Contrails - Research, comments and links

Contrails and Aviation-cirrus

Aviation-emissions (03)


02.7700.03/e External costs of aviation (Main report)
Jos Dings, Ron Wit, Bas Leurs, Marc Davidson (CE), W. Fransen (INTEGRAL Knowlegde Utilization)
Delft, 2002 (February)
Air traffic / Environment / Effects / Environmental damage / Climate / Prevention Costs / Analysis / Prognostication / Price fixing / Tariff /

This study, commissioned by the German Umweltbundesamt, aims at quantifying, within ranges as small as possible, external costs from environmental impacts of aviation. Benefits of aviation are important too, but they are generally, in contrast to the negative impacts, well captured by the market.
For the valuation of climatic impacts from aviation, both the damage cost and prevention cost approach is used, leading to a middle estimate of � 30 per tonne of CO2 equivalent, with sensitivities of � 10 and en � 50 per tonne. As contrails have a relatively large climatic impact and their formation can quite accurately be predicted, the climatic impact is differentiated for situations with and without contrail formation. For this analysis the most important assumption is hat contrails are formed during 10% of flight kilometres.
For the valuation of regional and local impacts, the damage cost approach has been followed. Avoidance or adaptation costs (e.g. costs of zoning around airports) have been included in the damage cost assessment.
For aircraft flying at distances up to a few hundred kilometres, external costs related to landing and take-off (LTO) impacts � especially noise - are dominant in the total picture. For flights over about 1,000 km, external costs of climatic impacts exceed those of LTO impacts, also in case no contrails are formed. New technology has more impact on LTO related costs than on costs related to climatic impact.
Contrail formation has a large influence on the climatic impact of aircraft, and thus on external costs related to this climatic impact. Based on a number of assumptions, a middle estimate is that the climatic impact of a contrail-causing aircraft km is, on average, about eight times as high as an aircraft km that does not lead to persistent contrails.
Expressed as a share of ticket prices, external costs (without contrail impacts) vary from roughly 5% of ticket prices (long-haul flights, new technology, no contrail formation) to roughly a quarter of ticket prices for 200 km flights with average technology. These figures rise sharply when contrails are formed during part of the trip.
Souce: Bron:

02.7700.04/e Externe kosten van de luchtvaart (achtergrondrapport)
J.M.W. (Jos) Dings, R.C.N. (Ron) Wit, B.A. (Bas) Leurs, S.M. (Sander) de Bruyn, M.D. (Marc) Davidson (CE), W. Fransen (INTEGRAL Knowledge Utilization)
Delft, 2002 (februari)

Luchtverkeer / Milieu / Effecten / Milieu effecten / Klimaat / Preventie / Kosten / Prognoses / Prijsstelling / Tarieven /

