Environ. Sci. Technol. 2007, 41, 6357-6362
Comparison of Carbonaceous Aerosols in Tokyo before and after Implementation of Diesel Exhaust Restrictions ,†
NAOMICHI YAMAMOTO,* ATSUSHI MURAMOTO,† JUN YOSHINAGA,† KEN SHIBATA,† MICHIO ENDO,† OSAMU ENDO,‡ MOTOHIRO HIRABAYASHI,§ KIYOSHI TANABE,§ SUMIO GOTO,§ MINORU YONEDA,§ AND YASUYUKI SHIBATA§ Institute of Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa-no-ha 5-1-5, Kashiwa-shi, Chiba 277-8561, Japan, National Institute of Public Health, Minami 2-3-6, Wako-shi, Saitama 351-0197, Japan, and National Institute for Environmental Studies, Onogawa 16-2, Tsukuba-shi, Ibaraki 305-8506, Japan
We compared the status of carbonaceous aerosols in Tokyo before and after the implementation of a diesel vehicle regulation intended to reduce the quantity of particulate carbon from diesel engines in one of the largest scale ever attempts at vehicle exhaust control. Radiocarbon (14C) in elemental carbon (EC) and total carbon (TC) were analyzed to identify fossil fuel carbonaceous particles emitted from diesel-powered vehicles. One-sided paired-month t-tests showed no distinct difference in the absolute concentrations of particles in terms of total mass (19.5 to 18.0 µg m-3; p ) 0.321), EC (3.6 to 3.3 µg m-3; p ) 0.272), and TC (6.3 to 6.2 µg m-3; p ) 0.418) for the finest particles (da < 1.1 µm) after the implementation of the regulation. The ratios of the concentrations of the chemical constituents were, however, altered after the regulation. EC/ TC was significantly decreased from 56.7% to 50.2% (p ) 0.039). Although it was not statistically significant, the percentage of fossil carbon in EC also decreased (67.8% to 63.8%; p ) 0.104). Since EC is predominantly of combustion origin, the observed decrease was likely due to the decrease in fossil EC emissions from diesel-powered vehicles. The decrease in EC/TC after the implementation of the regulation was also likely to have resulted from attachment to diesel vehicle exhaust systems of particulate filters as required as part of the regulation by the Tokyo Metropolitan Government. The EC/TC of fossil carbon of the finest particles decreased from 66.2% to 55.2% (p ) 0.066), but EC/TC of biomass carbon did not decrease but rose slightly from 43.6% to 44.5% (p > 0.5). Thus, the relative ratios of components of carbonaceous aerosol * Corresponding author tel: +81-463-93-1121 (ext. 4336); fax: +81463-90-2074; e-mail:
[email protected]. Present address: Department of Nursing, School of Health Sciences, Tokai University, Bohseidai, Isehara-shi, Kanagawa 259-1193, Japan. † The University of Tokyo. ‡ National Institute of Public Health. § National Institute for Environmental Studies. 10.1021/es070420p CCC: $37.00 Published on Web 08/15/2007
2007 American Chemical Society
particles, such as 14C, could provide a better understanding of the atmospheric pollution status, despite short-term fluctuations, than do measurements of absolute concentrations.
