Nitrous oxide is now top ozone-layer damaging emission

Many ozone-depleting substances (ODS) have been phased out as a result of the Montreal Protocol and are regulated by EU legislation. N2O is emitted from natural and anthropogenic sources, the latter including as a by product of agricultural fertiliser use and from fossil fuel combustion. Its role in ozone depletion has been known for some years and it is similar to CFCs in that it is stable when it is near the earth's surface but releases ozone-destroying active chemicals when transported into the stratosphere - between 10 and 50 km from the surface.

However, N2O is not defined as an ODS under the Montreal Protocol and, although it is a greenhouse gas (GHG) included in the basket of gases under the Kyoto Protocol, its emissions remain unregulated.

The study was the first to calculate the ozone-depleting potential (ODP) of N2O. The ODP is a measure of the amount of stratospheric ozone destroyed by a chemical relative to the amount destroyed by CFC-11, which is one of the most harmful ODS. Using a well-established methodology, the study calculated the ODP of N2O under current atmospheric conditions to be 0.017. This value is comparable to the ODP of many hydro chlorofluorocarbons that are currently being phased out under the Montreal Protocol, such as HCFC-123 and HCFC-124.

Although the ODP of N2O is just one sixtieth that of CFC-11, it is the most important anthropogenic ODS emission today because of the large amounts that are released into the atmosphere. Current global emissions of anthropogenic N2O are approximately
10 million metric tonnes per year, in comparison with annual emissions of one million metric tonnes of CFCs at the peak of their production in the 1980s.

These emissions of N2O also have a significant impact on the climate. Of all the non-CO2 greenhouse gases included under the Kyoto Protocol, N2O emissions have the second most powerful impact on the climate after methane.

Recent estimates of the effects of current mitigation policies indicate that levels of N2O emissions are likely to remain stable for the foreseeable future. The study suggests that both the ozone layer and the climate would benefit from reductions in N2O emissions. Strategies for reducing N2O emissions could include more efficient use of fertiliser on cropland and the capture and destruction of N2O from industrial processes, such as the manufacture of nitric acid.

Original source: Ravishankara, A.R., Daniel, J.S. & Portmann, R.W. (2009). Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century. Science. 326:123-125.

Source: EU commission

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