NASA has been studying various types of emissions from commercial aircraft to develop ways to reduce emissions and protect the environment. In recent years, fine particle emissions from aircraft have been identified as possible contributors to global climate changes and to lowering local air quality. These emissions are produced when a hydrocarbon fuel (such as modern jet fuel, which is primarily kerosene) does not burn completely. Incomplete combustion often occurs at the lower power settings used for aircraft descent, idling and taxiing. This produces fine carbon particles, or soot, as well as particles of nonvolatile organic compounds.
Using the NASA's DC-8 with CFM-56 engines, the team ran tests to investigate the effects of thrust and fuel type on particle emissions. Different engine operating settings were used to vary thrust, and three different fuels were used: a typical jet fuel, a fuel with high sulfur content, and a fuel with high aromatic compound content. In addition, the Environmental Protection Agency ran tests to simulate landing-takeoff cycles to study the emissions that would be created at an airport.
APEX1 took place in April 2004 in Palmdale, California.