Het doel van deze studie in opdracht van het UmweltBundesAmt (UBA) is het binnen zo smal mogelijke marges kwantificeren van de externe milieukosten van de luchtvaart. De studie schenkt ook aandacht aan de baten van de luchtvaart; de conclusie is echter dat de luchtvaartmarkt er zelf voor zorgt dat de baten van luchtvaart in de prijs van tickets en vrachtvervoer worden verdisconteerd, terwijl bij de negatieve effecten hiervoor overheidsingrijpen noodzakelijk is.
Voor de waardering van de effecten van de luchtvaart op klimaatverandering hebben we zowel de schade- als de preventiekostenmethode gebruikt. Dit leidt tot een middenschatting van � 30 per ton CO2-equivalent, met een bandbreedte van 10 tot 50 � per ton. We hebben de effecten van de luchtvaart op klimaatverandering gedifferentieerd voor situaties waarin w�l en g��n contrails (achterblijvende condensstrepen) optreden. Contrails leveren immers volgens het IPCC-rapport �Aviation and the global atmosphere� een substanti�le bijdrage aan klimaatverandering door de luchtvaart en bovendien kan het ontstaan van contrails goed worden voorspeld aan de hand van meteorologische en motortechnische gegevens. De belangrijkste aanname die we voor deze differentiatie hebben gemaakt is dat contrails gedurende 10% van de vliegtuigkilometers optreden.
Voor de waardering van regionale en lokale milieu-effecten hebben we de schadekostenmethode gebruikt. In de schadekostenramingen hebben we ook rekening gehouden met zogenoemde vermijdings- of aanpassingskosten, voornamelijk de kosten van suboptimale ruimtelijke ordening de het gevolg is van veiligheids- en geluidzonering rond luchthavens.
Bij korte vluchten �tot enkele honderden kilometers- zijn de regionale en lokale milieukosten als gevolg van landen en opstijgen, in het bijzonder geluid, dominant in het totaalplaatje van externe kosten. Bij vluchten boven ruwweg 1000 km is de invloed van klimaatverandering dominant boven de lokale en regionale effecten van landen en opstijgen. Toepassing van nieuwe technologie heeft meer invloed op de milieu-effecten bij landen en opstijgen dan op de klimaateffecten van de vliegtuigen.
Zoals gezegd heeft de vorming van contrails een grote invloed op de externe kosten als gevolg van klimaatverandering door luchtvaart. Op basis van een aantal aannemen schatten we dat een vliegtuigkilometer die tot contrailvorming leidt gemiddeld achtmaal zoveel invloed heeft op klimaatverandering als een vliegtuigkilometer zonder contrails.
Uitgedrukt als een percentage van ticketprijzen vari�ren de externe kosten -exclusief die van contrailvorming- ruwweg van 5% bij lange-afstandsvluchten met nieuwe vliegtuigen tot ruwweg een kwart op vluchten van 200 km met gemiddelde vliegtuigen. Deze percentages gaan sterk omhoog naarmate gedurende een groter deel van de vlucht contrails worden gevormd.

EarthTalk: Do airplanes contribute significantly to air pollution?


Tuesday, September 07, 2004
From the editors of E/The Environmental Magazine

Dear EarthTalk: Do airplanes contribute significantly to air pollution?
� Neil Gladstone, New York, New York

Airplanes do indeed create a great amount of air pollution. According to the Natural Resources Defense Council (NRDC), a nonprofit environmental group, "airport air pollution is similar in scope to that generated by local power plants, incinerators, and refineries, yet is exempt from rules other industrial polluters must follow."

Major airports rank among the top 10 industrial air polluters in cities such as Los Angeles, Washington, and Chicago, said NRDC. The hundreds of thousands of airplanes taking off, landing, taxiing, and idling each day across the country emit contaminants into the air and ground which have been linked to a wide range of human health problems, including asthma and cancer.

Beyond local environmental effects, air travel is contributing significantly to global warming. A 1999 report by the Intergovernmental Panel on Climate Change (IPCC) found that aircraft are responsible for 3.5 percent of greenhouse gas emissions worldwide; this could increase to 10 percent by 2050 as the popularity of air travel rises.

Meanwhile, contrails � those vapor condensation trails you see overhead that are formed when airplanes fly at high altitudes through extremely cold air � could be contributing to global warming as they turn into high thin cirrus clouds and trap heat from incoming sunlight within the atmosphere.

A recent agreement to cut 37 daily peak-hour arrivals at America's busiest airport, Chicago's O'Hare, should help to not only ease congestion and reduce delays but also to improve local air quality and reduce greenhouse gas emissions. Unfortunately, because of the increasing popularity of air travel, 60 of the 100 largest U.S. airports are proposing building more runways, thus expanding rather than reducing activity.

Because airplanes are considered part of interstate commerce, they are not subject to local and state pollution laws. Furthermore, the Federal Aviation Administration has the potentially conflicting responsibilities of monitoring pollution while promoting air travel.

