Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
ozone to test the physics and chemistry parameterized in these global models and identify areas of remaining uncertainty.
2.1.1.1. Aircraft Engine Emissions
Most present-day jet aircraft cruise in an altitude range (9-13 km) that contains portions of the UT and LS. Because these two atmospheric regions are characterized
by different dynamics and photochemistry, the placement of aircraft exhaust into these regions must be considered when evaluating the impact of exhaust species on
atmospheric ozone. Determination of the partitioning of exhaust into the two atmospheric regions is complicated by the highly variable and latitudinally dependent
character of the tropopause (i.e., the transition between the stratosphere and troposphere). Comparisons of aircraft cruise altitudes with mean tropopause heights has
led to estimates for stratospheric release of 20-40% of total emissions (Hoinka et al., 1993; Baughcum, 1996; Schumann, 1997; Gettleman and Baughcum, 1999).
Carbon dioxide CO2 ) and water vapor (H2O) are easily the most abundant products of jet fuel combustion (emission indices for CO2 and H2O are 3.15 kg/kg fuel
burned and 1.26 kg/kg fuel, respectively). However, both species have significant natural background levels in the UT and the LS (Schumann, 1994; WMO-UNEP,
1995). and neither current aircraft emission rates nor likely future subsonic emission rates will affect the ambient levels by more than a few percent. Future supersonic
aviation, on the other hand (which would emit at higher altitudes), could perturb ambient H2O levels significantly at cruise altitudes. Regardless of the magnitude of the
aircraft source, CO 2 does not participate directly in ozone photochemistry because of its thermodynamic and photochemical stability. It may participate indirectly by
affecting stratospheric cooling, which can in turn lead to changes in atmospheric thermal stratification, increased polar stratospheric cloud (PSC) formation, and
reduced ozone concentrations.
Aircraft water contributions, although relatively small in the troposphere, lead to the atmospheric phenomenon of contrail formation. Depending on the precise
composition of contrail particles-which is largely determined by the specific processes occurring in the aircraft plume and by the ambient atmosphere composition and
temperature-the particles may act as surfaces for a variety of heterogeneous reactions (Kärcher et al., 1995; Louisnard et al., 1995; WMO-UNEP, 1995; Schumann et
al., 1996; Danilin et al., 1997; Kärcher, 1997; Karol et al., 1997). The participation of contrails in atmospheric photochemistry is further addressed in Section 2.1.3.
NO x constitutes the next most abundant engine emission (emission indices range from 5 to 25 g of NO 2 per kg of fuel burned) (Fahey et al., 1995; WMO-UNEP, 1995;
Schulte and Schlager, 1996; Schulte et al., 1997). With respect to ozone photochemistry, NO x is the most important and most studied component; its aircraft emission
rates are sufficient to affect background levels in the UT and LS. Moreover, its active role in ozone photochemistry in the UT and LS has been well recognized (WMO-
UNEP, 1985, 1995). A great deal of the recent scientific literature has focused on aircraft NO x effects, and this chapter neccessarily reflects that focus. Aircraft carbon
moNO x ide (CO) emissions are of the same order of magnitude as NO x emissions (i.e., 1-2 g kg -1 for the Concorde aircraft and 1-10 g kg -1 for subsonic aircraft)
(Baughcum et al., 1996). Like NO x , CO is a key participant in tropospheric ozone production. However, natural and non-aircraft anthropogenic sources of CO are
substantially larger than analogous NOx sources, thereby reducing the role of aircraft CO emissions in ozone photochemistry to a level far below that of aircraft NOx emissions (WMO-UNEP, 1995).
Emissions of sulfur dioxide (SO2 ) and hydrocarbons from aircraft, at less than 1 g kg-1 fuel, are significantly less than the more prominent exhaust species discussed
above (Spicer et al., 1994; Slemr et al., 1998). Their primary potential impacts are related to formation of sulfate and carbonaceous aerosols that may serve as sites for
heterogeneous chemistry. This possibility is discussed in Section 2.1.3. Non-methane hydrocarbon (NMHC) emissions may also contribute to autocatalytic production
of HOx , provided that the reactivity of the NHMCs is sufficiently large relative to that of CH4 to overcome their numerical inferiority. However, model studies have
indicated that volatile organic emissions from aircraft have an insignificant impact on atmospheric ozone at cruise altitudes (Hayman and Markiewicz, 1996; Pleijel,
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