Airplane Emissions

Airplane emissions are a critical contributor to the greenhouse effect. Airplanes have revolutionized transportation and aviation has experienced rapid expansion as the world economy has grown. However, greenhouse gas emissions from aircraft, including CO2, NOx, and H2O, contribute significantly to global climate change. The airline industry contributes an estimated three percent of global greenhouse gas emissions – nearly as much as the entire African continent. ¹

Aircraft engines produce emissions that are similar to other emissions resulting from fossil fuel combustion. However, aircraft emissions are unusual in that a significant proportion is emitted directly into the upper troposphere and lower stratosphere where they have an impact on atmospheric composition. ²This makes airplane emissions particularly potent compared to other emissions.

Climate impact

The principal emissions of aircraft include the greenhouse gases carbon dioxide and water vapor (H2O). Other major emissions are nitric oxide (NO) and nitrogen dioxide (NO2), sulfur oxides (SOxO), and soot. According to the IPCC, because carbon dioxide has a long atmospheric residence time (ª100 years) and, as a result, becomes well mixed throughout the atmosphere, the carbon dioxide emitted from aircraft is indistinguishable from the same quantity of carbon dioxide emitted by any other source. ³ Thus, it is more difficult to quantify the climate impact of the carbon dioxide formed as a result of aviation than the emissions.

Ozone is another type of greenhouse gas and is a common air pollutant. Aircraft-emitted NOx participates in ozone chemistry. “Subsonic aircraft fly in the upper troposphere and lower stratosphere (at altitudes of about 9 to 13 km), whereas supersonic aircraft cruise several kilometers higher (at about 17 to 20 km) in the stratosphere. Ozone in the upper troposphere and lower stratosphere is expected to increase in response to NOx increases and methane is expected to decrease. At higher altitudes, increases in NOx lead to decreases in the stratospheric ozone layer. Ozone precursor (NOx) residence times in these regions increase with altitude, and hence perturbations to ozone by aircraft depend on the altitude of NOx injection and vary from regional in scale in the troposphere to global in scale in the stratosphere. ” ⁴

Aircraft jet engines also directly emit aerosol particles and condensable gases such as water vapor (H2O), sulfuric acid (H2SO4), and organic compounds. Other gas-phase species and charged molecular clusters (chemi-ions, or CIs) are also generated at emission. Under certain thermodynamic conditions, emitted water vapor condenses and freezes to form water-ice particles, thereby producing a condensation trail (contrail). An invisible aerosol trail is always left behind cruising aircraft. These line clouds evaporate rapidly if the ambient humidity is low but may change the size and chemical composition of the remaining liquid aerosol particles. If the humidity is above ice saturation, contrails persist and grow through further deposition of ambient water. ⁵

Solutions and proposals

There have been a number of efforts taken by different countries and on an international level to address the climate change impacts of airplane emissions.

Global efforts

The International Civil Aviation Organization (ICAO) is the United Nations (UN) specialized agency that has global responsibility for the establishment of standards, recommended practices, and guidance on various aspects of international civil aviation, including the environment. The Kyoto Protocol committed countries to work through ICAO in limiting or reducing emissions of greenhouse gases from aviation bunker fuels, however, international aviation emissions have not been specifically covered by emissions reduction targets in the Protocol. ⁶

In 1981, ICAO established aircraft engine emission standards for oxides of nitrogen (NOx). The NOx standard has been made more stringent by 20% in 1993 and a further 16% change to newly certified engines after 2003 was recommended in 1998. ICAO has established an approach that requires that any actions to mitigate the environmental effects of aviation show environmental need, technical feasibility, and economic reasonableness. ” ⁷

U.S. aviation greenhouse gas emissions make up two percent of the total inventory. (U.S. EPA, 2005)

United States

In the U.S., the rule of Control of Air Pollution from Aircraft and Aircraft Engines; Emission Standards and Test Procedures were published in November 17, 2005. Under this rule, the Environmental Protection Agency (EPA) is establishing new emission standards for oxides of nitrogen (NOX) for newly certified commercial aircraft gas turbine engines with rated thrust greater than 26.7 kilonewtons (kN). This action adopts standards equivalent to the NOX standards of the United Nations International Civil Aviation Organization (ICAO), and thereby brings the United States emission standards into alignment with the internationally adopted standards (ICAO standards for newly certified engines were effective beginning in 2004). ⁸