In lieu of government regulation to curb airplane emissions, economics sometimes prevail. In the wake of 9/11, consumers have been skittish about air travel, while fuel prices have risen to unprecedented levels. Ailing airlines are left with no choice but to scale back on flights as well as on engine idling, both of which benefit the environment. Analysts estimate that Delta Air Lines' voluntary reduction of engine idling, for instance, has cut ground-level emissions from its planes by as much as 40 percent.

Meanwhile, NRDC promotes taxes on jet fuel as a way to encourage airlines to increase their efficiency and encourages consumers to opt for alternative modes of transportation, such as high-speed rail when available, especially for shorter distances.

"Consumers can also help by demanding that airports be subject to the same rigorous standards and reporting requirements as their industrial neighbors," said the group.


July 6, 1996
Ten Thousand Cloud Makers

Is airplane exhaust altering Earth's climate?
By Richard Monastersky

When the small Saberliner jet carrying Bruce E. Anderson rolled almost completely upside down, the atmospheric scientist saw his dessert, rather than his life, pass before his eyes. Seconds earlier, the NASA researcher had been munching on some cookies when his plane entered the wake of a DC-8 jet just a few miles ahead. The backwash-a tight horizontal tornado whirling at more than 100 miles per hour-spun the light Saberliner 140 degrees and sent it into a dive, causing Anderson, his food, and everything else in the plane to go temporarily weightless.

"It seemed like forever," says Anderson, an atmospheric scientist at NASA's Langley Research Center in Hampton, Va., "but it was probably only 5 or 10 seconds before the pilots righted the plane and were back in control." Then they nosed up behind the DC-8 for some more punishment.

Although it sounds like military flight training, Anderson and his colleagues were actually conducting a high-tech emissions check-measuring the gases and particles spewing out of jet engines. Their mission resembles the pollution tests that states routinely perform on cars, except that the NASA-run experiment happened at 400 miles per hour, 40,000 feet above the ground. And whereas car emissions are well understood, scientists have little information on the pollution from jet engines. Toward that end, NASA gathered four planes and 120 scientists in Kansas during April and May to make the most detailed measurements yet of jet engine exhaust at cruising altitude.

This project and future ones are addressing the question of whether aircraft emissions are increasing the number of clouds and are perturbing atmospheric chemistry-both of which could affect the weather down on the ground, says project scientist Randall R. Friedl of NASA headquarters in Washington, D.C. "There are 10,000 large-size commercial aircraft in operation today. It's expected that this number will double by the year 2020. It's a natural question to ask whether these are having an environmental impact," says Friedl.

Fueling this investigation are several sketchy studies hinting that ground temperatures have shifted in the last few decades in regions beneath well-traveled jet routes. "There is some concern that aircraft may play a role in some of the changes that have been seen," notes Friedl.

Although commercial jets have been sailing through the skies since the 1950s, scientists have only just started wondering about their widespread effects on the atmosphere. NASA launched its environmental investigation of subsonic aircraft 2 years ago and plans to continue the $140 million program through 2001. European researchers began similar studies in 1992 and are running a project parallel to the NASA work.

Anderson may have lost his snack during the recent experiment in Kansas, but he can count himself fortunate. So many scientists on board another of NASA's research planes lost their lunch when their DC-8 entered the wake of a Boeing 757 that they ran out of air-sickness bags. Yet all but one of the 40 investigators climbed back on board for the next flight.

Like an airborne bloodhound, the DC-8 tracked the chemical scent left by the 757, enabling investigators to measure exhaust plumes at distances of up to 10 miles. The nimble Saberliner could approach planes much closer, at times tagging only 150 feet behind the larger jets to sample fresh emissions immediately after they had left the engines. From above, a high-flying ER-2 plane surveyed the scene and measured the optical properties of the exhaust.

In flights over the central United States, the Rocky Mountains, and the Pacific Ocean, the NASA team measured emissions of sulfur and soot, with the aim of understanding how these affect high-altitude clouds. The scientists also analyzed the makeup of condensation trails, or contrails, those long, straight clouds often created by jets. NASA, ever eager for a catchy acronym, labeled the mission SUCCESS, for Subsonic Aircraft: Contrail and Cloud Effects Special Study.

