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Down under flights produce most ozone

Pacific ozone Flights to and from Australia and New Zealand create the highest amount of atmosphere-damaging ozone, a new analysis has found.

The analysis of more than 83,000 aircraft trips using a global chemistry-transport model, is published in a recent issue of Environmental Research Letters.

The impact, say the researchers, is largely because flights in and out of Australia and New Zealand travel over a region that is particularly sensitive to ozone-generating aircraft emissions.

They found the most sensitive area is in the Pacific, around 1000 kilometres to the east of the Solomon Islands, but an area including a large part of Australia, southeast Asia and stretching all the way to Madagascar is also highly sensitive.

Lead author Professor Steve Barrett, of the Massachusetts Institute of Technology, says ironically, it is the relatively clean atmosphere in areas like the Pacific that causes the problem.

Barrett says aircraft emit oxides of nitrogen (NOx), which react with sunlight to form ozone.

"When there is already a lot of NOx in the atmosphere, the ozone produced from an additional tonne of NOx is lower than [from] the first tonne," he says.

"This means that new emissions in cleaner regions cause more ozone than new emissions in already polluted regions."

In the Pacific region, the researchers estimate one kilogram of aircraft emissions will result in an extra 15 kilograms of ozone being produced in one year.

This sensitivity to aircraft emissions is around five times higher than that of Europe and 3.7 times higher than that of North America.

Barrett and colleagues found the 10 highest ozone-producing flights were in and out of Australia or New Zealand.

As well as flying through sensitive areas, flights to and from these areas usually involve very long flight with large aircraft meaning more fuel is burnt and more NOx is emitted.

According to the study, a flight between Mumbai - India's capital - and Sydney was the most damaging, creating 25,300 kilograms of ozone.

Winter peak

The researchers also found that in the northern hemisphere, flights in October cause 40 per cent more NOx emissions than flights in April.

"It's basically because the ozone impact is a competition between how efficiently ozone is produced, and how long it lasts when it's created," Barrett says.

"Ozone is both produced and destroyed by sunlight, so when the sunlight is very intense (summer) it is produced efficiently, but does not last long.

"When there is low sunlight (winter), little is produced, but it lasts a long time. It so happens that the peak impacts occur right in the middle of these two."

Changing flight paths

Barrett says places that the sensitivities are highest now are the fastest growing regions in terms of civil aviation growth and the work opens the way for changes to flight paths to mitigate the damage.

"There could potentially be ways to achieve significant reductions in the climate impact of aviation by focusing on re-routing aircraft around the particular regions of the world where ozone formation is highly sensitive to NOx emissions," he says.

However Barrett admits the overall result for climate impact could be likened to robbing Peter to pay Paul.

"If you re-routed flights you could end up reducing short-term climate change (from the ozone), but because re-routing would mean burning more fuel and thus emitting more CO2, that would increase climate change in the longer term," he says.