Preprint, XII World Hydrogen Energy Conference, Buenos Aires, Argentina, 21-26 June 1998.


Paul Kruger, Mork Murdock
Civil Engineering Dept.
Stanford University
Stanford CA 94305
Toshihiro Hirai, Kazukiyo Okano
WE-NET Office - ENAA
1-4-6 Nishi-shinbashi
Minato-ku Tokyo 105


A revised estimation has been made of the local air quality improvement that can be achieved in Tokyo as a byproduct of a world-wide hydrogen energy economy based on global reduction of CO2 emission from fossil fuels. The study for the WE-NET Project in Japan examined the potential for reduction in nitrogen oxide emission with hydrogen fuel in the Tokyo metropolitan area where sufficient data exist on trends in population, vehicle registrations, traffic activity, and exhaust emissions. A dynamic model was developed to extrapolate the trends in these data to estimate future NOx emission rates in comparison to regulatory scenarios based on policy and technical decisions of the Tokyo Metropolitan Government for improvement of air quality in the metropolitan area. The results of the model show significant potential for reduction of NOx emission by the year 2020 with a concomitant reduction in mean roadside NOx concentration.


Epidemiological health benefit from reduced emission of hazardous air pollutants in automobile exhaust is a synergistic advantage of replacing hydrocarbon fuels with hydrogen fuel. Many heavily populated metropolitan areas suffer from above-standard ozone concentrations resulting from nitrogen oxides (NOx), primarily in vehicle exhaust gases. The potential for air quality improvement in three large metropolitan cities (Los Angeles, Mexico City, Tokyo) with hydrogen fuel was examined by Fioravanti and Kruger (1994). Further analysis of the potential for Tokyo was reported by Kruger (1996) from the extensive database provided by the Bureau of Environmental Protection of the Tokyo Metropolitan Government (BEP-TMG, 1992, 1994, 1995, 1996). The database included official data through 1996 and forecast expectations through 2010 on population, vehicle registration, traffic activity, and emission exhaust and proposed control measures to reduce pollution emissions from automobiles. Meteorological aspects of the Tokyo air basin were examined with respect to the history of roadside NOx air concentrations. The topography of the Tokyo Metropolitan Area is an open basin with hills only to the west in contrast to the closed basin of Mexico City and the semi-closed basin of Los Angeles. The database for Tokyo was examined in two time periods, historic data until 1995 and official estimates after 1995. From these data, trends were evaluated with a dynamic model to estimate the number of hydrogen-fueled zero-emission vehicles needed to reduce NOx emission consistent with expected regulatory and technical emission-control measures to come into compliance with the air quality standards of the Tokyo Metropolitan Government.

The database accumulated for the study consisted of:

  1. residential and daytime populations since 1965 and TMG forecasts for the year 2000;
  2. vehicle registrations from 1975 through 1995 for the official nine types of light and heavy vehicles grouped into five vehicle types;
  3. annual driving distances (vehicle-km traveled per year) in 1995 by vehicle type (and fuel) inboth the congested central area and the metropolitan area; and forecasts in 5-year intervalsthough the year 2010;
  4. emission inventories of NOx determined for 1995 and estimated in 5-year intervals through the year 2010;
  5. mean annual concentration of NOx since 1970 at both roadside monitoring stations and environmental monitoring stations in the air basin through 1995.
The residential population of the Tokyo metropolitan area in 1965 was 10.87 million and grew at a mean annual growth rate of 0.24 %/a through 1995. An additional daytime population of 2.6 million includes commuters and students from outside prefectures. Vehicle registrations in 1975 was 2.66 million and grew at a mean annual growth rate of 3.6 %/a for passenger cars and 3.1 %/a for all vehicles through 1995. Traffic data for the metropolitan area (BEP-TMG, 1992) were provided as annual driving distance (vkm/yr) in 5-year intervals from 1990 to 2010. The values for the central and suburban districts were calculated by BEP-TMG assuming a mean growth rate of 0.9 %/a for the period 1990 to 2000 with 71 % of the traffic in 1995 in the central district of which 65 % was by passenger cars. The data for the total fleet was 51.6 x 109 vkm/yr in 1995, expected to grow to 55.2 x 109 vkm/yr in 2000. Fuel aspects of the data show that about 97 % of the passenger cars are powered by LPG fuel and is forecast to remain constant during the next 20 years.

