The ozone produced there is too high to threaten health directly. Instead, its most important effect is as a greenhouse gas that traps thermal energy and may contribute to global warming. In the future, fleets of supersonic aircraft would have a different influence because they would emit nitrogen oxides and sulfuric acid in the stratosphere, where they trigger chemical reactions that destroy, rather than produce, ozone. Recent measurements and calculations by European and U.S. researchers indicate that current aircraft are responsible for about half the nitrogen oxides present in the Northern Hemisphere's midlatitudes at an altitude of 26,000 to 40,000 feet, says Schumann. Studies using computer models suggest that these nitrogen emissions could have boosted tropospheric ozone concentrations by several percent, especially over the heavily traveled North Atlantic.
Scientists point to many uncertainties that undermine the reliability of model results.
Current models include only some chemical reactions and may be missing important ones. In addition, researchers do not know what quantity of nitrogen oxides comes from other sources, such as lightning. Driven by this question, researchers plan to head once again into the skies next summer for a NASA-sponsored mission over the North Atlantic. This time, however, they will stock a more generous supply of air-sickness bags.