Seeking molecules that scrub the sky

An international team of scientists will drill into the Antarctic ice sheet this summer to reveal the extent to which natural ‘atmospheric detergents’ cleanse the air of harmful gases.

While the majority of greenhouse gas emitted is carbon dioxide (CO2), there are more than 40 other gases that contribute significantly to climate change and ozone depletion.

This season’s expedition to Law Dome in East Antarctica, led by CSIRO atmospheric physicist Dr David Etheridge* and Dr Vas Petrenko from the University of Rochester in the United States, seeks to understand the natural processes that remove these ‘other’ gases from the atmosphere.

The research team will drill ice cores to depths of 250 metres, to measure pre-industrial atmospheric levels of hydroxyl radicals (OH molecules with an unpaired electron) from the air bubbles trapped inside.

Hydroxyl is a naturally occurring, highly reactive oxidant, which acts as an ‘air purifier’ by chemically destroying greenhouse gases like methane, and industrial chemicals that deplete ozone.

“Understanding how much of these pollutants are removed naturally by hydroxyl is fundamental to our climate models, if they are to more accurately predict the levels of all greenhouse gases into the future,” Dr Etheridge said.

“For example, under a certain emissions scenario, what will be the amount of greenhouse gases or ozone depleting gases that remain in the atmosphere? What damage will they do to the ozone layer? How much warming will they cause?”

“Knowing how hydroxyl varies in the atmosphere is key to answering these questions.”

To find out, scientists and a support team will traverse more than 100 kilometres from Casey research station and set up a temporary laboratory at Law Dome for three months.

Three drills run by collaborators from the United States Ice Drilling Design and Operations will extract ice cores to be processed and analysed in the field laboratory.

“Law Dome is the best place on the planet to get trapped old air for this project, because its enormous rate of snowfall traps air quickly and preserves it at depth for centuries,” Dr Etheridge said.

The key challenge for the team, however, is that hydroxyl radicals cannot be sampled directly, because they are so reactive each lasts less than a second.

Instead, the researchers are seeking tracer molecules controlled by the chemical reaction with hydroxyl, like carbon monoxide (CO) that’s been tagged by cosmic rays.

“Cosmic rays are constantly bombarding the atmosphere and producing small amounts of the isotope carbon-14 (14C) that goes on to form carbon-14 monoxide (14CO),” Dr Etheridge said.

“This 14CO is removed by hydroxyl so the amount that remains in the atmosphere will tell us about the hydroxyl levels.”

While cosmic rays create a useful 14CO tracer in air, they also penetrate the upper layers of snow and can alter the 14CO levels in the snow-pack. This is where the high snowfall and rapid burial at Law Dome are important because this shielding minimizes any unwanted additional 14CO production.

“Once the ice is brought to the surface, it is once again exposed to cosmic rays, and the process of contamination starts,” Dr Etheridge said.

To get around this, the team will extract air from the ice cores as soon as they arrive at the surface. Each sample will melt hundreds of kilograms of ice to yield a few litres of air. This will be collected in canisters where the contamination process will stop.

Another challenge for the project is collecting enough material to analyse. The total mass of all the 14CO in the atmosphere amounts to about one kilogram. Each air sample will contain a vanishingly small amount of the 14CO tracer molecules, in the concentration of a few parts per hundred million trillion (1020).

“The concentrations we’re looking for are so miniscule it’s like trying to spot one particular sand grain on a beach,” Dr Etheridge said.

The amount of the 14C isotopes of CO in each sample will be measured by an accelerator mass spectrometer at the Australian Nuclear Science and Technology Organisation in Sydney, after initial analysis and preparation in the United States.

“Once we've measured these samples across the past 150 years, and quantified the trend in the tracer that tells us how hydroxyl levels have changed over that period, we can start to provide data for earth systems models that simulate the chemistry and the physics of the atmosphere,” Dr Etheridge said.

These tiny pieces of the puzzle will be crucial to improving the predictive powers of global climate models, and providing deeper insights into how the atmosphere will change in the future.

Mark Horstman
Australian Antarctic Division

*Australian Antarctic Science Project 4425

The Law Dome expedition is a US-Australian collaborative project involving glaciologists and atmospheric scientists from CSIRO, the Australian Nuclear Science Technology Organisation (ANSTO), the University of Rochester, Scripps Institution of Oceanography, the US National Science Foundation, and Australian Antarctic Division.