Introduction Adverse health impacts caused by airborne particulate matter are currently of concern. Numerous epidemiological studies have reported an association between fine particle concentrations and adverse health effects (1, 2). In particular, diesel exhaust particles are known to contain various mutagenic and/or carcinogenic substances. For example, diesel exhaust particles consist of elemental carbon (EC), organic carbon (OC), ash, sulfur compounds, and so on (3, 4), and many of the constituents (e.g., some polycyclic aromatic hydrocarbons such as benzo[a]pyrene) are known human carcinogens (5, 6). Therefore, the concentration of airborne particulate matter, including that emitted from diesel-powered vehicles, should be effectively regulated from the aspect of public health. According to the Tokyo Metropolitan Government (TMG), total road traffic in Tokyo in 1997 was 51.3 billion vehicle km, of which 11.7 billion vehicle km (23%) were contributed by diesel-powered vehicles (7). An emission inventory established by the TMG indicated that exhaust from vehicular traffic accounted for approximately 52% of suspended particulate matter emissions in 2000 (8). Assuming that diesel vehicles emit 10 times the quantity of particles than do gasoline vehicles (9), the contribution of diesel exhaust particles is calculated to be 39% of total particle emissions in Tokyo. This is nearly equal to the estimate from an earlier receptor-oriented study (10). Thus, approximately 40% of suspended particulate matter emissions in Tokyo are thought to come from diesel-powered vehicles. In December 2000, the Tokyo Metropolitan Assembly passed the ordinance to regulate diesel-powered vehicles, i.e., the Diesel Vehicle Regulation. The regulation, implemented as of October 2003, prohibits the use of dieselpowered vehicles violating particle emission criteria (11). Similar to the European standards for on-road vehicles (9), the regulation restricts particle emissions in terms of particle mass per travel distance and particle mass per energy consumption for light-duty and heavy-duty vehicles, respectively. Specifically, diesel exhaust emissions must not exceed 0.08 g km-1, 0.09 g km-1, and 0.25 g kW-1 h-1 for vehicles with weights of 2500 kg, respectively (11). The vehicles failing to meet these particle emission criteria were required to install diesel particulate matter reduction systems such as diesel oxidation catalyst (DOC) and/or diesel particulate filter (DPF) certified by the TMG. Furthermore, three of Tokyo′s neighboring prefectural governments, following the lead of the TMG, have concurrently implemented similar regulations (Figure S1). Radiocarbon (14C) is an ideal tracer to distinguish carbon from fossil fuels from that of modern biomass origin. Since 14C radioactively decays with a half-life of 5730 years, fossil fuels do not contain 14C whereas modern biomass will contain measurable amounts of 14C. Therefore, measurements of 14C in the samples can provide information on the relative amounts of carbon of fossil and biomass origin. Indeed, some earlier studies used 14C to determine mixing ratios of fossil and biomass carbons in aerosols in the Tokyo metropolitan area (12-14). Identification of the source of carbon in aerosol particles was a necessary part of determining the effectiveness VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of the regulations implemented for the control of vehicle particle emissions in the Tokyo metropolitan area, and 14C measurements were thought to be an appropriate way of achieving this. The aim of the research reported here was to compare the quantities and composition of urban aerosols before and after the implementation of the Diesel Vehicle Regulation in Tokyo. In particular, concentrations of EC and total carbon (TC) were analyzed to elucidate how the regulation affected carbonaceous urban aerosols. Besides measurements of the total particle mass and the carbon concentrations in air, ratios such as that of elemental carbon to total carbon (EC/TC), and the fossil fuel carbon fractions in EC and TC, were also determined. The concentrations were expected to fluctuate on a seasonal basis and with weather conditions making comparison of the aerosols before and after the implementation of the regulation difficult. Therefore, relative ratios rather than the absolute concentrations might be a more accurate way of characterizing any change in pollution status. In this study, the relative ratios as well as the absolute concentrations were determined to evaluate the air pollution in Tokyo for periods covering the implementation of the Diesel Vehicle Regulation.
