Changes in black carbon and PM_2.5 in Tokyo in 2003–2017

Abstract

Black carbon (BC) particles cause adverse health effects and contribute to the heating of the atmosphere by absorbing visible solar radiation. Efforts have been made to reduce BC emissions, especially in urban areas; however, long-term measurements of BC mass concentration (M_BC) are very limited in Japan. We report M_BC measurements conducted in Tokyo from 2003 to 2017, showing that M_BC decreased by a factor of 3 from 2003 to 2010 and was stable from 2010 to 2017. Fine particulate concentrations (PM_2.5) decreased by a much smaller factor during 2003–2010. The diurnal variations of BC size distributions suggest that the BC in Tokyo originates mainly from local sources, even after 2010. Our three-dimensional model calculations show that BC from the Asian continent contributes a small portion (about 20%) of the annual average M_BC in the Kanto region of Japan, which includes Tokyo. This indicates that continued reduction of BC emissions inside Japan should be effective in further decreasing M_BC.

1. Introduction

Air pollution is a major health risk linked to respiratory and cardiovascular diseases. Particulate matter (PM) is one of the most important elements of air pollution and is responsible for adverse health effects.^1)–3) Current air quality standards for PM are based on the concentrations of PM_2.5 or PM₁₀ (PM with effective diameters of less than 2.5 µm or less than 10 µm, respectively).¹⁾ Black carbon (BC) aerosols (or “soot”), a constituent of PM, are emitted from the incomplete combustion of fossil fuels and biomass.^4),5) Incomplete combustion also produces polycyclic aromatic hydrocarbons (PAHs),⁶⁾ and the molecular precursors of BC are considered to be heavy PAHs with molecular weights of 500–1,000 dalton.^7)–11) Thus, PAHs are co-emitted with BC and exist in gas and particle phases in the atmosphere.^12),13) Moreover, nitropolycyclic aromatic hydrocarbons (nitro-PAHs) are formed by the reaction between PAHs and NO₂ in the atmosphere. Many PAHs and nitro-PAHs are carcinogenic or mutagenic.¹¹⁾ They thus contribute to the adverse health effects caused by BC particles, including allergic, respiratory, and cancer-like diseases.^14)–18) Therefore, the reduction of PM_2.5 emissions, which contain BC particles, is beneficial to human health.^19)–21)

BC particles also contribute to atmospheric warming by absorbing solar radiation.^{4),5),22)–24)} They also act as cloud condensation nuclei and can influence cloud albedo, partially offsetting the atmospheric heating related to the direct radiative forcing of BC.⁴⁾ To mitigate the contribution of BC to global warming, measures aimed at reducing BC emissions are most effectively directed at BC sources, such as emissions from diesel engines.

Asia was the world’s largest source of BC aerosols in 2000,^25),26) and vehicular emissions generated the majority of BC aerosols in urban areas.^27)–29) Emissions from the transport sector (on-road and off-road vehicles) constitute about 17% of the global BC burden; however, these estimates have large uncertainties.⁴⁾

Tokyo, with a population of about 14 million, represents a typical urban environment. Earlier studies have suggested that vehicular emissions cause the majority of the atmospheric aerosol loading in Tokyo.^{26),28),30),31)} PM in Tokyo is mainly emitted from diesel vehicles, and Japan started regulations for these emissions in 1994. Subsequent regulations have required new vehicles to reduce PM emissions,^31),32) and emission factors of PM from heavy-duty diesel vehicles have decreased greatly in the last 25 years.

An earlier study showed that ambient mass concentrations of BC (M_BC) in Tokyo decreased by a factor of about 3 between 2003 and 2010.³¹⁾ However, the changes in M_BC after 2010 have not been reported. It is also important to investigate the changes in M_BC relative to PM_2.5 concentrations because the ratio of these two quantities is related to the single scattering albedo of aerosols; aerosols tend to cool the atmosphere as single scattering albedo increases. This study therefore estimated changes in BC/PM_2.5 concentration ratios in addition to updating M_BC data for Tokyo.

In addition, the relative contribution of BC particles transported from the Asian continent may have changed because of recent regulations in different countries in East Asia. Therefore, it is important to investigate the effect of long-range transport when planning further reductions in BC emissions in Japan. In reporting updated data on Asian BC emissions for the period of 2010–2017 and integrating them with earlier results, this study also underlines the need to measure BC and PM_2.5 with high accuracies on a long-term basis at key locations.

2. BC measurements

2.1. Observational sites.

M_BC measurements were made at the Komaba II campus (the Research Center for Advanced Science and Technology campus) of the University of Tokyo (35.66°N, 139.68°E) between 2003 and 2010 and at the main Hongo campus of the University of Tokyo after 2014 (35.71°N, 139.76°E). These sites are within the area with the largest BC emissions in the Kanto region (Fig. 1) and are strongly influenced by vehicular emissions.^28),30),31) BC and carbon monoxide were strongly correlated in measurements made in 2004 at Komaba.²⁸⁾ Carbon monoxide concentrations, measured at seven monitoring stations in Tokyo between 3 and 22 km from Komaba, were rather uniform and significantly correlated with M_BC.^28),31)

Fig. 1.

