| Nitrous Oxide: Top Ozone-Depleting Chemical of the 21st Century | |
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The Montreal Protocol (MP) on Substances That Deplete the Ozone Layer (adopted in 1987) was an outgrowth of the Vienna Convention for the Protection of the Ozone Layer (1985). The MP has been very successful at limiting the emissions of chlorine and bromine-containing halocarbons (halide elements combined with carbon) which have been, up until the end of the 20th Century, the dominant ozone-depleting substances (ODS). But in a new NOAA study (Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century, Ravishankara/Daniel/Portman, reported in Science, 2 October, 2009) analyzing and calculating the ozone-depleting potential (ODP) of various GHGs, it was N2O that was found to be the new, leading culprit of ozone (O3) depletion in the upper stratosphere. The ODP is defined as the ratio of O3 destroyed by a unit mass of a chemical at the Earth’s surface to the amount of O3 destroyed by a unit mass of CFC (the ODS standard gauge). Their calculations allowed them to also determine the global warming potential (GWP) of each chemical emission. Nitrogen oxides (NOx; any combination of nitrogen and oxygen), such as those created by the combustion of air and jet fuel, chemically react with ozone (O3) and then go through a short series of transformations, resulting in a single oxygen (O) molecule and one O3 molecule. But due to chemical affinities (oxygen in the air prefers its O2 state), this new O3 is short-lived, and quickly splits up and forms (2) O2 molecules. The main source of these nitrogen oxides is nitrous oxide emitted at the surface (within the troposphere) and which is then transported to the upper stratosphere via air current circulation (this transport usually takes from 4-5 months). N2O is similar to CFC in that both are stable near the surface and are easily transported to the stratosphere, where it is believed that solar radiation makes them chemical unstable and reactive, resulting in the splitting up of O3 molecules. But CFC tends to act most robustly on O3 in the mid latitudes, just below and above the ozone maximum, whereas N2O tends to concentrate and act on the region of the stratosphere just above the highest O3 concentrations, leading to more efficient ozone destruction from NOx. According to the author’s of the study: “Ozone depletion by NOx from N2O dominates the chemical control of ozone in the mid-stratosphere.” It was originally thought that most, if not all, N2O was a natural by-product of decomposition. However, industrial processes that require nitrogen-oxide catalysis have been found to contribute to this chemical’s abundance. Further, the authors assert that “N2O could be an unintended by-product of enhanced crop growth for biofuel production” and even geoengineering strategies such as iron fertilization of the oceans (to increase oceanic algae growth and mitigate CO2 build up). As we are coming to understand, “solutions” for mitigating climate change come with their own side-effects and unintentional consequences. Controlling O3 depletion via limiting ODS emissions is an important step towards the recovery of the ozone layer. The layer blocks much solar radiation from reaching the Earth’s surface. This solar input promotes short wave radiative forcing and is a major contributor to global warming There have been other studies linking N2O with ozone depletion, but this is the first study that has calculated the ODP for nitrous oxide. The authors conclude their report with an historical comparison of the total emission projected to result from the manufacture and continued use of 500 Super Sonic Transport jets (SSTs). “The total increase in stratospheric NOx by that fleet of SSTs is comparable to that from today’s total anthropogenic N2O emissions, indicative of the significance of anthropogenic N2O.” Source: GO Media - Written by Michael Ricciardi - diagram: NASA
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The destruction of the Earth’s protective ozone layer (and the growth of the “hole” in this layer over the South Pole) due to the action of human-made chemicals was the leading environmental issue of the last century (entering the public lexicon sometime in the mid 1980’s), and no doubt prompted wider concerns about “greenhouse” effects and global warming that occupy so much climate science reporting today. The main (or most publicized) culprit of this ozone loss was a chemical called chlorofluorocarbon (CFC). But now, there’s a new leader in ozone destruction: nitrous oxide (N2O, also known as “laughing gas”), and its increasing concentration in the atmosphere is no laughing matter.
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