In part 2, Eugene D Coyle examines space flight launch emissions; aviation gasoline (avgas); and the commitment to net zero.
Contrails
The International Panel for Climate Change (IPCC) adopted a metric of Radiative forcing (RF) to compare climate perturbations among different aviation scenarios and with total anthropogenic climate change.
RF is the global, annual mean radiative imbalance to the Earth's climate system caused by human activities. It predicts changes to the global mean surface temperature: positive RF leads to global warming.
The impact of aviation on climate follows several pathways. In addition to carbon dioxide and water vapour, both effective greenhouse gases, nitric oxides, which influence the chemical composition of the upper troposphere, soot and sulphuric oxides, add to the ambient aerosol and have an impact on cirrus formation and cloud microphysical properties.
Contrails (or condensation trails, vapour trails) are thin white lines produced by aircraft engine exhaust or by changes in air pressure, normally at cruising altitudes of 8km to 12km.
They result from the condensation of water vapour on soot particles (polynuclear aromatic hydrocarbons) to form ice crystals under certain conditions of temperature and humidity, that leads to saturation of air with water22. Contrails, and other clouds directly resulting from human activity, are collectively called homogenitus23.
IPCC informs that trails and the cirrus clouds they form, may have a climate impact comparable to the CO2 emissions from the combustion process.
The combination of water vapour in the engine aircraft exhaust and low temperatures that exist at high altitudes contributes to contrail formation.
Contrails can also be formed by changes in air pressure in wing-tip vertices or in air passing over the entire wing surface. The formation of persistent contrails can last from a few minutes to several hours and can form cirrus cloud coverage.
These additional clouds reduce the incoming solar radiation as well as the outgoing thermal radiation in a way that the mean net balance at top of the atmosphere is slightly positive, ie they add to the greenhouse effect24.
Current investigations of the extent and contribution of homogenitus contrails is largely data analytic based, coupled with weather forecast simulations and AI modelling.
One field of study proposes limiting the production of contrails by adapting aircraft flight level and pathways. Published research proposes “small-change diversion and technology adoption” to mitigate the climate forcing of aircraft contrails, offering some interesting insights into this growing field of study25.
Space flight launch emissions
Space flight is a complex branch of aviation, necessitating a detailed study beyond the scope of this paper. It is nevertheless important that a brief commentary be included of sectoral concern relating to airborne particulate pollution resulting from space missions by rocket launch to outer space.
Rockets operate with either solid or liquid propellants (comprising both fuel and oxidiser). The fuel is the chemical rocket’s burn, with the oxidiser providing the required oxygen for burning to take place. Solid rocket propellants contain both the fuel and the oxidiser combined in the chemical itself. The fuel is a mixture of hydrogen compounds and carbon and the oxidiser is made up of oxygen compounds.
Liquid rocket propellants are kept in separate containers, one for fuel and the other for the oxidiser. When the engine fires, the fuel and the oxidiser are mixed in the engine. The fuel of a liquid-propellant rocket is usually kerosene or hydrogen.
The propellants burn in the combustion chamber and build up high temperatures and pressures, and the expanding gas escapes through a nozzle at the lower end.
With rockets, weight is an important factor: the heavier the rocket the more the thrust required to get it off the ground26. At launch, the rocket engines provide the initial thrust to overcome the force of gravity to propel the spacecraft from the surface of the Earth.
Once in space, the motion of the craft is determined by gravitational and propulsion control. Most space launch vehicles have three stages: some may host four or five stages, depending on vehicle type. At the end of each stage, spent booster of rocket stages separate from the central launch vehicle27.
RP-1 (Rocket Propellant-1 or Refined Petroleum-1) is the most widely deployed propellant in powering orbital rockets. It is a highly refined form of kerosene, with high density levels that make it energetic and fuel efficient.
Recent literature on the study of the effect of rocket launches on Earth’s climate and ozone-layer impacts suggests that emissions from space flight play an increasing role in stratospheric aerosol pollution. This results from RP-1 emission of ‘black carbon particles’ directly into the stratosphere, where they accumulate, absorb solar radiation, and warm the surrounding air.
