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FIGURE 12.6 Total estimated fuel usage at various altitudes for an all-subsonic fleet and for a 2015 fleet that includes a modified subsonic fleet plus 500 Mach 2.4 HSCTs in the year 2015 (adapted from Stolarski et al., 1995).

at altitudes around 20 km, which is the optimum cruise altitude with respect to frictional drag for the HSCT flying at Mach 2.4. The residence time of compounds emitted at this altitude is quite long, of the order of months to years, so that emissions can have substantial effects on ozone and other stratospheric species (Stolarski et al., 1995).

Table 12.1 summarizes the potential effects of HSCT emissions on stratospheric ozone and on global climate through changes in radiative forcing (Stolarski et al., 1995) as discussed in Chapter 14. Since the effect of NOx on 03 is anticipated to be the most important stratospheric issue, we focus on this here. The other issues are treated briefly where appropriate.

Figure 12.7 shows the model-predicted changes in ozone as a function of altitude attributable to a fleet of 500 HSCTs with a so-called emission index of 5. The emission index (EINO ) is the number of grams of NOz emitted per kilogram of fuel burned (although the emissions are primarily in the form of NO, they are expressed as if they were N02). An EIN() of 5 or less is the goal of HSCT engine design. (It should be noted that at very high speeds, NO may produced not only by the engine but also in a thin layer surrounding the aircraft in which the air heats up due to viscous effects at the high speeds. For example, Brooks et al. (1993) show that at speeds of Mach 8 and above, this source could become important and, in fact, at Mach f6, may be about equal to that from combustion in the engine.) While there are clearly significant quantitative differences between the models (due in part to differences in the treatment of transport processes), they all predict ozone loss at altitudes above 24 km. Most models predict ozone increases from f4 to 20 km due to HSCT emissions for reasons to be discussed (Stolarksi et al., 1995).

The chemistry involving NOx is closely intertwined with that of the halogens (C10x and BrOx) and of HOx, so that the predicted effects of a given set of emissions from the HSCT depend on these species as well. Because halogen chemistry is treated in more detail in later sections, we shall focus here primarily on the reasons for the different effects of NOx emissions at different altitudes. How closely these chemistries are intertwined will be apparent in the treatment below of destruction of stratospheric ozone by chlorofluorocar-bons (CFCs) and brominated compounds.

The effect of NOx emissions on stratospheric ozone depends to a great extent on the competition between (If) and (12) for the fate of N02 once it has been formed in the NO + 03 reaction (fO):

Y71 Subsonic fleet

■ Combined subsonic and HSCT (500 M2.4) fleet

TABLE 12.1 Some Potential Effects of a Fleet of 500 HSCTs"

Pollutant

Potential perturbation

Potential atmospheric interactions noa.

Sulfur Soot

Hydrocarbons co co2 h2o

Sulfur

Soot no,

Peak 50-100% increase for an index of 5 g of N02/kg of fuel

Peak 10-20% increase

10-200% increase6 in surface area in sulfate particles Highly uncertain: 0-300% increase

~0.1% increase compared to CH4 5-20% increase

Current subsonic, ~3% of C02 from fossil fuel; HSCT, -1% Peak 10-20% increase

10-200% increase* in surface area of sulfate particles

Highly uncertain: 0-300% increase Peak 50-100% increase

On Ozone

Ozone depletion by NOA. catalysis at higher altitudes;

interference with C10v, HOv, and BrOA catalysis at lower altitudes

Increased HOA formation and hence ozone depletion; interference with NOv catalysis, enhanced C10A catalysis

Increased aerosol surface area, enhanced ozone depletion by C10A, decreased ozone depletion by NOv

Additional nucleation sites for aerosols and surface for catalyzed S02 oxidation to H2S04

Source of CO, HOA, and H20

Modification of catalysis by HOA. and NOA.

Radiative Forcing

Direct change in IR radiative forcing

Direct change in IR radiative forcing; NAT/ice condensation, cirrus cloud formation, change in radiative forcing

Increased aerosol mass loading, change in radiative forcing

Additional nucleation sites for aerosols, increased surface area change in radiative forcing

Ozone depletion, change in radiative forcing

6 Depends on assumptions concerning gas-to-particle conversion in the plume.

Reaction (11) destroys an oxygen atom that could otherwise add to 02 to form 03, reaction (2), and, in addition, generates another NO that will also react with 03 directly. The net cycle is then

O + 03 ^ 202, i.e., ozone loss. This cycle is the major source of NOx destruction of ozone in the middle and upper stratosphere. The reaction of N02 with 03 to form the nitrate radical, followed by its photolysis, can also contribute:

Net: 203 302

On the other hand, if the photolysis of N02, reaction (f2), predominates, there is a cycle formed by reactions (10), (12), and (2):

This cycle alone leads to no net change in 03.

However, an increase in 03 at lower altitudes can result due to interactions with the CIO,, BrOx, and HOx cycles. The reason for this is that at the lower altitudes, these species play a much larger role in determining 03 concentrations than the NOx family does (e.g., Garcia and Solomon, 1994; WMO, 1995). An example of the loss rates for 03 by NOx, (C10x + BrOx), and IIO, from 17 to 21 km calculated for 38°N in May 1993, based on measurements of OH, H02, CIO, NO, and 03, is shown in Fig. 12.8a (Wennberg et al., 1994). Below ~25 km, the major determinants of 03 loss are HOx and halogen chemistry, with NOx playing a much smaller role; as discussed earlier, the major source of 03 is photolysis of 02, reactions (1) and (2) in the Chapman cycle.

The HOx destruction of 03 shown in Fig. f2.8a is due to reactions (6) and (8):

For the conditions under which the measurements in Fig. 12.8a were made, this cycle accounts for about 30-50% of the total 03 removal rate. Increasing water concentrations have been observed in the stratosphere, so that the contribution of the HOx cycle in the upper

10 3

Predicted change in ozone (In units of 10 molecules cm )

FIGURE 12.7 Typical calculated changes in ozone concentrations as a function of altitude for a fleet of 500 HSCTs flying at Mach 2.4. These profiles are for 45°N latitude for the month of March using three different models (CAMED, CSIRO, and LLNL). Emissions of NOa. are assumed to be equivalent to 5 g of N02/kg of fuel burned (from Stolarski et al., 1995).

10 3

Predicted change in ozone (In units of 10 molecules cm )

FIGURE 12.7 Typical calculated changes in ozone concentrations as a function of altitude for a fleet of 500 HSCTs flying at Mach 2.4. These profiles are for 45°N latitude for the month of March using three different models (CAMED, CSIRO, and LLNL). Emissions of NOa. are assumed to be equivalent to 5 g of N02/kg of fuel burned (from Stolarski et al., 1995).

stratosphere particularly may be increasing (Evans et al., 1998).

Chlorine atoms produced by the photolysis of CFCs, and bromine atoms from halons, also destroy 03 by cycles that are discussed later. As seen in Fig. 12.8a, these cycles are similar in magnitude to the HOx cycle in terms of 03 removal.

Because NOx plays such a small role in the removal of 03 in the lower stratosphere, its effects in this region arise primarily because of its interactions with the halogen and HO, cycles that do control 03 loss here. Thus N02 interferes with the C!Ox and BrOx cycles by forming chlorine and bromine nitrate, C10N02 and Br0N02, respectively:

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