Experimental Section 1. Air Sampling. Air samples were taken bimonthly from April 2002 to August 2004, each for a duration of 7 days (Table S1), on the rooftop of the National Institute of Public Health (NIPH) building in Minato-ku, Tokyo (Figure S1) by a highvolume Andersen sampler (AH-600, Sibata, Tokyo; 7.0 µm of particle aerodynamic cutoff diameters (da)). The sampling flow rate was adjusted to 566 L min-1 as instructed by the manufacturer. The quartz filter substrates were prebaked-out at 850 °C before use. The quartz filters were weighed with a microbalance (LS-6, Shimadzu, Kyoto) before and after sampling to determine the total masses of particles retained during the 7 day sampling periods. The filters were analyzed for carbonaceous material as detailed below. 2. EC and TC Analysis. EC and TC on the filters after sampling were quantitated by a thermal and optical carbon analyzer (DRI Model 2001). In this study, the Interagency Monitoring of Protected Visual Environments (IMPROVE) protocol (15) was used. Portions of the quartz filter substrates (0.50 cm2) were punched out and introduced into the carbon analyzer to measure the quantity of carbon per unit area of the sampling substrates. Atmospheric concentrations were calculated based on the measured masses of carbon per unit area of the sampling substrates, the effective sampling areas, and the volumes of air in the samples. In the high-volume Andersen sampler, the effective areas of the impactor substrates (da ) 1.1-2.0, 2.0-3.3, 3.3-7.0, and >7.0 µm) and the after-filter substrate (da 0.05) even after month stratification, i.e., pairing the data of the same months in different years for interyear comparison. We suspected this was possibly because of additional temporal variations in concentrations. Although the present sampling data were stratified by month, they might not represent the monthly averages since the samples were taken only for 7 days. For instance, the weather conditions during the 7-day sampling periods might not have been representative for those of the entire months. Indeed, rainfall, a factor affecting aerosol concentrations, observed during the 7-day sampling periods in the same months in different years varied considerably, making paired-month comparison difficult (Table S6). Thus, the limited duration of our sampling periods (7 days) relative to the period of comparison (1 month) might obscure concentration differences before and after the implementation of the Diesel Vehicle Regulation. To characterize the polluting aerosols before and after the implementation of the Diesel Vehicle Regulation, regardless of the seasonal and/or other temporal fluctuations, the ratios of the carbonaceous components rather than their absolute concentrations were analyzed. Monthly variations in the percentage of EC in TC and of the percentages of fossil carbon in TC and EC are shown in Figures 3 and 4. EC contributions to TC were in the range 25% to 65%, which were comparable to values obtained from previous studies (21-23). The percentages of fossil carbon in TC were in the range 40% to 72%, which were consistent with our previous studies (12, 13), while those of fossil carbon in EC in the finest particles (da < 1.1 µm) ranged from 57% to 78%. Compared to the absolute concentration trends, these ratios were relatively stable throughout the sampling period, indicating that they were less influenced by seasonal and/or other temporal variations. In Table 2, we stratified the above-mentioned chemical constituent ratios by month, demonstrating a statistically significant decrease in EC as a percentage of TC after the implementation of the Diesel Vehicle Regulation, i.e., from 56.7% to 50.2% on average (p ) 0.039). While EC originates predominantly from combustion such as in diesel-powered vehicles, TC partly consisted of OC from noncombustion origins as well. Therefore, the relative elimination efficiency of EC from the atmosphere by the Diesel Vehicle Regulation is likely to be larger than that of TC. Although it was not statistically significant, the percentage of fossil carbon in the EC also decreased (from 67.8% to 63.8%) (p ) 0.104), indicating a decrease in EC from fossil fuel origins such as diesel-powered vehicles. Overall, we found significant decreases in EC/TC (Table S7) and no decrease of the percentage of fossil carbon in TC (Table S8) after the implementation of the Diesel Vehicle Regulation. The TMG required, as part of the Diesel Vehicle Regulation, the attachment of DPFs to all diesel-powered vehicles violating the particle emission criteria (7). This had the potential to, in particular, decrease the EC/TC of the diesel exhaust. For example, whereas the volatile OC fraction passes VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Monthly Comparisons of Concentrations of Mass, TC, and EC of the Finest Particles (da < 1.1 µm) before and after the Implementation of the Regulation (7-day sampling) concentration (µg m-3) mass montha
TC
EC
before
after
difference
before
after
before
after
Oct Dec Feb Apr Jun Aug
19.2 21.2 21.4 19.5 21.6 14.4
31.3 22.8 15.3 13.5 12.5 12.6
12.1 1.6 -6.1 -6.0 -9.1 -1.8
8.18 7.61 5.46 5.12 6.25 5.33
10.89 8.72 4.66 4.59 3.20 4.85
2.71 1.11 -0.80 -0.53 -3.05 -0.48
5.07 4.01 2.95 2.73 3.30 3.48
6.56 4.93 2.17 2.08 1.44 2.32
1.49 0.92 -0.78 -0.65 -1.86 -1.16
mean SDb one-sided paired-month t-test
19.5 2.7
18.0 7.6
-1.5
6.32 1.29
6.15 2.97
-0.17
3.59 0.85
3.25 2.02
-0.34
p ) 0.321
difference
p ) 0.418
difference
p ) 0.272
a The months before the regulation include Oct and Dec of 2002, and Feb, Apr, Jun, and Aug of 2003 while those after the regulation include Oct and Dec of 2003, and Feb, Apr, Jun, and Aug of 2004. b Standard deviation.
