gases – Cabot Institute for the Environment blog

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You may have noticed a story reported on widely recently on the discovery of 4 ‘new’ man-made ozone-depleting gases. This follows the publication of a study in the journal Nature Geoscience on the first measurements of these gases, their abundances in the atmosphere and estimated global emission rates. Responses to the reporting of this publication have ranged from the Daily Mail’s “Ozone Crisis” to the inevitable internet-based diatribe of “any research from UEA is clearly made up” in various comment sections. So just how concerned should we be about the emissions of these four gases?

Chlorofluorcarbons (CFCs)

The reason we care about CFCs is because they deplete ozone high up in the atmosphere, potentially exposing humans to harmful UV rays. Oh, they also happen to be extremely potent greenhouse gases, with each molecule of a CFC being equivalent to 1000s of molecules of CO₂, and they sit around in the atmosphere for 10s or 100s of years before being removed. Basically they’re pretty bad, and sure they might have been great refrigerants and aerosol propellants but at what cost?

The production of CFCs has now to all intents and purposes ceased, although that doesn’t mean that emissions have completely stopped; various banks of these gases exist in fridges for example. These might leak during use or when destroyed. So it’s not entirely surprising to read that this study has found that various CFCs are still being released.

Newly measured

In fact the reason this paper is important is more to do with the fact that these gases have never before been measured. Many of the media articles seem to lead with the fact these are ‘new’ ozone-depleting gases, which is a little misleading. They’re not new; they’ve been around for decades, only nobody has been able to measure them in the atmosphere before. Why’s that you might ask? Well much of it is to do with just how small their concentrations are in the atmosphere.

The fact of the matter is that the concentrations of these gases (CFC-112, CFC-112a, CFC-113a, HCFC-133a) are tiny. All four have atmospheric mixing ratios of less than 1 part per trillion (ppt). In other words, if you could isolate a trillion molecules of air (1 x 10¹²) then not even one of them would be one of these ’new’ CFCs. By contrast CO₂ in the atmosphere has a mixing ratio of hundreds of parts per million.

Compare these newly measured gases to the major CFCs (CFC-11, CFC-12, CFC-113) whose current atmospheric concentrations are hundreds if not thousands of times greater. Even though emissions of these major CFCs are now close to zero they will still be around in the atmosphere at these elevated concentrations for decades to come. This is shown in the plot below taken from the AGAGE network measurements of CFC-12. Although the concentration has reached a peak it will take at least one hundred years for levels to get back down to pre-1980 levels, with the current mixing ratio still over 500 ppt.

Plot taken from the AGAGE network measurements of CFC-12

So emissions of these newly measured gases would have to really pick up for a sustained period of time to add significantly to the ozone-depleting effect of what is already in the atmosphere. To say the measurement of these compounds has created some sort of ozone crisis is therefore a gross exaggeration. That’s not to say that this work was a waste of time; it’s vital that we know about these compounds and their atmospheric abundance so we can ensure their contribution to ozone depletion remains negligible.

Other factors influencing ozone recovery

There are other potentially more important causes for concern as well. Hydrochlorofluorocarbons (HCFCs) were introduced as replacements for CFCs but also contribute to ozone depletion, albeit in a less effective way. Although these are also being phased out many of these will have a greater impact on the recovery of the ozone ‘hole’ than these newly measured species. Just a few months ago the United Nations Environment Programme (UNEP) released a report saying another gas, Nitrous Oxide (N₂O), is now considered to be the biggest threat to the ozone layer over the next 50 years. Not to mention that one of the impacts of a rise in global surface temperatures could be a slowing in ozone hole recovery. There’s a genuinely interesting (honest!) explanation for why that is which I will cover in another blog.

The point is that there are lots of factors which affect the Earth’s ozone layer. Studies like the one recently published in Nature Geoscience are vital for our understanding of what the recent and current atmospheric composition is like. It might not be a problem now, but surely the key to looking after our planet, and ourselves, is to prevent things from becoming problematic in future. If we can take steps to find out where these emissions are coming from and why some of them are increasing then measures could be put in place to limit their future influence on ozone recovery.

