Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
modeled middle stratosphere photochemistry. However, confident predictions of stratospheric effects of future aviation will require resolution of discrepancies between
modeled and observed transport tracers.
2.3.2. Uncertainties in Observing Aviation Impacts
The data set resulting from ozonesondes is the only useful one for ozone trend analysis in the UT and LS. The error of an individual ozonesonde measurement has
been evaluated to be ~5% in the LS, based on several intercomparison campaigns (WMO, 1998). The error is larger in the UT, where ozone densities, hence
instrument signals, are substantially smaller. In addition, the background signals (i.e., dark current) of the sonde sensors have been checked relatively infrequently
during the measurement period, giving rise to further measurement uncertainty. If the measurement error is random, one can improve the statistical significance of
observed trends by increasing the observation frequency. Ozone densities vary greatly on time scales of days in the UT and LS, particularly in middle and high
latitudes during the winter and spring. The cause of this variability is believed to be related to active dynamic transport associated with weather disturbances. Because
the variability is largely random, it can be treated, to first order, as noise in the trend data. The variability is considered to be of the same order of magnitude (or larger)
as noise from instrument measurement errors. The frequency of ozonesonde observation-once a week at most stations-is not enough to document these variations
properly.
Long-term trends of external forcings other than aircraft greatly complicate analysis of ozone trends. The long-term variation of atmospheric chlorine loading is
relatively well-documented, allowing for the construction of credible models to predict stratospheric ozone depletion. However, changes in gases important in UT
photochemistry-such as NO x , oxygenated hydrocarbons, and water vapor-are much less well characterized. Feedbacks on tropospheric gases from climatic changes
(e.g., greenhouse warming) may also have an impact on ozone in the UT and LS, but even the sign of this effect on ozone levels is uncertain.
In summary, because the database for ozone observations in the UT and LS is still relatively limited and because uncertainties in observational data, as well as model
representations of non-aircraft ozone forcing phenomena, are quite large, it is presently impossible to associate a trend in ozone to aircraft operation with meaningful
statistical significance.
2.4. Conclusions and Overall Assessment of Present Aviation Impacts on Ozone
Currently, there is no experimental evidence for a large geographical effect of aircraft emissions on ozone anywhere in the troposphere. Furthermore, the only
evidence for an effect on NO x -the major ozone precursor in aircraft emissions anywhere outside the immediate vicinity (i.e., a few miles) of a jet engine's exhaust-has
been obtained during a stagnant meterological condition when exhaust products built up over several days. Nevertheless, our understanding of UT/LS chemical and
dynamical processes continues to improve and has progressed to a point where one can predict with some confidence the cruise-level effects of aviation.
Based on our current overall understanding of UT and LS processes, we are confident that NOx emissions from present subsonic aircraft lead to increased NOx and
ozone concentrations at cruise altitudes, especially in air traffic corridors between and over Northern Hemisphere continents and at altitudes of 9-13 km. Based on the
relatively large number of tropospheric model calculations, we are reasonably confident that tropospheric ozone increases from aircraft NOx have been on the order of
8 ppb, equivalent to 6% of the ozone density in the principal traffic areas.
Model studies, which have internal uncertainties associated with process parameterization and external uncertainties associated with the strengths of other very large
NO x sources, have returned effects as low as 2% of the ozone density in high-traffic areas and as high as 14% in those areas. One of the major current limitations to
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