Operational solutions

According to US EPA, jet fuel prices increased by about 140 percent from 2000 to 2007. The rising jet fuel prices tend to drive the aviation industry to decrease fuel usage and lower fuel usage can reduce GHG emissions. In the future, technology developments for lighter and more aerodynamic aircraft and more efficient engines which reduce aircraft fuel consumption and thus GHG emissions are expected to improve. ⁹

Air traffic management and operational changes are governed by FAA. The FAA, in collaboration with other government agencies, is developing the next generation air transportation system (NextGen). The goal of this project is to decrease aviation’s contribution to GHG emissions by reducing aviation system-induced congestion and delay and accelerating air traffic management improvements and efficiency. Those efforts, combined with technology changes may be an option to reduce GHG emissions in the near future. ¹⁰

Jet fuel

Jet fuel is a refined petroleum product used in jet aircraft engines. The major two types of jet fuel used in the U.S.. are “naphtha-based” jet fuels and “kerosene-based” jet fuels. ¹¹Burned in a similar way as other ground fuels, jet fuels emit greenhouse gases. Efforts are underway to develop alternative fuels for aircraft to reduce GHG emissions. Industry and FAA established the Commercial Aviation Alternative Fuels Initiative (CAAFI) to explore the potential use of alternative fuels. Jet biofuels are proposed as an option. ¹²Boeing estimates that biofuels could reduce flight-related greenhouse-gas emissions by 60 to 80 percent. The solution would be blending algae fuels with existing jet fuel. ¹³

Boeing and Air New Zealand are collaborating with leading Brazilian biofuels maker Tecbio ¹⁴and Aquaflow Bionomic of New Zealand and other jet biofuel developers around the world.
Virgin Atlantic successfully tested a biofuel blend made from 20% babassu nuts and coconut and 80% conventional jet fuel fed to a single engine on a 747 flight from London to Amsterdam. ¹⁵

Footnotes

¹:Carbon neutral airline NatureAir joins UNEP’s Climate Initiative , Terracurve.

²Richard Miake-Lye, Aerodyne Research David Fahey,NOAA Ian Waitz,MIT Chowen Wey,NASA-Glenn Howard Wesoky,FAA, AVIATION AND THE CHANGING CLIMATE,aiaa.org

³⁴:How Do Aircraft Affect Climate and Ozone? , IPCC Special Report: Aviation and the Global Atmosphere.

⁵: Chapter 3: Aviation-Produced Aerosols and Cloudiness, IPCC Special Report: Aviation and the Global Atmosphere.

⁶⁷: Chapter 10 : Regulatory and Market-Based Mitigation Measures,IPCC Special Report: Aviation and the Global Atmosphere.

⁸: Control of Air Pollution From Aircraft and Aircraft Engines; Emission Standards and Test Procedures, Federal register documents.

⁹¹⁰¹²: Environmental Protection Agency, Regulating Greenhouse Gas Emissions under the Clean Air Act.

¹¹: Documentation for Emissions of Greenhouse Gases in the United States 2006, EPA Office of Inegrated Analysis and Forecasting, US Department of Energy, Octorber, 2008.

¹³:A Promising Oil Alternative: Algae Energy - washingtonpost.com

¹⁴:Tecbio, NASA, and Boeing to Develop Biokerosene , Biodiesel and Ethanol Investing .

¹⁵:Crop this: Virgin takes off with nut-fuel - 26 Feb 2008 - NZ Herald: New Zealand Business, Markets, Currency and Personal Finance News

Resources

Federal Aviation Admistration, Regulations & Polices Topics
Technical Support for Development of Airport Ground Support Equipment Emission Reductions, Prepared by Sierra Research, Inc.for the Office of Mobile Sources, U.S. Environmental Protection Agency.
The Aviation Global Deal Group, a coalition of leading international airlines has launched a website, www.agdgroup.org, which includes information on the AGD initiative to address CO2 emissions from international aviation. The website contains background information, links to the Group’s policy proposal, press information and other documentation, including a presentation from AGD Group's recent UNFCCC Side Event on airline emissions mitigation in Bonn (2009). The AGD Group members are: Air France-KLM, British Airways, Cathay Pacific, Finnair, Qatar Airways, Virgin Atlantic, Virgin Blue, BAA, The Climate Group.

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