How are aircraft affecting our atmosphere?
Key words:contrails,climate change, NOx, free radicals
Ever-elevating numbers of planes traverse the globe year on year, injecting increased levels of aviation fuel into our atmosphere. This leads to the release of gaseous species, as well as solid particles, which cause a numberof environmental problems.With air travel and emissions increasing, it is essential that we understand the impacts they are having as well as possible. To do this, it is first necessary to look at what these emissions are, and what impacts they can have.
Aviation Emissions: Environmental Impacts
The emissions from aircraft primarily result from kerosene (jet fuel) combustion. A summary of the species released during this process is shown in Table 1.
Species / DescriptionCO2 / Carbon dioxide is produced when the jet fuel burns during complete combustion.
H2O / Water vapour is another product of complete combustion. This leads to the formation of contrails.
NOx / Nitrogen oxides are produced through the high temperature combustion in the jet engine, when nitrogen and oxygen present in the air combine.
1)NOxgases: NO + NO2
2)NOygases: NO + NO2 + NO3, + N2O5, + HNO2 + HNO3 + PAN (Peroxyacyl nitrates) + PANS + Nitrates
3)NOz gases: NOy -NOx
HC / Hydrocarbons (compounds of hydrogen and carbon only) are products of incomplete fuel combustion (insufficient oxygen in the engine to burn all of the jet fuel). Therefore, these are often referred to as unburned hydrocarbons.
SOx / Sulfur oxides are produced when the small quantities of sulfur in the jet fuel combine with the oxygen in the air during complete combustion. These include sulfur dioxide (SO2) and sulfur trioxide (SO3).
Particulate Matter / Incomplete combustion produces small particles of soot (or “black carbon), as well as aerosols from the condensed gases. These are referred to as particulate matter.
Aircraft emissions do have impacts at surface level. However, the majority of emissions take place at high altitude (1000 metres or above), and it is here where they can cause considerable changes to the composition of the atmosphere.One of the most globally significant impacts that aircraft emissions have is their contribution to climate change.
Why and how can aviation emissions affect the global climate?
The most notable climatic impact of aircraft emissions occur when injected into a climatically sensitive region of the atmosphere, which spans altitudes of 5 to 22 km. This is known as the upper troposphere/lower stratosphere (UT/LS) and encompasses the typical cruising altitudes of commercial aircraft (9-12 km).
Carbon dioxide emissions
Carbon dioxide (CO2) is the Earth’s most potent greenhouse gas. However, according to the International Civil Aviation Organisation (ICAO), aviation accounts for just 2% of total global CO2 emissions, despite it being the most abundant aviation-derived emission. Although a key player in the climate change arena, directly affecting the Earth’s radiative budget, (i.e. the balance between incoming solar energy and the outgoing longwave (infra-red) and reflected shortwave (ultraviolet) energy from the Earth),[AF1]many of the non-CO2 aviation emissions indirectly contribute to climate change.
[AF2]
Non-CO2 emissions: particulate matter, contrails and clouds
Climate impacts of non-CO2 emissions include the formation of contrails. These form when water vapour emissions from the aircraft can persist for several hours and condense onto what are known as ‘cloud condensation nuclei’.Black carbon (soot), which are also emitted by aircraft, are another source of cloud condensation nuclei. Contrail-cirrus clouds can cause both positive and negative effects on the Earth’s climate, making their overall environmental impact fairly complex.
Non-CO2 emissions: nitrogen oxides and ozone
Aviation nitrogen oxides (NOx) gases are now considered one of the more dominant emissions in terms of their contribution to climate change, more so than aviation CO2. These emissions play an important role in the atmospheric chemistry of the UT/LS region, as they increase the production of ozone (O3), which is the third largest contributor to global warming.However, NOxemissions also stimulate the destruction of methane (CH4), an extremely potent greenhouse gas, via the generation of hydroxyl (OH.) free-radicals. These free-radicals are highly reactive oxidising agents, reacting with many organic species; one of these being methane. So, not only can aviation NOx cause warming through the production of O3, but they can also lead to cooling by decreasing the atmospheric lifetime of CH4.
Aircraft emissions: a global concern for the future
These wide-ranging and complex atmospheric impacts, along with increasing demand for air travel, highlight the need for continued research into aircraft as a significant emissions source, as these could have potentially drastic implications in our planet’s future.
Amy Foulds is a postgraduate researcher at the School of Chemistry, University of Bristol. She is a member of theAtmospheric Chemistry Research Group. Amy is currently working on modelling the atmospheric impacts of biofuel use in aviation.
Acknowledgements
This piece is based on the Out of thin air: Atmospheric Pollution by Aircraft?
by Amy Foulds and Tim Harrison in Chemistry Review
How are aircraft affecting our atmosphere?
Questions
- What is the Earth’s most potent greenhouse gas?
- What is a free radical?
- Do passenger aircraft fly above the ozone maximum?
- Of what is a contrail composed?
- What chemical is the third largest contributor of global warming?
- What are the oxidation numbers of sulfur in sulfur dioxide and sulfure trioxide?
- Which
- Why?
Extension Question
[AF1]Added a definition for radiative budget, as requested ?
[AF2]Wasn’t sure how else to describe “C0” – if you want to write a question based on this, I guess it could be referred to as an initial concentration? (I.e. if you say the CO2 concentration was “…” in year “x” and is now at “…” the former would be the “C0” value.)
Not sure if I’ve put too much information in this box – let me know ?