World's most potent greenhouse gas detected in atmosphere
Low concentrations mean it doesn't contribute that much to global warming
A long-lived greenhouse gas more potent than any other — at least by one measure — has been detected in the atmosphere by Canadian researchers.
Perfluorotributylamine (PFTBA) has a radiative efficiency of 0.86 — the highest of any chemical found to date, reported chemists from the University of Toronto in a recent issue of Geophysical Research Letters. Radiative efficiency is one measure of a chemical's effectiveness at warming the climate, per part per billion of volume.
Greenhouse gases make the Earth warmer by absorbing and trapping heat that would otherwise escape from the atmosphere. The biggest contributor to climate change is carbon dioxide because its concentrations are so high — 393.1 parts per million in 2012 — and growing, thanks to human activity such as the burning of fossil fuels. However, many other gases with smaller concentrations also contribute.
Concentrations of PFTBA in the atmosphere are very small — about 0.18 parts per trillion by volume in the atmosphere, at least in Toronto, where it was detected.
"We recognize that it's very low," acknowledged Angela Hong, a PhD student at the University of Toronto's department of chemistry and the lead author of the paper.
But even though the overall contribution of PFTBA is small compared to that of major greenhouse gases such carbon dioxide and methane, because it is so powerful, its effect is "on the same scale as some of the gases that the monitoring community is aware of."
Those include gases recognized by the Kyoto protocol, such as nitrogen trifluoride, and various chloroflurocarbons (CFCs).
Chemical sold for 30 years
According to 3M, a producer of PFTBA, the chemical has been sold for more than 30 years, mainly for cooling semiconductor processing equipment and specialized military equipment such as electronic radar and guidance systems. It is effective at transferring heat away from electronic components, and is stable, non-flammable, non-toxic, and doesn't conduct electricity.
The chemical has long been known to have the potential to cause damage to the ozone layer. But up until now its ability to trap heat in the atmosphere had not been measured, nor had it been detected in the atmosphere.
The reason PFTBA is so potent compared to other greenhouse gases is that it absorbs heat that would normally escape from the atmosphere, Hong said.
Heat or infrared radiation comes in different colours, and each greenhouse gas, including carbon dioxide, only absorbs certain colours of heat. The other colours pass right through.
PFTBA happens to absorb the colours that other greenhouse gases don't.
"If you're suddenly going to add a greenhouse gas and it absorbs in that region," Hong said, "it's going to be very potent."
Hong said PFTBA is sometimes used as a solvent for specialized chemical reactions. When her supervisor, University of Toronto chemistry professor Scott Mabury found a bucket of it in the chemistry building, he wondered if it might be getting into the atmosphere.
Cora Young, a member of his research team who is now at Memorial University in St. John's started by studying the molecule's properties, discovering its record-breaking radiative efficiency.
Detected at 3 sites in Toronto
Hong then tried to find out if she could detect any of it in the air. She used a pump to suck in air from the University of Toronto campus, Mt. Pleasant Cemetery and Woodbine Beach. She condensed and concentrated the samples by cooling them to very cold temperatures. She then used a technique that separates chemicals roughly by weight.
Each molecule of PFTBA is about 15 to 25 times heavier than molecules of most gases in the atmosphere, including carbon dioxide. The separated gases were fed into a mass spectrometer, which breaks up the molecules into pieces that produce a characteristic fingerprint for that gas.
The PFTBA was found in all samples, including those upwind from the University of Toronto, suggesting that it wasn't just coming from the chemistry building.
However, Hong acknowledged that the measurements were local and not representative of the global average concentrations of PFTBA.
Its effect is far more intense if its effect per molecule is considered, since it is about 15 times heavier than carbon dioxide, and therefore there are far fewer molecules of PFTBA than carbon dioxide per kilogram.
Because many greenhouse gases are far lighter than PFTBA, some of them remain more potent greenhouse gases than PFTBA when a kilogram of each gas is compared, rather than a single molecule. For example, over 100 years, each kilogram of PFTBA is expected to have the potential to warm the climate as much as 7,100 kilograms of carbon dioxide, whereas each kilogram of nitrogen trifluoride can warm the atmosphere as much as 16,100 kilograms of carbon dioxide, the paper reported.
Unfortunately, PFTBA survives hundreds of years in the atmosphere, which means its effects are long-lasting.
Fortunately, its use has been regulated under a U.S. Environmental Protection Agency program that promotes alternatives to chemicals that deplete the ozone layer.
"That regulation stipulates that PFCs [the class of chemical that PFTBA belongs to] should be used only where there are no other alternatives on the basis of performance and safety," said a statement from 3M. "3M adheres to that policy globally."
It added that the company "has worked to limit the use of these materials to non-emissive applications" and emphasized that the concentration of PFTBA found in the atmosphere is very low.