Contrails develop when hot, humid fumes from a jet engine meet the cool air of the upper troposphere. Water vapor in the exhaust and atmosphere freezes to create tiny cloud particles, much like the mist that forms when a person exhales on a cold winter day. As turbulence in the upper atmosphere tears contrails apart, they can spread into wispy sheets essentially identical to natural cirrus clouds.

Engines can also stimulate cloud growth indirectly, by way of tiny aerosol particles within the exhaust. These aerosols-droplets of sulfuric acid and specks of soot-serve as seeds. They provide surfaces upon which water molecules can condense or freeze to create cloud particles, explains Eric J. Jensen, a participant in SUCCESS and a researcher at NASA's Ames Research Center in Mountain View, Calif.

Scientists do not know the fate of the aerosols once they leave the back end of a jet engine and start mixing with the ambient air. The specks and droplets may be among the ingredients necessary for creating contrails. They may also thicken natural cirrus clouds, rendering them more opaque to sunlight and making them last longer.

In fact, so little is known about the clouds produced by aircraft exhaust that researchers cannot say whether, on balance, they cool or warm the climate. The uncertainty exists because high-altitude clouds have numerous and contrary effects. Contrails and cirrus help cool the globe by reflecting sunlight that would otherwise hit Earth's surface. At the same time, they exert a warming influence because they absorb infrared radiation emitted by the ground, thus trapping energy and heating the atmosphere.

By studying what happens to engine exhaust immediately after it leaves the plane, SUCCESS aims to reveal how sulfuric acid and soot alter clouds. Although participants in the project are only now beginning to sift through the data, the sulfuric acid measurements have already shown some surprises. Previous engine tests conducted on the ground had suggested that most of the sulfur emitted by jets comes out as gaseous sulfur dioxide, with less than 1 percent in the form of sulfuric acid. But SUCCESS observations made at cruising altitude indicate that at least 10 percent of the sulfur in the exhaust appears as sulfuric acid droplets, making jet pollution an efficient producer of clouds. These results confirm observations from 1994, when 15 minutes after the supersonic Concorde passed, the ER-2 flew through its wake (SN: 10/7/95, p. 229).

The recent SUCCESS measurements made right behind jet engines reveal that the sulfuric acid forms either within the engine or immediately after it is ejected, says Anderson.

What this observation means for clouds and climate remains unclear. Yet many meteorologists think that increasing jet traffic in the last several decades has altered weather in noticeable ways.

In 1981, climatologist Stanley A. Changnon of the Illinois State Water Survey in Champaign reported that the Midwest had grown significantly cloudier during the 1960s and 1970s, with the greatest changes seen in areas of high jet traffic. He also noted a narrowing of the gap between high and low temperatures, possibly attributable to the increase in clouds.

More recently, Kuo-Nan Liou, an atmospheric physicist at the University of Utah in Salt Lake City, examined changes in high clouds. He found a 5 to 10 percent increase in cirrus cover over Salt Lake City, Denver, Chicago, St. Louis, and several other cities between 1948 and 1984. "Statistically, the high-level clouds appear to be increasing. So we speculate that there might be some potential relationship between aircraft activities and these high-level cloud increases," says Liou.

According to David J. Travis, a climatologist at the University of Wisconsin- Whitewater, who studies contrails, "there is a lot of circumstantial evidence that contrails are probably having an effect."

Travis and Changnon are collaborating in an attempt to document how contrails tweak climate. Because the artificial clouds trap heat predominantly at night, when they do not reflect sunlight, the researchers posit that contrails could be responsible for a trend toward reduced differences between daytime and nighttime temperatures, observed in the United States and elsewhere.

