3
Emissions

The combustion of hydrocarbon fuels—by aircraft engines as well as other types of internal combustion engines— produces carbon dioxide (CO2), water vapor, NOx, carbon monoxide (CO), oxides of sulfur (SOx), unburned hydrocarbons, particulates (primarily soot, which in high enough concentrations is visible as smoke), and other trace compounds. Aircraft emissions can affect climate, air quality, and ozone on global, regional, and local scales. Emissions can be reduced through improved engines (to create a smaller amount of emissions per pound of thrust), improved aircraft (to reduce the amount of thrust necessary to operate an aircraft with a given passenger and cargo capacity a given distance at a given speed), and improved operational systems and procedures (to use aircraft in a more fuel-efficient manner). NASA has established ambitious goals for reducing two emissions of particular interest: CO2 and NOx (see Table 1-4).

Over the years, technical advances in aircraft, engines, and operational systems and procedures have reduced the amount of emissions produced per unit of service provided (i.e., revenue-passenger-kilometer), but these advances have not kept pace with the increased demand for air transportation. Hence, the total amount of emissions from aircraft has increased, but at a rate comparable to emissions produced by other transportation modes and other sectors of the U.S. economy. However, a vigorous research program could increase the rate at which the efficiency of flight improves, provide better information on the significance of aircraft emissions, and help ensure that technical research and new regulatory standards are properly focused.

AIRCRAFT AND ENGINE TECHNOLOGY

Although aircraft fuel consumption is small relative to fuel consumption by other sectors, aircraft emissions are of increasing concern because they are deposited at altitudes where, with the exception of CO2, they affect the environment differently than ground-based emissions. Emissions from aircraft on or close to the ground are also a concern because, as with emissions from other industrial facilities, they are concentrated in specific localities (i.e., airports) where, over time, local air quality may be degraded.

Commercial aircraft have evolved from the propeller-driven craft of the 1940s and 1950s, through the early jet-powered craft of the 1960s, to contemporary airplanes with high-pressure-ratio engines. During this evolution, airframe aerodynamics and engine performance have been improved, and the weight of aircraft structures and system components has been reduced. These improvements were driven by economic requirements for longer range, higher fuel efficiency, larger capacity, and increased speed, and the net result was an air transportation system with aircraft that are more capable, yet consume less fuel and produce fewer emissions per revenue-passenger-kilometer than ever before. In the past 30 years, approximately 60 percent of the total improvement in fuel efficiency has been attributable to advances in engine technology, with the rest due to improvements in airframe design and more-fuel-efficient operations. Based on past trends, further improvements in engine and airframe efficiency seem likely to reduce fuel consumption per revenue-passenger-kilometer by about 1 percent per year for the next 15 to 20 years. This contrasts with anticipated long-term growth in commercial airline revenue-passenger-kilometers of approximately 3 to 5 percent per year (IPCC, 1999; Lee et al., 2001). In addition, improvements in engine efficiency do not reduce all types of emissions equally. Certification standards for aircraft engines recognize this disparity, allowing high-efficiency engines with high pressure ratios to emit more NOx than engines of the same size that have lower pressure ratios (and lower efficiency).

NASA has contributed significantly to technological advances in the past, and studies sponsored by NASA’s Ultra Efficient Engine Technology Program indicate that future advances could make up much, but not all, of the shortfall between future growth in demand and current projections of