FIGURE 3. Percentages of EC in TC obtained by sampling on the roof of the NIPH building, Tokyo, for periods before and after the implementation of the Diesel Vehicle Regulation (7-day sampling). through the DPF, the solid EC fraction is more efficiently eliminated by the DPF, resulting in a decrease in the EC/TC. Burtscher et al. (24) reported relative masses of EC and TC, assuming 100 for total mass of the particles emitted from diesel engines without filters, were 76 and 88, respectively, i.e., EC/TC ) 0.86, while those from the engines with filters were 0.82 and 1.79, respectively, i.e., EC/TC ) 0.46. Thus, while the attachment of DPF to the vehicles could reduce overall particle concentrations, it could also alter the emission characteristics of the carbonaceous components. The EC/TC of fossil and biomass carbons of the finest particles (da < 1.1 µm) are given, as percentages, in Figure 5. The EC/TC of fossil carbon decreased after the implementation of the Diesel Vehicle Regulation while that of biomass carbon was unchanged. Table 3 lists monthly comparisons of EC/TC of fossil and biomass carbons before and after the implementation of the regulation. Although there is no statistically significant difference, the table demonstrates the decrease in fossil fuel EC/TC from 66.2% to 55.2% (p ) 0.066) but almost no change for biomass EC/ TC. As described above, attachment of DPFs to vehicles could decrease EC/TC of diesel exhaust. Therefore, the EC/TC of airborne fossil carbon thought to come substantially from diesel-powered vehicles is likely to decrease as a result of the Diesel Vehicle Regulation requiring attachment of DPFs. The EC/TC of biomass carbon cannot, however, be changed by the regulation since no biomass carbon is emitted by dieselpowered vehicles. 6360
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FIGURE 4. Percentages of fossil carbon in TC and EC obtained by sampling on the roof of the NIPH building, Tokyo, for periods before and after the implementation of the Diesel Vehicle Regulation (7day sampling). It should be noted that dividing the dataset into pre- and postimplementation of the regulation was not to strictly measure the effect by the regulation but rather to compare the difference of the atmospheric pollution status attributable to the implementation of the regulation. The TMG reported 62% of the vehicles were installed with the particle reduction systems by the end of June 2003 (25) while 98% of the vehicles were found to meet the emission criteria in the early part of October 2003 based on the inspections at the roadside and logistic bases (26). Therefore, dividing the dataset into preand postimplementation of the regulation does not provide a strictly true measure of the effects since the installations of the systems were being started even before the imple-
TABLE 2. Monthly Comparisons of Ratios of EC/TC and Fossil Carbon in TC and EC of the Finest Particles (da < 1.1 µm) before and after the Implementation of the Regulation (7-day sampling) ratio (%) EC/TC montha Oct Dec Feb Apr Jun Aug mean SDb one-sided paired-month t-test
fossil carbon in TC
fossil carbon in EC
before
after
difference
before
after
before
after
difference
61.9 52.7 54.0 53.2 52.8 65.4
60.2 56.5 46.6 45.4 44.8 47.9
-1.7 3.8 -7.4 -7.8 -8.0 -17.5
62.9 57.7 60.7 50.1 47.9 69.4
54.1 54.6 63.0 62.5 54.0 66.2
-8.8 -3.1 2.3 12.4 6.1 -3.2
67.8 67.8 69.6 66.4 56.8 78.4
64.6 66.2 67.4 58.7 62.3 64.2
-3.2 -1.6 -2.2 -7.7 5.5 -14.2
56.7 5.5
50.2 6.5
-6.5
58.1 8.1
59.1 5.5
1.0
67.8 6.9
63.9 3.1
-3.9
p ) 0.039
difference
p > 0.5c
p ) 0.104
a The months before the regulation include Oct and Dec of 2002, and Feb, Apr, Jun, and Aug of 2003 while those after the regulation include Oct and Dec of 2003, and Feb, Apr, Jun, and Aug of 2004. b Standard deviation. c The p-value is greater than one-half because the result of the one-sided t-test was in the unexpected direction.
TABLE 3. Monthly Comparisons of Fossil EC/Fossil TC and Biomass EC/Biomass TC of the Finest Particles (da < 1.1 µm) before and after the Implementation of the Regulation (7-day sampling) ratio (%) fossil EC/fossil TC montha
biomass EC/biomass TC
before
after
difference
before
after
Oct Dec Feb Apr Jun Aug
66.7 61.9 62.0 70.5 62.5 73.8
71.9 68.5 49.9 42.7 51.7 46.4
5.2 6.6 -12.1 -27.8 -10.8 -27.4
53.8 40.1 41.7 35.8 43.8 46.3
46.4 42.0 41.0 50.0 36.7 50.8
-7.4 1.9 -0.7 14.2 -7.1 4.5
mean SDb one-sided paired-month t-test
66.2 5.0
55.2 12.1
-11.0
43.6 6.1
44.5 5.5
0.9
p ) 0.066
difference
p > 0.5c
a The months before the regulation include Oct and Dec of 2002, and Feb, Apr, Jun, and Aug of 2003 while those after the regulation include Oct and Dec of 2003, and Feb, Apr, Jun, and Aug of 2004. b Standard deviation. c The p-value is greater than one-half because the result of the one-sided t-test was in the unexpected direction.
rates of the reduction systems, dividing the dataset into preand postimplementation of the regulation could provide the approximate measure of the effect.
