Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Figure 7-28: The ICAO landing and take-off cycle (LTO).
7.6.4.2. Secondary Combustion Products (NO, NO 2, N 2O, SO 2, CO, stable HC)
Secondary products-such as NO, NO 2 , and SO 2 , as well as their oxidative products SO 3 , HONO, HNO 3 , and H 2 SO 4 -formed via reactions initiated with the reactive
radicals OH and O are the principal participants of interest in chemical and microphysical processes occurring soon after emission. Although OH and O are reduced
considerably by the engine exit, they continue to play an important role in global atmospheric processes (see Chapters 2 and 3). To understand the processes
occurring through the engine, relative and absolute levels of these secondary combustion products, their oxidative products (the acid gases), and the reactive radicals
need to be accurately characterized. Emissions indices for NO x , CO, and HC, as measured by ICAO procedures for stages in a standard LTO cycle, are documented
(ICAO, 1995b) for most in-use engines as part of the engine certification process; these emissions typically correspond to tens to hundreds of ppmv. SO x O emissions
are directly proportional to the level of sulfur in the fuel [a 400 ppmm fuel S level corresponds to an EI(SO x O) of 0.8]. Emissions of metals, whether from impurities in
the fuel or engine wear, are much smaller than the emissions discussed here but may be of interest in soot activation and condensation processes (Chen et al., 1998;
Twohy et al., 1998). NO x does not change significantly through the turbine and nozzle other than through changes resulting from dilution, although the NO 2 /NO ratio
may shift as a result of increased oxidation. Oxidation of NO and NO 2 to HONO and HNO 3 , respectively, is predicted to be on the order of a few percent or less,
occurring largely in the high-pressure turbine (Fahey et al., 1995a; Anderson et al., 1996; Lukachko et al., 1998). Although this change in the NO x level is not
significant, changes in HONO and HNO 3 represent important changes in trace species of NOy (see below). Ground-based and in-flight measurements indicate that
emissions of N 2 O are also small relative to NO x (Kleffmann et al., 1994; Fahey et al., 1995b). Further validation of NO x chemistry is warranted, but indications are that
current models can predict NO x evolution in the turbine and nozzle with sufficient accuracy for assessment needs (Dryer et al., 1993), and measurements of NO x with
a few percent accuracy are possible.
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