Environ. Sci. Technol. 2006, 40, 5355-5360
Size Distribution Characteristics of Elemental Carbon Emitted from Chinese Vehicles: Results of a Tunnel Study and Atmospheric Implications X I A O - F E N G H U A N G , † J I A N Z H E N Y U , * ,† LING-YAN HE,‡ AND MIN HU‡ Atmospheric, Marine and Coastal Environment Program and Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China, College of Environmental Sciences and Shenzhen Graduate School, Peking University, China
The size distribution characteristics of elemental carbon (EC) emissions from Chinese vehicles have not been previously described. In this study, we collected sizesegregated aerosol samples using a 10-stage MOUDI sampler (0.056-18 µm) in the Zhujiang tunnel, a roadway tunnel in the urban area of Guangzhou, China. The samples were analyzed for EC, organic carbon (OC), and inorganic ions. Fine particles had an OC/EC ratio of 0.57, indicating a dominant contribution of EC from diesel vehicles. Both EC and OC showed a dominant accumulation mode with a mass median aerodynamic diameter (MMAD) of 0.42 µm. In comparison, studies available in the literature typically reported a much lower MMAD for EC (∼0.1 µm) in vehicular emissions in North America. A theoretical analysis indicated that the larger EC particles observed in this study could not have resulted from after-emission growth processes (i.e., water accretion, coagulation, and vapor condensation). This leaves operating conditions such as high engine loads and low combustion efficiencies, which are more prevalent in diesel-fueled Chinese vehicles, as a more plausible inherent reason for producing the larger EC agglomerates. While fresh 0.1 µm EC particles are unlikely to act as cloud condensation nuclei (CCN), calculations showed that EC particles as large as 0.42 µm are effective CCN at atmospherically relevant critical supersaturation values of less than 1%. As a result, fresh EC particles from Chinese vehicle emissions could readily undergo cloud processing and form internal mixtures with sulfate in the residue droplet mode particles. This prediction is consistent with observations that EC frequently showed a dominant droplet mode in urban atmospheres in this region. The internal mixing of EC with highly hygroscopic sulfate would facilitate its removal by wet deposition and shorten its lifetime in the atmosphere. In addition, the light-absorbing capabilities of EC particles could also be enhanced due to their internal mixing with sulfate. Numerical aerosol models need to take these factors into consideration for better
* Corresponding author phone: 852-2358-7389; fax: 852-23581594; e-mail:
[email protected]. † Hong Kong University of Science and Technology. ‡ Peking University. 10.1021/es0607281 CCC: $33.50 Published on Web 08/05/2006
2006 American Chemical Society
predictions of the behaviors and effects of urban aerosols in China.
1. Introduction Elemental carbon (EC) is a ubiquitous component in atmospheric aerosols, with its concentrations ranging from 0.2 to 2.0 µg/m3 in rural and remote areas to 1.5-20 µg/m3 in urban areas (1). EC is the primary light-absorbing component in aerosols in the atmosphere and it plays an important role in the earth’s radiation balance (2-4). All carbonaceous fuel-based combustion processes lead to the formation of EC. In urban environments, motor vehicles are the major EC source (5-7). The past decade has seen a 15% annual increase in number of vehicles in China, and the total number of vehicles reached over 100 million in 2004 (http://www.mps.gov.cn). Domestically manufactured vehicles account for over 70% of the vehicles in Chinese cities (8). With the fast increase in vehicles traveling on roads, EC emissions in China have a potentially important impact on the regional and global climate because of the sheer size of such emissions. Data on the characteristics of EC particle emissions from vehicles in China are scarce. Numerous studies have been conducted on vehicles and driving conditions in the U.S. and Europe to obtain emissions characteristics of particles from vehicle exhausts, either through tunnel experiments or chassis dynamometer tests (e.g., refs 9-12). It is well-known that vehicle emissions characteristics depend strongly on fuels, vehicle technologies, and operating conditions (13). Consequently, emissions characteristics are not necessarily the same in China as they are in developed countries. Among the emissions characteristics, particle size distribution is particularly important. Particle size determines evolution behaviors of particles in the atmosphere, thereby determining the lifetime, and the optical and other properties of the particles. Particle size also determines where in the respiratory tract particles are deposited. In this work, we report measurements of EC particle size distributions in a roadway tunnel in China and explore the implications of the unusually large EC particles in our measurements.