While the pollutant quantities emitted by rocket launches relative to aircraft is small, it is important to note that aircraft release their pollutants within the troposphere and lower stratosphere, while rockets release their pollutants, starting at ground level and proceeding through the atmospheric layers to the mesosphere.
Lower atmosphere-emitted particulates
When particulate pollution is released into the upper layers it lasts for a longer time than lower atmosphere-emitted particulates. Studies, focusing principally on Rocket-Propellant-1, suggest that carbon emissions will increase with the expected rapid growth in space flight.
Such particles emitted by rockets are extremely efficient at retaining heat in the atmosphere. This could partially impact the recovery in the ozone layer experienced following the successful implementation of the 1987 Montreal Protocol28.
EPA informs that black carbon is the sooty black material emitted from gas and diesel engines, coal-fired power plants, and other sources that burn fossil fuel. It comprises a significant portion of particulate matter (PM), which is an air pollutant.
The complex role of airborne black carbon is now under intense study by the EPA. Scientists are conducting integrated and multidisciplinary research to improve understanding and determine more clearly the role of black carbon in air pollution and climate change29.
Aviation Gasoline (Avgas) Aviation gasoline (Avgas) is the fuel most used in piston-engine spark-ignited light aircraft within the general aviation community. Avgas is distinguished from conventional petrol (gasoline) used in motor vehicles, which is sometimes termed mogas (motor gasoline) in an aviation context.
Unlike motor petrol, which has been formulated since the 1970s to allow the use of platinum-content catalytic converters for pollution reduction, the most commonly used grades of avgas still contain tetra-ethyl-lead (TEL), a toxic substance used to prevent engine knocking (premature detonation).
Avgas, named 100 octane Low Lead, also known as 100LL, is the standard (leaded) fuel. It contains TEL as an additive to prevent engine damage at higher power settings30. There is continued concern of the use of avgas, as the lead contained poses a health risk. The principal concern is where take-off/landing strips are located close to congested urban environments.
There are ongoing experiments aimed at eventually reducing or eliminating the use of TEL in aviation gasoline. Some light aircraft are permitted a to use unleaded petrol (ethanol), conforming to standard EN 22831, but most are not.
Newly emerging sustainable aviation fuels
The avgas topic, however, is separate to the principal topic of discussion in this paper, that of contemporary aviation turbine fuels and newly emerging sustainable aviation fuels.
Several learned articles address the matters associating to the use of avgas, including an entry by Stephen Bridgewater in RAeS AEROSPACE, April 2023, and a National Academies of Science Engineering and Medicine Consensus Study Report32,33.
Aside from the topic of avgas, piston engines also create CO2 emissions, and they are likely to be replaced by e-powered aircraft in the coming decades.
Commitment to net zero airlines committed to net zero carbon emissions of CO2 by 2050 at the 77th International Air Transport Association (IATA) annual general meeting in 2021, and member states of the ICAO agreed to a long-term aspirational goal (LTAG) net zero CO2 emissions in 202234.
This commitment links to the United Nations Sustainable Development Goals35. It is estimated that the anticipated traffic of the aviation industry in 2050 would likely generate 1.8 billion tonnes of carbon emissions if fuelled by traditional jet kerosene alone.
To achieve net zero emissions, 65% of the total emissions reductions will need to be achieved using sustainable aviation fuel. This would represent 350 million tonnes (450 billion litres) of SAF annually by 2050, from every available sustainable feedstock36.
The scaled deployment of renewable energy is accelerating as the world looks to decarbonise its energy mix. SAF, as a ‘drop-in’ fuel, combines with conventional kerosene, in measures, currently, of up to 50%.
Predictions indicate that percentages of close to 90% may be achievable by 2030. PtL, power to liquid,’ has emerged as a promising e-SAF conversion technology pathway. Although in its maiden stage of research and plant development, innovations are emerging, offering hope in the drive to Net Zero 50.
Kerosene infrastructure
It is expected that e-SAF will be transportable via the existing networks of the kerosene infrastructure, including pipelines and filling stations.
