Temporal Variation of Mass Absorption Efficiency of Black Carbon at Urban and Suburban Locations

The mass absorption efficiency (MAE) of black carbon (BC) was measured for the ambient aerosols in urban and suburban areas and exhaust plumes from motor vehicles from September 2010 to September 2011 in Xiamen, China. Ambient aerosols, collected on quartz fiber filters, were analyzed for BC loadings using an OC/EC Carbon Aerosol Analyzer and measured for optical attenuations with a laser/LED transmissometer. The MAE of BC in Xiamen shows a large temporal and spatial variability, varying from 0.47 to 5.15 m/g. In the same month (May 2011), the average MAE of BC in suburban area (0.50 m/g) was one third that in urban area (1.50 m/g), suggesting that the variance of MAE was related to the specific emission sources of BC. Here we report the large variation in MAE of BC measured during different sampling times and for different sources in Xiamen. The average MAE was 1.69 m/g and the coefficient of variance was 52%. Factors contributing to the variability were also discussed, as well as the uncertainties caused by different carbon analysis methods, working wavelengths for optical attenuation measurements, and relative humidity when measuring the optical attenuation of the aerosol filters. The results suggest that if a constant value of MAE within the range of 0.47–5.15 m/g is used to quantify the BC concentration, then this would bias the results with a maximum factor of 11. Site-specific values of MAE are recommended for long-term stationary monitoring of ambient BC in air quality monitoring stations.


INTRODUCTION
Black carbon (BC), or elemental carbon (EC), particles originate mostly from incomplete combustion of fossil fuels, biomass and biofuels, is a major contributor to atmospheric light absorption (Hansen et al., 1984;Ahmed et al., 2009).BC is the second strongest climate forcer after carbon dioxide (Ramanathan and Carmichael, 2008;Chen et al., 2012), however its role in climate change has a large uncertainty (Bernstein et al., 2007).BC aggravates visibility degradation when suspended in the ambient air (Wolff, 1981;Deng et al., 2012;Han et al., 2012) and affects regional climate (Menon et al., 2002).In addition, BC is a risk factor for adverse health effects (Jansen et al., 2005).Because of these important effects, accurate determination of BC mass concentrations is crucial to better evaluate its impact on climate, environment and public health.
Currently, Thermal-Optical Analysis (TOA) and Optical Analysis (OA) are the two mostly used methods for measuring ambient BC aerosols.Optical methods provide relatively convenient and rapid measurement of BC mass concentration, but they require knowledge of site-specific mass absorption efficiency (MAE) (Ram and Sarin, 2009).A wide range of values for MAE (2-25 m 2 /g) have been reported in the literature (Liousse et al., 1993;Sharma et al., 2002;Bond and Bergstrom, 2006;Schwarz et al., 2008), however, it is common practice to use a constant value of MAE at a given wavelength for the determination of BC mass concentration by optical methods.For example, the Aethalometer uses a value of 19 m 2 /g while particle soot absorption photometer (PSAP) uses a value of 10 m 2 /g to convert measured absorption into BC mass concentration (Petzold et al., 1997;Sharma et al., 2002).Bond et al. (2006) suggested a value of 7.5 ± 1.2 m 2 /g at 550 nm for MAE for uncoated soot particles.
Undoubtedly, there is a large variation in the MAE of BC depending upon where and when the BC aerosols were sampled.The large variability for the MAE suggests the need of using site-specific MAE for optical methods to precisely quantify BC mass concentration in ambient aerosol (Martins et al., 1998;Schmid et al., 2006).In this paper, we report the results of MAE measurements for samples collected at an urban site, a suburban site and in the exhaust plume of a diesel-powered truck.MAE was determined by a laser/LED transmissometer and a Semi-Continuous OC/EC Carbon Aerosol Analyzer.The range of site-specific MAE is documented and factors that contribute to the variability are discussed.Uncertainty of MAE measurement using this method is also analyzed.These results will be helpful to evaluate the accuracy of optical methods for quantification of BC at the specific sites.