2. Experimental Section 2.1. Sample Collection. Sampling was carried out in the Zhujiang tunnel, a roadway tunnel crossing the Pearl River in the western part of Guangzhou (23.13°N, 113.25°E), a metropolitan city on the southeast coast of China. The tunnel has a total length of 1238 m, consisting of a 721-m flat underwater section, and two open slope sections of 517 m in length outside both ends of the tunnel. Each of the two tunnel bores has three lanes for traffic in one direction. The maximum and minimum speed limits in the tunnel are set to be 50 km/h and 15 km/h, respectively. The sampling events were carried out in September 2004. A 10-stage microorifice uniform deposit impactor (MOUDI, MSP Corp., Shoreview, MN) aerosol sampler was used to collect size-segregated samples in the range of 0.056-18 µm (14). The sampler was located 30 m from the outlet on a sideway in the tunnel, and operated at a flow rate of 30 L/min. The collection substrates were 47-mm quartz fiber filters (Whatman, Maidstone, England). Special spacers made of fiber filters of 0.05 in. in thickness (MSP) were used to compensate for the reduced space between two adjacent impact plates due to the use of the quartz fiber collection substrates. All the quartz fiber filters were baked at 550 °C VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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for 5 h before sampling to reduce organic impurities. Six sets of samples were obtained, and each set was collected for 6 h. A set of field blank samples was also collected by loading filters into the sampler without pulling air through. The ventilation system of the tunnel was turned off during the sampling periods, thus the dispersion of air pollutants in the tunnel was mainly brought about from the piston effect arising from the traffic flow (15). Meteorological parameters, including wind direction, wind speed, temperature, and relative humidity (RH), were recorded at the sampling site. A video camera recorded a total of 94 173 vehicles passing through the tunnel during the six sampling periods. The composition of the vehicles was 18% heavy-duty vehicles, 57% light-duty vehicles, and 25% motorcycles. The vehicles passing through the Zhujiang tunnel could be regarded as a random subsample representative of vehicles in Guangzhou. Ambient size-segregated aerosol samples were also taken at an urban site in Shenzhen, a city about 100 km to the east of Guangzhou. The sampling was carried out using the same MOUDI sampler on top of a four floor building on the campus of the Shenzhen Graduate School of Peking University. Nine sets of ambient samples were collected from July to August 2004, and 12 sets were collected from December 2004 to January 2005. Each sampling event lasted 2-3 days. 2.2. Sample Analysis. All filter samples were stored at -18 °C in a refrigerator before analysis. One-quarter of each quartz filter was used to determine the organic carbon (OC) and EC contents using a thermal/optical transmittance aerosol carbon analyzer (Sunset Laboratory, OR) (16). The temperature program for the thermal analysis was the same as that used in the NIOSH method to analyze diesel soot (17). Due to the nonuniform deposition nature of the MOUDI samples, laser correction did not work properly to set the OC and EC split point (18). For lack of a better alternative, we defined OC to be the fraction of carbon that evolved at or below 850 °C in the helium atmosphere, and EC to be the fraction of carbon that evolved after 1% oxygen was introduced to the carrier gas during an analytical cycle. This, in effect, fixed the OC/EC split time at 360 s into each analysis. The remaining three-quarters of each quartz filter were extracted with 5 mL ultrapure water in an ultrasonic bath for 40 min and then filtered with a 0.45 µm Teflon filter (Millipore, Billerica, MA). The resulting solutions were analyzed for ionic species using an ion chromatographic system (DX500, Dionex, Sunnyvale, CA). The anions (i.e., Cl-, NO3-, SO42-) were determined using an AS-11 column and a gradient elution solution of NaOH. The cations (i.e., Na+, NH4+, K+, Mg2+, Ca2+) were determined using a CS-12 column and the elution solution was methanesulfonic acid. The detailed method was described by Yang et al. (19).
3. Results and Discussion 3.1. Thermal Characteristics. Figure 1 shows a typical thermogram of carbonaceous materials in fine particles collected in the Zhujiang tunnel. The first two OC peaks (the OC1 and OC2 peaks in Figure 1), corresponding to the evolution of OC at the two lowest temperatures (250 °C and 500 °C) in helium, accounted for 80% of the total OC. This thermogram was quite different from those for ambient fine particles, in which the majority of OC typically evolves at the two higher temperature steps (650 °C and 850 °C) in the helium stage (i.e., the OC3 and OC4 peaks in Figure 1). The dominance of the OC1 and OC2 peaks in the thermogram indicated that organic compounds in the tunnel aerosols were mostly nonpolar species of relatively high volatility. Because of the relatively volatile and nonpolar nature of the tunnel aerosol OC, little charring was observed. As a result, the lack of charring correction had little influence on the accurate determination of OC and EC in the tunnel samples. 5356
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FIGURE 1. Typical thermogram of fine particles in the Zhujiang tunnel.
