Current Issue

CO is a colorless, odorless, and poisonous gas produced from incomplete combustion of carboncontaining compounds. This highly toxic gas strongly bonds to hemoglobin, inhibiting its oxygen-carrying ability. Automobiles are the major source of anthropogenic CO. Other sources of CO include woodburning stoves, incinerators, and industrial activities.

The EQS for CO was established in 1973. To satisfy the standard, the daily average for hourly values of CO concentrations must be 10 ppm (12 mg/m³) or lower, and the 8-h mean of 1-h mean CO concentrations must be 20 ppm (23 mg/m³) or lower. The trend for the annual average concentration of CO in air from 1970 to 2012 is shown in Fig. 2.

Fig. 2. 
Annual average concentrations of CO, SO₂, and NO₂ (1970-2012).

In 2011 and 2012, the EQS for CO was satisfied at all stations monitoring CO concentration. In 1975, the average CO concentrations at roadside stations were almost double those at ambient air-monitoring stations, but in 2011, the CO concentrations at the roadside and ambient air-monitoring stations were almost the same.

Combustion of fossil fuels is a major source of SO₂. Sources of ambient SO₂ include stationary sources (coal and oil combustion sites, for example, power plants, steel mills, oil refineries, nonferrous smelters, and pulp and paper mills) and mobile sources. SO₂ not only affects human respiratory organs, but also is a primary contributor to acid deposition (acid rain and dry deposition) and secondary aerosols.

The EQS for SO₂ was established in 1973. To satisfy the standard, the daily average for hourly values of SO₂ concentrations must be 0.04 ppm (105 μg/m³) or lower, and the 1-h mean SO₂ concentration must be 0.1 ppm (262 μg/m³) or lower. The trend for the annual average concentration of SO₂ in air from 1970 to 2012 is shown in Fig. 2.

In the 1970s, the SO₂ concentrations along roads and in the general environment dropped markedly. SO₂ emissions from stationary sources decreased owing to the use of desulfurization technologies and low-sulfur heavy oil. In the past, SO₂ from mobile sources has mainly been due to sulfur in diesel fuel, but refiners have reduced the sulfur content in diesel fuel on several occasions, and the SO₂ concentrations along roads and in the general environment have declined as a result. In 2011 and 2012, the EQS was satisfied at most stations monitoring SO₂ concentrations. Occasionally, the concentration of atmospheric SO₂ increased, mainly due to emission of SO₂ from volcanoes.

NO₂ is a highly reactive gas and an important precursor to O₃, secondary aerosols, and acid deposition. At high concentrations, NO₂ affects human respiratory organs. Most NO₂ arises from combustion of fuel. Gasoline-fueled vehicles (GV) and diesel-fueled vehicles (DV) and stationary sources such as electric and industrial boilers are major sources of NO₂ emission.

The EQS for NO₂ was established in 1973 and revised in 1978. To satisfy the standard, the daily average for hourly values of NO₂ concentrations must be 0.04-0.06 ppm (75-113 μg/m³) or lower. The trend for the annual average concentration of NO₂ in air from 1970 to 2012 is shown in Fig. 2.

Atmospheric NO₂ concentrations decreased from 1970 to 1985, but increased from 1985 to 1995. After 1995, NO₂ levels decreased, especially after 2006, when they dropped remarkably. NO₂ concentrations at roadside air-monitoring stations were notably higher than those at ambient air-monitoring stations from 1970 to 2012 (Fig. 2), mainly due to the short lifetime of NO₂ in the atmosphere.

In 2011 and 2012, the EQS was satisfied at most stations monitoring NO₂ concentrations, except roadside stations.

The atmospheric concentrations of the primary air pollutants, SO₂ and CO, have declined owing to emission-control technologies, but NO₂ is harder to control because it is produced during the combustion process and forms secondary products in the atmosphere. An especially difficult challenge is holding down emissions from diesel-powered, heavy-duty vehicles.

Owing to the NO_x/PM Law, the concentrations of NO₂ in the Tokyo metropolitan area, the Osaka-Hyogo area, and the Aichi-Mie area have fallen in recent years.

