Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
radical ClO:
ClO + NO 2 + M ClONO 2 + M. (18)
As a result of these coupling reactions, changes in the concentration of NO x can lead to
increased or decreased rates of stratospheric ozone destruction. When NO x is low-as it is in
most of the LS during winter, fall, and spring-most of the ozone loss occurs through HOx and halogen chemistry. Under these conditions, enhancements of NOx will decrease ozone
destruction. On the other hand, at higher altitudes and during summer, NO x -catalyzed
ozone loss (reactions 16-17) can dominate the removal of lower stratospheric ozone, so
enhancements in NO x will speed ozone loss (Brühl et al., 1998). These effects have been
demonstrated by direct measurements of free radicals in the stratosphere (Wennberg et al.,
1994; Jucks et al., 1997).
This chemistry is illustrated in Figure 2-2. A calculation is shown for typical mid-latitude
springtime conditions. This entire profile is within the stratosphere, where catalytic ozone
loss competes with and can exceed photochemical production. The left panel shows the
fraction of ozone destroyed during the month of March as a result of catalysis by NO x
Figure 2-2: A calculation of the rate of ozone loss in the lower
stratosphere for springtime mid-latitude conditions during March.
(squares), halogens (circles), and HO x (crosses). For this latitude and season, the loss is dominated by halogen and hydrogen oxides below 20 km, whereas above 25
km, nitrogen oxides are most important. To illustrate how changes in NO x perturb this chemistry, the right panel shows the effect of a uniform 20% increase in the
concentration of NOx . In regions where NOx is high, ozone destruction increases. On the other hand, the opposite occurs in the LS because the increased NOx decreases the loss of ozone by hydrogen and halogen radicals. Thus, as with the production rate of ozone in the troposphere, the response of ozone destruction with
changes in NOx is highly nonlinear. Because the photochemical lifetime of ozone in the LS is very long, the concentration of ozone in this region of the atmosphere is
strongly influenced by transport. The change in ozone loss rates illustrated in Figure 2-2 does not translate directly into a change in ozone. For example, for a uniform
20% increase in NO x , enhanced loss rates at high altitudes will reduce the transport of ozone to the LS. As a result, ozone concentrations in the LS can decrease even
when the local ozone loss rate slows. Thus, the change in the ozone column with added NO x is very sensitive to the altitude distribution of the perturbation. The
subsonic aircraft fleet adds NO x only to the lowermost stratosphere (< 13 km), where large-scale dynamics tend to prevent advection to higher altitude. As a result,
injection of NO x by the present fleet is thought to increase ozone in the LS.
2.1.2.6. Net Effects on Ozone
If the major direct impact of aircraft on the chemistry of the UT and lowermost stratosphere (below approximately 16 km) is an increase in the concentration of NO x , we
can say with high confidence that the ozone concentrations in this region will be higher than they would be in the absence of aviation. This increase occurs because
http://www.ipcc.ch/ipccreports/sres/aviation/023.htm (7 von 8)08.05.2008 02:41:39