All posts by mattrigby

35 years of AGAGE measurements

I’ve just returned from the 35th anniversary AGAGE meeting in Boston, MA. I’ve written a blog post for the Cabot Institute, summarising the changes AGAGE has seen during this time:

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

“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.

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.

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.

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 well 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!”

GAUGE kicks off!

On the 1st March, we officially began our new £3m NERC-funded consortium project Greenhouse gAs Uk and Global Emissions (GAUGE). The University of Bristol press release explains:

Scientists awarded grant to determine UK’s greenhouse gas emissions
Press release issued 1 March 2013

Researchers in the University of Bristol’s Atmospheric Chemistry Research Group (ACRG), in collaboration with scientists around the country, have been awarded funding from the Natural Environment Research Council (NERC) to provide an independent ‘top-down’ check on the UK’s greenhouse gas emissions estimates.

GAUGE schematic
GAUGE will involve measurements of greenhouse gases from the air, sea and space in an effort to better quantify UK emissions

The UK is required to estimate how much climate-warming carbon dioxide, methane and nitrous oxide it emits each year. However, at the moment, these estimates rely heavily on so-called ‘bottom-up’ accounting methods that may be subject to biases and inaccuracies.

The GAUGE project (Greenhouse gAs Uk and Global Emissions) is a three and a half year collaboration between several universities and research institutions across the UK.

ACRG’s Professor Simon O’Doherty, who also runs the UK greenhouse gas monitoring network funded by the Department for Energy and Climate Change, said: “It’s important that we expand our greenhouse gas observation capabilities in this country, if we’re really going to understand what we’re emitting. But it’s equally important that we begin exploring new types of measurement, which may help us understand emissions processes more fundamentally.”

GAUGE will bring together a more comprehensive suite of greenhouse gas observations around the UK than has ever been compiled. The project will determine emissions using information from satellites, aircraft, tall towers (including the BT tower in the middle of London), balloons and boats.

In addition to developing new measurements, GAUGE will use computer models to simulate how greenhouse gases travel through the air.

Dr Matt Rigby, a research fellow at ACRG, said: “By measuring the concentration of greenhouse gases in the atmosphere, and then using computer models to simulate where the air came from in the days before the measurements, we can determine emissions from the surrounding areas. This new funding will allow us to develop these methods with the help of the Met Office and GAUGE partners at other universities.”

Using the new measurements and modelling techniques, GAUGE researchers hope to make the UK’s emissions amongst the best-quantified in the world.

Visit to Westminster

On the 29th October, I visited Westminster, where I was shadowing Stephen Williams MP as part of the Royal Society Pairing Scheme. The week was a great opportunity to find out how policy makers get access to scientific research.

The University ran a press release about the event, saying:

Dr Matt Rigby from the University of Bristol will be swapping a lab coat for legislation, when he visits MP Stephen Williams at the House of Commons for a “Week in Westminster” commencing Monday 29 October as part of a unique ‘pairing’ scheme run by the Royal Society – the UK’s national academy of science.

During his visit Dr Rigby will shadow his MP pair and learn about his work, as well as attending a House of Commons Science and Technology Committee meeting and Prime Minister’s Question Time and meeting Professor Sir John Beddington, Government Chief Scientific Advisor. The visit will provide him with a behind-the-scenes insight into how science policy is formed as well as an understanding of the working life of an MP.

Dr Rigby said: “This will be a great opportunity to learn first-hand how science is translated into public policy. We’re often told that we need to get out of the lab and engage with policy-makers, and I think this will be fantastic way to see how we can interact more effectively.”

The Royal Society’s MP-Scientist pairing scheme aims to build bridges between parliamentarians and some of the best scientists in the UK. It is an opportunity for MPs to become better informed about science issues and for scientists to understand how they can influence science policy. Over 200 pairs of scientists and MPs have taken part in the scheme since it was launched in 2001.

Sir Paul Nurse, President of the Royal Society said: “We live in a world facing increasing challenges that can only be addressed with a clear understanding of science. From climate change to influenza outbreaks, GM food to nuclear power, our MPs have to make decisions about complex issues that will affect the lives of all those in the UK and, in many cases, more widely throughout the world. This means that MPs and scientists have a responsibility to engage with each other to get the best possible scientific advice into public policy making.

