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Over The Horizon

Aviation and Atmospheric Changes

Although noise pollution may be a nuisance for those living close to airports, there are increasing concerns about aviation's potential for causing significant atmospheric changes, says a visiting US expert.

Howard Wesoky is manager of NASA's Atmospheric Effects of Aviation project, which is looking at the types of emissions from aircraft and their effect on the Earth's atmosphere. The project is aiming to assess the impact of both subsonic and supersonic aviation.

It is generally accepted that photocatalytic reactions involving chlorine in the upper atmosphere leads to ozone depletion. Wesoky points out that reactions with oxides of nitrogen (NOx), produced by aircraft exhaust at high altitudes, have a similar destructive effect.

"Maybe less commonly known is that the exhaust of subsonic aircraft flying in the lower atmosphere contributes to ozone production in the same manner as automobile NOx emissions has contributed to the infamous chemical smog of Los Angeles and many other locations," he adds.

The crossover point between destruction and production of ozone is generally in the lower stratosphere, near the altitude where long-range subsonic aircraft cruise. With supersonic transports (SSTs), the faster the plane, the higher the altitude. Flying at Mach 3.2 at around 22 kilometres would place an SST near the peak ozone concentration level; slower SSTs would have less impact on ozone levels.

"Thus design Mach number for future SSTs may be directly influenced by considerations of atmospheric impact," Wesoky says. Already NASA and US aeronautical industry partners are examining the use of low NOx combustor technology in a Mach 2.4, 300-passenger vehicle, designed to have minimal impact on ozone levels.

By 2015, the number of revenue passenger kilometres is expected to triple over 1990 levels. Improvements in fuel efficiency are being made, but a doubling of fuel use is still predicted. At present, aviation contributes about 3% of the carbon dioxide produced by the burning of fossil fuels, as well as considerable quantities of water vapour and particulate matter involved in cloud formation.

The 12-year NASA project is part of studies designed to provide a scientific basis for setting acceptable emissions standards.

"Although aircraft may not be having any profound atmospheric impact today, in a few years the situation may be very different."