The Major Player in Ozone Depletion

The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer has resulted in the reduction of atmospheric concentrations and emissions of halocarbons – small organic molecules containing halogens such as chlorine or bromine. These halocarbons had been shown to be the leading cause of ozone depletion in the atmosphere and were the primary culprit for the ozone hole above Antarctica. Their reduction has lead to a measurable recovery of the ozone layer in recent years.¹

One such way that this is measured is in terms of ozone depletion potential (ODP). This is a measurement of ozone destroyed in the stratosphere per unit mass of a given chemical species released at the surface of the Earth. These values are normalized to chlorofluorocarbon 11 (CFCl₃), which is one of the predominant halocarbons responsible for ozone depletion. ODP values are critical to policy-making due to their simple description of ozone depletion, however they are quite difficult to determine due to the number of complex variables that must be taken into account. Furthermore, ODP values are dynamic as they depend greatly on the composition of the atmosphere at a given point in time. In addition, to get a better handle of the full effect of a given chemical, the ODP value can be corrected to include the emission history of the chemical in question. This value is referred to as ODP-weighted emission and gives arguable to clearest depiction of the effect a given chemical has on ozone depletion.¹

A 2009 publication by Ravishankara and co-workers in Science gave data to suggest that as a direct result of the Montreal Protocol, nitrous oxide (N₂O) was now the major species of concern in the atmosphere in terms of ozone depletion. Cornelia Dean of The New York Times picked up this information and wrote that there was a “New Culprit Seen in Ozone Depletion”. Dean correctly points out that while a significant portion of atmospheric N₂O is produced by naturally occurring bacteria, human activity in the form of fertilizer, biofuels, and livestock maintenance also produces the harmful gas. However, she is incorrect by suggesting that this is somehow a new issue.² Instead, the Science article clearly gives data saying that in 1987 N₂O had the fourth-highest ODP weighted emissions – significantly higher than many of the halocarbons listed in the Montreal Protocol. While the emissions and concentrations of all halocarbons listed have drastically decreased since then, including the three species with higher ODPs than N₂O, the ODP of N₂O has remained relatively unchanged.¹ As a result, the ozone depletion problem presented by N₂O is not new; it simply seems as though it is being recognized for the first time.

Overall, Dean does a really poor job of representing the work published in the Science article, as she does not give the proper depth necessary to truly represent the primary literature. In addition, she seems to cherry-pick points that are relevant to her narrative and leave out others that do not fit. For example, she comments that halocarbons can react with nitrous oxide to mitigate their ozone depletion capabilities, which the authors do mention in the Science paper, however she neglects their mention of the fact that nitric oxide can react with chlorinated byproducts to free up chlorine radicals, which go on to detrimentally react with ozone. As a result, there are both positive and negative reactions between nitrous oxide related species and halogenated species such that the effect is essentially offset.

The best aspect of Dean’s article comes when she links this knowledge about the ozone layer to the known effect that N₂O has on global warming. In my opinion, this is the key take-away for the general public – that this particular pollutant has a two-fold effect on the Earth’s climate. In addition, she does acknowledge the level of uncertainty that is still present in regards to the origin of atmospheric nitrous oxide. She represents well that policy decisions are well behind scientific understanding. Ultimately she ends her article on a poor note though by summarizing a statement from one of the experts saying that N₂O does not affect the ozone hole above Antarctica due to “unusual atmospheric chemistry”.² This is a completely vague way to end the article and may cause a layperson to wonder why N₂O is even a problem. She could have referenced the primary literature’s discussion of how halocarbons and nitrogen oxides are active in different levels of the stratosphere. I believe this is the main reason for the “unusual atmospheric chemistry”. Otherwise, she would have been much better off leaving out the last sentence entirely.

Overall, I score The New York Times article a 4/10, as there are some good take-away messages for the layperson, however the author cherry-picks to fit her narrative and lacks the necessary depth the fully represent the primary literature. This could easily result in a level of confusion in which the primary message is lost. 

1. Ravishankara et. al., “Nitrous Oxide (N₂O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century,” Science, 2009, 326, 5949, 123-125.

2. Dean C., “New Culprit Seen in Ozone Depletion,” The New York Times, 2009, online: http://www.nytimes.com/2009/08/28/science/earth/28nox.html?_r=0