“We set up the Royal Society’s MP Scientist pairing scheme in 2001 to provide the opportunity for MPs and scientists to build long-term relationships with each other and have now organised over two hundred pairings.

“I know many parliamentarians and scientists who have gained from the scheme, and the shaping of public policy can only improve over time as these relationships continue to grow.”

Using new measurement technology to estimate methane sources

MOZART simulation of 13C-CH4 in the atmosphere. The blue areas show parts of the atmosphere with a smaller fraction of the 13C-CH4 isotope than the atmospheric average, due to near-by emissions from microbial sources. In contrast, the red areas show air that is enriched in 13C-CH4, probably due to biomass burning.

Although methane is the second most important greenhouse gas its sources are quite poorly understood. However, new methods of measuring atmospheric methane may be able to help.

Methane is a molecule containing one carbon and four hydrogen atoms. These atoms usually have an atomic mass unit of 12 (carbon) and 1 (hydrogen). However, they also occur in higher masses in nature, called isotopes. Carbon-13 and deuterium (hydrogen with a mass of 2) occur in one atom for every few thousand atoms of regular carbon or hydrogen. This becomes potentially useful for us, because different sources of methane emit molecules with slightly different ratios of carbon-12 to carbon-13 or different ratios of hydrogen to deuterium. For example, methane emitted from wetlands has less 13C than the average in the atmosphere, whereas wildfires emit methane with a relatively high deuterium content.

So, by measuring methane concentrations and isotope ratios in the atmosphere, we can hope to learn something more about where the methane came from.

In the last few years, advances in laser spectroscopy have meant that we can now measure the isotopic composition of methane by shining lasers through a sample and measuring the absorption of certain wavelengths. However, the variations in methane isotope ratio that we expect to see in the atmosphere are very small. Therefore, to be able to resolve small changes, some people are proposing to also “pre-concentrate” air samples, which means that we remove a lot of the nitrogen, oxygen and other major components of air, to leave a more concentrated sample of methane that can be analyzed. Similar systems exist for measuring concentrations of other gases, but not yet for methane isotopes.

In this paper, we asked the question: “If we had these instruments at each AGAGE station, how much better would we be able to constrain methane emissions from different sources than we can at present?”. The answer we found was a little mixed. We found that these new measurements would provide additional information about the methane emissions to the atmosphere. However, the amount by which the uncertainties in our current estimates of methane emissions would be reduced is a little smaller than we hoped for. For example, for wetlands (the single largest source) and other microbial sources, we found that global uncertainty reduction would be reduced by only around 3%. For smaller sources that had a more “distinct” source isotope ratio such as biomass burning, larger relative uncertainty reductions were possible (9%).

Despite the relatively modest uncertainty reductions, my feeling is that, given the importance of methane in the global climate system, these new instruments will have a role to play in a future methane observing system. Given the complexity of the system, no single measurement (or modeling) strategy will be able to fully determine the causes of the strange changes we see in methane. However, by combining many measurement types, we should be able to understand the system better than we currently do.

Combining two models for emissions estimation

In the AGAGE network, we have a small number of monitoring stations, which measure greenhouse gases at high frequency. I’m interested in using these high-frequency measurements to estimate emissions from the countries surrounding the sites.  To connect the measurements to sources, we require chemical transport models (see some animations here). However, when we use global models, they take a lot of computer time to run, particularly at high resolution, which is needed when we’re trying to estimate emissions on national scales.  Sometimes it makes sense to run a model at very high resolution close to the measurement sites (where we have the most information about emissions) and low resolution everywhere else.  This was the problem we tried to tackle in this paper, co-written by colleagues at the UK Met. Office.

The method we developed takes the output from two different types of model and couples them together so that we could estimate emissions at very high resolution close to the monitoring sites, and low resolution further away.

Average 'footprints' around four AGAGE monitoring sites. Generated by the UK Met. Office NAME model.

We’ve used this method, along with the the Met Office NAME model and NCAR’s MOZART model, to determine SF6 emissions around four AGAGE sites (see the figure), and will be extending it to all the other AGAGE gases in the near future.