Photochemical oxidants are secondary air pollutants. Although O₃ in the stratosphere shields the Earth from harmful ultraviolet radiation from the sun, a high concentration of O₃ at ground level is a major health and environmental concern. High concentrations of O₃ irritate mucous membranes, affect respiratory organs, damage plants, and reduce agricultural harvests. In the United States, studies on the adverse effect of high concentrations of ground-level O₃ on human health began to emerge around 1945. In Japan, the first report of health damage attributable to OX in the Tokyo metropolitan area appeared in 1970. Ozone is a major component of OX in the urban atmosphere.

The EQS for OX was established in 1973. To satisfy the standard, the 1-h mean OX concentration must be 0.06 ppm (0.118 μg/m³) or lower. The trend for the annual average of the daily maximum OX concentrations from 1976 to 2012 is shown in Fig. 3. From 1980 to 2010, the annual mean of the daily maximum OX concentrations increased. In 2011 and 2012, the EQS was not satisfied at most stations monitoring OX concentrations.

Fig. 3. 
Annual average of daily maximum concentrations of Ox (1976-2012) and annual average concentrations of NMHC between 6 am and 9 am (1976-2012).

Volatile organic compounds (VOCs) undergo complex photochemical reactions with NO_x to form OX. These reactions are stimulated by sunlight and temperature, and peak OX levels typically occur during warm times of the year. Additionally, secondary aerosols may be formed in photochemical reactions involving VOCs. VOCs are emitted from both anthropogenic and biogenic sources. There are more than 100 kinds of VOCs in ambient air.

Guidelines for VOC levels in the atmosphere were established in 1976. GC-FID (gas chromatography with flame ionization detection) measurements of the total amount of atmospheric NMHC have been carried out in Japan since 1976, and VOC levels in the atmosphere should be reflected in those measurements. To satisfy the NMHC guideline, the 3-h (6-9 am) average NMHC concentration should be 0.20-0.31 ppmC or lower.

The trend for the annual average concentration of NMHC in air between 6 am and 9 am for the period 1976-2012 is shown in Fig. 3. NMHC concentrations have been decreasing since 1978. In 2011, NMHC concentrations at roadside air-monitoring stations were almost the same as those at ambient air-monitoring stations.

Particulate matter refers to dust, dirt, soot, smoke, and liquid droplets emitted into the air. Sources of PM include factories, power plants, cars, construction activities, fires, and natural windblown dust. Most PM arises from combustion of fossil fuel. In large urban areas, PM includes substantial amounts of secondary particles. Secondary particles are formed in the atmosphere by condensation or by the transformation of emitted gaseous pollutants. PM has a long residence time in the atmosphere, and at high concentrations, PM affects respiratory organs by adhering to the lungs and trachea.

Suspended particulate matter (SPM) is defined as airborne particles with aerodynamic diameters less than or equal to 10 μm.

The EQS for SPM was first established in 1973. To satisfy the standard, the daily mean of 1-h SPM concentrations must be less than or equal to 0.10 mg/m³, and the 1-h mean SPM concentration must be less than 0.20 mg/m³.

Research on the effects of particles with aerodynamic diameters less than or equal to 2.5 μm (PM_2.5) has been vigorously conducted in the United States. In Japan, the EQS for PM_2.5 was established in 2009. The annual standard for PM_2.5 is less than or equal to 15.0 μg/m³, and the 24-h standard is less than or equal to 35 μg/m³.

The US EPA defines PM₁₀ and PM_2.5 as particulate matter with aerodynamic diameters of 10 and 2.5 μm collected with 50% efficiency by PM₁₀ and PM_2.5 sampling devices, respectively. Because of differences in particle-size cutoff characteristics during measurement, SPM is not the same as PM₁₀. SPM particles are collected with 100% efficiency at the 10-μm cut point of aerodynamic diameter, whereas they are collected with 50% efficiency at the 7 μm cut point. On the PM₁₀ scale, Japan’s SPM is about PM₇. PM_2.5, SPM, and PM₁₀ are defined in Fig. 4.

Fig. 4. 
Definitions of PM_2.5, SPM and PM₁₀. SPM (Suspended particulate matter) is defined as airborne particles with a diameter smaller than or equal to 10 μm. The U.S. EPA defines PM₁₀ (PM_2.5) as particulate matter with a diameter of 10 (2.5) micrometers collected with 50% efficiency by a PM₁₀ (PM_2.5) sampling collection device.

The annual average SPM concentration in the environment has been generally decreasing since 1975 (Fig. 5). In 2011, the EQS was satisfied at 69.2% of ambient air-monitoring stations, and 72.9% of roadside air-monitoring stations. Attainment criteria for SPM environmental standards are as follows.

