Contrails

A contrail is the condensation trail that is left behind by a passing jet plane. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. Vapor pressure is just a fancy term for the amount of pressure that is exerted by water vapor itself (as opposed to atmospheric, or barometric, pressure which is due to the weight of the entire atmosphere above you). The mixing occurs directly behind the plane due to the turbulence generated by the engine. If condensation (conversion from a gas to a liquid) occurs, then a contrail becomes visible. Since air temperatures at these high atmospheric levels are very cold (generally colder than -40 F), only a small amount of liquid is necessary for condensation to occur. Water is a normal byproduct of combustion in engines.

Image courtesy NASA Langley Research Center

Air traffic and, therefore, contrails, are not evenly distributed around the globe. They are concentrated over parts of the United States and Europe, where local warming reaches up to 0.7 watts per square meter, or 35 times the global average. The contrails often turn into cirrus clouds, a thin, wispy type of cloud made of ice crystals. The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. While some clouds tend to help cool the globe and negate the affects of global warming, thin cirrus clouds are heat trappers, holding in more heat than they reflect back into space.

Present commercial aircraft fly at altitudes of 8-13 km. The emissions from such air traffic can change the atmospheric composition: Directly: by emitting carbon dioxide (CO2), nitrogen oxides (NOx = NO + NO2), water vapor, hydrocarbons, soot, and sulfate particles. Indirectly: by a chemical reaction chain similar to smog-formation the greenhouse gas ozone (O3) can be formed. In this reaction chain nitrogen oxides act as a catalyst under the influence of sunlight. As a result of these chemical reactions also the concentration of methane (CH4), another greenhouse gas, decreases. These changes can have effects on climate: Ozone, CO2, and water vapor are greenhouse gases and their increase has a warming effect. Methane is also a greenhouse gas and its decrease has a cooling effect. Aerosols (sulfate particles, soot) could have a cooling effect. Contrails formed due to the emission of particles and water vapor can increase the cloud cover in the upper troposphere. This may result in a cooling or heating depending on the size and optical depth of the ice crystals of which the contrails consist. Presently it is believed that contrails lead to a net warming effect. There may be changes in (non-contrail) upper level clouds: Most contrails decay after minutes to hours, but some continue to exist and are then not distinguishable from natural cirrus clouds .

Schematic based on DLR German Aerospace Center graphic and text

Schematic of aerosol and contrail formation processes in an aircraft plume and wake as a function of plume age and temperature. Reactive sulfur gases, water vapor, chemi-ions, soot aerosols, and metal particles are emitted from the nozzle exit planes at high temperatures. H2SO4 increases as a result of gas-phase oxidation processes. Soot particles become chemically activated by adsorption and binary heterogeneous nucleation of SO3 and H2SO4 in the presence of H2O, leading to the formation of a partial liquid H2SO4/H2O coating. Upon further cooling, volatile liquid H2SO4/H2O droplets are formed by binary homogeneous nucleation, whereby the chemi-ions act as preferred nucleation centers. These aerosols grow in size by condensation and coagulation processes. Coagulation between volatile particles and soot enhances the coating and forms a mixed H2SO4/H2O-soot aerosol, which is eventually scavenged by background aerosol particles at longer times. If liquid H2O saturation is reached in the plume, a contrail forms. Ice particles are created in the contrail mainly by freezing of exhaust aerosols. Scavenging of exhaust particles and further deposition of H2O leads to an increase of the ice mass. The contrail persists in ice-supersaturated air and may develop into a cirrus cloud. Short-lived and persistent contrails return residual particles into the atmosphere upon evaporation. The scavenging timescales are highly variable and depend on the exhaust and background aerosol size distributions and abundances, as well as on wake mixing rates

Types of Contrails

Short-lived contrails look like short white lines following along behind the plane, disappearing almost as fast as the airplane goes across the sky, perhaps lasting only a few minutes or less. The air that the airplane is passing through is somewhat moist, and there is only a small amount of water vapor available to form a contrail. The ice particles that do form quickly return again to a vapor state.

Persistent (non-spreading) contrails look like long white lines that remain visible after the airplane has disappeared. This shows that the air where the airplane is flying is quite humid, and there is a large amount of water vapor available to form a contrail. Persistent contrails can be further divided into two classes: those that spread and those that don't. Persistent contrails look like long, narrow white pencil-lines across the sky.

Persistent spreading contrails look like long, broad, fuzzy white lines. This is the type most likely to affect climate because they cover a larger area and last longer than short-lived or persistent contrails.

Contrail cousins are things that look like contrails but actually arise from a different physical process. For example, under the right conditions you will see vapor trails form from the wingtips of a jet on takeoff or landing. This phenomenon results from a decrease in pressure and temperature in the wingtip vortex. If conditions are right, liquid water drops form inside the vortex and make it visible. These evaporate very quickly after they form.

Contributing to Climate Change and Ozone Destruction

1 round trip from NY to LA or Trans Atlantic round trip = 2,000 pounds of CO2

In a year air travel releases 600 million tons of carbon dioxide into the atmosphere

NASA Graphic from The TERRA Program

Clouds play a complex role in the Earth's radiation budget. Low Clouds reflect much of the sunlight that falls on them, but have little Effect on the emitted energy. Thus, low clouds act to cool the Current climate. High clouds reflect less energy, but trap more of The energy emitted by the surface.

Aircraft engine emissions affect climate change in three ways that are expected to increase in concern as aviation grows:

From the burning of fossil fuels, aircraft produce about 3 percent of annual global emissions of carbon dioxide (CO2), the most important greenhouse gas. There is good scientific understanding of the impact of these emissions, which is the same as for CO2 at the earth's surface, such as from autos or power plants.
At high altitudes (25,000 to 50,000 feet), nitrogen oxide (NOx) emissions affect the production of ozone and the concentration of methane, both potent greenhouse gases for which a fair scientific understanding has developed.
The third effect results from emissions of aerosol and particulate matter at high altitudes, and can be observed by the apparent increased incidence of cirrus clouds and the persistence of contrails, which influence the radiative character of the atmosphere. There is increasing knowledge about these effects, but poor scientific understanding.

http://asd-www.larc.nasa.gov/GLOBE/science.html

http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgz

Credit: NASA, U.S. National Weather Service, U.S. Air Force