Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
showed a positive O 3 change slope. In general, the model-derived O 3 change is in better agreement among participating models at EI(NO x )=5, with a spread among
models from approximately 0% total column O 3 depletion (UNIVAQ) down to -0.4% (GSFC and SLIMCAT). At EI(NO x )=15, the overall spread in model-derived total
column O 3 change is much larger (+0.4% to -0.9%).
There is also a large spread in interhemispheric gradients in model-derived total column O 3 change (see Tables 4-11 and 4-12). For example, when the emission
scenario is S1c [EI(NO x )=5 with 500 supersonic aircraft], the AER, SCTM1, and CSIRO models derived a larger depletion in total column O 3 in the Northern
Hemisphere than in the Southern Hemisphere. The UNIVAQ model had very little change in column O 3 in either hemisphere. The LLNL and THINAIR models derived
a similar change in total column O 3 in both hemispheres. However, the ratio of Southern Hemisphere to Northern Hemisphere change in total column O 3 is greater
than 1.5 in the GSFC, LARC, and SLIMCAT models. This larger depletion in the Southern Hemisphere correlates with increased H 2 O and NOy being transported to
the Southern Hemisphere and the sensitivity of cold aerosol processes implemented within the GSFC, LARC, and SLIMCAT models.
4.3.3.4. Ambient Surface Area Density Sensitivity
Volcanic activity within the past several decades has greatly modulated ambient SAD. Two major volcanic eruptions-El Chichón (April 1982) and Mt. Pinatubo (June
1991)-are responsible for most of the observed SAD change. Satellite instruments (SAM, SAGE I, SAGE II, and SME; e.g., Thomason et al., 1997) have shown that
average SAD in the lower stratosphere between 1979 and 1995 was greater than that during volcanically clean periods (SA0; WMO, 1992) by a factor of 8 in the
equatorial region and a factor of 2 to 4 at higher latitudes. These enhancements decay with e-folding time scales on the order of 1 to 2 years (Yue et al., 1991;
Thomason et al., 1997).
For this assessment, we examined the impact of supersonic aircraft at a SAD of four times the volcanically clean condition (4xSA0). Results from these higher SAD
sensitivity studies are shown in Figure 4-7b. Unlike the volcanically clean ambient atmosphere (Figure 4-7a), when the EI(NO x ) varies between 0 and 15 (S3a-d), all
participating models derived a positive slope to the Northern Hemispherical change in total column O 3 . All models show a strong interference of the H 2 O only [EI(NO x )
=0] total O 3 depletion when NO x emissions are included [EI(NO x )=5]. This response is consistent with the idea that at higher ambient SAD, NO x catalytic processes
are less important, mainly as a result of conversion of active NO x radicals to nitric acid by heterogeneous reactions on sulfate aerosols. As supersonic aircraft NO x is
enhanced, the additional NO x interferes with ClO x , BrO x , and HO x odd-oxygen loss processes, reducing the net total odd-oxygen loss.
It should be noted that the model-derived impact on total column O 3 from increasing ambient SAD by natural causes (approximately 2-3% decreases in Northern
Hemisphere mid-latitude total column O 3 change; Solomon et al., 1996) exceeds the model-derived impact from supersonic aircraft (typically less than 1% Northern
Hemisphere mid-latitude total column O 3 change).
4.3.3.5. Sulfate Aerosols and Polar Stratospheric Cloud Sensitivities
Heterogeneous reactions are important in defining the O 3 removal rate in the stratosphere. These reactions occur on sulfate aerosols and PSC particles, though
sulfate aerosols have the greater influence because of their global distribution. New sulfate aerosols are produced by aircraft emissions; sulfate particles are generated
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