An international research team has tracked down and measured an elusive molecule that rapidly breaks down pollution in the atmosphere, turning it into clouds that actually help cool the Earth.
The compound is part of a class of molecules called "Criegee biradicals," named after the scientist Rudolf Criegee, who predicted their existence in 1949. The biradicals are intermediates in reactions, meaning they are steppingstones in processes where one compound becomes another.
In this case, they form naturally as ozone -- a high-energy oxygen molecule -- reacts with carbon chains that have double bonds, forming a compound that has two reactive pairs of electrons. The intermediates have high energies and are unstable, reacting quickly with other molecules, making them difficult to analyze.
Previously, if you wanted to study Criegee biradicals and how they reduce sulfur dioxide and nitrogen dioxide pollution in the atmosphere or how they make hydrocarbons spontaneously ignite, you had to do it with indirect measurements and observations.
But researchers at Sandia National Laboratories, partnering with scientists at the University of Manchester and the University of Bristol in the United Kingdom, not only have caught a glimpse of the molecule, but have observed it in action. With this knowledge, researchers can develop better climate models and improve fuel combustion efficiency.
"Almost everyone who dealt with them knew they existed. We discovered a way to make much more concentrated samples of Criegee intermediates. Because of that, now we can study measure the rates of the chemical reactions," said David Osborn, a combustion chemist at Sandia. He explained that his team initially detected Criegee intermediates in 2008 but didn't produce enough of them to see how they interacted with other compounds.
Caught in a powerful pulse of light
Using some new tricks, the researchers made enough Criegee molecules to see how fast they react. They created the biradical, carbonyl oxide, by bombarding diiodomethane with ultraviolet photons in a process known as photolysis.
To ensure that they found molecule they were looking for, the scientists measured the product's mass using a mass spectrometer. However, that only tells you that you have the right number and kinds of atoms in the molecule, not how they are arranged. Compounds with the same composition but different arrangements are called isomers.
Resolving the structure required the use of the Advanced Light Source at Lawrence Berkeley National Laboratory. The device is a synchrotron, a circular particle accelerator, and by shooting electrons around its ring near the speed of light, it generates X-rays and ultraviolet rays that are a billion times brighter than the sun. The light can also be precisely tuned to specific wavelengths, and when it strikes a compound, it causes it to develop a charge. This is called photoionization.
In these circumstances, different structures produce different signals, and by a process of elimination, the team concluded they had found a Criegee biradical. "We backed it up by doing calculations for ionization energy. The calculations agreed very well with our experiments and disagreed with all the other isomers," said Osborn.
Turning greenhouse gases into cooling aerosols
The team then mixed the intermediates with various compounds, like sulfur and nitrogen greenhouse gases, to see how fast they reacted. "The most important thing we found is that they react an awful lot faster than we predicted," said Carl Percival at the University of Manchester School of Earth, Atmospheric and Environmental Sciences.
He explained that the intermediates turn certain greenhouse gases into aerosols, which help form clouds that have a net cooling effect on the planet.
The findings will help researchers create better climate models, but Percival thinks Criegee biradicals cannot feasibly be used to lower temperatures.
"I don't think it's a geoengineering candidate, because it's already part of the Earth's system," he said. "When you make it in the lab, it falls apart. You can't exactly make a whole barrel of it and shove it up in the atmosphere."
The scientists said they will now look for Criegee intermediates in other processes. "The Criegee intermediates themselves are not new, but our understanding of them is much greater, and our understanding of autoignition chemistry and atmospheric chemistry is much greater," said Osborn.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500