FIGURE 2. Chemical compositions of size-segregated particles. The average OC/EC ratio was 0.56 for fine particles in the tunnel samples. This ratio was closer to ratios reported for diesel vehicles (0.28-0.92) instead of gasoline vehicles (>2) (12, 20, 21). The low OC/EC value suggested a dominant contribution of particles from diesel vehicles. EC and OC in the size bins in the fine mode were highly correlated (r2 ) 0.95, n ) 36), indicating that they were emitted by a common dominant source. 3.2. Size Distribution Characteristics. Figure 2 shows the mean chemical compositions of the size-segregated particles in the Zhujiang tunnel. The reconstructed mass, including EC, organic matter (OM), and the major ions, was on average 174 µg/m3 in the fine mode (0.056-1.8 µm) and 57 µg/m3 in the coarse mode (1.8-18 µm). The OM mass was obtained by multiplying OC by a factor of 1.2 to account for the non-carbon mass in the organic compounds from vehicle emissions (11). EC and OM were the two most abundant species in the individual size bins in the fine mode. Inorganic ions contributed only a small fraction (∼10%) to the aerosol mass. Such compositions were consistent with known chemical characters of particles emitted from vehicle tailpipes and were similar to observations made in the Caldecott tunnel by Allen et al. (12). The size distributions of EC and OM were consistent among the six sets of samples and showed a dominant fine mode that peaked at 0.32-0.56 µm (Figure 3). EC had a small presence while OM had a much larger presence in the coarse mode particles.
FIGURE 3. Measured and PMF-resolved EC and OM size distributions in the Zhujiang tunnel.
FIGURE 5. Reconstructed particle mass size distributions of the four sources identified by PMF.
FIGURE 4. Explained variations of PMF analysis of the tunnel samples. Multiple lines of evidence suggested that the carbonaceous materials in the fine particles in the tunnel were predominantly particles from vehicular emissions. First, the thermal characteristics of EC and OC described above were consistent with those of vehicular particles. Second, the EC and OC concentrations in the tunnel samples were highly elevated in comparison with those in the ambient atmosphere. In a study conducted in 2002/2003, the annual average concentrations of ambient EC and OC in PM2.5 in Guangzhou were measured to be 4.4 and 17.6 µg/m3, respectively (22). In comparison, the EC and OC in fine particles in the tunnel samples were on average 94 and 53 µg/m3, respectively. Last, additional quantitative evidence comes from statistical analysis of the chemical composition (i.e., EC, OC, and inorganic ionic species) of the size-segregated samples using positive matrix factorization (PMF) model (23). The PMF analysis was performed on a total of 60 samples (6 × 10, i.e., 6 sets of impact samples and 10 size-segregated samples in each impact sample set). The PMF analysis identified four sources that accounted for 99% of the reconstructed mass. Figure 4 shows the explained variations of four factors associated with the four sources. Factor 1 was dominated by EC, OM, NH4+, and sulfate, and was, therefore, identified to be vehicular emissions. The presence of NH4+ in this source could be explained by NH3 emissions from vehicles equipped with three-way catalysts while the presence of sulfate was likely due to combustion of S-containing fuels. Factor 2 was primarily loaded with NH4+, SO42-, NO3-, and K+, identified
to be secondary aerosols plus biomass burning aerosols. The two aerosol sources were not resolved by PMF likely due to their coexistence in the same particles considering that biomass burning particles serve as effective CCN to promote in-cloud sulfate formation. Factor 3 was primarily loaded with Cl- and Na+, indicating its association with fresh sea salt particles. Factor 4 was identified as aged sea salts plus suspended road dust on account of a large presence of NO3-, Na+, and Ca2+, and a deficiency in Cl- in comparison with fresh sea salt. The reconstructed mass size distributions of aerosols derived from the four sources are plotted in Figure 5. They are consistent with the respective source identifications. Sources 1-4 accounted for 75%, 14%, 1%, and 10% of the reconstructed aerosol mass in the fine mode, respectively. The dominance of vehicular emissions particles (i.e., source 1) among the fine particles was apparent. The source profile of vehicular emissions (source 1) obtained by PMF consisted of 61% EC, 36% OM, 1.2% SO42-, and 0.5% NH4+ by mass. The reconstructed size distributions of EC and OM derived from source 1 are plotted in Figure 3, together with those derived from actual measurements. The mass median aerodynamic diameters (MMADs) of EC and OM were determined by fitting the impactor data with log-normal distribution functions using the DISTFIT software (TSI, USA). Both species showed a unimodal distribution in the size range of 0.056-1.8 µm with an MMAD of 0.42 µm. The EC particles emitted by vehicles in the Zhujiang tunnel apparently had an MMAD value larger than those reported from vehicles in the U.S. and European countries. Measurements in tunnel studies