Aircraft emit a number of emissions into the atmosphere, such as CO2 and NOx, that lead to changes in the atmospheric composition. Analysis of emissions inventories show that aviation emissions are concentrated at altitudes between 10 and 12 km (Lee and Raper, 2003). Aircraft can give rise to condensation trails or contrails.
A contrail is a visible line cloud, consisting of tiny ice particles that form behind an aircraft if the ambient air is cold enough. The Schmidt-Appleman criteria describes the thermodynamics of contrail formation (Appleman, 1953), Schumann (1996). Contrail formation is dependent on physical parameters such as the atmospheric temperature and pressure, fuel energy content, aircraft-induced emission of water vapour and the aircraft propulsion efficiency. Contrails form below a threshold temperature, due to the increase in relative humidity occurring in an engine plume when the engine plume mixes with the surrounding atmosphere (Schumann, 2005), (Schumann, 1996).
In dry air, contrails dissolve immediately, thus having little to no impact on radiative forcing (RF). In moist air, contrails grow due to the intake of ambient water vapour. They may also persist in ice-supersaturated regions of the upper troposphere. Persistent linear contrails can spread in the atmosphere due to wind shear, mixing and uplift to form contrail-cirrus (Lee et al, 2010). There is also evidence that particles emitted in the engine exhaust plume influence cirrus formation. This is often referred to as soot-cirrus (Lee et al, 2010). Contrail-cirrus and soot-cirrus are collectively referred to as aviation-induced cirrus. Over congested airspace of areas such as central Europe and parts of the US, the cirrus coverage due to aircraft can reach up to 6-10% coverage (Burkhardt and Kärcher, 2011)
Contrails act in a similar manner to cirrus cloud in reducing the amount of short-wave (SW) solar radiation reaching the Earth’s surface and reducing the long-wave (LW) radiation emitted into space. The net positive contrail RF is a result of the opposing LW and SW radiation scattering. Uncertainty can arise in RF calculations as errors in the treatment of either LW or SW effects can result in overall uncertainty in the net RF (Myhre and Stordal, 2001), (Rädel and Shine, 2008). The RF of contrails and contrail-cirrus depends on:
The IPCC Aviation Report stated that contrails may have a climate impact comparable to that of CO2 (Prather et al, 1999). More recently studies such as Frömming et al (2010) and Lee et al (2009) estimate that the RF due to linear contrails is about one fifth of the total aviation RF. The RF from contrail-cirrus has been calculated as nine times as great as the RF from linear contrails alone. Contrail-cirrus may be the greatest contributor to the aviation-related RF (Burkhardt and Kärcher, 2011).
Prather et al (1999) predicted that the climate effects of contrails would increase more rapidly in the future than other climate perturbations such as CO2 emissions from aviation. This is due to a number of factors, including predicted increases in propulsion efficiency. There is a focus in the aviation industry on increasing the efficiency of propulsion technology to reduce fuel consumption (Lee, 2010). However, the increase in aircraft propulsion efficiency can result in an increase of linear contrails coverage. This suggests a significant future increase in coverage from linear contrails, since the demand for air transport is expected to increase by 5% per annum over the next 20 years.
Operational changes have been proposed to avoid or reduce the formation of contrails. Several studies have been carried out on operational changes in aircraft flight altitudes (Sausen et al, 1998), (Fichter et al, 2005). The results showed that contrails formed in mid-latitude areas are suppressed by flying higher in extra tropical regions, while contrails formed in tropical areas are avoided by flying in lower altitudes.
Technical mitigation options have also been proposed including changes in aircraft engine design (Haglind, 2008), use of alternative fuels (Ponater et al, 2006) and the use of fuel additives (Gierens, 2007). Using alternative fuels such as liquid hydrogen would allow contrails to form at a higher threshold temperature than kerosene-based fuel. However, the climate impacts from contrails formed may be different, as the physical properties of these contrails differ from those formed by kerosene combustion (Ponater et al, 2006).
The use of fuel additives has also been investigated (Montgomery et al, 2005). Early research showed that in theory, additives could decrease the potential for condensation to the extent that higher super-saturation would be necessary for the formation of contrails. However, more recent research found this method to be not effective. This is due to a number of factors including the inability of current technology to produce a sufficiently effective fuel additive (Gierens, 2007).
There are currently several on-going research programmes at CATE that are looking into the climate impacts of contrails. These studies used CATE’s suite of models, such as FAST [link to FAST model], COMA [link to COMA model], Edwards-Slingo Radiative Transfer Model (RTM) [link to E-S model] and LinClim [link to LinClim model].
