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Nitrogen oxides

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Modern jet engines release nitrogen oxide gases into the atmosphere. At the height aeroplanes cruise, these nitrogen oxides have two major climate effects - they create ozone and destroy methane. Both ozone and methane are greenhouse gases, so creating ozone has a warming effect on the climate and destroying methane cools the climate. But these two effects don't simply cancel out. Because the gases act over different geographical areas and timescales, ozone can still lead to warming and climate change on a regional scale.

Ozone is particularly potent at the altitude where most planes cruise. It's the coldest part of the atmosphere, where the warming effect of ozone has the biggest impact. Measuring precise levels of nitrogen oxides, ozone and other trace gases at cruising height is therefore crucial. But getting scientific kit thousands of metres into the atmosphere to measure these gases can be difficult. So scientists are taking advantage of the hundreds of planes that fly at these heights every day. They're working with aircraft manufacturers and airlines to fit measuring equipment on board commercial passenger planes.

One of these projects is a Europe-wide initiative called IAGOS. The team's major aim is to measure how much ozone and other trace gases there are in different parts of the atmosphere and to track how this changes over time. 'The great thing about using commercial aircraft to do this is that the aircraft are flying anyway, so it's very cost-effective,' says Jean-Pierre Cammas, one of the IAGOS team. 'They also spend lots of time in the most sensitive region of the atmosphere as well as a range of heights when they take off and land.'

Atmospheric scientists at Cambridge University, UK are building the lightweight ozone-testing instruments that fit on board planes. The measuring kit is fitted below the plane's cockpit and is entirely automatic. From takeoff to landing the system samples the air outside the plane and analyses and records the amounts of each gas present. The results are transmitted wirelessly back to the scientists once the plane lands. 'The first plane will be equipped to fly over Europe in 2009, but our aim is to eventually fly 20 aircraft over 20 years. We'd also like global coverage, so we're contacting more airlines to see if we can use other flight routes too,' says Jean-Pierre Cammas.

Scientists are also flying their own research aircraft to take extra measurements of ozone and methane in the atmosphere. A team from Cambridge University are using a remote-controlled glider to collect air samples. The glider launches from a balloon and can be programmed to stay up for days. The team are also sending up large balloons laden with kit to measure ozone levels. The biggest balloons carry sophisticated instruments that weigh hundreds of kilograms and can reach heights of up to 20 km - even higher than planes.

Measuring the exact amounts of nitrogen oxides that get pumped out of a plane's engines is also important in working out what effect these gases could have on the climate. Scientists are sending up research aircraft that can tail commercial planes and measure emissions as they're released. And both researchers and engine manufacturers carry out ground-based tests too - switching on a plane's engines and directly measuring the emissions that blast out.

So with all this research going on into the effects of nitrogen oxides - what's the verdict so far? 'Current research suggests that aircraft emissions of nitrogen oxides, through the production of ozone and destruction of methane, contribute to global climate change - with the biggest impact occurring in the Northern Hemisphere,' says Helen Rogers, a climate expert from Cambridge University. 'While this finding is worrying, it challenges engineers to simultaneously reduce both nitrogen oxides and carbon dioxide produced by aircraft.'