Reactive Chemistry in Aircraft Exhaust

Abstract

Exhaust emitted from jet engines contains high concentrations of combustion by-products, some of which are damaging to human and ecosystem health. As these pollutants mix with the surrounding air, they undergo chemical reactions that eventually break them down to their water-soluble or inert forms. To date, the reactivity of aircraft exhaust has been largely unexplored, and it is not known whether state-of-the-art models account for chemistry at the plume level, which directly relates to air quality downwind of airports. To a large extent, the concentrations of hydroxyl (OH) and hydroperoxyl (HO2) radicals—collectively called HOx—present in emitted plumes determine the rate at which components of the exhaust are oxidized. In January 2009, the authors quantified emissions of all HOx precursors including nitrous acid (HONO), formaldehyde (HCHO), acetaldehyde (CH3CHO), and ozone (O3) at the Alternative Aviation Fuels Experiment in Palmdale, California. The results indicate that (a) HOx production rate due to direct emission of these precursors is orders of magnitude faster in the exhaust plume than in “normal” urban air; (b) the concentration of pollutants in plumes does not reach typical ambient levels until it has been diluted by a factor of about 6,000; and (c) photolysis of HONO in these plumes is by far the biggest source of HOx during daytime. Analyses of the reactions involving HOx demonstrate that propagation of these radicals is favored over termination, which indicates that chemical reactivity will continue to be enhanced in these plumes even after being diluted to ambient levels.