Production costs are understandably very high during the early stages of plant development; however, costs will reduce accordingly should emerging prototype-plants achieve target goals in fuel production37,38.
On July 14, 2021, the European Commission presented a package of proposals to make the EU's climate, energy, land use, transport and taxation policies fit for reducing net greenhouse gas emissions by at least 55% by 2030, compared with 1990 levels: the 'fit for 55' package.
The package includes a proposal for a regulation to ensure a level playing field for sustainable air transport, also known as the ReFuelEU Aviation initiative.
The proposed regulation obliges fuel suppliers to distribute sustainable aviation fuels (SAF), with an increasing share of SAF (including synthetic aviation fuels, or e-fuels) over time, to increase the uptake of SAF by airlines and thereby reduce emissions from aviation.
It also obliges airlines to limit the uptake of jet fuel before departing from EU airports to what is needed for safe operation of flights, with the aim of ensuring a level playing field for airlines and airports and avoiding additional emissions relating to the extra weight of aircraft carrying excessive amounts of fuel.
Several companies have announced plans to enter the SAF market by 2030. Coupled with developments in SAF, innovations in engine and aircraft body design continue as high priority goals of leading global firms in the aviation and aerospace industries.
This includes improvements in engine efficiency and sustainability in manufacturing processes, with greater determination to reducing emissions and handling of waste materials, of which companies inclusive of General Electric, Rolls-Royce, Pratt and Whitney, are to the fore41,42,43. Research in the use of hydrogen as a future primary energy source, is also gaining momentum.
An Airbus-led ZEROe fuel-cell research consortium has a two-stream focus: hydrogen combustion for turbines with modified fuel injector systems, and hydrogen fuel cells for power of electric-drive motors.
The company plans to bring to market the world’s first commercial hydrogen-powered aircraft by 2035. The ZEROe craft will service the smaller regional 100-passenger market.
Partnerships may include Rolls-Royce in such innovative advanced projects44. The achievements by luminaries of the aviation industry, from those of the early pioneers in their endeavours to master flight, through to the innovative developments by today’s principal players in the industry, is truly inspiring.
It is extraordinary to ponder the short window of 100 years wherein so much has come to be and the role that air travel has played in helping shape the modern world.
The aviation industry is, nevertheless, facing arguably its greatest challenge to date, in its quest to achieve net zero aircraft emissions by the year 2050.
The introduction of SAF as a drop-in-fuel thus far represents a very small percentage (less than 1%) of global flights. Targets of 2% by 2025, reaching 60% by 2050, have been sanctioned.
The United Nations Environment Programme (UNEP) informs that the “Ozone layer is on track to recovery within four decades, with the single phaseout of ozone-depleting chemicals already benefitting efforts to mitigate climate change”45. This good news was disclosed by a UN-backed panel of experts, at the American Meteorological Society’s 103rd annual meeting, held in Nairobi in 2023.
Can we dare look forward with hope that similar good news will be forthcoming in 20 years’ time, with the reduction of carbon dioxide emissions to Earth’s atmosphere?
The ultimate drive to achieving net zero carbon emissions will require relentless innovation and determination. Responsibility rests with all sectoral players and all nations in the flight to Net Zero 50. Owing to the accelerated pace of climate change, there is no time to lose.
Author: Eugene D Coyle, Technological University Dublin, with the support of Mohamed Al Siyabi, Abdullah Al Shibli, Abid Ali Khan, Military Technological College Oman.
References
21) IPCC Intergovernmental Panel on Climate Change. Special Reports. Radiative Forcing. Aviation and Global Atmosphere. Potential Climate Change from Aviation.
