N2O: Not One of the Usual Suspects
Chlorine-free nitrous oxide now top ozone-depleting emission
Nitrous oxide is better known to the public for inducing giggles in dentists’ offices than for wreaking havoc on Earth’s protective ozone layer. It bears little resemblance to the perpetrators of ozone-layer destruction currently regulated under the international Montreal Protocol agreement— those chemicals all contain chlorine or bromine atoms, unlike nitrous oxide. But now that global actions have curtailed the use of those culprits, a new ESRL study has taken a fresh look at nitrous oxide (N2O). Nitrous oxide emissions have become the most significant ozone-depleting emission by people and are expected to remain so throughout the century, researchers in ESRL’s Chemical Sciences Division (CSD) concluded in a paper published in Science.
Nitrous oxide—also a greenhouse gas—is emitted from fertilized soils, livestock manure, sewage treatment, combustion and other industrial processes, and a very small amount comes from dental sedation. In nature, bacteria in soil and the oceans break down nitrogen-containing compounds, releasing nitrous oxide. Human activities account for about one-third of global nitrous oxide emissions today. The figure below shows historical and projected ODP-weighted emissions of the most important ozone-depleting substances (from Ravishankara et al. 2009).
The scientists evaluated, for the first time, how the human-related emissions of nitrous oxide stack up against the emissions of other ozone-depleting substances in terms of the potential impact on the ozone layer. With the emissions of chlorine- and bromine-containing chlorofluorocarbons (CFCs) and halons declining sharply as a result of the Montreal Protocol, nitrous oxide emissions have now emerged as the most significant among all anthropogenic ozone-depleting substances, the scientists found. Nitrous oxide emissions by people—from agriculture, industry, and other activities—are already more than twice as high (in terms of potential ozone impact) as the ozone-depleting substance with the next highest emissions, trichlorofluoromethane (CFC-11, a refrigerant).
“The dramatic reduction in CFCs over the last 20 years is an environmental success story,” said lead author A.R. Ravishankara, CSD Director. “But anthropogenic nitrous oxide is now the elephant in the room.”
A layer of ozone in Earth’s stratosphere protects the planet’s surface from damaging ultraviolet radiation from the Sun, and ozone-layer depletion is associated with skin cancer and crop damage. The Montreal Protocol of 1987 and its amendments phased out many ozone-depleting chemicals and have been credited with preventing millions of cancer cases—but the Protocol does not include nitrous oxide.
Even so, atmospheric scientists have long understood that the chemical is involved in the depletion of stratospheric ozone—it is the main source of oxides of nitrogen which can eat away at ozone. ESRL researchers Ravishankara, John Daniel, and Robert Portmann (CSD) took that knowledge several steps further, to evaluate nitrous oxide in the same way as the ozone-depleting substances regulated today. The research team first pinned a number on the N2O molecule’s ability to destroy the ozone layer, and then used that number to “weight” N2O emissions to quantify their potential to deplete the ozone layer.
Per molecule, nitrous oxide is comparable to the weakest of regulated ozone-depleting chemicals, the team found. The molecule’s “ozone depletion potential” in the year 2000 was 0.017. CFC -11 (the standard by which other ozone-depleting substances are measured) has an ozone depletion potential of 1, and some bromine-containing chemicals have ozone depletion potentials higher than 10. But when Ravishankara and his colleagues used N2O’s number and took its large emissions (today and projected throughout this century) into consideration, nitrous oxide’s importance emerged.
“We have demonstrated that N2O can be thought of as an ozone-depleting substance in many of the same ways as other gases that are currently regulated by the Montreal Protocol,” Portmann said.
While the emissions of many of the Protocol’s ozone-depleters are dropping, emissions of nitrous oxide are increasing, and N2O’s concentration in the atmosphere continues to rise: It was about 270 parts per billion (ppb) in pre-industrial times, and is roughly 325 ppb today. Though nitrous oxide emission is the single most important ozone-depleting emission by people today, the ozone depletion occurring today is still dominated by the CFCs and other regulated ozone-depleting chemicals that remain in the atmosphere in large amounts because of their long lifetimes and large historical emissions.
“Today’s human-caused N2O emissions represent a larger commitment to future ozone depletion than any other ozone-depleting substance humans are currently emitting,” Daniel concluded. “Soon, human-caused N2O emissions will represent a greater commitment than the anthropogenic emissions of all other ozone-depleting substances combined.”
More on the Web
- A. R. Ravishankara, John S. Daniel, and Robert W. Portmann (2009), Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century, Science, 10.1126/science.1176985.
- Podcast Interview with A. R. Ravishankara from Science