Particles are the byproduct of any form of incomplete combustion. They can come in a wide range of sizes ranging from the macro scale ash that is visible to the naked eye down to particles that nucleate from condensing gas phase molecules forming nanometer scale particles. At the high temperatures at which jet engines burn their fuel, one of the main byproducts are black carbon particles, or particles composed of carbon in its elemental form.
Figure 1. Global distribution of Black carbon emissions from aviation as calculated by Stettler et al using data from 2005 (Figure taken from Stettler et al. 2013.)
Aircraft emissions pose a very interesting problem as they emit not only at the ground level during take off and landing, but they also emit while at cruising altitudes, somewhere between 30,000 and 40,000 feet for typical commercial aircraft. Most emission inventories of black carbon for jet engines extrapolate data from wind tunnel or staged non realistic engine tests, but a recent study attempts to use data from in use aircraft as well as a new model to better estimate the global black carbon inventory from aircraft during takeoff, landing, and while cruising. The study concluded that the warming effects that these particles exhibit (by absorbing incoming solar radiation in the atmosphere) are approximately 1/3 as large as the warming effect from the CO2 emitted from the jet engine combustion itself.
Particles in the atmosphere can also help to induce climate change by reflecting incoming solar radiation, causing a cooling effect on the atmosphere, or serving as condensation sites for ice crystals and water droplets, which can then form clouds, depending on the meteorological conditions. The clouds can also cause a cooling effect by increasing the reflection of light from the sun as well causing a warming effect by trapping the heat from the Earth’s surface. While the study by Stettler et al only looked at the solar radiation absorption by black carbon, another study by Gettleman and Chen attempted to include the cooling effects into their study as well. They found that black carbon had very little effect on the negative (cooling) forcing from particles in the atmosphere, and the majority of the cooling, which was almost equal to the warming forcing of CO2, was due to inorganic sulfate particles also emitted by jet engines.
Alternative fuels for cars have been a big topic in the recent years, and it is not surprising alternatives to standard jet fuel have been investigated. In 2009 and 2011, NASA conducted two studies into using alternative fuels in commercial aircraft, named AAFEX for the Alternative Aviation Fuel Experiment. This study looked at alternative fuels synthesized from reactions of carbon monoxide and hydrogen to make long chain hydrocarbons suitable for aircrafts. These fuels, named Fischer-Tropsch (FT) fuels for scientists who developed the process, are different from standard jet fuel by having little to no sulfur or aromatic compounds. The study found that there are significant reductions in particulate emissions of both black carbon and sulfate by as much as 95% for pure FT fuels. While these results are encouraging, there is still significant work to be done before they could be adopted. The reduction in the particulate emissions effect on climate is likely to be offset by the added amount of CO2 produced in the synthesis process. Also, such aircraft would need significant overhauls to be able to handle the new fuels.
This November the 9th conference of parties (COP9) to the United Nations Framework Convention of Climate Change (UNFCCC) will meet to work towards a global climate treaty. Because aircraft emissions only account for less than 5% of global pollution, it is not a source that can be discounted, especially as the world continues to develop and air travel increases. More research needs to be done to better understand the black carbon emissions from aircraft.
Papers Discussed in this post:
Stettler, M. E. J., Boies, A. M., Petzold, A., & Barrett, S. R. H. (2013). Global Civil Aviation Black Carbon Emissions. Environmental Science & Technology, 130823150610008. doi:10.1021/es401356v
Gettelman, A., & Chen, C. (2013). The climate impact of aviation aerosols. Geophysical Research Letters, 40(11), 2785–2789. doi:10.1002/grl.50520
Beyersdorf, A. J., Timko, M. T., Ziemba, L. D., Bulzan, D., Corporan, E., Herndon, S. C., et al. (2013). Reductions in aircraft particulate emissions due to the use of Fischer–Tropsch fuels. Atmospheric Chemistry and Physics Discussions, 13(6), 15105–15139. doi:10.5194/acpd-13-15105-2013
Photo credit: Jon Franklin