European researchers have also detected signs of the influence of aircraft, says Ulrich Schumann of the DLR Institute in Oberpfaffenhofen, Germany. "There are certainly obvious indications that aircraft cause additional cloudiness regionally, say over mid-Europe and some parts of the United States. There is no doubt about that," says Schumann. "But whether this is just a minor change or whether it is an essential change is absolutely an open question."

Because so many factors influence weather, scientists have had a difficult time determining what effect-if any-aircraft have actually had on conditions at Earth's surface. "People have looked for changes in sunshine duration and for changes in temperature. Some of these results are suggestive, but none are conclusive. There are too many other possibilities to explain the same observations," Schumann cautions.

While European and U.S. researchers are just starting to tackle the cloud question, they have a longer history of addressing how aviation affects the chemistry of Earth's atmosphere.

Like anything that burns fossil fuel, airplanes emit carbon dioxide gas and thereby contribute to global greenhouse warming. Currently, planes account for only about 3 percent of the carbon dioxide produced by humans, far behind other emitters, such as automobiles. Surging air travel and transport are pushing fuel consumption steeply upward, however, and airplanes may outpace other carbon dioxide sources, especially if countries make good on their promises to limit greenhouse gas emissions.

Planes can also contribute to global warming through emissions of nitrogen oxides, which stimulate ozone formation in the lower level of the atmosphere, called the troposphere.

Ozone is best known for the protective role it plays higher up in the stratosphere, where it blocks out harmful ultraviolet radiation coming from the sun. Close to the ground, ozone is a pollutant that endangers the health of humans and plants; for that reason, the International Civil Aviation Organization sets standards for nitrogen oxide emissions during takeoff and landing.

Yet aircraft spend most of their time and release most of their nitrogen oxides at the top of the troposphere. The ozone produced there is too high to threaten health directly. Instead, its most important effect is as a greenhouse gas that traps thermal energy and may contribute to global warming.

In the future, fleets of supersonic aircraft would have a different influence because they would emit nitrogen oxides and sulfuric acid in the stratosphere, where they trigger chemical reactions that destroy, rather than produce, ozone.

Recent measurements and calculations by European and U.S. researchers indicate that current aircraft are responsible for about half the nitrogen oxides present in the Northern Hemisphere's midlatitudes at an altitude of 26,000 to 40,000 feet, says Schumann.

Studies using computer models suggest that these nitrogen emissions could have boosted tropospheric ozone concentrations by several percent, especially over the heavily traveled North Atlantic. But a 1994 report by the World Meteorological Organization warned that "little confidence should be put in these quantitative model results of subsonic aircraft effects on the atmosphere."

Scientists point to many uncertainties that undermine the reliability of model results. Current models include only some chemical reactions and may be missing important ones. In addition, researchers do not know what quantity of nitrogen oxides comes from other sources, such as lightning. Estimates of lightning's input could be off by several hundred percent, warns Howard L. Wesoky, of NASA headquarters in Washington, D.C.

"We simply don't know quantitatively how significant the effects of aircraft are," says Wesoky.

Driven by this question, researchers plan to head once again into the skies next summer for a NASA-sponsored mission over the North Atlantic. This time, however, they will stock a more generous supply of air-sickness bags



Production of Engine Emissions
The majority of NOx emissions are generated in the highest temperature regions of the combustor-usually in the primary combustion zone, before the products are diluted. The fundamental processes of NOx formation are well known and documented (reviewed and described in detail by Bowman, 1992). They are best expressed as a function of local combustion temperature, pressure, and time. Combustion zone temperature depends on combustor inlet air temperatures and pressure, as well as the fuel/air mass ratio. The dependence of NOx on fuel/air ratio is shown in Figure 7-15. As illustrated, peak NOx formation coincides with peak temperature, which occurs close to the stoichiometric fuel/air ratio (or equivalence ratio = 1). In current gas turbine engine combustors, there are always some regions of the flame that burn stoichiometrically, so NOx formation is very strongly linked to combustor inlet temperature.



Contrails are Bad News