Fig. 5. 
Annual average concentrations of SPM (1975-2012) and annual average concentrations of PM_2.5 at four stations in Tokyo (2000-2012).

• 1. Of the mean daily values obtained over one year, those within the top 2% (equivalent to values for 7 days, if daily measurements are made for 365 days) are excluded (2% exclusion value), and the highest remaining values are compared with the environmental standard.
• 2. If the mean daily values exceed the environmental standard on more than two consecutive days, they are “nonattainment values”.

Mineral dust from continental Asia often affects wide-area SPM air pollution in Japan. The impacts are especially pronounced in the western part of Japan in the spring, and year-round variation is large.

The trend for the annual average PM_2.5 concentrations at four monitoring stations in Tokyo is shown in Fig. 5. The environmental concentration of PM_2.5 has fallen in recent years, but still exceeds the EQS.

Automobiles are a major source of CO in the Tokyo metropolitan area. Fig. 6 shows the distribution of air quality monitoring stations in Tokyo for the year 2011. The history and impact of regulations focused on reducing CO emissions from automobiles in Tokyo from 1973 to 2012 are shown in Fig. 7.

Fig. 6. 
Distribution of air quality monitoring stations in Tokyo.

Fig. 7. 
History and impact of regulations focused on reducing CO emissions from automobiles in Tokyo. Trends for the yearly 98^th percentile values of the CO concentrations are shown.

In the Tokyo metropolitan area, sulfur in diesel fuel is a major source of SO₂ emissions. Since the 1990s, the sulfur content in diesel fuels in Japan has been reduced in several steps, as follows:

• ∙0.2% (2000 ppm) S limit beginning in 1992
• ∙0.05% (500 ppm) S limit beginning in 1997
• ∙mandatory 50-ppm S limit beginning in 2005; in practice, 50-ppm S diesel fuel was introduced nationwide in April 2003 through a voluntary effort of the Japanese petroleum industry
• ∙10-ppm S limit since 2007; the Japanese petroleum industry made a voluntary commitment to supply 10-ppm S fuel in January 2005 (nationwide, with the exception of certain island areas and Okinawa Prefecture)

The history of these reductions and their impact on SO₂ levels, together with the history and impact of regulations focused on reducing SO₂ emissions from factories and business sites during the 1970s, are shown in Fig. 8.

Fig. 8. 
History and impact of regulations focused on reducing SO₂ levels in Tokyo. Trends for the yearly 98^th percentile values of the SO₂ concentrations are shown.

All air-monitoring stations recorded decreases in environmental SO₂ concentrations corresponding to the stepwise reductions of sulfur in diesel fuel. Since 1999, the SO₂ concentrations recorded at roadside air-monitoring stations have been nearly identical to those recorded at ambient air-monitoring stations.

The history and impact of regulations reducing NO₂ emissions in Tokyo are shown in Fig. 9. The environmental concentration of NO₂ decreased slightly from 1985 to 2000. This tendency is different from the trend for all of Japan over the same period (Fig. 2). The environmental concentration of NO₂ decreased sharply after promulgation of the Automobile NO_x/PM Law in 2001.

Fig. 9. 
History and impact of regulations focused on reducing NO₂ levels in Tokyo. Trends For the yearly 98^th percentile values of the NO₂ concentrations are shown.

The history and impact of regulations focused on reducing OX levels in Tokyo are shown in Fig. 10. From 1978 to 2000, the annual maximum OX concentration in Tokyo showed an increasing trend. After VOC regulations were introduced in 2006, the annual maximum OX concentration in Tokyo decreased. In contrast, the annual mean concentration of OX increased slightly from 1978 to 2010.

Fig. 10. 
History and impact of regulations focused on reducing OX levels in Tokyo.

VOCs are photochemically oxidized to OX pollutants in the atmosphere, and some VOCs can adversely affect human health. Anthropogenic sources of VOCs include automobiles, chemical industries, dry cleaners, paint shops, and businesses using organic solvents. For benzene and other hazardous VOCs, annual averages are used for EQSs, because long-term exposure to such VOCs can be harmful.

Japan has routinely implemented measures to control VOCs from mobile sources, but measures to control VOCs from stationary sources have been implemented only recently. Measures aimed at stationary sources were launched in April 2006 under the Air Pollution Control Law as amended in May 2004.