Exhaust-gas emission factors (in g/km) were given (BEP-TMG, 1992) from measurements under controlled conditions in the laboratory. Air concentrations were measured in a network of monitoring stations distributed along roadways and throughout the air basin. Emission factors were computed as a function of speed for nine vehicle types for the year 1994. Reductions were forecast through 2010 (BEP-TMG, 1994) adjusted for expectations in increased emission controls, increased number of low-emission vehicles, and traffic control. NOx emission was expected to decrease from 52.2 kt/y in 1990 to 48.1 kt/y in 2000 without additional regulatory control and to 39.5 kt/y with forecast additional regulatory control. Measurement of NOx in the Tokyo air basin since 1970 by the Japan Environmental Agency (JEA, 1990) is in two forms of monitoring networks: automobile exhaust monitoring stations (at roadside) and general air quality monitoring stations (away from roadside). The environmental quality standard (EQS) for NO2 (JEA, 1990) is a daily average of hourly values within or less than the range 0.04 to 0.06 ppm. The roadside concentration in 1995 was 0.042 ppm and the areal concentration was 0.031 ppm, both the same as the mean concentrations during the last 5 years (1991-1995). The forecast for 2000 was 0.038 ppm roadside and 0.029 ppm basin-wide with no new additional regulations. Meteorological studies of NOx air quality (Uno, et al., 1996) show no simple correlation between automotive emission of air pollutants, even from a well distributed roadway system, and the resulting ambient air concentration. However, some means of metric evaluation is needed to obtain an indication of the value of introducing hydrogen-fuel zero-emission vehicles (HZEV) into the Tokyo air basin. To accomplish this, a dynamic model was developed in which the fractional reduction in emission was credited with a corresponding reduction in the ambient roadside air concentration.


Estimation of the potential for improved air quality in Tokyo from introduction of hydrogen-fuel zero-emission vehicles (HZEV) requires assumptions on two aspects of industrial decision-making and development. The first involves the limit for growth rate with which a fledgling industry can accelerate production to meet demand. The second aspect is the need to satisfy a "moving-target" air-quality improvement goal. As the annual emissions of oxidant-forming pollutants from gasoline- and diesel-fueled vehicles are reduced by regulatory or technical means, the number of HZEVs needed to achieve a given maximum emission level will increase proportionally. The choice of maximum emission level is an important TMG decision. Since the ambient air concentration is difficult to correlate with changes in emission rate, a more satisfactory goal for maximum emission level for Tokyo might be a maximum emission factor, such as for the 'Ultra-Low Emission Vehicle' (ULEV) for NO2 of 0.2 g/mi (0.124 g/km) in California.

A dynamic model (the Tokyo Vehicle NOx Emission Model) was developed by Murdock (1996) to examine the potential for reduced NO2 emission from hydrogen-fuel zero-emission vehicles under the two constraints. The model was developed with the commercially available Stella II program (High Performance Systems, 1994) useful in illustrating non-linear relationships in complex dynamic systems (Hannon and Ruth, 1994). The model evaluates a range of production growth rates of a new hydrogen-powered automobile industry and a range of plans of the Tokyo Metropolitan Government to reduce emission of NO2 from new production of fossil-fueled automobiles, trucks, and buses through the year 2010. The objective of the model was to identify the rate of development of HZEVs that would bring the automotive fleet in Tokyo into compliance with the Ultra-Low-Emission Vehicles (ULEV) emission standard of 0.124 g/km for nitrogen oxides by the year 2020. The model uses a classification of automotive vehicles in three categories based on emission level by fuel type: (1) gasoline-fueled vehicles, (2) diesel-fueled vehicles, and (3) hydrogen-fueled vehicles. Gasoline vehicles include private cars, LPG passenger cars, non-LPG passenger vehicles, light trucks, small trucks, and regular trucks. Diesel vehicles include buses, ordinary freight vehicles, and special trucks.

