Small-scale chemical evolution of aircraft exhaust species at cruising altitudes

The chemical transformation of exhaust trace species in aircraft plumes, both directly emitted by the airplane and generated in situ within the first few kilometers behind a single jet engine, is investigated. The chemical processes are strongly coupled to the turbulent dynamics of the hot jet which rapidly mixes with the surrounding atmospheric air. Key issues of the chemical reactions in the jet regime are worked out and the results of extensive parameter studies are presented. The evolution of hydrogen, oxygen, nitrogen, and sulfur species is discussed in detail and related to observations. Spatially resolved, two-dimensional simulations are compared with box model calculations to point out specific features that result from the coupling between mixing and chemistry and to answer the question under which conditions box models can be applied to give a reasonable description of the jet plume chemistry. Main results concern the conversion efficiencies of primary into secondary exhaust products, the oxidation potential within young aircraft plumes to produce nitrous, nitric, and sulfuric acid, the sensitivity of these processes under variation of exit plane conditions, as well as the production of new aerosol particles. Open questions concerning chemical processing in near-field exhaust plumes are addressed. The modeling framework presented in this study serves as a basis for further near-held studies on the physics and chemistry in aircraft wakes.