Study Sites and Aerosol Samples Collection
Xiamen is a coastal city in the southeast part of Fujian Province, China.Jimei District is one of the six districts (Siming, Huli, Jimei, Haicang, Tong'an and Xiang'an Districts) under the administration of Xiamen.Jimei District is located in the geometric center of Xiamen, with convenient transportation and experiencing rapid urbanization.The field sampling for BC particles occurred at an urban site, Shigu Road, downtown of Jimei District and a suburban site, Institute of Urban Environment, Chinese Academy of Sciences (IUE, CAS) during September 2010 to September 2011 (Fig. 1).These two sites are 8 km from each other and had similar climate conditions during the same sampling periods.PM 2.5 was collected with a high-volume air sampler at a flow rate of 67.8 m 3 /hr ± 10% (Model HiVol-3000, ECOTECH, Victoria, Australia) on quartz fiber filters (20.3 cm × 25.4 cm, Whatman, Maidstone, England) at IUE (24°36′N, 118°03′E), the sampling site was located on the rooftop of a 3 m-high building, 40 m away from an intersection of two highways (Jimei Road and Haixiang Avenue).Another sampling site is on Shigu Road, downtown of Jimei District, which was located on a balcony of a second-floor flat of a 10 m-high building, 50 m away from one bus station.PM 2.5 was collected with a mid-volume air sampler (Model TH-150C III, Tianhong, Wuhan, China) on quartz fiber filters (Ф90 mm, Whatman, Maidstone, England).
The sampling durations in May 2011 at site IUE were 12 hours, i.e., 9:00 to 21:00 or 21:00 to 9:00 the next day.Other samples were collected with different sampling durations ranging from 3 hours to 24 hours in order to obtain different ATN values.The average RH during the entire sampling period was 67 ± 13%.For the fresh vehicle emissions, quartz fiber filters were mounted to the opening of the exhaust pipe of a diesel-powered heavy duty truck 4.26 L,9.3 ton) to directly sample particles.All the quartz fiber filters were pre-baked at 550°C for 6 hours to minimize the background carbon in each filter to avoid any overestimation of its carbonaceous content.

Measurements of Optical Attenuation and BC Loading
Optical attenuation method measures the light-absorbing component of the aerosol particles (Yasa et al., 1979), therefore the absorption of filter-based BC particle can be represented by optical attenuation (ATN) and is governed by the Beer-Lambert's law, according to Eq. (1): 0 100 ln where I 0 is the intensity of incident light and I is the transmitted light through the particle deposited filter.
All known optical methods are based on the assumption that light transmission through a particle-loaded filter is only sensitive to light absorption (Hansen et al., 1982).ATN of particle-loaded filters were measured using a custom laser/LED transmissometer at 532 nm and 880 nm wavelength respectively (The 532 nm wavelength laser was used as a substitute for 880 nm LED source when the LED was not available).The laser transmissometer consists of a laser module (Model GM-CF04-5, 532 nm, Beta Electronics, Ohio, USA) and a photon detector (Model DET36A, Thorlabs, New Jersey, USA) (Du et al., 2011).The LED transmissometer consists of a LED module (Model SL2420, 880 nm, Advanced Illumination, Rochester, UK) and the photon detector.In a dark room, a blank filter was placed between the laser/LED source and the detector, with the filter being in immediate contact with the detector (Fig. 2).The voltage output from the detector was measured.Subsequently, the same operation was repeated with a particle-loaded filter.The ATN of the particle layer was then calculated from the ratio of the two voltage outputs.
The light absorption of BC deposited on the filter is sensitive to different wavelength, so experimentally determined values of ATN differ considerably due to different wavelength.The wavelength dependence of MAE can be represented by a power-law relationship using an Ångstrom exponent (Å): where K is a constant, λ is the wavelength.The Ångstrom exponent (for short wavelengths 355/400/532 nm) ranged mostly from 1 to 1.6 for Southeast Asian particles, from 0.8 to 1.4 for North Indian pollution, and from 0.6 to 1 for South Indian air masses (Franke et al., 2003).A value of Å = 1.2 was used in this study according to our experimental results.Then MAE measured at a given wavelength (MAE λ ) could be used to calculate the MAE for another wavelength (MAE λ' ) with Eq. ( 3): Theoretically, any wavelength of light source may be used to determine MAE, but 550 nm and 880 nm are the most common wavelengths used in practice.In this study, a 532 nm laser transmissometer and an 880 nm LED transmissometer were used to quantify the ATN of the particle layer on the filter.MAE at 532 nm was converted to 880 nm using the Eq. ( 3).
Particulate carbon measurements are essential to the quantification of MAE.There are multiple methods to quantify carbon mass on filters: Oregon Graduate Institute thermal optical reflectance method (TOR) (Johnson et al., 1981), Interagency Monitoring of Protected Visual Environments (IMPROVE) TOR (Kim et al., 2011) and thermal optical transmittance method (TOT) (Chow et al., 1993(Chow et al., , 2001)), National Institute of Occupational Safety and Health (NIOSH) TOT (Chow et al., 2001), ACE-Asia TOT (Mader et al., 2003) etc. Different carbon analysis protocols yield different OC/EC fractions.BC loadings (L BC , μg/cm 2 ) in this study were determined using Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.).The temperature protocol we used in this study is NIOSH TOT.