An Ultra-Low-Emission Vehicle (ULEV) is defined by the State of California as any vehicle that emits nitrogen oxides at less than or equal to 0.2 g/mi (0.124 g/km). All vehicle technologies, including gasoline, electric-battery, fuel-cell, or hydrogen-powered vehicles could be considered ULEVs. A new standard of Near (essentially) Zero Emission for internal-combustion hydrogen-powered vehicles where a negligible (compared to 0.2 g/mi) amount of NO2 may be formed from the high-temperature combustion with air. Hydrogen as fuel in a fuel cell results in zero emission of NO2. For model purposes, it was assumed that all hydrogen powered vehicles (HZEV)are zero emission vehicles.

The Tokyo Vehicle NOx Emission Model is shown schematically in Figure 1. The model calculates the total fleet emission of NOx annually from the baseline for 1995 through the WE-NET project date of 2020 which goes 10 years beyond the predicted values to 2010 published by BEP-TMG. The modeled components are population, vehicle ownership, traffic activity, vehicle fleet composition, and emission factors. The input data for the model are those of the Tokyo Metropolitan Government for population, vehicle registrations, traffic activity, and emission factors for NOx . The mean values of these parameters for the initial year of 1995 are summarized in Table 1.

The population component of the model utilizes data for the residential population of the Tokyo metropolitan area. With an initial population of 11.77 million in 1995 and a mean population growth rate of 0.08 %/a, the model calculated a value of 12.0 million people for 2020. The vehicles component is calculated from the vehicle ownership component, which avoids making large extrapolations with large uncertain mean growth rates by expressing future fleet size as a saturation parameter (in vehicle/capita ratio) based on expected economic growth. The vehicle per capita ratio for Los Angeles (Kruger, 1996) was constant at 0.74 veh/cap from 1980 to 1990 while the value for Tokyo grew from 0.266 veh/cap in 1980 to 0.392 veh/cap in 1995. The model calculated the vehicle fleet with a mean annual growth rate of 2.07 %/a and a saturation value of 0.6 veh/cap over the long-term period beyond 2020. With an initial value of 0.392 veh/cap in 1995, the model calculated a value of 0.48 veh/cap by 2020.

Vehicle traffic was defined as the total distance traveled by the vehicle fleet per year in the Tokyo metropolitan area. The value for 1995 was 51.6 x 109 v-km/yr, which yielded a mean vehicle activity of 11,165 km/v-yr. The model assumed that vehicle activity will be constant over the 25-year period to 2020, consistent with BEP-TMG (1992) control scenarios. Short term emission controls with respect to NOx were defined mainly for diesel vehicles through 1995 with longer-term controls for vehicles built after 1995. Trucks were slated for greater emission control after 1995. Further control was planned with traffic volume reduction measures. The model was used to examine the effect of introduction of hydrogen (HZEV) vehicles in relation to the emission control scenarios of BEP-TMG, listed in Kruger, et al.(1998).

The Vehicle Fleet Composition component simplifies the distribution of travel by the‘official nine types of vehicles into three general types by fuel: (1) gasoline-fueled vehicles; (2) diesel-fueled vehicles; and (3) hydrogen fuel-cell vehicles. The distribution for 1995 was 76 % gasoline vehicles, 24 % diesel vehicles, and 0 % hydrogen vehicles. A summary of the distribution is given in Table 2.

BEP-TMG provided estimates of traffic by gasoline and diesel fuel types through the year 2010. The ratio of gasoline to diesel vehicle travel was assumed to remain constant, although this ratio could change as the relative prices and availability of the two fossil fuels change with time. Also summarized in Table 2 are the activity-weighted means of the 1994 Emission Factors values based on the emission factors for vehicle speed of 25 km/hr provided by BEP-TMG (1996) through 2010 and extrapolated to 2020. The weighted averages by fuel type were used in the model to provide the emission factors annually for the two vehicle categories through the model end-time of 2020.