FAST is used to generate 4D inventories of distance travelled from global aircraft movement data. The COMA model uses these distance inventories, fleet propulsion efficiency, meteorological data and satellite observations of contrail coverage to calculate global contrail coverage. The coverage data are then transferred to the Edwards-Slingo Radiative Transfer Model (RTM), to produce RF estimates due to these coverages. LinClim is then applied to produce the temproral evolution of contrails RF and their related contribution to temperature increase.
The following figures show an example results from these models. This study investigated the effects of increases in propulsion efficiency, η, and the growth of air travel on contrail coverage (Lee, 2010).
In another application, RF (left panel) and temperature response (right panel) from linear contrails were estimated for years between 2000 and 2100 (Lee, 2010).
Appleman, H. (1953). The formation of exhaust contrails by jet aircraft. Bulletin of the American Meteorological Society. 34. pp 14 – 20.
Burkhardt, U., Kärcher, B. (2011). Global radiative forcing from contrail cirrus. Nature Climate Change. 1. January 2011. pp 54 – 58.
Fichter C, Marquart S, Sausen R, Lee DS. (2005). The impact of cruise altitude on contrails and related radiative forcing. Meteorol Z. 14(4). pp 563-72.
Frömming, C., Ponater, M., Burkhardt, U., Stenke, A., Pechtl, S., Sausen, R. (2011). Sensitivity of contrail coverage and contrail radiative forcing to selected key parameters. Atmospheric Environment. 45(7). pp 1483 – 1490.
Gierens K. Are fuel additives a viable contrail mitigation option? (2007). Atmos Environ. 41(21). pp 4548-52.
Haglind, F. (2008). Potential of lowering the contrail formation of aircraft exhausts by engine re-design. Aerospace Science and Technology. 12. pp 490 – 497.
Lee, D. S. (2010). DfT Aviation Environment and Atmospheric Expert Technical Support, Final Report to the Department of Transport March 2010. Centre for Aviation Transport and the Environment (CATE), Manchester Metropolitan University (MMU).
Lee, D. S., Pitari, G., Grewe, V., Gierens, K., Penner, J.E., Petzold, A., Prather, M. J., Schumann, U., Bais, A., Berntsen, T., Iachetti, D., Lim, L. L., Sausen, R. (2010). Transport impacts on atmosphere and climate: Aviation. Atmospheric Environment. 44. pp 4678 – 4734.
Lee, D. S., Fahey, D. W., Forster, P. M., Newton, P. J., Wit, R. C. N., Lim, L. L., Owen, B., Sausen, R. (2009). Aviation and global climate change in the 21st century. Atmospheric Environment. 43 pp 3520 – 3537.
Lee, D. S., Raper, D. (2003). The global atmospheric impacts of aviation. In: Upham, P., Maughan, J., Raper, D., Thomas, C. (eds). Towards Sustainable Aviation. Earthscan. London
Montgomery CJ, Sarofim AF, Preciado I, Marsh ND, Eddings EG, Bozzelli JW. (2005). Experimental and numerical investigation of soot-reducing fuel additives. AIAA paper 2005-4472. 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Tucson. Arizona. July 10-13 2005.
Myhre G, Stordal F. (2001). On the tradeoff of the solar and thermal infrared radiative impact of contrails. Geophys Res Lett . 28(16). pp 3119-22.
Ponater, M., Pechtl, S., Sausen, R., Schumann, U., Hüttig. (2006). Potential of the cryoplane technology to reduce aircraft climate impact: A state-of-the-art assessment. Atmospheric Environment. 40. pp 6928 – 6944.
Prather, M., Sausen, R., Grossman, A. S., Haywood, J. M., Rind, D., Subbaraya, B. H. (1999). Potential Climate Change from Aviation. In: Penner, J. E., Lister, D. H., Griggs, D. J., Dokken, D. J., McFarland, M. (eds). Aviation and the Global Atmosphere. A special report of IPCC Working Groups I and II. Intergovernmental Panel on Climate Change. Cambridge University Press. Cambridge. UK.
Rädel, G., Shine, K. P. (2008). Radiative forcing by persistent contrails and its dependence on cruise altitudes. Journal of Geophysical Research. 113. D07105.
Sausen R, Gierens K, Ponater M, Schumann U. (1998). A diagnostic study of the global distribution of contrails, Part I. Present day climate. Theor Appl Climatol. 61. pp 127-41.
Schumann U. (2005). Formation, properties and climatic effects of contrails. C R Phys 2005. 6. pp 549-65.
Schumann, U. (1996). On conditions for contrail formation from aircraft exhausts. Meteorologische Zeitschrift. N.F. 5. pp 4 –23.
