5 January 2010
A paper authored by A.R. Ravishankara, John Daniel, and Robert Portmann of CSD has made the list of the Nature journal's "top ten" research highlights of 2009.
At year-end, the editors of the journal Nature publish their picks of the top papers published in other journals during the year. The paper by Ravishankara, Daniel, and Portmann garnered the top paper spot in the category of atmospheric science. It was published in Science in October 2009 (Nitrous oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century Science, 2009).
The authors calculate the Ozone Depletion Potential of nitrous oxide, a substance long known to play a role in depleting the stratospheric ozone layer but not included in the Montreal Protocol. The ODP gives a measure of the ability of the substance to destroy ozone, relative to chlorofluorocarbon-11. ODPs have long been calculated for chlorine- and bromine-containing substances, but until this paper the concept had not been applied to a non-halogenated substance such as nitrous oxide.
The authors used the ODP to "weight" the emissions of nitrous oxide, and compared the result to the weighted emissions of other ozone-depleting substances. They found that nitrous oxide now tops the list of anthropogenic ozone-depleting emissions, and will remain the frontrunner for the rest of the century.
Background: Based on the seminal work of Paul Crutzen and Harold Johnston, it has been known for nearly 40 years that nitrogen oxides cause ozone reductions and are involved in maintaining natural levels of ozone. Emission of nitrogen oxides from supersonic transport (aircraft) and the ensuing ozone depletion was the very first reason for the concern about anthropogenic impacts on the ozone layer. It has been known for over 30 years that nitrous oxide (N2O) is the primary source of nitrogen oxides in the stratosphere.
Nitrous oxide (N2O) is produced by both natural and human-related sources, with about one-third of the sources being human-related. Primary human-related sources of N2O are related to agriculture (agricultural soil management, animal manure management); others include sewage treatment, mobile and stationary combustion of fossil fuel, etc. Nitrous oxide is also produced naturally from a wide variety of biological sources in soil and water, particularly microbial action in wet tropical forests. The preindustrial level of N2O was about 270 parts per billion (ppb), and the current level is roughly 325 ppb. Its concentration in the atmosphere continues to increase.
Significance: This study is the first to suggest that nitrous oxide could be considered to be an ozone-depleting substance in the same way as other gases already regulated under the Montreal Protocol. Considered in this light, the paper shows that anthropogenic N2O emissions were the fourth largest emissions even at the height of the chlorofluorocarbon (CFC) emissions in 1987 prior to the Montreal Protocol. It also shows that N2O is now, and is expected to be for the next century, the largest ozone depletion gas emission if the anthropogenic N2O emissions are unabated. The authors note that because N2O is also a greenhouse gas, there would be climate and ozone layer benefits to reductions in N2O emissions – a "win-win" for both ozone and climate.