The truth is out there: Atmospheric forensics

Helium balloons

The supporting balloon (left) and the main balloon (right)

9 December 2011 by Johannes Laube

Just when you thought CFCs had gone away, new ozone-depleting compounds and greenhouse gases are turning up in the atmosphere. Air detective Johannes Laube used helium balloons and a former Russian spy plane to find them.

My life as an organic chemist took an exciting turn in 2005 when I started a PhD at the University of Frankfurt, researching halocarbons in the atmosphere. I was thrilled by this opportunity, but little did I know how far into the unknown my research would take me.

Halocarbons are compounds of carbon and halogens (fluorine, chlorine, bromine and iodine), and include the notorious chlorofluorocarbons (CFCs) once used as coolants and propellants.

They are found in the atmosphere in tiny amounts - usually less than one in a billion air molecules. But they can have large environmental impacts - notably the massive ozone hole that appears over the poles every year.

My main interest was in halocarbon distributions in the stratosphere, which is between about 10 and 50 kilometres up (where the ozone loss occurs). I had a special focus on the tropics, where most of the air enters the stratosphere before being transported pole-wards.

Despite its significance, there are still very few observations of halocarbons in this region - it's just not easy to get to. The stratosphere is at least 14 kilometres up at the tropics, which means most aircraft can only just touch it.

Close-up of an air sampler

Close-up of an air sampler ready for take-off

One of the few options is to use the large helium balloons launched by the French space agency CNES. Measuring up to 100 metres across, these balloons can carry gondolas weighing up to a ton to altitudes of about 35 kilometres - perfect for sampling stratospheric air.

Filling some canisters with air might sound easy, but retrieving them is not. Even though the balloon comes down on parachutes it reaches up to 20mph as it descends and often crashes into trees (it could easily crush a roof).

As outside pressure drops to below a hundredth (compared to sea level) and temperatures reach -80°C, an air sampler has to be quite robust, and to bring back a big enough sample the air has to be frozen using liquid neon.

I analysed air samples from two balloon flights launched near Teresina, which is at the heart of the tropics. It actually claims to be the hottest city (temperature-wise, that is) in Brazil.

Working with researchers at the University of East Anglia (UEA) I found and measured 28 different ozone-depleting halocarbons that had reached the stratosphere; a degree of completeness which had not been achieved before.

To our surprise, we also found several additional 'signals' - signs that there were unidentified halocarbons in the stratosphere making an unknown contribution to ozone depletion. UEA already had expertise in detecting new trace gases in the atmosphere, so this was the place to go to investigate.

Fate and halocarbons

While I was writing a proposal for funding for this next piece of research, fate struck again. To demonstrate that there are lots of unidentified halocarbons in the atmosphere, I collected a sample at the local observatory in the Taunus Mountains near Frankfurt.

At first sight, its halocarbon content looked very strange. There were several compounds which I had never seen before, and their concentrations were above one part per billion which is very rare even for known halocarbons. It soon became clear that we had caught a plume of pollution from the densely populated Rhine-Main area.

The compounds we were seeing belonged to a new type of CFC. CFCs are usually inert in the atmosphere - they don't react with other chemicals - which is why vast quantities of them reach the stratosphere. They are also non-toxic.

This new type was quite the opposite. It contained a double bond which made it quite reactive, and was also very toxic. Several days of reading existing research revealed that these compounds can be formed by burning certain types of plastic, and further air samples confirmed that they are widespread, though at much smaller concentrations than I'd found in the plume. They were the perfect ingredient for my proposal.

By the end of 2008 I had started my new role as an 'air detective' at UEA, searching for unidentified halocarbons in the atmosphere, and especially strong greenhouse gases or ozone destroyers.

I looked at air samples from all over the world - taken from passenger aircraft above Europe and Africa, from balloons in the stratosphere, and from a high-flying research aircraft (formerly a Russian spy plane). I also consulted the archives - a man-made archive of samples collected at a station with very clean air in Tasmania, Australia, and the natural archives of air trapped in deep snow in Antarctica and Greenland.

A balloon gondola

A balloon gondola before launch

It does ruin the surprise a bit, but of course I would not have analysed that many samples if there was nothing new. So yes, there are plenty more (but so far minor) greenhouse gases in the atmosphere and many of them are increasing in concentration.

Some of the gases I found are known by-products of industrial processes, so it wasn't a great surprise to see them in the samples. But I also discovered unexpected compounds, including man-made, ozone-depleting CFCs.

The increase in these compounds is especially worrying as this is very likely to be in violation of what is probably the most successful international treaty ever signed, the Montreal Protocol, which led to a ban on ozone-depleting substances.

One example of a newly discovered greenhouse gas is 2H-heptafluoropropane, or HFC-227ea as we call it. It's a replacement for the banished CFCs, used for instance in fire extinguishers and as a propellant in asthma inhalers.

Unfortunately it is also a very strong greenhouse gas, several thousand times more powerful than carbon dioxide. It is still not very abundant but emissions are growing, which is worrying, if not unexpected. HFC-227ea is not covered by the Montreal Protocol.

In the end I was finding so many new halocarbons that I had to stop my search and admit I had reached my limit for a three-year research fellowship. So my main conclusion has to be that there is a very large number of unknown fluoro-, chloro-, bromo- and iodocarbons in the atmosphere, and industry is introducing new compounds at tremendous speed.

It's important we identify them and understand what their effects might be, and I hope I'm lucky enough to stay in this field for a while longer; it doesn't look like 'atmospheric forensics' will get boring any time soon.

Dr Johannes Laube is an atmospheric chemist in the school of environmental sciences at the University of East Anglia.