BC sampling locations in Tokyo (35.5–35.9°N, 138.9–139.9°E). The gridded total BC emission rates in Kanto (35.0–37.0°N, 138.8–140.9°E), surrounded by dashed lines, for the year 2000 are also shown.³⁰⁾

PM_2.5 concentrations were measured at 40 roadside stations in central and suburban Tokyo and at 48 non-roadside stations in central and suburban Tokyo during 2000–2012.³²⁾ These stations were classified into four categories: roadside stations in central Tokyo, roadside stations in suburban Tokyo, non-roadside stations in central Tokyo, and non-roadside stations in suburban Tokyo. Komaba and Hongo are non-roadside stations in central Tokyo.

2.2. Measurement techniques.

M_BC was measured at Komaba during 2003–2005 and 2006–2010 (Table 1). During 2003–2005, we used a semi-continuous elemental carbon-organic carbon analyzer (model RT3052, Sunset Lab., OR, U.S.A.) based on the thermal-optical transmittance (TOT) method with a time resolution of 1 h.^28),33) The overall accuracy of M_BC measurement using this method was estimated to be 22% based on the uncertainties in the sensitivity calibration, aerosol sampling, and temperature protocol used in the present study.^28),34)

Table 1. Mean and median values of BC mass concentrations (M_BC)

Period	Observation sites	Observation days	Measurement	Number of data	MBC (µg m−3)
					Mean (± 1σ)	Median (25%, 75%)
May 2003–Aug 2005	Komaba	308	TOT	7,386 (hourly data)	2.30 ± 0.45	2.29 (2.05, 2.56)
Dec 2006–Dec 2010	Komaba	540	COSMOS	12,965 (hourly data)	1.12 ± 0.48	1.00 (0.88, 1.23)
Feb 2014	Hongo	16	SP2	21,365 (1-min data)	0.71 ± 1.04	0.35 (0.18, 0.69)
Jul–Aug 2014	Hongo	20	SP2	26,775 (1-min data)	0.41 ± 0.31	0.31 (0.20, 0.56)
May 2016	Hongo	7	COSMOS	8,765 (1-min data)	0.94 ± 0.86	0.61 (0.28, 1.53)
Sep 2016	Hongo	8	COSMOS	11,380 (1-min data)	1.10 ± 1.41	0.79 (0.37, 1.27)
May 2017	Hongo	24	COSMOS	30,290 (1-min data)	0.70 ± 0.83	0.54 (0.33, 0.81)

From 2006 to 2010, we used the continuous soot monitoring system (COSMOS), which is based on a filter-based absorption photometer with a heated inlet.^34)–37) The accuracy of M_BC measurements by COSMOS was estimated to be about 10% based on comparisons with measurements by a single particle soot photometer (SP2) (r² = 0.97).^{34),36)–38)} The M_BC measurements by COSMOS and TOT instruments agreed to within about 10% (r² = 0.92) at various sites in Asia.³⁴⁾

After 2014, we measured M_BC at Hongo via SP2 and COSMOS during intensive measurement campaigns that lasted 1–4 weeks (Table 1). Because M_BC measurements by TOT, COSMOS, and SP2 agree to within ∼10%, in this paper we use the M_BC data without reference to the technique used.

PM_2.5 concentrations were based on the measured mass of aerosols collected using filters.³²⁾

3. Temporal variations

3.1. BC and PM_2.5 during 2003–2017.

The annual average PM_2.5 concentrations at the stations in the four categories agreed to within about 10% (Fig. 2b). From this, we conclude that PM_2.5 data in central Tokyo are representative of the whole Tokyo metropolitan area, consistent with the uniform distribution of carbon monoxide concentration and its strong correlation with M_BC (section 2.1). It is therefore likely that all aerosol measurements at Komaba and Hongo are representative of the Tokyo metropolitan area.

Fig. 2.

Temporal variations of (a) BC mass concentration (M_BC) observed at Komaba (black circles) and Hongo (black squares), (b) PM_2.5 concentrations for four station categories (roadside and non-roadside stations in central Tokyo and suburbs of Tokyo), reported by Wakamatsu et al. (2013),³²⁾ and (c) BC/PM_2.5 and BC/(PM_2.5 − BC) ratios at non-roadside stations in central Tokyo. The error bars in (a) represent the standard deviation of the annual means at Komaba and the monthly means at Hongo.

