Past Antarctic air informs our future

By Fiona Brown

28 March 2019

7 minute read

An international team of highly specialised scientists and engineers are researching the history of a compound called hydroxyl in an effort to improve the accuracy of climate models. Credit: CSIRO

Take a deep breath and exhale. Do you know what exactly is in the air you’ve just breathed in?

How can you even measure what’s in the air around you, let alone have it give you an idea of the past?

That’s what an international team of highly specialised scientists and engineers are doing in an exciting Antarctic project, including one of our own David Etheridge, who is the Co-chief Investigator.

They are researching the history of a compound called hydroxyl. Hydroxyl is a powerful oxidant and a natural ‘air purifier’ found in the atmosphere. It breaks down and cleans the air of greenhouse gases, like methane, and industrial chemicals that deplete the ozone layer.

“We’re looking to understand how much of these pollutants have been removed naturally by hydroxyl and whether the amount of hydroxyl in our atmosphere has changed since pre-industrial times, more than 140 years ago,” said David.

“This information is key to improving the accuracy of our climate models, enabling us to better predict the impact of pollutants in the future.”

After six years of planning and three months working in one of the most remote and inhospitable places on Earth, the team is one step closer to achieving what was once considered an impossible task.

The hunt for hydroxyl

Measuring hydroxyl on its own is no easy task. Hydroxyl radicals are so reactive that each molecule lasts less than a second before it reacts with a compound. This makes it impossible to sample directly.

And measuring hydroxyl in 140-year-old air is almost impossible.

But there’s a solution.

The researchers will instead measure tracer molecules, such as carbon-14 monoxide (14CO). This acts as a measurable proxy for hydroxyl and also gives a meaningful average level for hydroxyl within the atmosphere.

14CO is removed from the atmosphere by hydroxyl so the amount that remains in the atmosphere tells us about the hydroxyl levels.

“However, the 14CO tracer molecule isn’t easy to measure either. There’s only a very small amount of it in the Earth’s entire atmosphere – about 10 kilograms or less in total. So measuring 14CO is a bit like finding a particular grain of sand on an entire beach,” said David.

But the team aren’t looking for this tracer molecule in today’s atmosphere. To measure its impact on the atmosphere and changes in its concentration, they need to find 14CO in the air going back in time. In other words, they are looking for that grain of sand that existed centuries ago. And that’s where the fun really starts…

The team set up a make-shift lab in a trench dug into the snow. Credit: Sharon Labudda AAD

Something in the air (bubbles)

Antarctic ice may not be the first place you’d think to hunt for clues about atmospheric changes. But David and team know that the only place where we can look back to past atmospheres, before direct measurements began, is the air captured in ice.

Now take Law Dome in Antarctica, a site 130km from Australia’s Casey research station that has long been studied by the Australian Antarctic Program. It is a unique place, with very high snowfall. This rate of snowfall traps air and buries it at depth quickly where it is preserved in the ice for centuries. Because of this, it’s the only known place on the planet where we can search for past hydroxyl levels.

By drilling deep (up to 240 metres deep!) into the ice and then melting the ice cores, scientists can extract bubbles of air that were trapped as far back as the late 1800s. This means we can sample the atmosphere from pre-industrial times, before increases in greenhouse gas emissions from human activity began.

Person sitting in snow

The team’s tents after a blizzard. Did we mention it snows a lot at Law Dome? Credit: Sharon Labudda AAD

One in a ten million trillion

So how did the team begin the process of measuring tiny concentrations of 14CO to measure hydroxyl?

The team extracted 0.5 tonnes of ice for each sample and used three different ice drills from US Ice Drilling Program, including one installed in a specially prepared trench. The ice had to be melted at the site to extract the air bubbles containing the precious 14CO.

This process had to be done within 24 hours of the core being brought to the surface as 14CO is not only rare, but also highly sensitive to cosmic rays (which can penetrate metres of rock, let alone centimetres of ice).

One half -tonne of melted ice resulted in 50 litres of air, in which the concentration of 14CO tracer molecules will be a few parts per 1019 molecules of air. CSIRO scientist David Thornton had an extended stay at Law Dome preparing the ice cores for melting extraction and sub-sampling them for other gases too.

Outside the drilling trench (the trench is under the wood). Credit: Sharon Labudda AAD

Predicting the future from the past

Now that the team has returned from their three-month expedition, they can start to see what this ancient air tells us.

Dr Vas Petrenko, co- Chief investigator from the University of Rochester prepares ice cores for melting. The air ice cores had to be melted extracted from the ice on site to minimise the impact of cosmic rays on the air samples. Credit David Etheridge CSIRO

These precious air samples are now journeying by ship to the University of Rochester in the United States, where the US team members will use a highly specialised technique to extract the CO from the air and convert it to CO2. The samples need to be transported by sea as air transportation would increase their susceptibility to those cosmic rays we mentioned earlier, which could irreversibly alter them.

Each sample will then return to Australia in a small ampule, where a team at the Australian Nuclear Science and Technology Organisation (ANSTO) in Sydney will turn the CO2 into about 10 micrograms of graphite.

From here, the 14CO can be measured to reveal any historic changes in the concentration of hydroxyl in the atmosphere.

“It’s a huge achievement just to get this far,” David said.

“Even in summer, Law Dome is a very harsh environment in which to undertake such highly sensitive work. But the team did an amazing job, and from the preliminary analysis we’ve done, everything seems to have gone to plan.”

“But we’re of course very excited to see what the samples hold.”

It may be a small and fleeting compound, but if we know more about how hydroxyl removes pollutants from our atmosphere, we can improve our global climate models. And from this, we can gain ice-borne insights into the future levels of greenhouse gases and ozone-depleting gases that affect our planet.

The project was supported by the Australian Antarctic Division (AAD), Australian Nuclear Science and Technology Organisation (ANSTO), University of Rochester and the US National Science Foundation.