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For Greener Skies: Reducing Environmental Impacts of Aviation 3 Emissions The combustion of hydrocarbon fuels—by aircraft engines as well as other types of internal combustion engines— produces carbon dioxide (CO2), water vapor, NOx, carbon monoxide (CO), oxides of sulfur (SOx), unburned hydrocarbons, particulates (primarily soot, which in high enough concentrations is visible as smoke), and other trace compounds. Aircraft emissions can affect climate, air quality, and ozone on global, regional, and local scales. Emissions can be reduced through improved engines (to create a smaller amount of emissions per pound of thrust), improved aircraft (to reduce the amount of thrust necessary to operate an aircraft with a given passenger and cargo capacity a given distance at a given speed), and improved operational systems and procedures (to use aircraft in a more fuel-efficient manner). NASA has established ambitious goals for reducing two emissions of particular interest: CO2 and NOx (see Table 1-4). Over the years, technical advances in aircraft, engines, and operational systems and procedures have reduced the amount of emissions produced per unit of service provided (i.e., revenue-passenger-kilometer), but these advances have not kept pace with the increased demand for air transportation. Hence, the total amount of emissions from aircraft has increased, but at a rate comparable to emissions produced by other transportation modes and other sectors of the U.S. economy. However, a vigorous research program could increase the rate at which the efficiency of flight improves, provide better information on the significance of aircraft emissions, and help ensure that technical research and new regulatory standards are properly focused. AIRCRAFT AND ENGINE TECHNOLOGY Although aircraft fuel consumption is small relative to fuel consumption by other sectors, aircraft emissions are of increasing concern because they are deposited at altitudes where, with the exception of CO2, they affect the environment differently than ground-based emissions. Emissions from aircraft on or close to the ground are also a concern because, as with emissions from other industrial facilities, they are concentrated in specific localities (i.e., airports) where, over time, local air quality may be degraded. Commercial aircraft have evolved from the propeller-driven craft of the 1940s and 1950s, through the early jet-powered craft of the 1960s, to contemporary airplanes with high-pressure-ratio engines. During this evolution, airframe aerodynamics and engine performance have been improved, and the weight of aircraft structures and system components has been reduced. These improvements were driven by economic requirements for longer range, higher fuel efficiency, larger capacity, and increased speed, and the net result was an air transportation system with aircraft that are more capable, yet consume less fuel and produce fewer emissions per revenue-passenger-kilometer than ever before. In the past 30 years, approximately 60 percent of the total improvement in fuel efficiency has been attributable to advances in engine technology, with the rest due to improvements in airframe design and more-fuel-efficient operations. Based on past trends, further improvements in engine and airframe efficiency seem likely to reduce fuel consumption per revenue-passenger-kilometer by about 1 percent per year for the next 15 to 20 years. This contrasts with anticipated long-term growth in commercial airline revenue-passenger-kilometers of approximately 3 to 5 percent per year (IPCC, 1999; Lee et al., 2001). In addition, improvements in engine efficiency do not reduce all types of emissions equally. Certification standards for aircraft engines recognize this disparity, allowing high-efficiency engines with high pressure ratios to emit more NOx than engines of the same size that have lower pressure ratios (and lower efficiency). NASA has contributed significantly to technological advances in the past, and studies sponsored by NASA’s Ultra Efficient Engine Technology Program indicate that future advances could make up much, but not all, of the shortfall between future growth in demand and current projections of

