“Contrails might be a punch line in the culture these days, thanks to the imaginative folks who have rechristened them “chemtrails” and embroidered them with elaborate theories involving government and corporate misdeed. But contrails are pretty serious business for a less conspiratorial reason: scientists believe these ice clouds generated by water exhaust gases from aircraft engines could have a real impact on the climate, perhaps by cooling temperatures during the day and warming them at night. When contrails spread to form cirrus clouds, they can persist for hours and extend over areas of several square kilometers. These “contrail cirrus,” which artificially increase Earth’s cloudiness and become almost indistinguishable from natural cirrus, are among the most uncertain contributors to the Earth’s radiative forcing. (Radiative forcing is defined as the change of the net radiating flux resulting from changes in the atmospheric composition. A measure of the perturbation of Earth-atmosphere energy budgets, it is widely used as a climate metric).” 
Although the emission performance of gas-turbine engines burning renewable aviation fuels have been thoroughly documented in recent ground-based studies, there is still great uncertainty regarding how the fuels effect aircraft exhaust composition and contrail formation at cruise altitudes. To fill this information gap, the NASA Aeronautics Research Mission Directorate sponsored the ACCESS flight series to make detailed measurements of trace gases, aerosols and ice particles in the near-field behind the NASA DC-8 aircraft as it burned either standard petroleum-based fuel of varying sulfur content or a 50:50 blend of standard fuel and a hydro-treated esters and fatty acid (HEFA) jet fuel produced from camelina plant oil. ACCESS 1, conducted in spring 2013 near Palmdale CA, focused on refining flight plans and sampling techniques and used the instrumented NASA Langley HU-25 aircraft to document DC-8 emissions and contrails on five separate flights of approx.2 hour duration. ACCESS 2, conducted from Palmdale in May 2014, engaged partners from the Deutsches Zentrum fuer Luft- und Raumfahrt (DLR) and National Research Council-Canada to provide additional scientific expertise and sampling aircraft (Falcon 20 and CT-133, respectively) with more extensive trace gas, particle, or air motion measurement capability. Eight, muliti-aircraft research flights of 2 to 4 hour duration were conducted to document the emissions and contrail properties of the DC-8 as it 1) burned low sulfur Jet A, high sulfur Jet A or low sulfur Jet A/HEFA blend, 2) flew at altitudes between 6 and 11 km, and 3) operated its engines at three different fuel flow rates. This presentation further describes the ACCESS flight experiments, examines fuel type and thrust setting impacts on engine emissions, and compares cruise-altitude observations with similar data acquired in ground tests. 
Three different fuel types are discussed: a low-sulfur JP-8 fuel, a 50:50 blend of JP-8 and a camelina-based HEFA fuel, and the JP-8 fuel doped with sulfur. 
Contrail cirrus properties can be influenced by changing aircraft emissions, either by introducing a next generation of cleaner aircraft engines or by using alternative fuels. Box model studies as well as ground and flight measurements have shown that the emitted soot particle number is significantly reduced by the use of alternative fuels such as Fischer-Tropsch (FT) fuels based on coal or natural gas or biofuels like hydroprocessed esters and fatty acids. Measurements of contrails using a 50:50 blend of FT and conventional kerosene (ACCESS and ECLIF-I) indicate a reduction in in-flight soot number emissions by 50%–70% and in the initial ice crystal number by around 50%. There are several possibilities to reduce soot number emissions further, for example, using pure alternative fuels or a blend with a lower aromatics content, using bio kerosene together with liquid natural gas/liquid hydrogen, or improving engine designs like going toward lean combustion.. 
Contrails during day cause cooling because of reflecting of sunlight back into space. During night, they trap infrared heat causing heating. So it is a balance between the two time intervals. We would like to have more CICs (contrail-induced cirrus clouds) during day and none during night. 
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