Extract from ABC News
If you look up at the sky on a clear sunny day, chances are you'll see fluffy white trails left behind by airplanes.
Key points:
- Contrails are largely made of water vapour that comes out of aircraft engine exhaust
- When it's really cold this can form clouds of ice crystals that hang around for hours and trap heat in the Earth's atmosphere, adding to global warming
- Non-fossil fuel blends can reduce problem contrails but some experts question how much difference this will make.
These condensation trails — or contrails — are produced by exhaust from aircraft engines, and while they may seem harmless they actually contribute to global warming.
But using different jet fuels could help clean up the skies, and reduce the warming effect of contrails, say researchers in a new study out today in Communications Earth and Environment.
"More than half of the climate impacts from aviation come from contrails," said study co-author Christiane Voigt, an atmospheric physicist at the German Aerospace Center just outside of Munich.
A warming blanket of cloud
Contrails form when the hot, humid exhaust from an aircraft engine is expelled into the cold outside air.
Most commercial planes cruise at an altitude of 35,000 to 42,000 feet where air temperatures can drop below minus 50 degrees Celsius.
Under these chilly temperatures, the exhaust forms tiny ice crystals as water vapour rapidly cools and latches onto soot particles.
Like natural clouds, these human-made clouds can affect the climate in two ways.
On the one hand, they have a cooling effect by reflecting sunlight away from the Earth.
But on the other, they can also trap in heat radiating back into space from the Earth. This is the essence of the warming greenhouse effect.
Despite a recent fall in air travel during the pandemic, the skies have generally become more crowded with planes over the past two decades.
And as a result, more problem contrails are blanketing the sky.
A recent study found that warming associated with contrails increased by 64 per cent between 2005 and 2018.
And some experts predict that this contrail-induced warming is going to see a three-fold increase by 2050.
Reducing contrails
One strategy for reducing contrails involves re-routing aircraft to avoid routes and altitudes that are colder where they are likely to form.
But the problems with this approach is that it can lead to longer flights and more fuel being burned.
Professor Voigt and her colleagues are exploring another solution: using fuel that results in exhaust with less soot, which is less likely to form ice crystals and contrails.
The researchers tested out five different types of aviation fuel on the German Aerospace Center's A320 research aircraft.
Two of these were standard petroleum-based fuels, while the other three were a blend of conventional fuel with either synthetic or bio-based varieties.
The three alternative fuels contained less naphthalene and fewer aromatic compounds, which generate soot.
They blended each of these fuels with the standard fuels, and measured the soot particles and ice crystals in the contrails left behind by the plane with the help of NASA's DC8 research aircraft.
The contrails produced by the pure standard fuels contained high levels of soot, with over 80 per cent of them leading to ice crystals.
But
when these fossil fuels were mixed with the alternative fuels, the soot
levels in the contrails were halved, resulting in 45 to 74 per cent
fewer ice crystals.
The ice crystals produced by the fuel blends were also less likely to hang around.
The fuel blends formed larger ice crystals, which were heavier and evaporated faster, leading to shorter-lived contrails.
"That adds to the reduced warming, because their lifetime is reduced," Professor Voigt said.
"Biofuel blends with the least amount of naphthalene and aromatics showed the largest reduction."
Can reducing contrails really make a difference?
Steven Sherwood, who studies climate change and atmospheric processes at the University of New South Wales, said that while the findings are interesting, reducing CO2 emissions should still be top priority for reducing climate change in the long run.
One issue is that CO2 remains in the atmosphere for centuries, while contrails disappear within hours, said Professor Sherwood, who was not involved in the study.
"Air travel is growing very fast, and if that continues in the future and remains fossil-fuelled, the CO2 from that will keep accumulating until it exerts a far greater warming effect, whereas the contrails will not," he said.
"If aviation fuel becomes renewable, that will be great because it will reduce or eliminate both contributions. But getting rid of the CO2 contribution is the most important one for helping to limit future global warming."
The next step for Professor Voigt and her team is to try out different fuel blends on different types of aircraft engines to find the cleanest combination.
"The question is whether there is an optimum that we can achieve in order to reduce emissions even further," she said.
No comments:
Post a Comment