VOCs can form ozone to varying degrees, depending on their components, and therefore emphasis is usually placed on highly reactive components. However, the approach depends on what measure is used to assess ambient ozone concentration. If a highly localized concentration is the issue, high reactivity is singularly important, but if the concentration over a broad area is the issue, substances with low reactivity can make a significant contribution and are also important. The aim should be an overall reduction in the mathematical product of the reactivity and the abundance of VOC components. From this perspective, initiatives for the overall reduction of hydrocarbons have been proposed to reduce VOCs in the environment.

The idea of maximum incremental reactivity (MIR, the ozone-forming potential of a VOC component per unit weight) was used to decide which substances to regulate. The ozone-forming potentials of the individual VOC components were assessed by using a simulation model. In view of Japan’s VOC sources and meteorological conditions, VOC components with ozone-forming potentials lower than the ozone-forming potential of methane were exempted from regulation.

Controlling VOC emissions is primarily meant to prevent secondary air pollution, and many small-and medium-size enterprises (for example, paint shops) have traditionally controlled their hydrocarbon emissions as a way of limiting foul odors. At plating shops, dichloromethane, trichloroethylene, and other chlorinated organic solvents have been widely used for degreasing and finishing. These hazardous chemicals have primarily been controlled through strict environmental regulations.

The newest regulations for controlling VOC emissions were the first regulations to incorporate the idea of a “best mix” of legal regulations and voluntary cuts by businesses. Many experts from industry participated in discussions on the specific provisions of the regulations, which was a first for the environmental policy-making process in Japan. The goal was a 30% reduction of the emissions in 2000 (1,850,000 tons; 1,420,000 tons from stationary sources) by 2010.

In 2005 and 2009, VOC emissions from stationary sources were 1,110,000 tons and 820,000 tons, respectively. A reduction of more than 42% in VOC emissions from stationary sources in Japan was achieved from 2000 to 2009.

Fig. 11 shows trends for the annual mean concentrations of NO, NO₂, NO_x, OX, and NMHC in Japan. In recent years, the annual mean concentrations of NO, NO₂, NO_x, and NMHC have been decreasing, whereas the annual mean concentration of OX has been increasing.

Fig. 11. 
Trends for annual mean concentrations of NO, NO₂, NO_x, OX, and NMHC in Japan (MOE, 2012).

Formation of OX is highly dependent on meteorological factors such as temperature, total solar radiation, and wind speed. Wakamatsu et al. (1996) found a linear relationship between daily maximum temperature and daily maximum OX concentration in the Tokyo metropolitan area (Metropolis of Tokyo, Saitama, Tochigi and Gumma Prefectures).

Annual changes in percentile values for OX concentrations were analyzed by using daily data from July and August for each year to eliminate meteorological bias as follows:

• ∙Daily maximum temperature: 25-34˚C
• ∙Total solar radiation: 18-25 MJ/m²
• ∙Average wind speed from 0500 to 1200 JST: 1.5-2.5 m/s

The numbers of days selected in 2000 to 2010 were 10, 9, 7, 4, 8, 8, 8, 10, 14, 6, and 11, respectively. Fig. 12 shows percentile values for OX concentrations in the TMA from 2000 to 2010 (MOE, 2012).

Fig. 12. 
Percentile values for OX concentrations in theTMA (metropolis of Tokyo, Saitama, Gumma,and Tochigi Prefecture) from 2000 to 2010. Daily data for each year were selected underthe following fixed ranges to eliminatemeteorological bias: daily maximumtemperature, 25.34˚C; total solar radiation,18.25 MJ/m²; average wind speed from 0500 to 1200 JST, 1.5.2.5 m/s (MOE, 2012).

Geographically, Tochigi Prefecture is in the northern part of the TMA, Saitama Prefecture is in the middle of the TMA, and the metropolis of Tokyo is in the southern part of the TMA. In general, sea breezes move OX into the northern inland areas of the TMA (Wakamatsu et al., 1996, 1990, 1983).

