Air Pollution as a Climate Forcing: A Workshop
Summary D. Technology
The technology session addressed the questions: (1) are technologies available for reduction of relevant global air pollution, and, anticipating a positive answer, (2) what are real-world expectations based on case studies for different cities, states and countries, and what are the barriers that limit technology implementation?
Carbon dioxide. It was essential to bring CO2 into the technology discussion. The reason is apparent from Figure 1, which shows that most air pollutants arise from fossil fuel use, which is also the main source of CO2. Furthermore, the three principal elements that must be considered in any carbon management approach, energy efficiency, decarbonization of energy sources, and sequestration of CO2, can be generalized to air pollutants.
Pollution management. The three elements in pollution management are shown in Figure 2. The first element in pollution management is energy efficiency which can be defined as the energy (BTUs) used per unit gross domestic product (GDP). Other things being equal, emissions are reduced in proportion to improvements of energy efficiency [A notable exception discussed during the workshop is diesel engines, which are more energy efficient than gasoline engines but increase particulate pollution.]. Energy efficiency by itself cannot clean the air and yield declining global CO2 emissions on the long run, as illustrated by the electricity and transportation examples below. However, efficiency has a crucial role to play in both the near-term and long-term in reducing air pollution and in slowing the growth rate of CO2. Figure 3, the energy intensity in different global regions, illustrates the potential for improved "productivity", i.e., GDP per unit energy, in the developing world. Moreover, as discussed below, there still remains great potential for increased energy efficiency in developed countries.
The second element in pollution control concerns the amount of emissions per unit energy. Emission per unit energy is a function of the fuel employed, as illustrated quantitatively for CO2 below. The amount of air pollution released is also a function of the technology employed in "burning" the fuel. Thus reductions in emissions can be achieved via fuel switching, technology improvements, and introduction of new energy sources including renewable energies.
The third element in pollution management is pollutant capture. The technology for capture of true air pollutants, at power plants and mobile sources, is steadily improving. Implementation and maintenance of technology on small-scale sources and vehicles can be challenging. Capture and sequestration of CO2 can occur via enhancement of biological uptake of CO2 by the land (vegetation and soils) and ocean or by capture of CO2 at power plants with subsequent injection into the deep ocean or into geological formations. The cost, long-term effectiveness, and environmental acceptability of CO2 sequestration require further study.
Two key pollutant sources. Two sources of emissions that are growing rapidly and thus present our primary technology challenges are electricity and transportation. Forest fires and other biomass burning are also large and possibly increasing sources of emissions. Land clearing and agricultural practices are subject to technology improvements but were not a focus of workshop discussion.
The fact that electricity has become a larger and larger fraction of total energy use in the developed world, and will grow in the developing world, is a boon for clean air. As household and small scale uses of fossil fuels and biofuels are replaced, the emission of air pollutants can be greatly reduced. The effect of electrification on energy efficiency is less clear, as typical present-day central power stations use about 3 units of energy for every 1 unit delivered. This ratio can be reduced to 2:1 with newer technologies, i.e., an efficiency improvement from 33% to 50%. Less centralized cogeneration plants potentially can operate at about 70% efficiency. A potential near-term alternative or adjunct to electrification in developing countries is improvements in cookstove technology.
Electricity generation was the source of 40% of CO2 emissions in the United States in 2000 (2). Figure 4 shows the trend in per capita electricity use in the United States and California. Although the per capita use in California remained constant over the past 25 years, the population grew by 50%. In the United States as a whole, electricity use per capita increased almost 50% in that period, or about 1.5% per year. In contrast, total energy use per capita in the United States was the same (350 million BTUs) in 2001 as in 1976, i.e., the growth rate was 0% per year (2).
The second key pollutant source, in the United States and worldwide, is transportation vehicles. Transportation now accounts for 33% of CO2 emissions in the United States (2) and almost 60% in California (Figure 5). The International Energy Agency projects that global emissions from transportation will increase 75% in the next 25 years. In the period 1990 to 2000 the change in CO2 emissions from transportation in the European Union was +18%, while the change in European CO2 emissions from all other sources was -6% (3). In the United States the corresponding numbers were +18% for transportation and +9% for all other sources (2). The growth rate of transportation emissions in key developing countries is even more rapid. The emission of air pollutants per vehicle is decreasing, at least in the developed world, but vehicles continue to be a dominant source of pollution because of the rapid increase in the number of vehicles.
Electricity. Staunching the growth of emissions from electricity requires attention primarily to energy efficiency and choice of energy sources. The merit in addressing efficiency is illustrated by the energy use of refrigerators produced in the United States (Figure 6). Energy use per refrigerator increased rapidly after World War II, but since 1975 it has declined by more than two-thirds. The success in refrigerator efficiency improvements has dropped residential refrigerators to 10th place in the list of peak energy uses in California (top uses: commercial air conditioning 15%, residential air conditioning 14%, industry assembly 11%, commercial lighting 11%, commercial miscellaneous 7%, residential miscellaneous 6%). Air conditioning offers an opportunity for significant further improvement, but, because of the large number of electricity uses, efficiency must be addressed across the board to minimize pollution and CO2 emissions.