Two assumptions required in the model concern the ability of the automobile industry to produce HZEVs in response to future industrial and regulatory decisions. The assumptions are when to initiate production and at what growth rate over the period through 2020. Although it is not clear when introduction of low-emission or zero-emission vehicles into the Tokyo vehicle fleet could be mandated, the number of such vehicles will not be significant in this present century. One possibility is the TMG (1992) plan for low-emission vehicles (LEV) with a target of 300,000 vehicles by 2000 and 900,000 vehicles by 2010. The standard for LEV could be met by reformulated hydrocarbon fuels as well as electric-battery and hydrogen-fueled vehicles. These TMG targets represent about 5% of the total fleet in 2000 and 15% of the total fleet in 2010. The initiation date incorporated in the model was from the revised mandate of the California Air Resources Board which requires that 10% of all new cars sold in California starting in the year 2003 will be zero-emission vehicles.

The second assumption was the production capacity for HZEV manufacture. Selection of a basis that is technically, regulatory, and market driven is very difficult. Industrial companies are considered successful if their growth rate exceeds 15-20 %/a over a long growth period and public demand could stimulate growth rates to values of 25-50 %/a over short time periods. A large growth rate was achieved over a 17-year period from 1903 to 1920, exactly 100 years earlier, by the Ford Motor Company. Vehicle production by the Ford Motor Company (private communication, 1996) grew from 1700 vehicles in 1903 to almost 1 million vehicles in 1919 at a mean annual growth rate of 41%/a. To estimate the potential for HZEV production over an initial 17-year period to 2020, the model used a range of mean annual growth rates of 20, 30, 40, and 50 %/a from an assumed first manufacturing capability in the year 2003 to meet a 10 % new HZEV mandate.

With these data, the model calculated the annual total emission of NOx with annual travel activity by vehicle type multiplied by its respective emission factor for each year interval and the products were summed to obtain a value for total annual NOx emission. The model then stepped all factors forward and recalculated annual emissions for each year through 2020. The forecast of roadside NOx concentration was calculated from the forecast emission based on the 1995 data for total NOx emission and the corresponding mean annual roadside NOx concentration. BEP-TMG (1994) attributed 71 % of the observed NOx concentration to be generated by vehicles and 29 % by stationary sources (industry and residential). Based on the mean value of 0.042 ppm for the period 1990 to 1995, a linear relationship was used in which the 1995 total vehicle NOx emission of 45.3 kt/yr was assumed to generate 0.030 ppm of the mean NOx roadside concentration. For the meteorology conditions in the Tokyo air basin, the relationship was assumed to remain constant. The stationary source contribution of 0.012 ppm was also assumed to remain constant over the model time. The calculated annual concentration produced by the total vehicle fleet was added to the fixed industrial contribution for total roadside concentration.


The results of the model simulations were obtained in three forms:
  1. estimation of the potential growth in the number of HZEVs that could be produced in the period from 2003 to 2020 over the range of industrial growth rate from 0 to 50 %/a compared to the forecast growth of the total vehicle fleet in Tokyo;
  2. estimation of the corresponding reduction in NOx emission resulting over that potential range of industrial growth rates for producing HZEVs for use in the Tokyo metropolitan area.
  3. estimation of the roadside NOx concentration through 2020 also over that range in growth rate.
The results are shown in Figures 2, 3, and 4. The output for the potential number of HZEVs solely for the Tokyo vehicle fleet (Figure 2) shows that the total number of vehicles in the TMA over the 25-year model period will reach about 6.4 million by the year 2020. The output for the size of the Tokyo fleet was 5.02185 million vehicles in 2003 and 5.06867 million vehicles in 2004. Thus, the initial value for number of new HZEV vehicles registered in 2003 was 4682. The results show that the attainable fraction of HZEVs in the total fleet could reach a modest 0.4 to 1 percent by the year 2010, the attainable fraction could become significant by 2020. For a 40 %/a growth rate starting with 4682 HZEVs in 2003, it would be possible to supply more than two million HZEVs in 2020, representing about 30 percent of the total fleet forecast for that year.

