Nitrous Oxide: Top Ozone-Depleting Chemical of the 21st Century
The ozone-oxygen cycle in the Earth’s stratosphere
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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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. 
Model of a nitrous oxide molecule’s (N2O) dimensions - Blue spheres are nitrogen atoms, red sphere is oxygen.
Nitrogen oxides (NOx; any combination of nitrogen and one or more oxygen atoms), 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 chemically 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 (the band of the stratosphere where O3 is in its highest concentration), 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 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 five hundred 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.”
* Other fluorocarbons, such as hydroclhlorofluorocarbon (HCFC) and hydrofluorocrabon (HFC) were/are also the target ed for regulation at the time; these O3-destroying chemicals were used primarily for insulation and cooling in industry.
top diagram: NASA
Can you please send the research report or tell me where I can find it.
Thanks
The CFC’s products are forbidden in many countries but now the ozone’s (O3) protection is destructed by the N2O emissions.
I’ve read before that the jet fuel combusted at cruising altitude (42,000 ft) was much worse for the ozone layer than the same amount of fuel done at sea level. I couldn’t tell from this article, but is it saying that isn’t true?
The Author responds:
1st poster: The report on N2O can be found in the October 2, 2009 edition of Science Magazine (Ravishankara et al, ‘Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century’).
2nd poster: Yes, but there are still large “banks” of trapped CFC (and HCFC, HFC) in the form of insulation foam, fire extinguishing equipment (halons) and additional medical uses (CFC). These sources–not yet emitted into the atmosphere–are considered “non cost-effective” to recover.
3rd poster: What the article is saying, and what the paper seems to say, is that the ODP of a chemical (here N2O or NOx) depends upon its proximity to the Ozone maximum band of the stratosphere. Even though CFC has a much higher ODP than N2O, due to where it tends to concentrate in the atmosphere (above and below the O3 max., not just above it/adjacent to it, like NOx chemicals), it’s impact on O3 depletion is less. 42K feet is still within the troposphere, so, I would say that jet fuel combustion at this level is probably more harmful than lower altitude combustion, but not as harmful to the ozone layer as if it were dispersed even higher up. Still, NOx in the atmosphere can be carried up to higher altitudes, so, the higher up the emissions of NOx, the more likely they are to be up-circulated nearer to the ozone maximum.
Note: increased sulfate aerosols in the stratosphere (due to volcanic eruptions) were found to decrease the effectiveness of N2O in destroying O3, but also increase the ODP effectiveness of CFCs (Mickley et al)…which is another one of those geo-engineering ‘trade-offs’ So, it’s a good thing that most industrial nations have banned CFCs (and halocarbon chlorine/bromine compounds).
NO2 is reported to form downtown in the western cities during very hot days of summer.
I am confused on how such a heavy molecule of N2O can go up to the Ozone layers that are hundred Km above the surface.
Could the author of the article explain how this happens ??
Thanks
“Heavy” is a relative term when one is speaking about molecules. Fluorocarbons are also “heavy” relative to methane and CO2, etc., yet they manage to be transported to regions above the ozone maximum.
My guess is that upward circulation of the air (from long-wave radiative forcing, or, due to heat agitation from evaporative processes) does the trick. Plus, there are normal, ever-present currents in the atmosphere that can move molecules higher into the stratosphere. Note that it takes 4-5 months for this “up-circulation” to occur.