Nitrous Oxide Threat To Ozone

Nitrous oxide emissions are now the single most important threat to the ozone layer, which protects Earth and its inhabitants from ultraviolet radiation, NOAA scientists report (Science, DOI: 10.1126/science.1176985).

The chemistry of atmospheric N₂O is well-established: It is stable in the lowest level of the atmosphere, the troposphere, where it has a lifetime of about 100 years and acts like a greenhouse gas. When N₂O migrates up to the stratosphere, it is converted to NO, which reacts with O₃ to produce NO₂ and O₂. NO₂ in turn reacts with O to re-form NO.

The new work uses this N₂O chemistry to calculate nitrous oxide’s “ozone-depletion potential,” which compares the O₃ destroyed per unit mass of N₂O and per unit mass of CFCl₃.

The result shows that the ozone-depletion potential of N₂O is comparable with that of several hydrochlorofluorocarbons, the industrial use of which is set to be phased out by 2030 in accordance with an international treaty, the Montreal Protocol on Substances That Deplete the Ozone Layer.

When the NOAA group weighted current atmospheric emissions by their respective ozone-depletion potentials, N₂O emerged as the most critical ozone-depleting substance.

“In the ozone-depletion story of the past few decades, the focus has been solely on chlorine and bromine compounds,” says Martyn Chipperfield, professor of atmospheric chemistry at the University of Leeds, in England. “This paper neatly demonstrates that we are now at a point where N₂O/NO_x is the biggest threat to the ozone layer.” Ironically, atmospheric chlorinated compounds reduce the ozone-depletion effects of N₂O, which should therefore worsen as chlorine levels drop.

Nitrous oxide in the atmosphere comes from both natural and anthropogenic sources, says A. R. Ravi­shankara, who led the NOAA group. About one-third of atmospheric N₂O comes from human activities, in particular agriculture. Scientists estimate that anthropogenic sources of N₂O are increasing at a rate of about 1% per year, Ravishankara says.