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For Greener Skies: Reducing Environmental Impacts of Aviation technological improvements. New airframe technologies have the potential to reduce current fuel consumption by 25 percent, and new engine technologies could provide an additional improvement of 15 percent over the next 15 years. Improvements in aircraft fuel efficiency have been similar to fuel-efficiency advances demonstrated by the automobile: in 2000, the average new car used 41 percent less fuel per mile than the average new car in 1973; fuel efficiency of new aircraft (per passenger-seat-kilometer) improved about 34 percent over the same time period. Additional improvements in aircraft fuel efficiency could be achieved by continued advances in the following areas: improvements in airframe aerodynamics from a combination of high-resolution numerical simulations of airflows around aircraft; wind tunnel testing techniques; laminar flow technology; and integrated design of the wing, fuselage, and propulsion system reductions in the weight of airframe and engine structures (such as the nacelle, which supports the engine) from lighter and stronger materials, and high-fidelity finite-element models for more accurate analyses of safety and strength load-factor margins improvements in the aerodynamics of engine nacelle flows and changes in the shape and length of the engine inlet to reduce local drag effects and increase efficiency thrust reversers with higher efficiency to reduce propulsion-system weight fly-by-wire and electrical actuation systems to reduce or eliminate the need for heavy hydraulic systems, and fly-by-light systems to replace electrical wiring with lighter-weight fiber optics advanced engine technology to increase engine bypass ratio (for lower exhaust jet velocity and higher propulsive efficiency) and to increase engine pressure ratio (for higher thermal efficiency) advanced air traffic control and air traffic management systems and procedures to improve operational efficiency—for example, through more direct routing of flights Even with improvements such as those listed above, most or all new commercial aircraft will not have significantly greater cruise speed, altitude, or range. Large commercial jet aircraft have had cruise speeds of about 500 knots (Mach 0.80 to 0.85) for about 30 years. Typical cruise altitude has also changed little. This trend could change, however, if ongoing Boeing design studies lead to production of a new class of commercial aircraft with cruise speeds of Mach 0.95 or greater. For long-range aircraft, average cruise altitudes have remained fairly constant, at 35,000 to 38,000 feet, over the past 35 years. Although maximum cruise capability has slowly increased and some aircraft can now cruise at altitudes up to approximately 43,000 feet, subsonic aircraft are not expected to see much change in cruise speed or altitude in the foreseeable future. Maximum range is also unlikely to increase significantly, because commercial aircraft can already provide nonstop service between almost any two cities in the world; there is little demand for aircraft with longer ranges. NASA is the only federal agency with research programs focused on the reduction of emissions from commercial aircraft. NASA’s emissions goals, which are focused on CO2 and NOx, are commendable, but research funding to achieve these goals has been greatly reduced. Figure 3-1 shows the magnitude of emissions research funded by the High Speed Research (HSR) Program, the Advanced Subsonic Technology Program, and the Ultra Efficient Engine Technology Program. A major thrust of the HSR Program was to develop low-NOx combustor technology for future supersonic aircraft. Component tests demonstrated a reduction of 80 to 90 percent, achieving an NOx emission index of 5 grams per kilogram of fuel (which was the program goal). However, the HSR Program was canceled before the low-NOx technology could be integrated in a test engine to characterize transitory and steady-state performance and demonstrate programmatic goals such as low noise and long life. NASA also conducted extensive combustor emissions research under the Advanced Subsonic Technology Program before it was terminated and replaced with the Ultra Efficient Engine Technology Program. The goals of the latter program are to reduce NOx by 70 percent (with hardware demonstrations at TRL 5) and to reduce CO2 emissions by 15 percent (with hardware demonstrations at TRL 4). Figure 3-2 shows how funds from all three programs have been allocated. As with any carbon-based fuel, the major combustion products of conventional jet fuel are CO2 and water vapor. Reducing the emission of CO2 and water requires either reduced fuel consumption (through the development of more efficient engines, aircraft, and operational systems and procedures, as discussed above) or the use of alternative fuels. Even though contemporary commercial jet aircraft are designed to operate exclusively with aviation kerosene as a fuel, gas turbine engines can operate with a wide variety of liquid and gaseous fuels. In fact, derivatives of several operational aircraft engines are used in marine and industrial applications using natural gas, diesel fuel, alcohol, and many other fuels. Current and future aircraft engines could also be configured to operate with alternative fuels, such as natural gas or hydrogen. Natural gas would reduce CO2 emissions on the order of 20 percent relative to kerosene. With hydrogen, zero CO2 emissions would result. However, both fuels, especially hydrogen, would increase emissions of water vapor. Because aircraft have limited volumes available to store fuel, natural gas or hydrogen would have to be in liquefied form. Although the energy density of hydrogen by weight is nearly three times that of conventional aviation fuels, the energy density by volume is one-fourth that of conventional

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For Greener Skies: Reducing Environmental Impacts of Aviation FIGURE 3-1 Funding for emissions research (adjusted to constant year 2000 dollars). FIGURE 3-2 Allocations of NASA’s emissions research funding (adjusted to constant year 2000 dollars).