The choice of energy sources for electricity also strongly affects CO2 emissions. Figure 7 shows that CO2 emission per kilowatt-hour varies tremendously with energy source, as do air pollutants. CO2 emission associated with renewable energies is not negligible, especially for solar energy, but more efficient construction of solar power materials is anticipated. Nuclear power is especially effective in minimizing CO2 emissions. Black carbon aerosols and ozone precursors are minimized by nuclear power and by most renewable energies, but not, of course, with residential burning of wood or other biofuels.
The increasing role of electricity makes it more feasible to capture undesirable products of fossil fuel burning. However, it remains to be demonstrated that capture and sequestration of CO2 on a large scale will be both economically feasible and environmentally acceptable.
Transportation. The number of new vehicles produced per year increased by a factor of 10 in the past 50 years and is still increasing. In Europe, in the United States, and in developing countries, transportation vehicles cause a growing proportion of CO2 emissions. Vehicles are also a major source of air pollutants, including black carbon and ozone precursors.
In the near-term, even though the number of vehicles will be increasing, air pollutant emissions could decrease and the growth rate of CO2 emissions could be slowed as available and developing technologies are implemented. As for the climate effect of diesel engines, it is unclear whether their added efficiency (above gasoline engines) is outweighed by the increased emission of black carbon. Although particle trap technology exists for capture of most of the black carbon emissions, a small proportion on non-conforming vehicles is capable of dominating the total emissions.
On the long-term, achievement of declining emissions and a clean atmosphere probably requires introduction of vehicles powered by a source other than fossil fuels, for example, by hydrogen. The rationale for requiring a small proportion of so-called zero emission vehicles (ZEVs), as planned in California, is the aim of spurring relevant technological development. Renewable energies such as solar and wind power, available in large but intermittent amounts in certain regions, are well-suited for producing hydrogen. However, despite a significant research effort, practical storage and transportation technologies for hydrogen remain a challenge. There is still much to be learned about the potential environmental, engineering, and economic aspects of a hydrogen economy. Practical storage and transportation technologies are challenges that continue to be addressed in research efforts.
Case studies. The case studies demonstrate the potential of more efficient and less polluting technologies. China provides a recent demonstration, with a rapid reduction in energy use per GDP (Figure 3) and a recent modest reduction in some air pollution (4). In the United States the increased efficiency of refrigerators (Figure 6) is a triumph of the "Energy Star" program of the Environmental Protection Agency. It illustrates the potential of the government promoting energy-efficient practices by working with industry and educating the buying public. The successes in China and the United States show the possible gains from technologies that are developed with industrial acceptance, cognizance of local cultures, and ease of use by the public.
California provides an example in the United States of aggressive policies to reduce emissions. The near constant use of electricity per capita in California in recent decades (Figure 4), is a large part of the reason that the carbon intensity for California is now more like that in Europe than like the rest of the United States (Figure 8). Another reason for the lower emissions is that the mix of power sources in California (Figure 9) includes only 16% coal, with 19% from large hydroelectric plants and a non-negligible amount (12%) from other renewable sources. Downward emission trends are continuing, as electricity use was reduced 8% in 2001 via aggressive short-term conservation measures, and there are plans for long-term conservation measures as well as increased contributions from renewable energies.
Barriers. There are a number of barriers to the insertion of new technologies into the energy marketplace. One societal barrier is the tendency of industry and the public to ignore life-cycle costs and impacts, and instead make purchases based on initial costs. This barrier has reduced the market penetration of many high-efficiency technologies.
There are also regulatory barriers. Current regulations in the United States often favor central power station generation over distributed renewable generation, and utility rewards are designed such that profits are usually greater when more energy is sold. There are also out of date workplace regulations that impede the incorporation of new industrial process technologies.
A practical barrier, which most candidate new technologies fail to span, is the so-called "valley of death" in which the technology fails to achieve sufficient acceptance by industry and the public to be economically viable. For example, hydrogen automobiles may require a huge investment in fuel distribution infrastructure, if they are to be accepted by the public.
Scientific uncertainties. Technology has great potential for reducing air pollution emissions, but assignment of goals and priorities would be aided by more quantitative scientific information. Better understanding of the climate impact of black carbon and other aerosols is needed. We do not have a good database for aerosol emissions, especially black carbon, from different energy systems. Similarly, as natural gas is used for an increasing fraction of electricity production, we need quantitative understanding of methane emissions from the integrated infrastructure.
- U.S. EPA 1998 Emissions Trends Report; U.S. DOE-EIA Emissions of Greenhouse Gases in the United States.
- Annual Energy Review 2000 and 2001, DOE/EIA-0384 (2000), August 2001.
- Walsh, M.P. (ed.), CAR Lines, issue 2002-3, June 2002 (http://walshcarlines.com).
- Streets, D. et al., Science, 294, 1835, 2001.
- UNDP, World Energy Assessment: Energy and the Challenge of Sustainability, United Nations Development Programme,
- United Nations Department of Economic and Social Affairs, World Energy Council, Jose Goldenberg (ed.), New York, 2000.
- California Energy Commission, Inventory of Greenhouse Gas Emissions for California (draft), Sacramento 95814 (2002).
- Dermaut, J. and B. Geeraert, A better understanding of greenhouse gas emissions by different energy vectors and applications, 17th World Energy Congress, Houston, September, 1998.