Analysis of air pollution trends suggested a change in the mechanism of formation of photochemical ozone during the summer in the TMA. Wakamatsu et al. (1999) analyzed the relationship between the concentration of photochemical ozone and emissions of its precursor NO_x and hydrocarbons in the Kanto area (Tokyo and surrounding prefectures). From 1978 to 1990, the NO_x concentration increased, while the NMHC/NO_x ratio and NMHC concentration decreased. These changes altered where the maximum ozone concentration occurred in the Kanto area. To clarify the temporal and areal distributions of photochemical air pollution in the Kanto area, the urban airshed model (UAM) of Wakamatsu and Schere (1991) was applied. Assuming a 25% decrease in NMHC emission intensity and a 50% increase in NO_x emission intensity, the UAM simulation yielded almost the same maximum O₃ value as that obtained for the base-case simulation for 1981. However, the time of maximum O₃ concentration shifted from 1400 JST to 1600 JST. High concentrations of OX were usually observed near the shore in the morning, and moved inland following sea breezes. Under these meteorological conditions, the time shift obtained from the UAM simulation corresponds to the observed areal shift of the daily maximum O₃ concentration. Since 1990, the annual mean concentration of NO_x in the metropolis of Tokyo has been decreasing, and the annual maximum OX concentration has been increasing. In contrast, after 2000, the decreasing gradient in NO₂ concentration increased, and the OX maximum concentration showed a decreasing trend (Figs. 9 and 10). After 2006, the year VOC regulations were implemented in Japan, the decreases in high-percentile OX concentrations became clearer (Fig. 12).

The history and impact of regulations focused on reducing SPM levels in Tokyo are shown in Fig. 13. From 1980 to 1990, SPM concentrations increased. After promulgation of the Automobile NO_x Law in 1992, the 98^th percentile values of the SPM concentrations in Tokyo decreased.

Fig. 13. 
History and impact of regulations focused on reducing the SPM concentration in Tokyo. Trends for the yearly 98^th percentile values of the SPM concentrations are shown.

In 1999, the 98^th percentile value of the SPM concentration decreased significantly, mainly due to the meteorological conditions of 1999 and the shutdown of a small incinerator (concerns about dioxins led to the shutdown of the incinerator). After promulgation of the Automobile NO_x/PM Law in 2001, the quality of the air along roadsides improved, and after 2005, the SPM concentrations measured at ambient air-monitoring stations were not much different from those measured at roadside air-monitoring stations.

Fig. 14 shows trends for the yearly 98^th percentile values of the PM_2.5 concentrations in Tokyo. PM_2.5 concentrations have decreased since 2000, but are still above the EQS. The PM_2.5 concentrations were similar at roadside and ambient air-monitoring stations during the period 2000-2012.

Fig. 14. 
Trends for yearly 98^th percentile values of the PM_2.5 concentrations in Tokyo.

Air quality monitoring stations should continue to collect continuous hourly data on primary air pollutant levels. Such data are useful for deciding when to issue official air pollution warnings and declare air pollution emergencies; for evaluating the efficacy of air quality standards in limiting air pollution; for understanding mechanisms of formation of secondary air pollutants; for determining the impact of regulations on controlling air pollution; and for identifying trends in air pollutant levels. Long-term trend analysis must take into account changes in sampling methods, monitoring methods, and calibration procedures due to advancing technology. Three-dimensional information is necessary to analyze the horizontal and vertical distributions of air pollutants. Remotesensing and -sounding data for meteorological elements and pollutants will be useful for trend analysis. To understand fate and transport of pollutants in the atmosphere, wide-area data analysis should be conducted. Chemical species in PM_2.5 should be identified and quantified, which is especially important in understanding the relationship between emissions of air pollutants and the environmental concentration of PM_2.5.

Secondary air pollutants are formed through complex, nonlinear chemical reactions. Meteorological parameters also play an important role. To understand the formation and fate of these pollutants in the atmosphere, a three-dimensional air pollution modeling study should be undertaken. The model must be validated periodically with actual three-dimensional air pollution and meteorological data.

Studies to precisely identify anthropogenic and biogenic sources of air pollutants should be continuously carried out. Changes in the chemical composition of PM_2.5 (i.e., the proportions of organic, inorganic, carbonaceous, and elemental components) over time will be especially useful for evaluating the effect of air pollution countermeasures on trends in source-receptor relationships.

Transboundary air pollution has become an important issue, especially for ozone and PM_2.5, underscoring the importance of areal representativeness and coverage of monitoring stations. In 2012, the Japan Society for Atmospheric Environment became an incorporated body of the Public Utility Association. International cooperation and cooperation between the academic, government, and private sectors will be required to improve regional and global atmospheric environmental conditions.