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For Greener Skies: Reducing Environmental Impacts of Aviation aviation fuels. In addition, the potential weight savings of hydrogen fuel is offset by the additional weight of the liquid cryogenic fuel storage and handling systems and associated aircraft structures. The engineering challenges associated with accommodating low-density, cryogenic fuels in aircraft fuel tankage and supply systems are so substantial that their use can probably only be considered in new aircraft specifically designed for such fuels. Other major impediments, especially with respect to hydrogen, include cost, availability, and infrastructure (for production, transportation, storage, and aircraft servicing). Natural gas is readily available, but hydrogen must be produced. One approach for producing hydrogen would be to release and collect hydrogen from hydrocarbon fuels, but that process releases 2 to 4 times more CO2 than simply using hydrocarbon fuels directly as an aircraft fuel. Another alternative for producing hydrogen would be electrolysis of water, but that assumes the availability of large amounts of electricity not produced by power plants powered by fossil fuels. Burning hydrocarbon fuels to produce electricity to produce hydrogen to replace the use of hydrocarbons as a jet fuel would release more CO2 than continuing to use conventional hydrocarbon jet fuel. Given the magnitude of these challenges and the long time it would take to develop and deploy significant numbers of new commercial aircraft equipped to operate with alternative fuels, it seems highly likely that commercial aviation will be dominated by aircraft powered by conventional jet fuels for the foreseeable future. Finding 3-1. Gap Between Technology and Demand. Continuation of ongoing technology research will reduce fuel consumption per revenue-passenger-kilometer by about 1 percent per year over the next 15 to 20 years. During the same time, the demand for air transportation services is expected to increase by 3 to 5 percent per year. An aggressive, broad-based technology program that encompasses propulsion systems, the airframe, and operational systems and procedures could significantly close this gap. Existing allocations of research funding and funding trends within NASA and the FAA do not support such a program. Finding 3-2. Gap Between NASA Goals and Programs. NASA funding to achieve its goals for reducing CO2 and NOx emissions is insufficient to reach the specified milestones on time. Little or no funding is available for research related to other emissions, such as hydrocarbons, particulates, and aerosols, which may also have significant effects on the atmosphere locally, regionally, or globally. ATMOSPHERIC ASPECTS Aircraft emissions can affect the atmosphere on global, regional, and local scales. Aircraft emissions include primary emissions (that are present in the engine exhaust as it leaves the aircraft) and secondary emissions (that are produced in the atmosphere by chemical reactions that use the primary emissions either as a reactant or a catalyst). The primary emissions of greatest concern are CO2, water vapor, NOx, particulates (primarily soot), and SOx. Primary emissions of less concern include CO and unburned hydrocarbons. Secondary emissions include aerosols and some types of particulates. Typical levels of primary emissions are shown in Table 3-1. Global Effects Globally, the major concerns with aircraft emissions are (1) the potential for subsonic aircraft operating in the upper troposphere to contribute to climate change and (2) the effects of aircraft operations in the troposphere and stratosphere in altering the concentration of ozone (see Table 3-2). TABLE 3-1 Typical Aircraft Turbine Engine Exhaust Gas Composition at Cruise Operating Conditions   Concentration   Constituent Emission Index (g/kg fuel) (vol-%) (ppm) Combustion products CO2 3,200 4.1     Water 1,200 3.7 Pollutants NOx as NO2 15   190   CO 1 20 SOx 1 9 HC (as CH4) 0.20 7 Soot (as C) 0.02 1 Note: For Jet A fuel (CnH1.8n) and an overall fuel/air ratio of 0.020.

