"The overall impact of this compound is much lower [than that of CO2] when you can take into consideration atmospheric levels," says Cora Young, a chemist at Memorial University, in Newfoundland, who was a lead author on the recent study published in the journal Geophysical Research Letters. But while PFTBA's impact is small, there may be more compounds like it. "Even though this one by itself doesn't have a large impact," Young says, "if there are a lot of fluorinated compounds out there like this, then together, they might have some impact, and that, I think, has not been widely appreciated."
So how can such a chemical be so much worse than CO2, and why didn't we know about it?
Greenhouse gases warm the planet by trapping heat within the atmosphere. CO2 is one of the most familiar and most problematic greenhouse gases because humans produce so much of it. However, per molecule, its warming potential is relatively low. Other naturally-occurring greenhouse gases, such as methane, have a higher impact per molecule on global warming, and human-synthesized compounds containing carbon-fluorine bonds, such as the infamous chlorofluorocarbons used in refrigeration that were banned because they harm the ozone layer, have even higher warming potential.
This new compound, PFTBA, trumps all the other known greenhouse gases in terms of its per molecule heat-absorbing capacity, or radiative efficiency. Yet it has been overlooked in the past because it is difficult to measure. In addition to its high radiative efficiency, PFTBA stays in the atmosphere for 500 years. That combination of radiative efficiency and long "life" gives PFTBA its enormous warming potential: In a century, the warming caused by one molecule of PFTBA should be equivalent to the warming caused by 7100 CO2 molecules.
PFTBA's high radiative efficiency has to do with the way carbon-fluorine bonds absorb infrared radiation, or heat, in a particular part of the energy spectrum called the atmospheric window, Young says.
"[The atmospheric window] is the region where few other compounds in the atmosphere absorb, and that's where most of the heat from the earth normally escapes," Young says. "These compounds that contain a lot of carbon-fluorine bonds, like perfluorotributylamine, absorb in this region, and that's why they are such potent greenhouse gases."
Parts Per Trillion
Scott Mabury, the head of Young's former lab at the University of Toronto, decided to look for PFTBA after he noticed a container of the chemical sitting around in a lab and thought that, based on its chemical structure, it might be a greenhouse gas. Researchers in Mabury's lab collected samples from the atmosphere, separated the air samples into components using what's called a gas chromatograph, and identified PFTBA using a technique called mass spectroscopy. They also measured the compound's ability to absorb infrared radiation and found that it absorbed within the atmospheric window.
Despite its high warming potential per molecule, PFTBA exists in low concentration in the atmosphere. Young and colleagues found that PFTBA exists at 0.18 parts per trillion. CO2, by contrast, exists at 365 parts per million, or 365 x 106 parts per trillion).
"A lot of these compounds that contain carbon-fluorine bonds have high radiative efficiencies, but because they are present in the atmosphere at low concentrations and they have unique properties, you have to specifically look for each individual compound to perceive them," Young says. "And no one's really been doing that."
Another reason that this greenhouse gas went undetected is that it has "unique properties," says Young, which kept it hidden during previous analyses of chlorofluorocarbons. "When people measure chlorofluorocarbons, they would not have detected this molecule." Specially, the new compound is much larger and less volatile than chlorofluorocarbons, which is why they are not detected at the same time, Young says.
The Maburg lab has identified several other compounds within this group, perfluoroalkylamines, that they predict might also be present in the atmosphere, and they plan to do similar analyses on them, Young says. Perfluoroalkylamines are used as heat transfer agents in the electronics industry. "We know that other perfluorinated amines are being used by industries, so we'd like to look for some of those compounds and see what their concentrations in the atmosphere might be."
Steven Smith, a climate scientist at Washington State's Pacific Northwest National Laboratory, who works at intersection of climate science and policy, tells Popmech by email: "Given that this appears to be one of the strongest greenhouse gases emitted by human activities, we are fortunate that PFTBA is currently at low concentrations (with, as the authors note, "approximately 10,000 times" smaller impact than CO2 at present. This study provides useful information to global policymakers."