or dynamometer tests conducted in the U.S. and Europe have mostly reported a unimodal distribution with an MMAD of ∼ 0.1 µm (e.g., refs 10-12, 24). It is significant to explore whether such a difference in the MMAD values was due to the inherent emissions characteristics of Chinese vehicles or caused by after-emissions growth in the tunnel. We discuss below the known possible mechanisms that could lead to particle growth in the tunnel, i.e., hygroscopic growth, coagulation, and vapor condensation. (1) Hygroscopic growth could be excluded on the grounds of the hydrophobic nature of freshly emitted soot particles. The relative humidity in the tunnel was below 70% during all the sampling periods. At this RH, freshly emitted soot particles absorb little water (25, 26). (2) Diffusion coagulation between VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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smaller particles and a larger particle could deplete smaller particles but could not cause any significant particle growth. Self-coagulation between particles of similar sizes was a possible mechanism for effective particle growth (27). However, the time scale was on the order of several days for the lower mode (0.18-0.32 µm) particles at the mass concentration level of around 32 µg/m3 observed in the Zhujiang tunnel to grow to the peak mode particles (0.320.56 µm) (28). The residence time of particles in the tunnel, estimated by the tunnel length divided by the inside wind speed, was on the order of several minutes. The short residence time, therefore, excluded self-coagulation as a pathway leading to the larger EC particles. (3) Gaseous semivolatile organic compounds inside the tunnel were expected to be more abundant relative to those in the ambient atmosphere and therefore could lead to enhanced condensation and partitioning onto the EC particles. The particles in the peak size mode (0.32-0.56 µm) had an OC/EC ratio of 0.48. If the particles were regarded to consist of an EC core and an adsorbed organic layer surrounding the EC agglomerates, then the organic layer only accounted for 14% in diameter. Condensation by semi-volatile organic compounds, therefore, could not be responsible for the significant presence of EC in the size bin of 0.32-0.56 µm. In summary, the above analysis suggests that the large soot particles (0.320.56 µm) could not possibly be produced through known particle growth mechanisms, despite the favorable particle growth conditions in the tunnel (e.g., high concentrations of particles and condensable semivolatile organic species). On the other hand, certain characteristics of Chinese vehicles, such as engine parameters and fuel quality, tend to support the observation that direct emission of the large EC particles was possible. Engine load was reported to be an important factor affecting the concentrations and size distribution patterns of diesel soot particles (29-31). Virtanen et al. (31) found that with increasing engine load, the particle number distribution became broader and the peak of the particle volume distribution could move up to 0.4 µm. Under the higher engine load conditions (80 km/h + 50% load), the combustion temperature and the amount of injected fuel were highest while the air/fuel ratio was lowest. These conditions were favorable to initial soot formation and also to an accelerated agglomeration process (31). If some diesel vehicles traveling in the Zhujiang tunnel run on high engine loading, this could partly explain the observed large accumulation mode of EC. Most diesel vehicles (e.g., buses and heavy-duty trucks) are domestically made (8) and their engines are generally believed to be inferior to similar engines from developed countries in combustion efficiency and endurance. In fact, many buses and heavy-duty trucks were observed to emit dark smoke during our sampling events. We speculate that the poor engine performance of some domestically made diesel vehicles may also induce more incomplete fuel combustion and thus promote soot production and agglomeration. This needs to be investigated in future studies. 3.3. Implications. The observation that EC particles from Chinese vehicles were significantly larger than those from vehicles in the developed countries (0.42 µm versus 0.1 µm) has significant atmospheric implications. Hygroscopic studies showed that in order for water to condense on freshly emitted soot particles 0.1 µm in size, a critical supersaturation (Sc) of >1% is necessary, which is higher than that encountered in atmospheric environments (26, 32). EC particles of 0.1 µm were therefore unlikely to act as CCN. Since the Kelvin effect of an insoluble particle decreases with increasing particle size, larger particles of the same chemical composition require lower critical supersaturations to become CCN. Laboratory experiments also showed that the Sc of soot particles decreased to ∼0.5% from >1% when the diameter of the soot 5358
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FIGURE 6. Typical EC size distributions in the urban atmosphere in Shenzhen. (a) a summer sample, (b) a winter sample. particles increased to 0.4 µm from