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For Greener Skies: Reducing Environmental Impacts of Aviation TABLE 3-2 Global, Regional, and Local Effects of Aircraft Emissions Constituent Global Effects Regional Effects Local Effects Carbon Dioxide (CO2) CO2 is a greenhouse gas that, like water vapor, is an unavoidable by-product of the combustion of fossil fuels. When the concentration of CO2 in the atmosphere changes, the time to reach equilibration ranges from about 5 years (with respect to the biosphere and surface layers of the ocean) to hundreds of years (with respect to the full ocean). The relative importance of CO2 produced by aviationis equal to the amount of fossil fuel used by aviation compared with that consumed for other uses. In spite of the enormous gains made in the fuel efficiency of aircraft, total fuel consumption by U.S. commercial aviation continues to increase, but at a rate comparable with overall U.S. growth in the use of petroleum and other fossil fuels. None None Water Vapor, Contrails, and Cirrus Clouds Water vapor is a greenhouse gas, but the vast majority of water vapor in the atmosphere comes from evaporation of water. The emission of water vapor from aircraft is not significant except, perhaps, in the upper troposphere or lower stratosphere, where it can lead to the formation of contrails and cirrus clouds. The effect of aviation-induced cirrus clouds is highly uncertain. They may have very little or no effect, or they may affect the global radiation budget more than CO2. The effect may also vary with latitude and season. More research is needed to adequately understand the effects of contrails and aviation-induced cirrus clouds and to determine if technology goals and programs should be established to mitigate their environmental impacts. Even if the effects of contrails and aviation-induced cirrus clouds are small when averaged globally, they may have significant climatological effects in some regions. None Oxides of Nitrogen (NOx) Nitric oxide (NO) and nitrogen dioxide (NO2) together are referred to as NOx. Both are generated by high temperatures in jet engines, primarily in the combustor. Nitrogen in jet fuel will contribute to the formation of NOx, but most NOx is formed from nitrogen and oxygen in the air. The highest levels of NOx are produced at the highest engine power settings, and the most practical approach for reducing NOx is to reduce flame temperatures at high power conditions. This can be accomplished using complex, staged combustors or combustors with variable geometry that provide good performance at both high and low power settings. Low-NOx combustor technology developed by NASA and industry is now adequate to satisfy current regulatory standards. In fact, some advanced combustors can reduce NOx as much as 60 percent below the ICAO standard, but they currently have a limited market because (1) they cost more and weigh more than simpler combustors (that reduce NOx to about 35 percent below current standards) and (2) they provide no economic benefits to offset the higher cost and weight. NOx emissions in the upper troposphere increase the amount of ozone (by roughly 6 percent in 1992). This increase may have decreased UV radiation at the Earth’s surface by 1 percent at 45 degrees latitude, and growth in aviation could double the double the effect by 2050. However, NOx emissions in the stratosphere (which can occur with some flights during winter or at high latitudes) can reduce ozone, although the net effect of subsonic aircraft is uncertain. Assessing the effect of aircraft NOx on ozone is complex because NOx has a short lifetime (days to weeks), high variability, and many sources other than aviation, including lightning and mixing from lower levels of the atmosphere. Additional laboratory tests, field observations of aircraft emissions, and detailed studies of the chemistry involved in ozone production are needed to develop a better understanding. NOx emissions can also produce regional variations in ozone—ozone resulting from aircraft emissions of NOx is concentrated in the Northern Hemisphere and along major flight routes. NOx is a local concern because it contributes to the formation of photochemical smog and/or ozone in the lower atmosphere. A few major European airports have implemented landing fees that reward operators who use ultralow-NOx combustors while penalizing operators using standard combustors. The cost differential does not appear to be a sufficient financial incentive to most international air carriers, for which operations at these airports represent a very small fraction of their total operations.