Determination of MAE
The MAE of BC is calculated from the measured ATN and BC loading using Eq. ( 4): MAE has been reported to be dependent on the fraction of BC in the particles (Petzold et al., 1997), and the particle mass loading (Liousse et al., 1993) or ATN (Virkkula et  al., 2007).In addition, the specific environment where the particles were sampled (Liousse et al., 1993), such as urban, or rural, or coastal, also influences MAE value, probably because the aerosol mixing status and aging that are unique for each environment affect the optical properties of BC.In infrared spectrum, a linear relationship was found to hold between L BC and ATN when ATN fell below a certain level, for instance, ATN ≤ 200 (Gundel et al., 1984).However, the relationship becomes non-linear at high BC loadings and the degree of nonlinearity varied with location, season, and type of aerosol samples (Park et al., 2010).The measured ATN and L BC exhibit a significant linear relationship when ATN is below 200, indicating the validity of Beer-Lambert's law and BC as a principal absorbing component in aerosols.However, this linearity does not extend for ATN exceeding 200.Thus, samples with ATN more than 200 were not included in this study.
The shadowing effect of "filter-based" methods is a factor that has positive impact on MAE.As a result, an increased underestimation of the measured light signals occurs with increasing filter loads.One of the commonly used correction approaches was developed by Weingartner and Saathoff (2003) and Weingartner et al. (2003): (5) Fig. 2. Scheme of 880 nm wavelength LED transmissometer.
The factor C is used to correct for multiple scattering within a relatively clean filter.Weingartner and Saathoff (2003) and Weingartner et al. (2003) found that multiple scattering in the nearly unloaded fiber filter is responsible for enhanced light absorption by a factor of about 2.14.R(ATN) corrects for the shadowing effect, which is empirically defined by Eq. ( 6): where R(ATN) depends on parameter f.According to previous studies, for higher values of optical-attenuation (ATN > 10%), f = 1.103 is used during wintertime (December-March) and f = 1.114 for rest of the seasons to calculate R(ATN) (Sandradewi et al., 2008).However, for ATN < 10%, R(ATN) can be taken as unity (Weingartner et al., 2003).All the MAE values in this study were corrected using Eq. ( 5) and ( 6).