Measurements at Komaba show that M_BC decreased by a factor of 3 during 2003–2010, with the rate of change apparently slowing in 2009 (Fig. 2a). Similarly, the annual average PM_2.5 concentrations for each of the four station categories decreased by about 35% from 2003 to 2010 (Fig. 2b). Our data from Hongo during 2014–2017 were rather stable and similar to M_BC observations at Komaba in 2010. We infer that M_BC did not decrease significantly after 2010. These reductions in BC and PM_2.5 are probably due mainly to the regulation of emissions from diesel vehicles.³²⁾

3.2. Diurnal BC variations.

Diurnal BC variations are useful for analyzing the effects of local BC emissions on M_BC. Figures 3 and 4 show the diurnal variations of M_BC and the size distribution of BC measured by SP2 in July and August 2014. M_BC increased in the morning, which is likely due to emissions from diesel vehicles.^28),31) The average number and mass size distributions shifted to smaller diameters with this increase in M_BC (Fig. 4). The mass median diameter (MMD) reached its minimum when M_BC reached its maximum in the morning (Fig. 3). Sizes of freshly emitted BC particles in urban areas are generally smaller than those of aged BC particles because they are less influenced by coagulation. Similarly, BC particles observed downwind from the Asian continent are considerably larger than those emitted from Japan.^39),40) It is thus likely that locally emitted BC influenced M_BC in Tokyo strongly even in 2014, when M_BC had decreased by a factor of 3 since 2003.

Fig. 3.

Average diurnal variations of (a) M_BC and (b) mass median diameter (MMD) measured by SP2 at Hongo in July and August 2014.

Fig. 4.

Hourly mean size distributions of BC by (a) number and (b) mass measured by SP2 at Hongo in July and August 2014.

In 2014, M_BC reached its daily maximum value around 10:00 local time (Fig. 3a), whereas the maximum occurred around 07:00 local time during 2003–2005.²⁸⁾ These diurnal trends are further evidence that BC emissions from diesel vehicles had decreased as a proportion of the total BC emissions.

3.3. BC/PM_2.5 ratio.

BC and PM_2.5 concentrations at non-roadside stations in central Tokyo decreased by 70% and 35%, respectively, during 2003–2010. This resulted in a decrease in the BC/PM_2.5 ratio by a factor of 2.3 (Fig. 2c). Kondo et al. (2010)⁴¹⁾ showed that light-scattering particles, which include organic and inorganic aerosols, constituted about 80–90% of aerosol mass concentrations for PM with diameters less than 1 µm (PM₁) at Komaba in 2003. PM₁, in turn, is the dominant contributor to scattering coefficients for visible radiation in urban atmospheres.⁴²⁾ In this study, we used the BC/PM_2.5 ratio as a qualitative measure of aerosol optical properties. BC/(PM_2.5 − BC) ratios were similar to the BC/PM_2.5 ratios (Fig. 2c). Reducing the BC/PM_2.5 ratio leads to a corresponding increase in single scattering albedo, which contributes to net radiative cooling by aerosols. Quantitative estimates of these effects require radiative transfer calculations, which are beyond the scope of this study.

Murphy et al. (2011)⁴³⁾ reported reductions of more than 25% in BC and PM_2.5 at stations in national parks and other remote regions in the United States during 1990–2004. Despite the decrease in BC, these changes were estimated to have caused warming in the United States because the rate of decrease in BC was comparable to that for light-scattering particles during the period.

4. Emission of BC in Tokyo and Japan

A previous study³¹⁾ estimated the emission flux of BC (EF_BC) in Tokyo for the period 2000–2011 on the basis of EF_BC determined in the Kanto region for the year 2000.³⁰⁾ The annual EF_BC was derived from the annual number of vehicles for each category (cars, buses, medium- and heavy-duty diesel trucks, and light-duty diesel trucks) for which statistical data were reported by the Ministry of Environment, Japan.⁴⁴⁾ The use of diesel particulate filters and low sulfur fuels beginning in 2003 has led to large reductions in EF_BC,⁴⁴⁾ which accounted for the decrease in total EF_BC from all sources during this period.³¹⁾ The estimated decrease in total EF_BC is consistent with the observed changes in M_BC reported here (Fig. 2a); however, the study found that EF_BC from sources other than on-road vehicles did not substantially change in 2000–2011.³¹⁾

Estimated EF_BC values for all of Japan and the Kanto region decreased by factors of 3.0 and 2.8, respectively, from 2003 to 2015 (Fig. 5).⁴⁵⁾ The change in estimated EF_BC in Kanto agrees well with the change in observed M_BC in Tokyo. Although the EF_BC from transport in Kanto (including diesel vehicles) decreased greatly during 2003–2015, the EF_BC from industry slightly increased after 2011, which slowed the rate of decrease in total BC emissions after 2011.

Fig. 5.