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For Greener Skies: Reducing Environmental Impacts of Aviation Particulates and Aerosols Solid particulates in aircraft exhaust generally consist of carbon, sulfates, and metals. The most prevalent particulates are carbon particles (soot), which cause visible smoke if the concentration is high enough. Soot is caused by locally rich fuel-air mixtures within the combustor primary zone and, to a lesser extent, by high combustor operating pressures. The turbine breaks up some large soot particles, while some small particles agglomerate in the exhaust plume to form larger particles The highest level of soot typically occurs during takeoff and climbout when fuel f lows and pressures are at their highest. As the atmosphere cools aircraft exhaust, particulates, water vapor, and other constituents in the exhaust of hydrocarbon-fueled engines can form liquid aerosols, which consist of a colloidal suspension of liquid particles in a gas. (Fog is an example of a liquid aerosol.) The atmospheric effects of soot and other particulates are uncertain—the effects of soot have been determined to only within a factor of 2—and may be important. Particulates provide nuclei for the formation of liquid droplets in the atmosphere and may be involved in the formation of contrails. Soot absorbs and, to a lesser extent, scatters incoming solar radiation. If soot particles are absorbed by aerosol droplets, soot absorbs more solar radiation, which could contribute to climate change. Soot also has the potential to alter the abundance of atmospheric trace constituents by facilitating chemical reactions that would not otherwise be possible. Particulates and aerosols can contribute to changes in atmospheric visibility and ozone on a regional scale, but this has not been observed as a significant problem with regard to aircraft emissions. Visible smoke emissions are highly objectionable, especially in and around airports, but low-smoke combustors were incorporated into operational engines beginning more than 30 years ago, and visible smoke is no longer an issue. Particulates, however, may contribute to the formation of photochemical smog and may be a health hazard. While solid carbon is relatively inert, it tends to absorb unburned hydrocarbons, which are potentially carcinogenic and might be absorbed in the lungs. Some particulates might also be able to block some air passages in the lungs. In addition, aerosol droplets can contain combustion by-products in a much more concentrated form than the exhaust gases from which they are formed, which may pose a health risk if they are ingested into the lungs and absorbed into the body. Available research is inadequate to assess the validity of these health concerns. Recent speculation about community health problems in the vicinity of airports suggests that further research may be warranted. However, health problems associated with particulates and aerosols (to the extent that there is a problem) would not be unique to emissions from aircraft, and corrective action, if required, should address other regional and local sources of particulates and aerosols, such as automobiles. Oxides of Sulfur (SOx) SOx is generated in the combustor from sulfur in the fuel. Aircraft generally produce very little SOx because the sulfur content of aviation fuels is low—generally well below the regulatory limit. However, SOx emissions even in minute quantities may increase the level of particulates and aerosols. The processes by which these particul ates and aerosols are formed is not well understood, and their ultimate significance in terms of atmospheric effects is uncertain.

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For Greener Skies: Reducing Environmental Impacts of Aviation Constituent Global Effects Regional Effects Local Effects Carbon Monoxide (CO) CO emissions are the result of incomplete combustion and are produced primarily at taxi and engine idle conditions on the ground. At higher engine power settings the combustor operates with essentially 100 percent combustion efficiency, effectively eliminating the production of CO. Between 1975 and 1985, combustor designs were modified to improve combustion efficiencies at low power conditions. Today, almost all modern engines have CO emissions that are well below the regulatory limits. CO results in higher concentrations of ozone and methane and thus acts as an indirect greenhouse gas, but the CO in aircraft emissions is a very small fraction of all anthropogenic CO. None Early investigations showed low levels of CO near airport terminals, with ground support vehicles being the major contributor. Subsequent improvements in the combustion efficiency of aircraft and ground support vehicles have essentially eliminated CO as a concern. Unburned Hydrocarbons (HCs) Emissions of unburned HC are composed of many components. Although present in only trace quantities, some components are classified as hazardous air pollutants (see below). Like CO, HC emissions are caused by incomplete combustion and have been reduced by the same improvements in engine design that have reduced CO emissions. Airborne HC may also result from vaporization of liquid fuel from spills, venting of fuel tanks, or engine startup. The global effects of HC emissions are similar to those of CO, and their effects are estimated to be negligible. Unburned HCs are a precursor to photochemical smog and/or ozone. However, unburned HCs are also created by other users of hydrocarbon fuels. The extent to which unburned HCs from aircraft pose a particular problem regionally or locally is uncertain. Hazardous Air Pollutants The EPA defines hazardous air pollutants (also referred to as toxic air pollutants) as pollutants that are known or suspected t o cause cancer or other serious health effects, such as birth defects, or adverse environmental effects. One study that analyzed the exhaust from two jet engines (one an older design, one a newer design) detec ted benzene, formal dehyde, and several other chemicals that appear on the EPA's list of hazardous air pollutants (Spicer et al., 1984). The measurements were made at idle power in order to obtain measu rable amounts—at higher power levels the chemicals were not produced in easily measurable chemical reactions that may take place. The emission levels detected were typical of the emissions produced by other types of engines that use hydrocarbon fuels. concentrations. Also, the measurements were taken in the exhaust stream and therefore did not include any dilution or any subsequent The pollu tants in the exhaust of the newer engine were approximately half the levels of the older engine, with the exception of formaldehyde (which was about the same). Better data is needed to assess what effect s, if any, may result from emissions of hazardous air pollutants by the current fleet of commercial aircraft.

