To understand how the sky cleanses itself, a team of Australian and US researchers is heading to Antarctica to track down the atmosphere’s main detergent. By drilling deep into polar ice, the scientists hope to determine how the sky’s capacity to scrub away some ozone-depleting chemicals and potent greenhouse gases has changed since the Industrial Revolution — information that could help to improve global-warming projections.
The first members of the project travelled to Law Dome, their drilling site in East Antarctica, this week. There, they hope to capture the first historical data on concentrations of the dominant atmospheric detergent, the hydroxyl radical. This highly reactive molecule, made of an oxygen atom bonded to a hydrogen atom, breaks down about 40 gases in the air. They include methane and hydrofluorocarbons, but not the most prevalent greenhouse gas — carbon dioxide.
Although studies1 of other atmospheric gases have been used to infer the abundance of hydroxyl over the past four decades, atmospheric chemists still refer to the chemical as ‘the great unknown’.
“We have been more or less in the dark when it comes to how hydroxyl has evolved from pre-industrial times to present day,” says Apostolos Voulgarakis, an environmental scientist at Imperial College London. “This new research endeavour can provide unprecedented information on hydroxyl variations in the deeper past, which is exciting.”
Over two and a half months, the team will drill at least two ice cores — three if time allows — down to depths of about 230 metres. They will then melt the cores to extract bubbles of air that were trapped as the ice froze. The samples will represent the atmosphere back to about 1880, before emissions of greenhouse gases from human activity started to increase.
Hydroxyl radicals form naturally in the atmosphere in a reaction involving ultraviolet rays, ozone and water vapour. But because the radicals last about a second before they react with other gases and break them down, as a proxy, the team will instead measure the tiny fraction of carbon monoxide that contains the carbon-14 isotope.
Carbon-14 in carbon monoxide is produced in the atmosphere by cosmic rays at a known rate, and is almost entirely removed by hydroxyl. Because of this, scientists can use the trend in its abundance to infer the trend of the radical, says David Etheridge, an atmospheric chemist at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Aspendale, Australia, and a co-leader of the drilling project.
But measuring levels of carbon monoxide that contain carbon-14 is tricky, because there are only a few kilograms of it in the atmosphere, says Etheridge. “And we’re trying to measure a bit of that over the last 150 years in the Antarctic ice.”
There is also a risk that the ice cores will become contaminated with external sources of carbon-14 from cosmic rays. This high-energy radiation cannot penetrate the ice, but the moment the cores are removed, they are at risk of exposure. This would interfere with the signal the team is trying to measure, says co-leader Vasilii Petrenko, an ice-core scientist at the University of Rochester in New York. To avoid that risk, the researchers will melt the ice and extract the air on-site.
A long haul
Organizing the equipment to do this and transport it to a remote ice sheet has been a huge logistical challenge, says team member Peter Neff, an ice-core scientist at the University of Washington in Seattle.
Tractors pulled giant sleds loaded with equipment to the Law Dome drilling site, which is more than 130 kilometres from the nearest research station. And it will take the team 36 days to melt the ice they need to get enough air samples. “It's a marathon, not a sprint,” says Neff.
The project is co-funded by the Australian Antarctic Division and the US National Science Foundation.
Once the researchers return from Antarctica, to assess the levels of the carbon-14 in carbon monoxide, the team will convert the gas into carbon dioxide and then into graphite, from which the isotope can be measured. The scientists can then use the information to infer how hydroxyl levels in the Southern Hemisphere have changed over time.
Up to now, information on historical trends in hydroxyl levels has come solely from atmospheric models; these simulations suggest2 that concentrations remained fairly stable from 1850 until the 1970s, when they started to rise. The increase was mainly because of a boost in atmospheric warming at the time, says Voulgarakis.
The data collected from Law Dome will help to determine whether the atmospheric models have captured this trend correctly, says Matt Woodhouse, a climate modeller at CSIRO, who will use the information to improve Australia’s global chemistry-climate model, called ACCESS. “Our ability to resolve hydroxyl won’t revolutionize climate models, but it’ll increase our confidence in them.”
And accurate pictures of hydroxyl’s historical and current atmospheric concentrations are essential for developing better projections of its future levels, says Voulgarakis. This will then enable more-accurate projections of the future abundance of gases that affect climate — such as methane, ozone in the lowest layer of the atmosphere, and aerosols — that hydroxyl scrubs from the sky, he says. This would make it easier to determine the gases’ potential contribution to global warming.
Nature 563, 455-456 (2018)