Bae JH, Avedisian CT. (2004). Effect of TPGME blending on soot emissions from JP8 fuel droplets burning with spherical symmetry. AIAA paper 2004-5680. 2nd International Energy Conversion Engineering Conference. Providence. Rhode Island. Aug 2004. pp 16-19.
Boucher O. Air traffic may increase cirrus cloudiness (1999). Nature. 1999. 397. pp 30-31.
Busen R, Schumann U. (1995). Visible contrail formation from fuels with different sulfur contents. Geophys Res Lett. 22. pp 1357-60.
Chen JP (1999). Particle nucleation by recondensation in combustion exhausts. Geophys Res Lett. 26. pp 2403-06.
Fichter C, Marquart S, Sausen R, Lee DS. (2005). The impact of cruise altitude on contrails and related radiative forcing. Meteorol Z. 14(4). pp 563-72.
Gierens K, Kärcher B, Mannstein H, Mayer B. (2006). Aerodynamically induced formation of contrails. International Conference on Transport, Atmosphere and Climate. Oxford. UK. June 2006.
Gierens K. Contrails, contrail cirrus, and ship tracks (2006). International Conference on Transport, Atmosphere and Climate. Oxford. UK. June 2006.
Gierens K, Sausen R, Schumann U. (1999). A diagnostic study of the global distribution of contrails. Part II. Future traffic scenarios. Theor Appl Climatol. 63. pp 1-9.
Green J. E. (2003). Civil aviation and the environmental challenge. Aeronautical J. 107. pp 281-99.
Grewe V, Dameris M, Fichter C, Lee DS. (2002). Impact of aircraft NOx emissions. Part 2: effects of lowering the flight altitude. Meteorol Z. 11. pp 197-205.
Jensen EJ, Toon OB, Kinne S, et al. (1998). Environmental conditions required for contrail formation and persistence. J Geophys Res. 103. pp 3929-36.
Kärcher B, Busen R, Petzold A, Schröder FP, Schumann U, Jensen EJ. (1998). Physicochemistry of aircraft generated liquid aerosols, soot, and ice particles. 2. Comparison with observations and sensitivity studies. J Geophys Res. 103. pp 17129-48.
Kärcher B, Yu F, Schröder FP, Turco RP. (1998). Ultrafine aerosol particles in aircraft plumes: analysis of growth mechanisms. Geophys Res Lett. 25. pp 2793-96.
Klima K. (2005). Assessment of a global contrail modeling method and operational strategies for contrail mitigation. MIT. Cambridge. Massechusetts. pp. 172.
Krebs W. (2006). Analyse des Einflusses des Flugverkehrs auf die natürliche Zirrusbewölkung über Europa, Nordafrika und dem Nordatlantik. Ludwig-Maximilians Universität München. pp 211 (in German).
Kristensson A, Gayet JF, Ström J, Auriol F. (2000). In situ observations of a reduction in effective crystal diameter in cirrus clouds near flight corridors. Geophys Res Lett. 27. pp 681-84.
Lewis, J. S., Niedzwiecki, R. W., Bahr, D. W., Bullock, S., Cumpsty, N., Dodds, W., DuBois, D., Epstein, A., Ferguson, W. W., Fiorentino, A., Gorbatko, A. A., Hagen, D. E., Hart, P. J., Hayashi, S., Jamieson, J. B., Kerrebrock, J., Lecht, M., Lowrie, B., Miake-Lye, R. C., Mortlock, A. K., Moses, C., Renger, K., Sampath, S., Sanborn, J., Simon, B., Sorokin, A., Taylor, W., Waitz, I., Wey, C. C.,Whitefield, P., Wilson, C. W., Wu, S. (1999). Chapter 7 Aircraft Technology and Its Relation to Emissions. In Penner JE, Lister DH, Griggs DJ, Dokken DJ, McFarland M. Aviation and the Global Atmosphere. IPCC. Cambridge University Press. Cambridge, UK.
Liscinsky DS, Colket MB, Hautman DJ, True B. (2001). Effect of fuel additives on particle formation in gas turbine combustors. AIAA paper 2001-3745. AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit.37th. Salt Lake City. UT. July 8-11, 2001.
Liou KN. (1992). Radiation and Cloud Processes in the Atmosphere. Theory, Observation and Modeling. Oxford University Press. New York. Oxford.
Mannstein H, Schumann U. (2007). Corrigendum to “Aircraft induced contrail cirrus over Europe”. Meteorol Z . 16. pp 131-32.
Mannstein H, Spichtinger P, Gierens K. (2005). A note on how to avoid contrail cirrus. Transp Res Part D. 10. pp 421-26.
Mannstein H, Schumann U. Aircraft induced contrail cirrus over Europe (2005). Meteorol Z. 14. pp 549-54.