MAE of BC in Xiamen: Temporal and Emission Sources Variability
The variability of MAE has been reported in literature depending on location, composition, and the mixing state of BC in aerosols (Liousse et al., 1993;Avino et al., 2011).For example, Sharma et al. (2002) have reported that the range of median values for site-specific MAE is from 6.4 to 20.1 m 2 /g for different locations in Canada and different sampling time.Sharma attributed this variability to the distribution of sources and processes contributing to carbonaceous species at the sampling sites.However, Bond and Bergstrom (2006) suggested a value of 7.5 ± 1.2 m 2 /g at 550 nm for uncoated soot particles.The MAE of BC in Xiamen showed a large temporal and spatial variability, varying from 0.47 to 5.15 m 2 /g (the average MAE is 1.69 m 2 /g with a coefficient of variance of 52%) and was related to the difference in chemical and optical properties of carbonaceous species emitted from different sources.In addition to the ambient sampling, ten quartz fiber filters were mounted to the exhaust pipe of a diesel-powered truck (YC4E140-33, 4.26 L, 9.3 ton) to investigate MAE of diesel-powered vehicle emissions.The monthly average MAE values for samples of ambient aerosols and average MAE value of the samples of the diesel-powered truck are showed in Table 1 and Fig. 3.
The monthly average values of MAE at IUE during the period of September 2010-September 2011 showed a large variability.The minimum monthly average value that occurred in May 2011 was 0.50 m 2 /g and the maximum value that occurred in September 2010 was 2.63 m 2 /g.The average MAE at IUE during the year is 1.78 ± 0.98 m 2 /g.If using a constant value of MAE within the range of 0.50-2.63m 2 /g to quantify BC concentration, this temporal variability would bias the result by a factor of 5 maximum.
The monthly average MAE at IUE in May 2011 (0.50 m 2 /g) is one third of that on Shigu Road (1.50 m 2 /g) during the same month, suggesting the sources of BC were quite different at these two sampling sites.The sampling site at IUE is located on the rooftop of a 3 m-high building, which is 40 m away from the intersection of Jimei Road and Haixiang Avenue.Vehicles on these two roads travel rapidly at a constant speed of 60-80 km/h and make few or no stops.On the contrary, the average speed of vehicles on Shigu Road is slow (about 20-30 km/h) and vehicles stop frequently due to the frequent traffic jams and buses picking up and dropping off passengers.The statistics of traffic volume monitored at these two sites are showed below (Fig. 4).
Experiments were carried out to study the impact of operating conditions of vehicles on the concentration of BC in their exhaust plumes.The results showed that when vehicles was idling, speeding up or slowing down, they emitted much higher concentration of BC than moving at a constant speed (Fig. 5).Average BC concentration of emissions by a gasoline-powered car in idling condition was approximate 1.6 times of that when it moving at a constant speed (60 km/h).Average BC concentration of emissions by a diesel-powered truck when speeding up from 40 km/h to 70 km/h was approximate 1.3 times of that when it moving at the speed of 40 km/h.Thus, vehicles emissions had larger effect on MAE at Shigu Road site than that at IUE site.
The panels of a, b, c, d, e, f, g, h, i and j in Fig. 6 show monthly MAE values for BC samples collected during the entire sampling period in Xiamen.
It is noteworthy that the intercept of the linear regression   plot for IUE, August 2011 is the smallest (9.56) among all the plots, which is less than one-tenth of the monthly average ATN (123.87).This suggests that BC was the principal absorbing component in the ambient atmosphere at this sampling site during this month.In contrast, the intercept of regression plot for IUE, May 2011 (33.3) is higher than samples from Shigu Road (intercept of regression plot is 28.1 in May and 18.1 in Jun, respectively).The relatively high intercept in linear regression analysis observed for the data from IUE sites could be attributed to the presence of absorbing species other than BC.IUE is located in a place enclosed by many construction sites.Samples could be influenced by mineral dust generated from construction activities.Mineral dust is one of absorbing species presents in aerosols (Schnaiter et al., 2003), hematite (Fe 2 O 3 ), a constituent of desert dust, is another absorber found commonly in the atmospheric aerosol (Bodhaine, 1995), thus mineral dust could be another factor contributing to the higher intercept of regression plot for IUE than Shigu Road.However, absorption due to mineral dust is very low compared to that of BC (Bodhaine, 1995).Yang et al. (2009) apportioned total light absorption to BC, brown carbon, and dust, their MAE at 550 nm were estimated to be 9.5, 0.5, and 0.03 m 2 /g, respectively.The contributions of mineral dust and other component of aerosol to MAE need to be investigated in future work.
Climate models typically treat BC as the only lightabsorbing aerosol component (Kirchstetter and Novakov, 2004).However, light-absorbing species other than BC have been observed to date in atmospheric particles (Jacobson, 1999).Freshly emitted OC, which particularly originated from biomass burning (Kirchstetter and Novakov, 2004;  Kirchstetter et al., 2004; Bergstrom et al., 2007; Zhang et al.,  2010) can contribute to total aerosol absorption at a shorter wavelength region of the spectrum (Andreae and Gelencser, 2006;Yang et al., 2009).Residential coal combustion (Bond, 2001), biogenic materials (Andreae and Crutzen, 1997), and atmospheric reactions (Gelencser et al., 2003;Hecobian et al., 2010) are also regarded as sources of light-absorbing organic aerosols.The OC EC ratio of fresh emissions of a diesel-powered heavy duty truck (YC4E140-33, 4.26 L, 9.3 ton) is 33, much higher than that of the ambient samples (2-16), which results in a relative low MAE for fresh vehicle emissions.The monthly average MAE for samples collected during November 2010 is 1.12 m 2 /g, which is much lower than those collected during September (2.63 m 2 /g) and October (2.30 m 2 /g).BC samples collected during November could be dominated by emissions from biomass combustion, which is known to emit a large amount of light absorbing organic aerosol.The absorbing organic aerosol is usually termed brown carbon, which cannot be directly measured by the currently used thermal optical method.Wonaschutz et al. (2009) presented an integrating sphere method to correct the BC signal for the contribution of brown carbon and to obtain an estimate of brown carbon concentrations.This approach does not measure absorption coefficients.It uses a calibration curve obtained with a test substance to convert the optical signals to BC concentrations.Based on this optical method, Reisinger et al. (2008) found that thermal and thermal-optical method overestimated BC concentration when the aerosol contains appreciable amounts of brown carbon.This may cause underestimation of MAE.However, Alexander et al. (2008) estimated a mean MAE of brown carbon to be 3.6-4.1 m 2 /g and MAE of BC to be 4.3-4.8m 2 /g, indicating brown carbon is less absorbing than BC.As a result, brown carbon from biomass burning source is expected to decrease the value of MAE.Thus the large amount of brown carbon in the atmosphere during November, which was mainly emitted from biomass burning, could be the factor leading to the lower MAE values compared with those for other months.

