Contrails and Aviation-cirrus
Chemische omzettingen van vliegtuigemissies in de atmosfeer
Op woensdag 21 november 2001 promoveerde ir. Ernst Meijer aan de TU
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.
In order to understand what is being done, we first have to understand some basic concepts. A jet engine is an internal combustion engine, just like an automobile engine is. In a jet engine, the fuel and an oxidizer combust (or burn) and the products of that combustion are exhausted through a narrow opening at high speed. Modern jet engine fuel is primarily kerosene, the same fuel used to heat homes in portions of the U.S. Kerosene, a flammable hydrocarbon oil, is a fossil fuel. Burning fossil fuels primarily produces carbon dioxide (CO2) and water vapor (H2O). Other major emissions are nitric oxide (NO) and nitrogen oxide (NO2), which together are called NOx, sulfur oxides (SO2), and soot.
While atmospheric scientists are still learning about how aircraft engine emissions affect us and our world, researchers at Glenn are leading the efforts to reduce them at their source.
"We cannot rule out that the observed change in cirrus amount is not due solely to
aviation, or inversely that other climate processes have masked an even larger aviation
impact on cirrus cloudiness. This study does not demonstrate undoubtedly that aviation is
the cause for the observed changes in cirrus cloudiness, but taken with other studies, the
results presented here are consistent with the hypothesis that cirrus cloudiness is
affected by aviation,"
With more than 62 million commercial and military flights weaving trails of jet exhaust across the skies above the United States each year, there's an emerging question on the radar screens of some climatologists.
And the short answer, according to Steven Ackerman, a professor of atmospheric science, is that jet exhaust plumes -- commonly referred to as contrails -- can indeed influence regional climate.
Addressing scientists at a meeting of the American Geophysical Union, Ackerman detailed the results of a year-long study of contrails and their fate in the upper atmosphere.
"Contrails are of concern in climate studies because increased jet traffic may result in an increase in cloud cover," said Ackerman. "It's been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20 percent."
Patrick Minnis, J. Kirk Ayers and Steven P. Weaver, Surface-Based Observations of Contrail Occurrence Frequency Over the U.S., April 1993--April 1994 , NASA RP-1404, December 1997, pp. 83, (4MB).
Keywords: Contrails; Cirrus clouds; Surface observation; Subsonic assessment; Aircraft effects; Climate change
Abstract: Surface observers stationed at 19 U.S. Air Force Bases and Army Air Stations recorded the daytime occurence of contrails and cloud fraction on an hourly basis for the period April 1993 through April 1994. Each observation uses one of four main categories to report contrails as unobserved, non-persistent, and indeterminate. Additional classification includes the co-occurence of cirrus with each report. The data cover much of the continental U.S. including locations near major commercial air routes. The mean annual frequency of occurrence in unobstructed viewing conditions is 13 percent for these sites. Contrail occurrence varied substantially with location and season. Most contrails occurred during the winter months and least during the summer with a pronounced minimum during July. Although nocturnal observations are not available, it appears that the contrails have a dirunal variation that peaks during mid morning over most areas. Contrails were most often observed in areas near major commerical air corridors and least often over areas far removed from the heaviest air traffic. A significant correlation exists between mean contrail frequency and aircraft fuel usage above 7 km suggesting predictive potential for assessing future contrail effects on climate.
H. JAEGER, V. Freudenthaler, F. Homburg, and R. Sussmann