The results for the potential of the HZEV fleet for reducing emission of NOx in the Tokyo air basin are given in Figure 3. The curves through 2010 show the early decrease expected from reduction of diesel-fuel emission factors through 2005. The number of HZEVs after 2005 becomes important after 2010 with reductions shown as a function of mean annual growth rate. The reduction in NOx emission by 2020 ranges from 2 % at a m.a.g.r. of 20 %/a to more than 30 %/a at a m.a.g.r. of 40%/a. For a rate approaching 50 %/a, the entire vehicle fleet of the Tokyo metropolitan area could be converted to zero-emission vehicles.

The simulation for the roadside NOx concentration was carried out from 1995 based on the ratio of vehicle emission of 45.3 kt/y to the observed air concentration of 0.042 ppm adjusted for the 71 % of observed concentration from vehicle traffic (0.030 ppm) reported by BEP-TMG (1994). The non-traffic contribution of 0.012 ppm (29 %) was kept constant and added back to the annual value calculated each year. The resulting forecast of mean annual roadside NOx concentration is shown in Figure 4. The results show a gradual drop in concentration as the TMG regulatory controls take effect and the roadside concentration falls rapidly after 2010 as the rate of introduction of HZEVs increases. The concentration would drop even faster as regulatory control of stationary sources of NOx emission is also carried out. A discrepancy is noted between the input conditions to the model and the NOx concentration reported for 1995. The calculated NOx emission for 1995 is 35.9 kt compared to the reported 45.3 kt (a 20 % difference). The resulting concentration is 0.036 ppm compared to the reported 0.042 ppm. Input factors that could account for this difference include: (a) actual travel distance greater than 51.55 billion v-km/y; (b) a different distribution of vehicle travel by fuel type; (c) larger emission factors than given in the BEP-TMG tables; and (d) vehicle emission of NOx less than 45.3 kt in 1995.


One objective of this study was to evaluate whether a credit for air quality improvement in the Tokyo air basin could be claimed as an argument for early acceptance of hydrogen as a transportation fuel. The magnitude of the credit for reduced cost of epidemiological health is difficult to ascribe to individual sources, but it is clear for large-scale traffic activity, that any reduction in exhaust pollution would be helpful. The approximate value for Los Angeles indicated from medical costs of air-pollution sources of diseases and traffic activity in the air basin (Kruger, 1966) was of the order of 4.5 ¢/mi. From evaluation of available air-pollution costs in Tokyo, the epidemiological health value for Tokyo would be in excess of 0.4 ¥/km (0.6 ¢/mi). The results of the simulation of introduction of HZEVs into the growing Tokyo fleet show that even with growth rates of 20 to 40 %/a, only a small improvement in the air quality of Tokyo can be expected by the 2010 forecast period of the Tokyo Metropolitan Government. At growth rates of 30-40 %/a, a very significant improvement in air quality could be demonstrated by 2010.

The results of the study indicate that hydrogen-fuel zero-emission vehicles could be of public value in the Tokyo air basin and an important consideration in public acceptance of hydrogen as a fuel. The WE-NET project (based on reduction of global CO2) needs to integrate the epidemiological health value of hydrogen as a fuel into the overall economic evaluation where price is considered the major factor. The driving force to replace fossil-fuel combustion in metropolitan air basins with large traffic activity is reduction of vehicle exhaust. Control by local governments have taken a variety of forms, but they can be grouped in basic control philosophies of Do Without and Do Better. A mixture of both philosophies is prevalent in current TMG proposals, which lean strongly to curtailing traffic. With respect to the WE-NET program, the problem for achieving marked success is the long lead time (20-30 years) needed to demonstrate the potential of hydrogen as an economic fuel for improving air quality and the even longer lead time (50-60 years) needed to establish a large publicly accepted infrastructure for distribution of hydrogen fuel. The key issue for TMG is whether the government administration has the perseverance to initiate and accelerate the needed industrial production over the next 20-30 years to achieve the potential results, even in poor economic times.


This study was supported by the United States Department of Energy (USDOE) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan.


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