Sunlight Triggers Nitrous Oxide Formation

Nitrous oxide, an important greenhouse gas that also destroys ozone in the stratosphere, comes from a variety of natural and manmade sources including agriculture and nylon production. But even as the amounts entering the atmosphere increase yearly, scientists don't know where nearly one-third of N₂O emissions come from. Now researchers describe an unexpected mechanism for N₂O formation: light interacting with ammonium nitrate on airborne particles (Environ. Sci. Technol., DOI: 10.1021/es103295v).

Increasing amounts of N₂O can have serious climate impacts. Its global warming potential is more than 300 times carbon dioxide's, according to the Environmental Protection Agency, and its lifetime in the atmosphere is more than a century. However, researchers cannot account for 30% of N₂O sources globally.

One known source is ammonium nitrate fertilizer. Microbes chew through the compound, emitting N₂O from farm lands. But Vicki Grassian of the University of Iowa and her colleagues wondered if there was a non-biological route to the gas from ammonium nitrate. They knew that particles in the atmosphere can be composed of ammonium nitrate from fertilizers applied to agricultural fields. The surface of those particles could provide surfaces for chemical reactions in the atmosphere, Grassian thought.

To simulate these conditions, the researchers coated alumina particles with ammonium nitrate and exposed them to light. They tagged each ion in the fertilizer with different nitrogen isotopes to determine which ion was the source of the nitrogen atoms in N₂O.

The researchers determined that the mechanism started with light reacting with the nitrate and the ammonium oxidizing. Eventually, the process yielded water and N₂O, with one nitrogen atom from each fertilizer ion. The team also quantified how much N₂O formed under different levels of humidity. (Later, they also tested particles wholly composed of ammonium nitrate, which spawned N₂O at lower rates.)

The modelers on Grassian's team then could estimate the amount of N₂O released across the U.S., at varying heights above the earth's surface. They calculated that this photochemical mechanism could account for the formation of about 4,000 metric tons of N₂O in the atmosphere annually. Current EPA estimates put emissions of the gas at close to 966,000 metric tons in the U.S., of which more than two-thirds comes from agricultural soils treated with nitrate fertilizers. When they included other sources of nitrate, such as industrial emissions, in their calculations, the researchers found that the light reaction could account for 9,300 metric tons, or about 1% of total emissions in the U.S.

Grassian's proposed mechanism for creating N₂O is "straightforward" but has been "really hard to measure," says Mark Thiemens, an atmospheric chemist at the University of California, San Diego. By using isotope tagging to follow the reactions triggered by light, Grassian's experimental method gives an unambiguous result, he says. Armed with these results, he says, researchers now can hone their monitoring strategies to track this potent greenhouse gas.

"We talk about the carbon footprint; nitrogen footprints are important too," Thiemens says. "If you know what the problem is, you have a chance of dealing with it."