Aerosols play many roles in the atmosphere, including seeding cloud formation and cooling the planet by scattering sunlight. Researchers have found a potential new, unlikely source of precursors to atmospheric aerosols: fatty acids.
Although fatty acids exist in the environment, scientists long thought these molecules didn’t participate in atmospheric chemistry because they’re photochemically inactive at wavelengths beaming through the atmosphere.
That may not have been a good assumption.
A team led by D. James Donaldson of the University of Toronto and Christian George of France’s National Center for Scientific Research and the Claude Bernard University Lyon 1 now show that fatty acids are indeed photochemically active at environmentally relevant wavelengths—if the fatty acid is at a high enough concentration (Science 2016, DOI: 10.1126/science.aaf3617). Such concentrations can exist at the interface between water and air, the researchers found.
The team used nonanoic acid—a nine-carbon carboxylic acid—to represent compounds found in a microlayer of organic molecules that floats on the ocean surface. Nonanoic acid doesn’t absorb ultraviolet-visible radiation longer than 280 nm in the gas phase or in dilute aqueous solution. But when George’s team irradiated a reactor half-filled with a dilute nonanoic acid solution, they detected aldehydes in the gas phase above the solution. This suggested that the fatty acid collects and concentrates at the water’s surface and then undergoes a reaction.
Based on quantum calculations, Donaldson’s group thinks that, at the surface, the fatty acid absorbs light that excites it to a triplet electronic state. The excited state might cleave to form a hydroxyl radical and an acyl radical. Or, it might abstract a hydrogen atom from a neighboring molecule.
Donaldson says the team’s lab experiments can’t distinguish between the two routes. “The quantum chemistry calculations indicate that both are possible under the conditions in the experiment,” he says.
Both routes lead to chemically active products with double bonds that are susceptible to reactions with ozone and hydroxyl radicals—reactions that can lead to aerosol formation. Donaldson expects that the same kind of chemistry would happen with other fatty acids. Molecules with larger alkyl groups may be more susceptible to this process, he says, because they’re even less soluble than nonanoic acid and therefore more likely to accumulate at the water’s surface.
Vicki H. Grassian, an atmospheric chemist at the University of California, San Diego, notes that nonanoic acid has been observed in sea spray aerosol, but other fatty acids are more common. “I’m interested to see if this same photochemistry applies to these more abundant fatty acids,” she says. “The nature of the triplet state—for example, how it becomes more photochemically active at the interface—is really quite a puzzle that should be further explored from both theoretical and experimental perspectives.”
That’s what Donaldson and George intend to do next. “Stay tuned for further developments,” Donaldson says.