The main purpose is to support NASA’s goal of reducing aviation impacts on the environment. In order to achieve that goal, we need to conduct these experiments to better understand how current and future aviation technology affect engine emissions, their evolution in the atmosphere, and how they influence the formation and microphysical characteristics of condensation trails (contrails) and cirrus clouds.
The Inter-governmental Panel on Climate Change (IPCC) has concluded that climate change is real and that it is caused primarily by human activities. In particular, direct aircraft emissions (aerosols and gases) and aircraft-induced cirrus clouds are identified as two of its many causes. These concerns have motivated a great deal of research over the last 20 years, aimed at gaining an understanding aircraft emissions, characterizing contrail radiative effects, and developing strategies for mitigating aviation environmental effects.
Additionally, many European nations as well as the International Civil Aviation Organization have instituted or will institute greater restrictions on source emissions than those imposed by the US EPA. Because the aviation industry operates on a global scale, US airlines and engine and aircraft manufacturers will have to comply with ICAO and European regulations or lose market share.
Because the fuels are produced from renewable organic carbon feedstocks (e.g. algae or camelina), they produce much lower effective CO2 emissions and thus may mitigate changes in climate caused by build-up of greenhouse gases in the atmosphere. The fuels can be manufactured here in the U.S., which will stimulate job growth and increase fuel security. Because the fuels are simple hydrocarbons and do not contain aromatics and sulfur, they produce much lower soot, organic carbon, and sulfate (i.e. PM) emissions and hence may reduce local air quality impacts.
The airline and Air Force test flights simply demonstrate that aircraft operate normally when burning blends of alternative fuels; nothing has been learned about how the fuels influence engine emissions or contrail formation at altitude. It is impossible to replicate cruise operating conditions in ground tests in open air venues; full engine tests in something like the NASA Propulsion System Laboratory are much more expensive than flight tests.
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Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane and some is added by the exhaust of the aircraft.
In order for the contrail to form, there must be enough moisture in the high levels of the atmosphere for the ice crystals to form around the airplane exhaust. If the upper atmosphere is very dry, contrails will not easily form, or will be short-lived. More helpful information about contrails can be found at http://science-edu.larc.nasa.gov/contrail-edu/science.php.