FIGURE 5. Percentages of EC in TC of fossil and biomass carbon of the finest particles (da < 1.1 µm) collected by sampling on the roof of the NIPH building, Tokyo, for periods before and after the implementation of the Diesel Vehicle Regulation (7-day sampling). mentation of the regulation. Nevertheless, given the majority of the vehicles were being fitted with the reduction systems within a few months before the implementation of the regulation, as demonstrated in the above-mentioned fitting
In summary, monthly comparisons showed no distinct difference in the absolute concentrations of particles before and after the implementation of the Diesel Vehicle Regulation. However, the ratios of chemical constituents of the particles were altered. For example, the EC/TC decreased from 56.7% to 50.2% (p ) 0.039) while the percentage of fossil carbon in the EC decreased from 67.8% to 63.8% (p ) 0.104). While EC is predominantly from combustion origins, TC, partly composed of OC, comes also from noncombustion sources. Consequently, EC is expected to be eliminated from the atmosphere more effectively than is TC by the Diesel Vehicle Regulation, which might have caused the observed decrease in EC/TC. Some decrease in EC/TC possibly resulted in the change in emission characteristics caused by attachment of DPFs as required by the TMG. Although the EC/TC of fossil fuel carbon decreased from 66.2% to 55.2% (p ) 0.066), the EC/TC of biomass carbon did not decrease. Thus, analyses of the relative ratios of carbonaceous components such as 14C in particular could provide a better understanding of atmospheric pollution despite short-term fluctuations in the absolute concentrations. VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Acknowledgments This study was partly funded by a grant from the Steel Industry Foundation for the Advancement of Environmental Protection Technology.
Supporting Information Available Eight tables and one figure show additional details. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) Dockery, D. W.; Pope, C. A., III; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M. E.; Ferris, B. G.; Speizer, F. E. An association between air pollution and mortality in six U.S. cities. N. Engl. J. Med. 1993, 329, 1753-1759. (2) Pope, C. A., III; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F. E.; Heath, C. W., Jr. Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults. Am. J. Respir. Crit. Care Med. 1995, 151, 669-674. (3) Kittelson, D. B. Engines and nanoparticles: a review. J. Aerosol Sci. 1998, 29, 575-588. (4) Shah, S. D.; Cocker, D. R., III; Miller, J. W.; Norbeck, J. M. Emission rates of particulate matter and elemental and organic carbon from in-use diesel engines. Environ. Sci. Technol. 2004, 38, 25442550. (5) Albert, R. E. Comparative carcinogenic potencies of particulates from diesel engine exhausts, coke oven emissions, roofing tar aerosols and cigarette smoke. Environ. Health Perspect. 1983, 47, 339-341. (6) Stenberg, U.; Alsberg, T.; Westerholm, R. Emission of carcinogenic components with automobile exhausts. Environ. Health Perspect. 1983, 47, 53-63. (7) Tokyo-to kankyo hakusyo 2000 (in Japanese); Tokyo Metropolitan Government, 2000. http://www2.kankyo.metro.tokyo.jp/kikaku/ hakusho/2000/index.htm (accessed May 22, 2007). (8) Tokyo-to kankyo hakusyo 2004 (in Japanese); Tokyo Metropolitan Government, 2004. http://www2.kankyo.metro.tokyo.jp/kikaku/ hakusho/2004/index.htm (accessed May 22, 2007). (9) Gertler, A. W. Diesel vs. gasoline emissions: Does PM from diesel or gasoline vehicles dominate in the US? Atmos. Environ. 2005, 39, 2349-2355. (10) Yoshizumi, K. Source apportionment of aerosols in the Tokyo metropolitan area by chemical element balances. Energy Build. 1991, 16, 711-717. (11) Tomin no kenkou to anzen wo kakuho suru kankyo ni kansuru jyourei (Tokyo-to Jyourei Dai 215 Gou) (in Japanese); Tokyo Metropolitan Government, December 2000. http://www2. kankyo.metro.tokyo.jp/soumu/jyourei_2000/index.htm. Last updated Dec 22, 2000 (accessed May 22, 2007). (12) Endo, M.; Yamamoto, N.; Yoshinaga, J.; Yanagisawa, Y.; Endo, O.; Goto, S.; Yoneda, M.; Shibata, Y.; Morita, M. 14C measurement for size-fractionated airborne particulate matters. Atmos. Environ. 2004, 38, 6263-6267.