Marquart S, Ponater M, Ström L, Gierens K. (2005). An upgraded estimate of the radiative forcing of cryoplane contrails. Meteorol Z. 14. pp 573-82.
Marquart S, Ponater M, Mager F, Sausen R. (2003). Future development of contrail cover, optical depth and radiative forcing: Impacts of increasing air traffic and climate change. J Climate. 16. pp 2890-2904.
Minnis P, Ayers JK, Palikonda R, Phan D. Contrails, Cirrus Trends, and Climate. (2004). J Climate. 17. pp 1671-85.
Minnis P, Ayers JK, Palikonda R, Doelling DR, Schumann U, Gierens K. (2001). Changes in cirrus cloudiness and their relationship to contrails. Proceedings American Meteorology Society. Boston, USA. 11(9). pp. 239-42.
Meerkötter R, Schumann U, Doelling DR, Minnis P, Nakajima T, Tsushima Y. (1999). Radiative forcing of contrails. Ann Geophys. 17. pp 1080-94.
Noppel F, Singh R. (2007). An overview on contrail and cirrus cloud avoidance technology. J Aircraft . 44. pp 1721-26.
Noppel F, Singh R, Taylor M. (2006). Novel engine concept to suppress contrail and cirrus cloud formation. International Conference on Transport, Atmosphere and Climate. Oxford, UK.June 2006.
Penner J. E., Lister D. H., Griggs D. J., Dokken D. J., McFarland M. (1999). Aviation and the Global Atmosphere. IPCC. Cambridge University Press. Cambridge. UK.
Rogers, H. L., Lee, D. S., Raper, D. W., Forster, P. M. de F, Wilson CW, Newton PJ. (2002). The impacts of aviation on the atmosphere. Aeronaut J 2002. 106. pp 521-46.
Sausen, R., Isaksen, I., Grewe, V., Hauglustaine, D., Lee, D. S., Myhre, G., Köhler, M. O., Pitari, G., Schumann, U., Stordal, F., Zerefos, C. (2005). Aviation radiative forcing in 2000: An update on IPCC (1999). Meteorologische Zeitschrift. 14. pp 555 – 561.
Schumann U. On conditions for contrail formation from aircraft exhausts (2006). Meteorol. Z. 1996; 5: 4-23.
Stordal, F., Myhre, G., Stordal, E. J. G., Rossow, W. B., Lee, D. S., Arlander, D. W., Svendby, T. (2005). Is there a trend in cirrus cloud cover due to aircraft traffic? Atmos Chem Phys. 5. pp 2155-62.
Ström L, Gierens K. First simulations of cryoplane contrails. (2002). J Geophys Res. 107. 4346. doi:10.1029/2001JD000838.
Ström J, Ohlsson S. (1998). In situ measurements of enhanced crystal number densities in cirrus clouds caused by aircraft exhaust. J Geophys Res. 103. pp 11355-361.
Stubenrauch CJ, Schumann U. (2005). Impact of air traffic on cirrus coverage. Geophys Res Lett . 32. L14813.
Stuber N, Forster P, Rädel G, Shine K. (2006). The importance of the diurnal and annual cycle of air traffic for contrail radiative forcing. 441. pp 864-67.
Suzanne J, Ferry D, Popovitcheva O, Shonija NK. (2003). Ice nucleation by kerosene soot under upper tropospheric conditions. Can J Phys. 81. pp 423-29.
Svensson F, Hasselrot A, Moldanova J. (2004). Reduced environmental impact by lowered cruise altitude for liquid hydrogen-fuelled aircraft. Aerosp Sci Technol. 8. pp 307-20.
Taylor MD, Noppel FG, Singh R. (2007). A gas turbine engine. EP1852590, Nov. 2007.
Tompkins AM, Gierens K, Rädel G. (2007). Ice supersaturation in the ECMWF integrated forecast system. Q J Roy Meteor Soc. 133. pp 53-63.
Williams V, Noland RB (2005). Variability of contrail formation conditions and the implications for policies to reduce the climate impacts of aviation. Transp Res Part D. 10(4). pp 269-80.
Williams V, Noland RB, Toumi R. (2003). Air transport cruise altitude restrictions to minimize contrail formation. Clim Policy. 3. pp 207-19.
Williams V, Noland RB, Toumi R. (2002). Reducing the climate change impacts of aviation by restricting cruise altitudes. Transp Res Part D. 7. pp 451-64.
Zerefos CS, Eleftheratos K, Balis DS, Zanis P, Tselioudis G, Meleti C. (2003). Evidence of impact of aviation on cirrus cloud formation. Atmos Chem Phys. 3. pp 1633-44.;