Factors Contributing to the Uncertainty of MAE Measurement
There are factors that contribute to the uncertainty of the experimentally quantified MAE, which include the different ) are the two most widely used instruments, which are based on the two different protocols i.e., IMPROVE and NIOSH, to determine particulate carbon (Chow et al., 2011; Zhi et al., 2011).Measured L BC values vary significantly among the various instruments and temperature protocols (Schauer et al., 2003; Subramanian et  al., 2006; Cheng et al., 2010).Temperature of IMPROVE, TOT/TOR charring correction and residence time at temperature plateau lead to the difference of these methods (Fung et al., 2006).Based on our experimental results of 17 samples, L BC determined by the Thermal Optical Carbon Analyzer (DRI Model 2001A) IMPROVE temperature protocol is 1.6 times the value of that determined by Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.) NIOSH, which is near the range (1.2-1.5)observed during the comparison studies for the two protocols conducted in North America and Europe (Cheng et al., 2010).Three factors primarily contribute to the uncertainty of results of L BC .When measuring L BC with a Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.), a piece of filter sample of 2.0 cm 2 is punched off to analyze for BC loading as mass per unit area.The piece of sample is assumed to be representative for the whole filter.However, non-uniform deposition of particles on the filter would incur biases to L BC for the whole filter.Too much or too little particle deposition on filters would result in uncertainty to pyrolysis correction in EC/OC measurements.Furthermore, non-uniform deposition of BC particles biases optical monitoring and charring.10 punches of the same filter were analyzed to determine the L BC uncertainty caused by non-uniform deposit.The overall relative uncertainty in L BC is 5.9%.According to Chow's study (Chow et al.,  2001), minerals can oxidize EC at high temperatures in the helium (He) atmosphere and can lower EC decomposition by catalytic reactions.EC is oxidized by mineral oxides at 700-900°C in He in NIOSH protocol and catalyze EC oxidation at 550°C in O 2 /He in IMPROVE protocol.Thus, content of mineral in particle samples could be another uncertainty contributing factor for L BC measurement.
The light absorption of BC on filter is sensitive to wavelength, and thus experimentally determined values of ATN are wavelength dependent.Three particle-loaded filters with transmittance ranging from 15.6% to 89.7% were measured for ATN at 532 nm, 850 nm, 880 nm and 940 nm wavelengths respectively.It showed that light absorption of BC is more sensitive at shorter wavelength (532 nm) than at longer wavelengths (850 nm, 880 nm, 940 nm), which can be illustrated in Fig. 7.
A sample with transmittance of 50% (measured at 532 nm) was investigated to compare the difference between the measured MAE values at different wavelengths and the converted MAE using Eq.(3).All MAE values were corrected by Eq. ( 5) before the comparison.Calculated MAE values of this sample at 850 nm (0.452 m 2 /g) and 880 nm (0.436 m 2 /g) are very close to the measured values (0.449 m 2 /g at 850nm, 0.443 m 2 /g at 880 nm), with the relative difference of 0.7% and 1.5% respectively.However, the calculated MAE value at 940 nm (0.401 m 2 /g) is approximate 12% larger than the measured value (0.357 m 2 /g).These results indicate that it is reliable to convert MAE value from 532 nm to 850 nm or 880 nm using Eq.(3) as we did in this research, but when convert it to 940 nm, a relative larger error would occur.Optical transmission method is frequently used to quantify the ATN of filter-based BC.To investigate the uncertainty associated with the measurement of ATN using transmission method, a filter with transmittance of 62.5% at 880 nm was measured ten times using the 880 nm LED transmissometer.The coefficient of variance of the results was 2.2 %, without consideration of the instrument instability.
Relative humidity (RH) affects the liquid water content in aerosols, altering the scattering and absorption of visible light (Brem et al., 2012).In Arnott's study (Arnott et al., 2003), the photoacoustic measurements of light absorption exhibited a systematic decrease when the RH increased beyond 70%, but this decrease was deemed as bias caused by instrumental artifacts.In order to quantify the influence of RH on absorption of BC on filters, ATN values of a particle filter were determined at one temperature (20°C), but at different RH values (55%, 60%, 70%, 80%, 90%).RH was controlled by a constant temperature and humidity chamber (JingHong, HWS-080, Shanghai, China).ATN showed an ascending tendency as RH increases, but the coefficient of variance of the ATN values measured at the five RHs was 2.8%.However, influence of temperature to ATN was not investigated in this study because of constrained experimental conditions and working temperature for laser/ LED transmissometer.
The uncertainties of individual independent factors, such as the L BC analysis method, ATN measurement method, RH, etc., contribute to the overall uncertainty of the optical method to quantify MAE.These three factors were considered independent of each other, and the associated relative uncertainties are denoted as U Loading , U ATN , U RH , respectively.Hence, the total uncertainty (U Total ) can be calculated with Eq. ( 7): As a result, measurement of MAE using an 880 nm LED transmissometer and a Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.) at RH 65% will have an uncertainty of 6.9%, suggesting an optical and thermal-optical combined method to quantify MAE is practical and convenient, nevertheless uncertainty resulting from different measurement methods still requires further investigation.

