Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
1998).
2.1.1.2. Plume and Wake Processing of Engine Emissions
Although jet exhaust spends a relatively short time in the immediate vicinity behind the aircraft, a number of important processes occur during that time that influence
exhaust gas and aerosol composition, hence the ozone-forming or ozone-depleting potential of the exhaust. The near-field evolution of jet aircraft exhaust wake can be
divided into three distinct regimes-commonly termed jet, vortex, and plume dispersion. The time scales associated with these regimes are 0-10 s for the jet, 10-100 s
for the vortex, and 100 s to tens of hours for plume dispersion-the latter time period defining the effective "lifetime" of the aircraft plume. The jet and vortex regimes are
closely related; they are initiated at the exit plane of the engine nozzle and ended by atmospheric shear forces at distances of approximately 10-20 km behind the
aircraft (Hoshizaki, 1975; Schumann, 1994).
Several fluid dynamic models are now available to study wake dynamics-namely, two-dimensional (2-D) jet mixing codes (Miake-Lye et al., 1993; Beier and Schreier,
1994; Kärcher, 1994; Garnier et al., 1996) and codes that capture the jet/vortex interaction and vortex break-up (Quackenbush et al., 1993; Lewellen and Lewellen,
1996; Schilling et al., 1996), some of them using vortex filament methods combined with large eddy simulations (LES) (Gerz and Ehret, 1997).
The small spatial and temporal scales of exhaust species distributions in near-field wakes hamper a robust comparison of model simulations with in situ observations
of exhaust effluents. Nevertheless, the dynamic models have been successful in explaining the few observations of near-field tracer concentration, temperature, and
humidity (Anderson et al., 1996; Garnier et al., 1996; Gerz and Ehret, 1997; Gerz and Kärcher, 1997). The data and calculations reveal a strong suppression of plume
mixing and dispersion during the vortex regime. Vortex systems are composed of cylindrical core regions, not well mixed radially and entraining only small amounts of
ambient air. As a result, vortex plume temperatures and associated H 2 O concentrations may be well defined from fluid dynamic simulations and known emission
indices. Within the vortex, high concentrations of exhaust species interact with each other and with small amounts of ambient gases and particles over a range of
temperatures that differ from those in the background atmosphere. It is likely that some of the chemical interactions occurring in the vortex regime will influence the
eventual composition of aircraft-derived aerosol particles and gases.
The plume dispersion regime begins after disintegration of the wake vortex and extends to an area where the primary exhaust gas concentrations (i.e., NOx , H2O, CO,
CO2 ) are of the same order of magnitude as the corresponding ambient background levels. Results from modeling and observational studies of aged plumes (Karol et
al., 1997; Meijer et al., 1997; Schlager et al., 1997; Schumann et al., 1998) show that most plumes mix with the background atmosphere according to a simple dilution
law that can be approximated with a Gaussian plume model that includes estimated and measured atmospheric shear and diffusion parameters (Konopka, 1995;
Schumann et al., 1995; Durbeck and Gerz, 1996). The key observables for these models have been ice particles in visible contrails and measured CO2 that serve as
tracers of the plume mixing process. All of the studies have indicated that during the 10-20 hrs of plume dispersion, the plume cross-section may grow to 50-100 km in
width and 0.3-1.0 km in height, with a corresponding exhaust species dilution ratio (R-the ratio of the plume mass to fuel mass) up to 10 8 as a result of ambient air
entrainment. From analysis of more than 70 aircraft plume crossings by research aircraft in the North Atlantic flight corridor, Schumann et al. (1998) proposed that R
can be approximated by R=7000 (t/t 0 )0.8, where t 0 = 1s for 0.006 < t < 10 4 s. The relative rate, (dR/dt)/R, of ambient air entrainment into the plume is on the order of
10 -3 s -1 in the first minutes of plume dispersion but decreases to on the order of 10 -4 s -1 over a 1-2 hr period (Durbeck and Gerz, 1996).
In the plume dispersion stage, aircraft-derived gas and particle concentrations are still highly elevated over background levels, but they interact with large volumes of
ambient species under temperature and pressure conditions of the background atmosphere. The composition and reactive characteristics of aircraft-derived particles
fully evolve in the vortex and plume dispersion regions as a result of aerosol-precursor photochemistry and particle condensation, coagulation, and agglomeration
processes. These particle-forming processes are described in further detail in Chapter 3. In addition, chemistry process model calculations indicate that a significant
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