Despite all the grumbling at the time, European aviation authorities were justified in grounding planes following the eruption of Iceland's Eyjafjallajökull volcano last year, new research shows.
A team of researchers from the University of Copenhagen and the University of Iceland base their conclusions on a protocol they've developed to give air traffic authorities the data they need.
They say that danger arises when particles are small enough to travel high and far, sharp enough to sandblast the windows and bodies of airplanes, or if they melt inside jet engines. And the ash from the Eyjafjallajökull eruption was dangerous on all counts, they say.
Some 10 million travellers were affected by the ash plume, which cost an estimated two and a half billion Euros.
"Aviation authorities were sitting on a knife-edge at the centre of a huge dilemma," says Professor Susan Stipp from the Nano-Science Centre of the University of Copenhagen.
"If they closed airspace unnecessarily, people, families, businesses and the economy would suffer, but if they allowed air travel, people and planes could be put at risk, perhaps with tragic consequences."
Her team collected and analyzed ash samples from the eruption, using facilities Stipp describes as 'unique'.
"Some of the analytical instruments needed are standard equipment in Earth science departments and some are commonly used by materials scientists, so with our protocol, aviation authorities ought to be able to get fast, reliable answers," she says.
Stipp says her protocol can provide information for safety assessment in less than 24 hours. Within an hour of receiving samples, scientists can tell how poisonous they are, while half a day enables them to predict the danger of sandblasting on aircraft and assess the risk of fouling jet engines.
Within a day they can tell the size of the particles, providing data for predicting where and how far the ash cloud will spread.