This blog is written by Mark Lunt, Atmospheric Chemistry Reseach Group, Cabot Institute, University of Bristol, .

Mark Lunt

I work on an experiment that began when the Bee Gees’ Stayin’ Alive was at the top of the charts. The project is called AGAGE, the Advanced Global Atmospheric Gases Experiment, and I’m here in Boston, Massachusetts celebrating its 35-year anniversary. AGAGE began life in 1978 as the Atmospheric Lifetimes Experiment, ALE, and has been making high-frequency, high-precision measurements of atmospheric trace gases ever since.

At the time of its inception, the world had suddenly become aware of the potential dangers associated with CFCs (chlorofluorocarbons). What were previously thought to be harmless refrigerants and aerosol propellants were found to have a damaging influence on stratospheric ozone, which protects us from harmful ultraviolet radiation. The discovery of this ozone-depletion process was made by Mario Molina and F. Sherwood Rowland, for which they, and Paul Crutzen, won the Nobel Prize in Chemistry in 1995. However, Molina and Rowland were not sure how long CFCs would persist in the atmosphere, and so ALE, under the leadership of Prof. Ron Prinn (MIT) and collaborators around the world, was devised to test whether we’d be burdened with CFCs in our atmosphere for years, decades or centuries.

Fig 1. The AGAGE network

ALE monitored the concentration of CFCs, and other ozone depleting substances, at five sites chosen for their relatively “unpolluted” air (including the west coast of Ireland station which is now run by Prof. Simon O’Doherty here at the University of Bristol). The idea was that if we could measure the increasing concentration of these gases in the air, then, when combined with estimates of the global emission rate, we would be able to determine how rapidly natural processes in the atmosphere were removing them.

Fig 2. Mace Head station on the West coast of Ireland

Thanks in part to these measurements, we now know that CFCs will only be removed from the atmosphere over tens to hundreds of years, meaning that the recovery of stratospheric ozone and the famous ozone “hole” will take several generations. However, over the years, ALE, and now AGAGE, have identified a more positive story relating to atmospheric CFCs: the effectiveness of international agreements to limit gas emissions.

The Montreal Protocol on Substances that Deplete the Ozone Layer was agreed upon after the problems associated with CFCs were recognised. It was agreed that CFC use would be phased-out in developed countries first, and developing countries after a delay of a few years. The effects were seen very rapidly. For some of the shorter-lived compounds, such as methyl chloroform (shown in the figure), AGAGE measurements show that global concentrations began to drop within 5 years of the 1987 ratification of the Protocol.

Figure 3. Concentrations of methyl chloroform, a substance banned under the Montreal Protocol, measured at four AGAGE stations.

Over time, the focus of AGAGE has shifted. As the most severe consequences of stratospheric ozone depletion look like they’ve been avoided, we’re now more acutely aware of the impact of “greenhouse” gases on the Earth’s climate. In response, AGAGE has developed new techniques that can measure over 40 compounds that are warming the surface of the planet. These measurements are showing some remarkable things, such as the rapid growth of HFCs, which are replacements for CFCs that have an unfortunate global-warming side effect, or the strange fluctuations in atmospheric methane concentrations, which looked like they’d plateaued in 1999, but are now growing rapidly again.

The meeting of AGAGE team members this year has been a reminder of how important this type of meticulous long-term monitoring is. It’s also a great example of international scientific collaboration, with representatives attending from the USA, UK, South Korea, Australia, Switzerland, Norway and Italy. Without the remarkable record that these scientists have compiled, we’d be much less informed about the changing composition of the atmosphere, more unsure about the lifetimes of CFCs and other ozone depleting substances, and unclear as to the exact concentrations and emissions rates of some potent greenhouse gases. I’m looking forward to the insights we’ll gain from the next 35 years of AGAGE measurements!This blog was written by Dr Matt Rigby, Atmospheric Chemistry Research Group, University of Bristol.

Matt Rigby

Tag: gases

‘New’ man-made gases: Ozone crisis or hoax?

Chlorofluorcarbons (CFCs)

Newly measured

Other factors influencing ozone recovery

35 years monitoring the changing composition of our atmosphere