Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
fraction of emitted NO x is converted to other reactive nitrogen (NO y ) species in the plume dispersion region during the daylight (Karol et al., 1997; Meijer et al., 1997;
Petry et al., 1998). Observations of NO y in aircraft plume compositions are consistent with these results (Schlager et al., 1997).
2.1.1.3. Atmospheric Ozone
Most interactions between ambient ozone and ozone-controlling gases and aircraft exhaust occur in the days and weeks following emission. Dispersion of exhaust on
regional and global scales is dictated by the same large-scale atmospheric dynamic processes that control mixing of other natural and anthropogenic sources of gases
and particles. During that time, aircraft-derived gases and particles participate in the natural chemical cycles that control ambient ozone levels. The following
subsections provide an overview of ozone chemistry.
2.1.1.3.1. Stratospheric ozone
Approximately 80% of atmospheric ozone resides in the stratosphere, where it is produced via in situ photochemistry occurring predominantly in the tropical middle
stratosphere, albeit with significant contributions from mid-latitudes. Stratospheric circulation patterns transport ozone from the tropical stratosphere poleward and then
downward from the mid-stratosphere predominantly in the winter hemisphere. Stratospheric ozone is not only transported but also destroyed via photochemical
reactions over the whole stratosphere. In addition, about 7-25% of the total ozone mass (WMO-UNEP, 1985; Wauben et al., 1998) is transported to the extratropical
troposphere; this type of transport occurs most intensively in the winter and in the Northern Hemisphere. Different transport modes correspond to different time scales,
ranging from days to years.
Ozone formation and destruction rates increase with height and change with latitude in the stratosphere. Consequently, ozone "lifetime" decreases with height from
about a year in the LS to minutes in the upper stratosphere. At the uppermost altitudes, ozone lifetime is sufficiently short that its abundance is in local photochemical
equilibrium (WMO-UNEP, 1985).
At lower altitudes, ozone is not in photochemical steady-state, and ozone transport by air motions of various scales becomes increasingly important. The primary
mechanism for mean global stratospheric transport is referred to as the Brewer-Dobson circulation, with rising motion in the equatorial belt of the LS and air mass
spreading to the poles in the middle and upper stratosphere, with more intensive transport into the winter hemisphere.
In summary, stratospheric ozone distributions are determined mainly by atmospheric motions in the nightime polar regions, by a mixture of transport and
photochemistry in the lower and middle stratosphere, and by photochemistry in the upper stratosphere.
2.1.1.3.2. Tropospheric ozone
Sources of ozone in the troposphere are more numerous than in the stratosphere, as are the photochemical reactions participating in ozone production and loss.
Although in situ photochemistry is the dominant source of tropospheric ozone, downward flux of stratospheric ozone represents a significant source, especially in the
UT and in winter over high latitudes. Removal of tropospheric ozone occurs predominantly by photochemistry, with some contribution from surface deposition.
The lifetime of ozone in the troposphere varies with latitude and altitude; in general it is of the order of 1 month, a value that is smaller than the time scale for transport
between the Northern and Southern Hemisphere troposphere, which is typically about 1 year (WMO-UNEP, 1985).
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