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(13) Shibata, K.; Endo, M.; Yamamoto, N.; Yoshinaga, J.; Yanagisawa, Y.; Endo, O.; Goto, S.; Yoneda, M.; Shibata, Y.; Morita, M. Temporal variation of radiocarbon concentration in airborne particulate matter in Tokyo. Radiocarbon 2004, 46, 485-490. (14) Kumata, H.; Uchida, M.; Sakuma, E.; Uchida, T.; Fujiwara, K.; Tsuzuki, M.; Yoneda, M.; Shibata, Y. Compound class specific 14C analysis of polycyclic aromatic hydrocarbons associated with PM10 and PM1.1 aerosols from residential areas of suburban Tokyo. Environ. Sci. Technol. 2006, 40, 3474-3480. (15) Chow, J. C.; Watson, J. G.; Crow, D.; Lowenthal, D. H.; Merrifield, T. Comparison of IMPROVE and NIOSH carbon measurements. Aerosol Sci. Technol. 2001, 34, 23-34. (16) Kume, H.; Shibata, Y.; Tanaka, A.; Yoneda, M.; Kumamoto, Y.; Uehiro, T.; Morita, M. The AMS facility at the National Institute for Environmental Studies (NIES), Japan. Nucl. Instrum. Methods Phys. Res. B 1997, 123, 31-33. (17) Tanaka, A.; Yoneda, M.; Uchida, M.; Uehiro, T.; Shibata, Y.; Morita, M. Recent advances in 14C measurement at NIES-TERRA. Nucl. Instrum. Methods Phys. Res. B 2000, 172, 107-111. (18) Stuiver, M.; Polach, H. A. Discussion: reporting of 14C data. Radiocarbon 1977, 19, 355-363. (19) Mizuno, T.; Kondo, H.; Matsukawa, M. Meteorological condition of extremely high concentration of suspended particulates in early winter in Kanto plain. J. Jpn. Soc. Air Pollut. 1990, 25, 143-154 (in Japanese). (20) Yoshikado, H. Meteorological structure of high-level air pollution in early winter over the Kanto plain. J. Jpn. Soc. Air Pollut. 1994, 29, 351-358 (in Japanese). (21) Ho¨ller, R.; Tohno, S.; Kasahara, M.; Hitzenberger, R. Long-term characterization of carbonaceous aerosol in Uji, Japan. Atmos. Environ. 2002, 36, 1267-1275. (22) He, Z.; Kim, Y. J.; Ogunjobi, K. O.; Kim, J. E.; Ryu, S. Y. Carbonaceous aerosol characteristics of PM2.5 particles in Northeastern Asia in summer 2002. Atmos. Environ. 2004, 38, 1795-1800. (23) Hagino, H.; Kotaki, M.; Sakamoto, K. Levoglucosan and carbonaceous components for fine particles in early winter in Saitama. J. Aerosol Res. 2006, 21, 38-44 (in Japanese). (24) Burtscher, H. Physical characterization of particulate emissions from diesel engines: a review. J. Aerosol Sci. 2005, 36, 896-932. (25) Hodo happyo shiryo, September 2003 (in Japanese); Tokyo Metropolitan Government, 2003. http://www.metro.tokyo.jp/ INET/CHOUSA/2003/09/60d9a100.htm. Last updated Sep 9, 2003 (accessed May 22, 2007). (26) Hodo happyo shiryo, October 2003 (in Japanese); Tokyo Metropolitan Government, 2003. http://www.metro.tokyo.jp/INET/ OSHIRASE/2003/10/20da6300.htm. Last updated on Oct 3, 2003 (accessed on May 22, 2007).
Received for review February 19, 2007. Revised manuscript received May 23, 2007. Accepted June 25, 2007. ES070420P
