1.3	0.7	9.9	7.3	8.8	7.3
fBC (%)	38.3	5.7	48.6	6.9	47.4	7.6
Organics (μg m^−3)	13.8	6.8	82.1	49.7	73.8	46.8
SO4^2– (μg m^−3)	2.9	1.4	31.3	23.2	18.3	15.3
NO3^− (μg m^−3)	2.9	1.8	17.0	8.6	13.6	7.6
NH4^+ (μg m^−3)	3.7	1.5	23.5	13.0	16.7	10.0
Cl^− (μg m^−3)	1.4	0.9	8.7	5.4	7.6	5.4
PM2.5 (μg m^−3)	31.9	17.3	213.6	115.8	191.2	123.9

Clean periods: visibility greater than 10 km.

Polluted periods: visibility less than 10 km.

SD: standard deviation.

FIG. 1. (a) Hourly variations of rBC mass concentration and (b) number fraction of thickly coated rBC particles (f_BC). In each panel, the lower and upper edges of the boxes denote the 25% and 75% percentiles, respectively. The black and grey lines indicate the median and mean values, with vertical bars showing the 10th and 90th percentiles. LST stands for local standard time.

FIG. 2. (a) Contributions of organic/rBC, SO₄²⁻/rBC, NO₃⁻/rBC, NH₄⁺/rBC, and f_BC in the three-factor PMF model; (b) time series of factor contributions to the f_BC; (c) contributions of each factor to f_BC during clean and polluted periods.

FIG. 3. (a) The linear relationship between light absorption and rBC concentration during the campaign. (b) Frequency distribution of rBC mass absorption cross section (MAC_BC).

Table 3 Comparison of mass absorption cross sections of BC (MAC_BC) from different studies

			Measurement technique^a
Location	Type of sample	Sampling period	σabs	BC	λ (nm)^b	MAC (m^2 g^−1)	Reference
Xi’an, China	Urban	December 2012–January 2013	PAX	SP2	870	7.6	This study
Shenzhen, China	Urban	August–September 2011	PASS	SP2	532	6.5	Lan et al. 2013
Fresno Supersite, USA	Urban	August–September 2005	PASS	EC/OC analyzer	532	8.1	Chow et al. 2009
Pasadena, CA, USA	Urban	May–June 2010	AE	EC/OC analyzer	532	5.7	Thompson et al. 2012
The Jungfraujoch, Switzerland	Remote	February–March 2007	MAAP	SP2	630	10.2	Liu et al. 2010
United Kingdom	Plumes	April 2008	PSAP	SP2	550	15.4	McMeeking et al. 2011
Mexico City	Plumes	March 2006	PSAP	SP2	660	10.6	Subramanian et al. 2010

^aPAX: Photoacoustic extinctiometer; PASS: photoacoustic soot spectrometer; PSAP: particle soot absorption photometer; MAAP: multi-angle absorption photometer; AE: aerosol extinction; SP2: single particle soot photometer.

^bλ stands for wavelength of light.

FIG. 4. The rBC mass absorption cross section (MAC_BC) as function of number fraction of thickly coated rBC (f_BC).

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