Temporal variations of total BC emissions (dots) and BC emissions from different sectors in (a) Japan and (b) Kanto during 2003–2015. Data from Kurokawa et al. (2018).⁴⁵⁾

In a study that measured concentrations of nine PAHs in the total PM in Tokyo, and in Kanagawa Prefecture (in the Kanto region), from 1997 to 2014, the total concentration of all nine PAHs (ΣPAH) in winter decreased by a factor of about 3.6 from 1997 to 2005 and then remained steady from 2008 to 2014.⁴⁶⁾ The initial large decrease in ΣPAH was interpreted to result of a corresponding decrease in PAH emissions from vehicles.⁴⁶⁾ The similarity in the temporal changes in ΣPAH and M_BC in Tokyo is consistent with the similarities of their sources, as discussed in section 1.

5. Transport of BC from the Asian continent

Morino et al. (2017)⁴⁷⁾ calculated the distribution of aerosols, including BC, in East Asia for the year 2012 using the three-dimensional chemistry and transport model of the Models-3 Community Multiscale Air Quality modeling system. They used EF_BC data from the Auto-Oil Program^48),49) for anthropogenic sources in Japan (∼1 and 10 km resolutions for vehicles and other sources, respectively), from the Regional Emission Inventory in Asia⁵⁰⁾ for anthropogenic sources of other Asian countries (0.25° resolution), and from the Global Fire Emission Database for biomass burning (0.5° resolution).^51),52) The effects of possible localized sources in Tokyo that were not included in these estimates should not influence the present results considering the spatial uniformity of observed PM_2.5 concentrations in Tokyo. Their calculated average M_BC values for winter, spring, and summer for all of Japan and for Kanto are shown in Fig. 6, along with the contributions from emission sources in Asia and Japan.

Fig. 6.

Contributions of BC emitted in Japan and Asia to M_BC for (a) all of Japan and (b) Kanto in winter, spring, and summer 2012. Data from Morino et al. (2017).⁴⁷⁾

The M_BC value of about 0.4 µg m⁻³ for Kanto in 2012 (Fig. 6) is about half that observed at Komaba and Hongo during 2010–2017 (Fig. 2a). BC emitted within Japan contributed about 60% of the average M_BC in Japan and about 80% of the average M_BC in Kanto, which has large local as well as internal sources of BC (Fig. 6). This analysis shows that M_BC in Kanto can be further decreased by continuing to reduce BC emissions in Japan as well as Kanto as long as BC emissions in East Asia do not increase enough to offset those reductions. To further reduce domestic BC emissions, it will be necessary to reduce emissions from other sources besides on-road vehicles. To improve estimates of these contributions, further analyses using the latest emission datasets with higher spatial resolutions are needed.

6. Conclusions

Our measurements near the urban center of Tokyo show that M_BC decreased by a factor of 3 during 2003–2010 and remained steady during 2011–2017, whereas annual average PM_2.5 concentrations in Tokyo decreased by only 35% during 2003–2010. As a result, BC/PM_2.5 ratios decreased by a factor of 2.3, leading to corresponding increases in single scattering albedo and the cooling effect of aerosols. The diurnal variations of M_BC and the size distribution of BC in 2014 suggest the significant contribution from BC emitted within Tokyo. The stability of M_BC during 2010–2017 indicates that EF_BC from sources other than vehicular emissions in Tokyo did not decrease substantially. The pattern of the temporal changes in M_BC was similar to that of ΣPAH in Tokyo, consistent with the similarities of their sources. Three-dimensional modeling of M_BC values in Kanto, the region that includes Tokyo, showed that the contribution of domestic BC emissions to M_BC was about 80% in Kanto as compared with about 60% for all of Japan. These results indicate that M_BC in Tokyo can be further lowered by reducing emissions of BC inside Japan as well as in Kanto.

Acknowledgements

We thank Jun-ichi Kurokawa of the Asia Center for Air Pollution Research for providing the BC emission rate data from different sectors in Japan. We also thank Kazuichi Hayakawa of Kanazawa University for providing the PAHs data for Kanto. This work was supported by the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT), the Environment Research and Technology Development Fund (2-1403 and 2-1703) of the Environmental Restoration and Conservation Agency of Japan, Japan Society for the Promotion of Science KAKENHI Grant (JP16H01770), and the Arctic Challenge for Sustainability project. The data used in this study are available at URL (https://ads.nipr.ac.jp/dataset/A20191212-002).

Notes

Edited by Atsuhiro NISHIDA, M.J.A.

Correspondence should be addressed: Y. Kondo, National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan (e-mail: [e-mail]).

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Abbreviations

BC

black carbon

COSMOS

continuous soot monitoring system

EF_BC

emission flux of BC

M_BC

BC mass concentration

MMD

mass median diameter

PAHs

polycyclic aromatic hydrocarbons

PM

particulate matter

SP2

single particle soot photometer

TOT

thermal-optical transmittance