Hydroxyl hunters

How do you measure the abundance of a molecule that disappears from the atmosphere almost as soon as it is created? Scientists on the hunt for hydroxyl have found a way to measure another trace gas — itself present in minuscule but measureable concentrations — that acts as a proxy for hydroxyl. As a natural ‘air purifier’ the amount of hydroxyl in the atmosphere is critical to the removal of pollutants, including greenhouse gases and ozone depleting chemicals.

Century-old air extracted from up to 240 metres below the Antarctic ice sheet will help scientists determine how much of a natural atmospheric ‘air purifier’ is available to scrub the Earth’s atmosphere of pollutants.

Samples totalling some 500 litres of air, extracted from five tonnes of melted ice cores, were collected at Law Dome in East Antarctica last season, by an international team of scientists led by Dr David Etheridge of CSIRO* and Dr Vas Petrenko from the University of Rochester in the United States.

The extracted air samples cover about 140 years of the Earth’s atmospheric history, back to about 1875. This will allow the team to measure pre-industrial levels of the hydroxyl radical (OH), which chemically destroys gases like methane and ozone-depleting chemicals.

“The majority of greenhouse gas emitted is carbon dioxide, but there are more than 40 other gases that contribute to climate change, ozone depletion and pollution,” Dr Etheridge said.

“Hydroxyl radicals act like natural atmospheric scrubbers, by chemically destroying many of these gases. But we don’t know how they have withstood the demand of increased emissions.

“Variations in hydroxyl could have implications for future levels of greenhouse gases, so this information is key to improving the accuracy of our climate models and predicting the impact of pollutants in the future.”

The air samples will be processed at the University of Rochester, and then analysed at the Australian Nuclear Science and Technology Organisation (ANSTO) to see whether the concentration of the radical has changed over time.

But there’s a catch. The hydroxyl radical lasts less than a second in the atmosphere before it reacts with gases. So the analysis will focus on a tracer molecule of carbon-14 monoxide (14CO). 14CO is removed by hydroxyl, so the amount that remains in the atmosphere provides information about the hydroxyl levels — as previously reported in Australian Antarctic Magazine 35: 17–18, 2018.

Because the amount of 14CO in the atmosphere is so small (analogous to finding one particular grain of sand on a beach), hundreds of kilograms of ice had to be collected to provide enough air to measure its concentration in each sample. Three different drilling systems were used to drill a total of 1010 metres of ice core at different depths (time periods), by the United States’ Ice Drilling and Design Operations. This provided 11 samples spanning from the pre-industrial era to today.

“The ice samples were melted immediately after drilling on site using a large vacuum tank in a ‘melter shelter’ to extract the air containing the 14CO,” Dr Etheridge said.

“We also took around 240 metres of continuous ice core samples for ice dating and climate reconstructions.”

The air samples are now at the University of Rochester where the 14CO will be converted to CO2. The team at ANSTO in Sydney will then convert the CO2 into graphite (elemental carbon) and use an accelerator mass spectrometer to count the 14C atoms amongst the ‘regular’ C atoms.

“Once we've measured these samples across the past 140 years, and quantified the trend in the tracer that tells us how hydroxyl levels have changed, we can begin providing data to Earth systems models that simulate the chemistry and the physics of the atmosphere,” Dr Etheridge said.

The unique glaciology of Law Dome continues to provide ice to help scientists understand more about Earth’s atmospheric environment.

“It the only known place on the planet to find air for reconstructing hydroxyl trends. After six years of planning and three months of working in one of the highest snowfall areas in Antarctica, it’s a huge achievement to get this far.”

Wendy Pyper
Australian Antarctic Division

*Australian Antarctic Science Projects 4167 and 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.

CSIROscope video

Law Dome Mission video