CONCLUSIONS
Temporal and spatial variations of black carbon MAE were studied for ambient aerosols in urban and suburban areas in Xiamen and for fresh vehicle emissions during September, 2010 to September, 2011.MAE of BC in Xiamen showed a large temporal and spatial variability, varying from 0.47 to 5.15 m 2 /g.Monthly average MAE from different sampling sites (IUE and Shigu Road) in the same month (May 2011) was quite different (0.50 m 2 /g and 1.50 m 2 /g), suggesting that the variance of MAE was related to the different BC emission sources.Small intercept of regression plot of MAE indicated BC as the principal absorbing component in ambient atmosphere at the specific sampling site and sampling time.In contrast, relatively high intercept in linear regression analysis for suburban samples could be attributed to the presence of absorbing species other than BC.Mineral dust is one of absorbing species presents in aerosols, thus absorbing component other than BC should also be taken into account when quantifying MAE at areas with heavy construction activities near around.
Besides the different sources of particles that affect the optical properties of the sampled aerosols, different measurement methods of L BC and ATN also influence the variance of measured MAE values.Measurement of L BC is dependent on the specific carbon analyze instruments and protocols.For example, L BC determined by the Thermal Optical Carbon Analyzer (DRI Model 2001A) and IMPROVE temperature protocol was about 1.6 times the value of that determined by Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.) and NIOSH protocol.Light absorption of BC is more sensitive at shorter wavelength than at longer wavelength.Corrected MAE were very close to the measured values at 532 nm, 850 nm and 880 nm, but when MAE was converted to 940 nm, a relative larger difference should be taken into consideration.RH is another factor that contributes to the variance of MAE by influencing ATN determination.In this study, MAE was measured using an 880 nm LED transmissometer and a Semi-Continuous OC/EC Carbon Aerosol Analyzer (Sunset Laboratory Inc.) at RH 65%, giving an uncertainty of 6.9%.These results suggest that site-specific values of MAE should be used for long-term stationary monitoring of ambient BC in air quality monitoring stations.

Fig. 3 .
Fig. 3. Average MAE of three types of samples.

Fig. 4 .
Fig. 4. Statistics of traffic volume at monitoring sites of IUE and Shigu Road.

Fig. 5 .
Fig. 5. Relationships between concentration of BC in vehicle exhaust plume and vehicle speed for a gasoline-powered car (A) and a diesel-powered truck (B).

Fig. 6 .Fig. 6 .
Fig.6.Linear relationship between ATN and L BC (μg/cm 2 ): data in panels of a, b, c, d, e, i, j were from IUE site duringSeptember, 2010, October, 2010, November, 2010, January, 2011, May, 2011, August, 2011 and September, 2011   respectively.Data in panels f and g were for Shigu Road site during May, 2011 and June, 2011 respectively.Data in panel h were for fresh diesel-powered vehicle emissions.

Fig. 7 .
Fig. 7. ATN values of three samples measured at four different wavelengths.

Table 1 .
Site-specific mass absorption efficiency of BC in Xiamen.