NOAA Study Identifies Nitrous Oxide as Top Ozone-Depleting Emission in 21st Century
28 August 2009
| Comparison of annual N2O ODP-weighted emissions from the 1990s with emissions of other ODS in 1987 and in 2008. Even at the height of ODS emissions in the 1980s, annual anthropogenic N2O emissions were the fourth most significant. Currently, anthropogenic N2O emissions represent the largest contribution to ozone depletion. Source: Ravishankara et al. 2009. Click to enlarge. |
Nitrous oxide (N2O) has become the most important ozone-depleting substance (ODS) emitted through human activities, and is expected to remain the largest throughout the 21st century, according to a new study by scientists at the National Oceanic and Atmospheric Administration (NOAA). N2O is unregulated by the Montreal Protocol.
The study, published online 27 August in the journal Science, for the first time evaluated nitrous oxide emissions from human activities in terms of their potential impact on Earth’s ozone layer. As chlorofluorocarbons (CFCs), which have been phased out by international agreement, ebb in the atmosphere, nitrous oxide will remain a significant ozone-destroyer, the study found.
Though the role of nitrous oxide in ozone depletion has been known for several decades, the new study is the first to explicitly calculate that role using the same measures that have been applied to CFCs, halons and other chlorine- and bromine-containing ozone-depleting substances. Nitrous oxide is also a greenhouse gas, so reducing its emission from manmade sources would be good for both the ozone layer and climate, the scientists said.
Nitrous oxide is emitted from both natural and anthropogenic sources; the latter accounts for about one-third of global N2O emissions. In addition to soil fertilization, nitrous oxide is emitted from livestock manure, sewage treatment, combustion and other stationary and mobile sources.
The US Environmental Protection Agency reports that of the US anthropogenic N2O emissions (311.9 TgCO2 equivalents in 2007), the predominant source was agricultural soil management (207.9 TgCO2e, 67%), followed by mobile sources (30.1 TgCO2e, 9.7%). The transportation end-use sector accounted for 67% of N2O emissions from fossil fuel combustion in 2007.
In nature, bacteria in soil and the oceans break down nitrogen-containing compounds, releasing nitrous oxide. About one-third of global nitrous oxide emissions are from human activities.
Nitrous oxide, like CFCs, is stable when emitted at ground level, but breaks down when it reaches the stratosphere to form other gases (nitrogen oxides) that trigger ozone-destroying reactions. Today, nitrous oxide emissions from human activities are more than twice as high as the next leading ozone-depleting gas.
Limiting future N2O emissions would enhance the recovery of the ozone layer from its depleted state, and would also reduce the anthropogenic forcing of the climate system, representing a ‘win-win’ for both ozone and climate.
—Ravishankara, et al. 2009
The team, A.R. Ravishankara, J.S. Daniel and Robert W. Portmann of the NOAA Earth System Research Laboratory (ESRL) chemical sciences division, calculated the ozone depleting potential (ODP) of N2O to be 0.017. This value is comparable to the ODPs of many HCFCs such as HCFC-123 (0.02), -124 (0.022), -225ca (0.025), and -225cb (0.033) that are currently being phased out under the Montreal Protocol.
Even though N2O’s ODP is only 0.017, roughly one sixtieth of CFC-11’s, the large anthropogenic emissions of N2O more than make up for its small ODP, making anthropogenic N2O emissions the single most important of the anthropogenic ODS emissions today.
—Ravishankara, et al. 2009
The ozone layer serves to shield plants, animals and people from excessive ultraviolet light from the sun. Thinning of the ozone layer allows more ultraviolet light to reach the Earth’s surface where it can damage crops and aquatic life and harm human health.
Resources
A. R. Ravishankara, John S. Daniel, Robert W. Portmann (2009) Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century. Science doi: 10.1126/science.1176985
Stop burning things!
Posted by: Ben | 28 August 2009 at 07:22 AM