Idling airplanes produce more harmful pollution than previously thought

Credit: Magnus Rosendahl,

( -- A group of researchers from Carnegie Mellon University in Pittsburgh, have shown that the emissions produced by aircraft idling at the gate, or lining up for takeoff, contain tiny oil droplets, that when exposed to ordinary sunlight, undergo a chemical reaction that causes them to solidify into tiny particles that can infiltrate the lungs and eventually the brain.

In a paper published in , the team led by Allen Robinson, describe how they collected samples of exhaust from an idling KC-135 military cargo plane, into large Teflon coated bags, and how they then exposed that exhaust to sunlight and/or to initiate photo-oxidation (when a polymer surface degrades in oxygen or ozone). The result, they say, was that the original droplets of oil were converted into multiple minute solid particles, small enough to penetrate the lungs and brain of people working or living near airports.

In contrast, exhaust emissions from airplanes running at speed, such as when in-flight, tend to be mostly comprised of solid particles, which would not be effected by sunlight in the same way, and thus would pose no additional health hazards over what is already known about such types of pollution.

The paper highlights the fact that airplane pollution, and specifically the kind produced at airports, is in stark contrast to other types of pollution emitters such as cars and manufacturing plants, in that little to nothing has been done to reduce the amounts spewed into the environment. It also shows that the type of pollution produced at airports is far more hazardous than was previously thought; but, because these findings are so new, it’s likely no research has yet been conducted to ascertain what exactly happens to the human body when these are breathed in on a regular basis.

The authors also mention in their paper, the folly of studying emissions from aircraft (or presumably any other pollution emitters for that matter) without following up to find out what becomes of such pollutants as they enter the environment and are affected by such forces as UV radiation, temperature, or other substances.

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More information: Secondary aerosol formation from photochemical aging of aircraft exhaust in a smog chamber, Atmos. Chem. Phys., 11, 4135-4147, 2011 doi:10.5194/acp-11-4135-2011

Field experiments were performed to investigate the effects of photo-oxidation on fine particle emissions from an in-use CFM56-2B gas turbine engine mounted on a KC-135 Stratotanker airframe. Emissions were sampled into a portable smog chamber from a rake inlet installed one-meter downstream of the engine exit plane of a parked and chocked aircraft. The chamber was then exposed to sunlight and/or UV lights to initiate photo-oxidation. Separate tests were performed at different engine loads (4, 7, 30, 85 %). Photo-oxidation created substantial secondary particulate matter (PM), greatly exceeding the direct PM emissions at each engine load after an hour or less of aging at typical summertime conditions. After several hours of photo-oxidation, the ratio of secondary-to-primary PM mass was on average 35 ± 4.1, 17 ± 2.5, 60 ± 2.2, and 2.7 ± 1.1 for the 4, 7, 30, and 85 % load experiments, respectively. The composition of secondary PM formed strongly depended on load. At 4 % load, secondary PM was dominated by secondary organic aerosol (SOA). At higher loads, the secondary PM was mainly secondary sulfate. A traditional SOA model that accounts for SOA formation from single-ring aromatics and other volatile organic compounds underpredicts the measured SOA formation by ~60 % at 4 % load and ~40 % at 85 % load. Large amounts of lower-volatiliy organic vapors were measured in the exhaust; they represent a significant pool of SOA precursors that are not included in traditional SOA models. These results underscore the importance of accounting for atmospheric processing when assessing the influence of aircraft emissions on ambient PM levels. Models that do not account for this processing will likely underpredict the contribution of aircraft emissions to local and regional air pollution.

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Citation: Idling airplanes produce more harmful pollution than previously thought (2011, May 12) retrieved 20 July 2019 from
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May 12, 2011
Good research, but almost totally irrelevant to current air pollution concerns. It needs to be repeated with modern rather than 50 year old engine designs. The KC-135 is a variant of the Boeing 707 and first deployed by the USAF in 1957. The current engines have been upgraded somewhat, but the original engine design drank oil. Since the plane could be refueled itself, its maximum air duration was limited by oil consumption (like the B-52).

The shaft bearings are in effect sleeve bearings with oil injected under pressure, so that a thin film of oil prevents metal to metal contact. Modern jet engines tend to use air bearings instead. Why not use ball or roller bearings? They do--at either end of the shaft. But in the middle, the high temperatures make such bearings impractical. Why not just support the shaft at the ends? Fine until the airplane turns, or something sticks to one of the blades. Now you need the center bearings to damp vibration modes.

May 12, 2011
I'm assuming that zagwee is a bot because all of his posts are like this, have a website, and are general statements that only vaguely relate to the topic at best.

May 13, 2011
eachus your are only partially correct...The test were conducted on a CFM56 engined airframe. This the same engine used on the 737, A340, A318/319/320/321 and there are about 15,000 of these engines in daily use.

Thanks for the correction. I thought all modern jet engines used air bearings instead of ball and roller bearings (filled with oil under pressure) today. But the CFM56 seems frozen in time. The KC-135R refit used the CFM56-2 variant, but even the most recent variants designed to reduce pollution and increase efficiency apparently use the same old bearing design. On page 300 of this pdf for the CFM56-5C: http://www.scribd...c-Engine you can find the er, smoking gun. The bearing sumps contain both high-pressure air and oil, and use a centrifugal separator. Then the air is bled into the bypass air, not into the high-pressure turbine intake, where it would get burned.

May 13, 2011
On the subject of fluid (air or oil) bearings vs. ball or roller bearings, here is an ASME brochure honoring one of the first major fluid bearing installations: http://files.asme...5583.pdf This bearing was originally installed in 1912, and the last estimate was it may need maintenance around 3232.

Note also that the roller bearings in the CFM56 (there are also ball bearings) act as fluid bearings in normal operation. The rollers can't push the oil aside, so they quickly (like a quarter turn) end up with an oil film between the bearing and raceway. The problem with oil-filled fluid bearings is in the details. Oil under pressure and air under pressure means very fine oil droplets which have to be combined, separated from the air and recirculated.

How much of a design change would it be to vent the oil sumps into the high-pressure air stream instead? For the forward oil sump, not much all. The rear sump would be too hard to refit.

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