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For Greener Skies: Reducing Environmental Impacts of Aviation FIGURE 3-3 Radiative forcing caused by the global fleet of commercial subsonic aircraft as of 1992. The vertical line embedded in each bar depicts a two-thirds uncertainty range, meaning that there is one chance in three that the true value falls outside the ranges shown. Available information on cirrus clouds was judged to be insufficient to determine either a best estimate or an uncertainty range; the dashed line indicates a range of possible best estimates. The adjectives below each bar are relative appraisals of the level of scientific understanding associated with each component. SOURCE: IPCC, 1999. Ozone directly affects the level of ultraviolet (UV) radiation reaching the Earth’s surface and, through photochemical reactions, alters the abundance of reactive gases in the atmosphere, such as methane. The net effect of aircraft emissions on climate change is shown in Figure 3-3. (An increase in radiative forcing tends to cause higher temperatures.) Aviation accounts for perhaps 3.5 percent of anthropogenic changes to radiative forcing and is expected to account for about 5 percent in 2050. Beneficial effects are also possible, however, in the form of reduced exposure to UV radiation caused by higher levels of ozone in the troposphere (which result from the emission of NOx). In any case, estimates of the future impact of aviation are imprecise because of uncertainties about (1) the total amount of emissions that commercial aviation will produce in the future and (2) the accuracy of current methods for quantifying the impact of aviation emissions. The latter uncertainty is reflected in the range of uncertainty shown for each of the emissions in Figure 3-3. Reducing these uncertainties is important to ensure that technology programs are properly directed. In particular, NASA’s environmental goals are focused on reducing CO2 and NOx, but, as shown by Figure 3-3, contrails and cirrus clouds could affect climate change as much or more than CO2 and NOx emissions. Regional Effects On a regional scale the emissions of potential interest are water vapor, NOx, particulates, and aerosols. However, little effort has been made to assess regional effects, and available data are insufficient to estimate either the current or future effects of aircraft. Whatever effects do or will exist, however, are an extension of either global or local effects, and efforts made to reduce those effects should also mitigate regional effects. Nonetheless, additional research is needed to determine if those efforts will ensure that regions with large amounts of air traffic (such as the northeastern United States or Western Europe) will not experience unacceptable changes to the environment either at the surface or at higher altitudes. Local Effects At the local level, aircraft emissions have been a concern for about 30 years—since the use of gas turbine engines in commercial service first became widespread. Federal regulations to limit the effects of aircraft on local air quality were first issued in 1973 by the FAA and EPA. These regulations required the use of low-smoke combustors, prohibited the