22) EPA Environmental Protection Agency. Polynuclear Aromatic Hydrocarbons (PAHs) Factsheet. https://www.epa.gov/sites/default/files/2014-03/documents/pahs_factsheet_cdc_2013.pdf
23) WMO World Meteorological Organisation. Aircraft Condensation Trails. Manual on the Observation of Clouds and Other Meteors. [WMO-No. 407]
24) IPCC Intergovernmental Panel on Climate Change. Contrails. https://www.ipcc.ch/site/assets/uploads/2018/03/av-en-1.pdf
25) MIT: Man-Vehicle Laboratory. Department of Aeronautics and Astronomics. Rocket Principles. https://web.mit.edu/16.00/www/aec/rocket.html
26) NASA National Aeronautics and Space Administration. Rocket Principles. Editor: Tom Benson. https://www.grc.nasa.gov/www/k-12/rocket/TRCRocket/rocket_principles.html
27) EPA Science in Action. Black Carbon Research and Future Strategies: Reducing Emissions, Improving Human Health, and Taking Action on Climate Change. https://www.epa.gov/sites/default/files/2013-12/documents/black-carbon-fact-sheet_0.pdf
28) United Nations Environmental Programme (UNEP) Oxone Secretariat. 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. 1522 UNITS 3, 26 ILM 1541, 1550 (1987). Date of Adoption: 16/09/1987. Montreal, Canada, Conference of Plenipotentiaries on the Protocol on Chlorofluorocarbons to the Vienna Convention for the Protection of the Ozone Layer.
29) IATA Fact Sheet: US and EU Policy Approaches to Advance SAF Production. Airlines committed to net-zero carbon emissions of CO2 by 2050 at the 77th International Air Transport Association (IATA) Annual General meeting in 2021. https://www.iata.org/contentassets/d13875e9ed784f75bac90f000760e998/fact-sheet---us-and-eu-saf-policies.pdf
30) FAA US Department of Energy: Aviation Gasoline. https://www.faa.gov/about/initiatives/avgas
31) Irish Aviation Authority (IAA). Aviation Safety. EN 228. IGA 9 R3. Using Unleaded Petrol (Mogas) in Aircraft. using-unleaded petrol (mogas)-in-aircraft.pdf.
32) Bridgewater S. Unleaded Aviation Fuel. Pumping lead – GA’s dirty little secret. Royal Aeronautical Society. Aero Space. General Aviation.
33) National Academies of Sciences, Engineering, and Medicine. 2021. Options for Reducing Lead Emissions from Piston-Engine Aircraft. Washington, DC: The National Academies Press. https://doi.org/10.17226/26050.
34) ICAO document. NTL CORSIA 2020 Emissions. https://www.icao.int/environmental-protection/CORSIA/Documents/CORSIA%202020%20Emissions_Oct2022.pdf
35) United Nations Sustainable Goals. Goal 13: Climate Action. https://www.undp.org/sustainable-development-goals
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37) Airbus P&W SAF+ Airbus, Pratt & Witney, SAF+: Consortium Collaboration on Developing Next Generation Sustainable Fuels. https://airwaysmag.com/consortium-to-develop-e-saf-canada.
38) US Department of Energy. Energy.Gov. Office of Energy Efficiency and Renewable Energy. Bioenergy Technologies Office. SAF Sustainable Aviation Fuels.
39) PACE. Pace Academy. A Guide to Refuel Initiative for the Aviation Sector. https://www.pace.esg.com/academy/
40) Rolls Royce Plc. 2024. Ultra Fan – the Ultimate Turbo Fan.https://www.rolls-royce.com/innovation/ultrafan.aspx
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42) Airport Corporation Research Programme. Preparing Your Airport for Electric Aircraft and Hydrogen Technologies. National Academies Science Engineering and Medicine. https://nap.nationalacademies.org/download/26512
43) Rolls Royce Plc. ACCEL Accelerating the Electrician of Flight. https://www.rolls-royce.com/innovation/accel.aspx
44) Airbus. Developing a global eco-system to support hydrogen-powered flight. https://www.airbus.com/en/newsroom/stories/2024-09-developing-a-global-ecosystem-to-support-hydrogen-powered-flight
45) UN Environmental Programme. Ozone layer recovery is on track, helping avoid global warming by 0.5C. 103rd Annual Meeting of the American Meeting 2023. https://www.unep.org/news-and-stories/press-release/ozone-layer-recovery-track-helping-avoid-global-warming-05degc