Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
source of HO x is limited to the reaction of O1D with H 2 O and CH 4 . This assumption is made in many (but not all) of the chemical transport models used to assess the
effect of aviation in Section 2.2.1 and Chapter 4. Case 1 includes an additional source of HO x from acetone photolysis (Singh et al., 1995), assuming acetone is
present at a concentration roughly consistent with recent measurements in the UT (Singh et al., 1995; Arnold et al., 1997). Case 2 assumes that in addition to acetone,
peroxides and formaldehyde are transported to the UT by convection (Chatfield and Crutzen, 1984; Jaeglé et al., 1997). It is clear from observations of HO x obtained
from the DC-8 (Brune et al., 1998) and the ER-2 (Wennberg et al., 1998) that HO x sources in addition to O1D + H 2 O are needed to explain measured concentrations of
OH and HO 2 . The effect of these HO x sources is most pronounced in UT air when the water vapor mixing ratio is less than 100 ppmv. At median NO x concentrations
observed during these campaigns-50-100 pptv, typical of the UT (Brunner et al., 1998)-the net ozone production rate is calculated to be 1-2 ppbv per day. This rate is
significantly faster than would be calculated assuming only the simple O1D HO x chemistry.
The sensitivity of the net ozone production rate to assumptions about the sources of odd hydrogen is high and remains an area of significant uncertainty. The budget of
acetone, for example, is poorly understood, and relatively few measurements of its concentration have been made in the UT. Observational constraints on the HOx chemistry of the UT are just now becoming available; the number of measurements is expected to increase greatly over the next few years.
2.1.2.5. The Response of Stratospheric Ozone Destruction to NO x
The concentration of NO x also influences the rate at which ozone is destroyed in the atmosphere, particularly in the stratosphere and in the lower troposphere. In the
stratosphere, because of the abundance of ozone and lower pressures, the concentration of atomic oxygen is sufficiently large that NO x can catalytically destroy
ozone:
O 3 + sunlight O + O 2 (6b)
O + NO 2 NO + O 2 (16)
NO + O 3 NO 2 + O 2 (17)
Net: 2 O 3 3 O 2
Reactions of the HO x family also destroy ozone. In particular, reaction 15 leads to significant ozone loss in the LS and in the troposphere. Reaction 3 is in competition
with reaction 15, so the rate of ozone loss by HO x decreases with increasing NO.
Finally, ozone loss by halogen chemistry is important in the stratosphere. During winter,
particularly in polar regions, it can dominate all other chemical destruction mechanisms. As
with HO x chemistry, NO x interferes with this chemistry by binding to the reactive chlorine
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