New Scientist 15 Feb 97 Aircraft Destroy Ozone Layer
CONCORDE, the supersonic airliner, may be destroying the ozone layer far faster than anyone anticipated, by leaving a hitherto unnoticed fog of sulphuric acid in its wake. Worse still, subsonic aircraft are probably doing the same thing, adding an unexpected threat to both stratospheric ozone and climatic stability. The findings are worrying for both atmospheric chemists and aircraft engineers. "For twenty years, we have asked engineers to design engines that minimise production of nitrogen oxides. We regarded them as the main hazard from aircraft emissions," says David Fahey of the US government's National Oceanic and Atmospheric Administration in Boulder, Colorado. "Now it seems aerosols of sulphur particles may be equally important." Three years ago Fahey sent a research plane to chase Concorde as it flew high over the ocean near New Zealand, sampling exhaust gases in Concorde's slipstream 10 minutes after the supersonic airliner had passed. He discovered much more sulphuric acid, in the form of a very fine aerosol, than expected. And, using a model of atmospheric chemistry developed by fellow researcher Bernd Kiircher of the University of Munich, Fahey has shown how Concorde's exhaust produces so many particles. Aircraft fuel contains trace impurities, including sulphur. Previously, researchers had imagined that most of the sulphur that burns in aircraft engines forms sulphur dioxide. Once in the atmosphere, this would be gradually converted to sulphuric acid and condense onto particles in the air, with little effect on vital stratospheric chemical processes. But Fahey's research has shown that, in Concorde at least, the burnt sulphur emerges from the exhaust pipe as sulphur trioxide. This form converts much more rapidly to sulphuric acid. And when there is a dearth of particles in the atmosphere on which to condense, the acid creates a fog of tiny new particles. The number of particles present in the stratosphere is a vital factor in ozone loss because it is on the surface of these particles that chlorine pollutants destroy ozone. "A fleet of 500 supersonic aircraft like Concorde could double the surface area of tiny particles present in the stratosphere. That would have a direct effect on stratospheric ozone," says Fahey, whose research is reported in the latest issue of Geophysical Research Letters (vol 24, p 389). The handful of Concordes flying today may not matter a lot, he says. But large subsonic aircraft on long-haul flights also spend a lot of their time in the stratosphere. "We haven't done the measurements yet, but we see no reason why their emissions should be very different from Concorde," says Fahey. With subsonic aircraft most frequently intruding into the stratosphere on Arctic routes, this raises the prospect that they could contribute to the ozone holes 12 that have emerged over the Arctic during the 1990s.
Fahey believes that aircraft could also have a big impact on the more complex chemistry at lower altitude, in the troposphere, where subsonic aircraft fly most of the time. Here, the aerosols are likely to encourage the formation of cirrus clouds, which could warm the global climate. "Controlling aerosol formation in aircraft plumes may be required to minimise ozone loss and climate change," says Kdrcher. That may bring calls for the production of jet fuel with a lower sulphur content, and for a redesign of aircraft engines to reduce production of sulphur trioxide.