Subsonic aircraft emissions represent the most quantitatively known direct source of NOt in the upper troposphere. Aircraft engines use an extremely efficient, high temperature combustion process that primarily produces C02, H20, and a few percent of other compounds. Like lightning, the initial NOy content of these emissions consists primarily (>85%) of NO. Estimates of NOx production are derived from assessments of the emission indices for various engines under different flight conditions and the annual amount of air traffic in terms of the kilograms of fuel consumed. The total strength of this source for both scheduled commercial and nonscheduled (e.g., military and chartered) air traffic in 1992 has been estimated at approximately 0.46 TgN/yr for all altitudes with approximately 65% of the emissions occurring in the upper troposphere (>8km), and, of that, approximately 45% has been assessed as occurring between 20° and 45°N with another 36% at latitudes further north (i.e., 55% of total emissions were at altitudes >8 km and latitudes > 20°N). This source of NOc has also been projected to increase to approximately 1.3 TgN/yr by the year 2015 (NASA, 1991). Because about 90% of the emissions occur in the free troposphere over the Northern Hemisphere, the impact from this source on the budgets and distributions of NOx and ozone should be substantially different than in the Southern Hemisphere. In addition, this source may have a larger relative impact on upper tropospheric, winter time Northern Hemispheric NOt distributions. This reflects the fact that during this period lightning and convection of surface NOx emissions are both significantly reduced.
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