Visibility Reduction due to Jet-Exhaust Carbon Particles

James E. McDonald Institute of Atmospheric Physics, The University of Arizona, Tucson, Ariz.

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Abstract

Pyrolysis of hydrocarbon fuels leads to emission of free carbon in the exhaust of aircraft turbojet engines, visible as a faint dark trail. Carbon formation rises markedly when water injection is employed to augment thrust by 20–30 per cent in takeoffs under heavy loads or at high temperatures, the enhanced pyrolysis resulting from the lower combustion efficiency on such “wet takeoffs.” During the takeoff run, while the aircraft is still moving at low speeds but with maximum thrust, dense dark smoke fills the exhaust wake, reducing visual ranges to as little as a few hundred feet. To crow-check recent measurements indicating carbon particulate emissions of the order of 15 lb per ton of fuel consumed in wet takeoff, Mie extinction coefficients have been computed for carbon particles of the size known to form as a sequel to pyrolytic freeing of carbon. These are used to make theoretical estimates of the maximum visual range to be expected if the carbon loading measurements were correct. A discrepancy costs in the sense that the predicted visual ranges are found to be some five times larger than the observed. It is concluded that a large fraction of total carbon emission leaves the tailpipe still unaggregated into large soot particles, whence the reported carbon loadings may be too low by a factor of as much as five. Consequently the aircraft operational hazards as well as the air pollution problems implicit in rising volumes of jet traffic at certain terminals may become rather more serious than has been predicted.

Abstract

Pyrolysis of hydrocarbon fuels leads to emission of free carbon in the exhaust of aircraft turbojet engines, visible as a faint dark trail. Carbon formation rises markedly when water injection is employed to augment thrust by 20–30 per cent in takeoffs under heavy loads or at high temperatures, the enhanced pyrolysis resulting from the lower combustion efficiency on such “wet takeoffs.” During the takeoff run, while the aircraft is still moving at low speeds but with maximum thrust, dense dark smoke fills the exhaust wake, reducing visual ranges to as little as a few hundred feet. To crow-check recent measurements indicating carbon particulate emissions of the order of 15 lb per ton of fuel consumed in wet takeoff, Mie extinction coefficients have been computed for carbon particles of the size known to form as a sequel to pyrolytic freeing of carbon. These are used to make theoretical estimates of the maximum visual range to be expected if the carbon loading measurements were correct. A discrepancy costs in the sense that the predicted visual ranges are found to be some five times larger than the observed. It is concluded that a large fraction of total carbon emission leaves the tailpipe still unaggregated into large soot particles, whence the reported carbon loadings may be too low by a factor of as much as five. Consequently the aircraft operational hazards as well as the air pollution problems implicit in rising volumes of jet traffic at certain terminals may become rather more serious than has been predicted.

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