Using biofuels to help power jet engines can reduce particle emissions in their exhaust by as much as 50 to 70 per cent, according to a new NASA study that bodes well for airline economics and Earth's environment. "We show that, compared to using conventional fuels, biofuel blending reduces particle number and mass emissions immediately behind the aircraft by 50 to 70 per cent," researchers said. The observations quantify the impact of biofuel blending on aerosol emissions at cruise conditions and provide key microphysical parameters, which will be useful to assess the potential of biofuel use in aviation as a viable strategy to mitigate climate change, they said. During flight tests in 2013 and 2014 near NASA's Armstrong Flight Research Centre in California, data was collected on the effects of alternative fuels on engine performance, emissions and aircraft-generated contrails at altitudes flown by commercial airliners. Contrails are produced by hot aircraft engine exhaust mixing with the cold air that is typical at cruise altitudes several miles above Earth's surface, and are composed primarily of water in the form of ice crystals. Researchers are most interested in persistent contrails because they create long-lasting, and sometimes extensive, clouds that would not normally form in the atmosphere, and are believed to be a factor in influencing Earth's environment. "Soot emissions also are a major driver of contrail properties and their formation," said Bruce Anderson, project scientist at NASA's Langley Research Centre. "As a result, the observed particle reductions we have measured during ACCESS should directly translate into reduced ice crystal concentrations in contrails, which in turn should help minimise their impact on Earth's environment," said Anderson. That is important because contrails, and the cirrus clouds that evolve from them, have a larger impact on Earth's atmosphere than all the aviation-related carbon dioxide emissions since the first powered flight by the Wright brothers, NASA said. The tests involved flying NASA's workhorse DC-8 as high as 40,000 feet while its four engines burned a 50-50 blend of aviation fuel and a renewable alternative fuel of hydro processed esters and fatty acids produced from camelina plant oil. A trio of research aircraft took turns flying behind the DC-8 at distances ranging from 300 feet to more than 32 kilometres to take measurements on emissions and study contrail formation as the different fuels were burned. "This was the first time we have quantified the amount of soot particles emitted by jet engines while burning a 50-50 blend of biofuel in flight," said Rich Moore, lead author of the study published in the journal Nature.
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