Ozone study hits new level

七月 9, 1999

This month, developing countries will begin phasing out chlorofluorocarbon production. International regulation of CFC production was prompted by research in the 1970s and 1980s that established that CFCs were depleting the ozone in the stratosphere, which protects us from harmful ultraviolet radiation.

Despite consensus that damage was occurring and subsequent international agreements, many important scientific questions about ozone and its role in our climate system are unresolved. The Natural Environment Research Council is funding a programme to shed light on some of the remaining issues.

"It is very likely that changes due to man-made emissions have taken place in polar regions, but we need to understand changes in other parts of the world such as the mid-latitudes, where we live," said programme manager Kathy Law, of the department of chemistry at Cambridge University.

The NERC programme concentrates on the upper troposphere, the region 5km-10km from the earth's surface, and the lower stratosphere, 10km-20km up. This is the region where ozone changes have their largest impact on climate. The UTLS (upper troposphere, lower stratosphere) programme, which is in the second year of its five-year Pounds 9.5 million run, is concentrating on understanding ozone changes at mid-latitudes, from 30° to 60° north.

Previous research concentrated on the effects of ozone-depleting chemicals in the stratosphere. Traditionally, scientists have considered the lower stratosphere separate from the troposphere. The UTLS project will treat the two together. "We've become very aware that ozone has been changing in the very low stratosphere, and that is the region where the connections with the higher troposphere are very important," said John Pyle, also of Cambridge's chemistry department.

"There is a lot of exchange of air between the two regions," Dr Law said. "Particularly at mid-latitudes, there are various theories about why lower stratospheric ozone is decreasing." It could be due to the importation of air from the troposphere, which has much lower ozone concentrations than the stratosphere. Or perhaps it is that ozone-depleted air from the poles is being transported to mid-latitudes. The depletion could also be accounted for by changes in the circulation patterns in the stratosphere; or by chemical changes taking place in the mid-latitudes.

Early research was largely theoretical, but the UTLS programme will collect much new data from the atmosphere itself in an effort to rank these different theories. Using aircraft, it will measure chemical compounds to discover where ozone comes from in the upper troposphere, where changes in ozone concentrations have also been found, and it will look at how the gases that affect ozone move from the troposphere to the stratosphere.

Balloons will carry novel instruments up to about 25km on flights that can last for up to three weeks to investigate how gases are transported through the atmosphere. Instruments for measuring water vapour and other trace gases, including ozone, are also being developed for flying on commercial aircraft.

Commercial aircraft are part of the picture not only because of their research input. Their emissions of trace gases, including nitrogen oxides, are suspected of producing ozone in the upper troposphere. "The aircraft often fly in very clean parts of the atmosphere, where you have very low concentrations of nitrogen oxides - the key gases that affect the ability to produce ozone in the troposphere," Dr Law said. "Computer models of the atmosphere suggest the ozone they produce could be between 2 and 10 per cent.

"People want to quantify the effect aircraft are having because of the predicted increase in air transport over the next 50 years.

To understand more about ozone and climate, the UTLS project will have to try to unpick a very difficult skein. Ozone is a powerful greenhouse gas, and its changes in the lower stratosphere are predicted to have a reasonably significant impact on surface climate. The chemistry that regulates ozone is very dependent on temperature - but ozone levels help determine those very temperatures. And climate will affect how the ozone layer at mid-latitudes recovers, as it is expected to do as CFCs are phased out.



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