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For Greener Skies: Reducing Environmental Impacts of Aviation intentional venting of fuel from the engine manifold after normal shutdown, and described prospective standards for virtually all pollutants; with some modifications, those standards would ultimately appear in regulations issued in 1983. Also in the early 1980s, ICAO developed similar standards and recommended practices to protect local air quality in the vicinity of airports. Since 1997, airport construction projects that require FAA approval or support have had to show that all emissions resulting from the project, both directly and indirectly, would be consistent with state implementation plans for meeting federal air quality standards. As a result, localities and regions with chronic air quality problems would especially benefit from the availability and use of technology that increases fuel efficiency and reduces aircraft emissions. Current needs also include better understanding of the health concerns, if any, posed by aircraft emissions of hazardous air pollutants (Ozone Transport Committee, 2001 and Holzman, 1997). Very few data exist for characterizing aircraft exhaust with regard to hazardous air pollutants, many of which are mutagenic and carcinogenic, or for comparing the possible effects of aircraft exhaust with those of other potential sources of hazardous air pollutants, such as automobiles. Although hazardous air pollutants are present in aircraft emissions only in small concentrations, environmental challenges that cite these emissions may be hard to deflect without better data. Atmospheric Research Two decades of research have demonstrated the importance of laboratory studies, field observations, and numerical modeling for understanding the effects of aircraft emissions on global climate issues. The federal government continues to support several small research programs, such as NASA’s Atmospheric Effects of Aviation Project, but funding for this effort has been reduced from about $12 million to about $4 million per year. NASA has a stratospheric chemistry program, which studies some aspects of tropospheric chemistry that are important for understanding the stratosphere. NASA also has a tropospheric chemistry program, which is funded at about $4 million per year, but the focus is on the mid- and free troposphere, which encompass altitudes below the region of primary interest to commercial aviation. The Department of Energy’s Atmospheric Radiation Measurement Program studies the effects of aerosols on climate and has provided some information relevant to aviation, but it is not focused on aerosols of particular interest to aviation. The Atmospheric Chemistry Program of the National Science Foundation funds some basic studies of atmospheric and chemical processes that will help assess the effects of aviation. Recommendation 3-1. Research on Global, Regional, and Local Emissions. NASA should continue to take the lead in supporting federal research to investigate the relationships among aircraft emissions (CO2, water vapor, NOx, SOx, aerosols, particulates, unburned hydrocarbons, and other hazardous air pollutants) in the stratosphere, troposphere, and near the ground, and the resulting changes in cirrus clouds, ozone, climate, and air quality (globally, regionally, and locally, as appropriate). Other agencies interested in aircraft or the environment should also support basic research related to these programmatic goals. Recommendation 3-2. Eliminating Uncertainties. NASA should support additional research on the environmental effects of aviation to ensure that technology goals are appropriate and to validate that regulatory standards will effectively limit potential environmental and public health effects of aircraft emissions, while eliminating uncertainties that could lead to unnecessarily strict regulations. REFERENCES IPCC (Intergovernmental Panel on Climate Change). 1999. Aviation and the Global Atmosphere. Cambridge, United Kingdom: Cambridge University Press. Pp. 210. Holzman, David. 1997. Airports and the Environment. Environmental Health Perspectives 105(12):1300-1305. Available online at <http://ehpnet1.niehs.nih.gov/docs/1997/105-12/focus.html>. March 11, 2001. Lee, J., S. Lukachko, I. Waitz, and A. Schafer. 2001. Historical and future trends in aircraft performance, cost, and emissions. Annual Review of Energy and the Environment 26:167–200. Ozone Transport Committee. 2001. Press release. OTC advocated air pollution reductions from airport and clean energy initiatives. Washington, D.C.: State Services Organization. Available online at <http://www.sso.org/otc/Press%20Releases/PressRelease010730_airport&energy_final.PDF>. March 11, 2002. Spicer, C., M. Holdren, T. Lyon, and R. Riggin. 1984. Composition and photochemical reactivity of turbine engine exhaust. Report ESL-TR-84-28. September. Tyndall Air Force Base, Florida.