By Jonathan Amos
Science reporter, BBC News, San Francisco
The dramatic springtime collapse of surface ozone in the Arctic has been documented by scientists.
The Tara platform has made a number of fascinating observations
Observations from a boat that drifted with the ice across the North Pole show the gas can disappear in just days.
Dr Jan Bottenheim told a US conference that the precise chemical reactions involved were not fully understood.
However, he said any changes to these processes as the Arctic warmed might limit the region's ability to deal with pollutants in the atmosphere.
"Ozone is the source for the 'vacuum cleaner of the atmosphere' - the molecule OH. So if we don't have as much ozone, we can't make as much hydroxyl. If we then pump pollutants from mid-latitudes into the Arctic, they may just stay there," explained Dr Bottenheim.
"But a lot of this is speculation at the moment because so much of this information is new and we are not sure what to make of it."
The ozone studied by Dr Bottenheim and colleagues at Environment Canada is distinct from the gas high up in the stratosphere that has been damaged through the release of reactive chlorine compounds by industrial society. The group's ozone exists at ground level - or, in the case of the Arctic, at ice level - in the first 100-200m of air.
In the winter, the concentration of the three-atom oxygen-molecule in this still air is at normal levels; but as the sunlight returns to the polar north in the spring, chemical reactions are set in train that reduce the ozone in dramatic fashion.
Dr Bottenheim told BBC News: "In a city, in the evening the ozone will react with exhausts from cars and can go down from, say, 40 to five or even one [units of ozone]; but in the Arctic we've seen it go to 100 times less than one, which is an incredibly low level that I don't think has been seen anywhere else.
"And once it goes, it can go very fast. It can go in almost a day."
The ozone instrument on the Tara schooner observed one period in late April of this year when there was virtually no ozone for a period of more than 15 days.
Whereas stratospheric ozone is depleted though an unnatural process involving chlorine; the ice-level ozone falls victim to reactive bromine atoms released quite naturally from briny Arctic waters. In perfect conditions, the chemistry produces an explosion of bromine oxide (BrO), which is detectable by over-flying satellites.
The form the ice takes in future could impact ozone loss
Scientists are now trying to determine how the ozone behaviour might change in a rapidly warming Arctic.
Conditions that lead to more slushy ice, which could assist the release of bromine, might result in more extensive periods of ozone loss.
"It's a possibility, but as I say this is still speculative," stressed Dr Bottenheim.
The consequences of any change needed to be understood, he added. It is known from observations, for example, that the depletion of ozone at the same time also causes a depletion of gaseous mercury, a major toxic chemical.
The ozone study is just one of a number of fascinating outcomes from the Tara expedition. The vessel is a privately owned polar schooner that was loaned to the European research community for the duration of the expedition.
It is a key project in Damocles (Developing Arctic Modelling and Observing Capabilities for Long-term Environmental Studies), a European-led effort to gather much needed new information on the changing Arctic.
TARA ARCTIC DRIFT EXPEDITION
The 36m, 130-tonne boat is currently locked in ice
The schooner provides habitation for about 10 people
Scientists deploy and monitor a number of instruments
Experiments study air, ice and ocean behaviour
The two-masted vessel was sailed into the pack-ice in September 2006 and allowed to drift with the ocean currents and wind - but it has moved further, faster than anyone expected.
"The boat is full of sensors to monitor the ocean, the atmosphere and the ice. It also serves as a logistics platform for Damocles to put observational beacons all around the Arctic," explained Christian de Marliave, Tara's scientific director.
"When you are on a boat at least you are floating. This is a big security. More and more I think, it will be a ship that will be used as an ice camp. It's become very dangerous. This year, the Russians were unable to find ice thick enough to place their camps."
Tara is approaching the edge of the pack-ice and is likely to emerge into the open ocean between Greenland and Svalbard in the next few weeks.
With just over half of International Polar Year (IPY) still left to run, Tara is likely to be sent to the Bering Strait to study how warm waters from the Pacific are entering the Arctic Ocean and contributing to ice shrinkage.
Dr Dave Carlson, the director of IPY, said the Tara platform had provided many remarkable insights during its 15-month drift.
He has been taken by its observations of "frazil ice".
"We think of ice as forming at the interface of ocean and atmosphere, but Tara has seen a lot of this ice that foams 20m down and then floats up to the surface.
"You can think of it as a reverse snowfall - crystals that form at depth and then make their way to the surface because of their density. It tells us the layers in the ocean don't work in quite the way we thought they did."