The code for the project can be found at Google code:

Four new HFCs

Hydrofluorocarbons (HFCs) are replacements for chlorofluorocarbons (CFCs), whose use is being phased out because they are primarily responsible for depleting the ozone layer. While HFCs don’t destroy ozone, they are often very powerful greenhouse gases, so it is important that we monitor their concentration and emissions. One difficulty in doing this is that there are many HFCs emitted into the atmosphere, and new ones are appearing all the time.

To keep track of these gases, my colleagues in the AGAGE network have developed a system that can measure gas concentrations using mass spectrometry. The system is able to detect gases at very low concentrations, by removing most of the nitrogen and oxygen from the measured samples, increasing the concentration of the pollutants they want to measure. This means that we can now detect ‘new’ gases very soon after they appear in the atmosphere.

In this paper, Martin Vollmer from Empa in Switzerland describes the measurement of four HFCs that have appeared in the atmosphere over he last decade or so (HFC-227ea, HFC-236fa, HFC-245fa, HFC-365mfc). Using a combination of in situ measurements, and new measurements of archived air samples, we can determine the entire air history of the four gases, from the year they first appeared in detectable amounts.

Measured mixing ratios of four new HFCs

Using these observations, and a two-dimensional model of the atmosphere, we calculated the annual global emission rates. As is often the case, the emissions we found differed from inventory estimates by substantial amounts, highlighting the value of these sort of ‘top-down’ verification techniques.

Deriving emissions time series from sparse observations

Over the last couple of years, a few of us at MIT and Scripps have been thinking about how we could estimate trace gas emissions over several decades using archived air samples.  For example, the Cape Grim Air Archive, from Tasmania, is a really nice collection of air samples that have been taken under carefully controlled conditions over many decades.  The archive was set up so that as measurement techniques improved, we could monitor the history of important atmospheric gases that were difficult to measure in the past.  We’ve used these observations to determine global emissions of species like the perfluorocarbons (see Jens Muhle’s paper), and some newly-observed HFCs (in Martin Vollmer’s recent paper), all of which are very potent greenhouse gases.

However, one problem that we kept encountering was that, using our usual methods for determining emissions, we would get large ‘jumps’ in our derived emission, which we knew were unlikely to have occurred in the real world.  These fluctuations were probably caused by slight biases in the observations, or inaccuracies in the independent emissions inventories that we have to use as a first guess.  To get around this problem, we tried to think about how we could derive emissions using information on how we would expect emissions to change from one year to the next.  For example, if a gas is emitted by Aluminium smelters, then emissions are not very likely to change by more than a few percent per year, because the number of smelters, or the emissions processes, are unlikely to change very fast.

We found some techniques that addressed similar problems in other fields (notably Oceanography), and modified them so that they could easily be applied to atmospheric trace gas emissions estimation.  The result is a method that firstly allows you to estimate the emissions growth rate, based on some prior information, and the uncertainty that you expect this estimate to have.  Then a chemical transport model is used to produce an emissions estimate that best matches both the measurements, and our emissions growth constraint.  The paper, written with Anita Ganesan and Ron Prinn, can be found here.

Measurements of C3F8, a powerful greenhouse gas that stays in the atmosphere for thousands of years. Emissions derived using these observations and a growth-based estimation scheme are shown in the bottom panel.

History of Atmospheric SF6

Sulfur hexafluoride (SF6) is a particularly potent greenhouse gas.  It is used in large electrical equipment, and leaks into the atmosphere during maintenance.  Once it is in the atmosphere, it is only destroyed if it reaches very high altitudes, making it last for hundreds to thousands of years.  It is also a very strong absorber of infra-red radiation.  These factors result in it being one of the most potent greenhouses yet discovered.  One tonne of emissions is thought to be equivalent to releasing over 22,000 tonnes of CO2.

Global SF6 emission rate from 1970 to 2008. The solid line shows the estimate using AGAGE measurements.

In collaboration with many of my AGAGE and NOAA colleagues, we examined how concentrations of this gas have increased in the atmosphere since the 1970s, and determined global and regional emissions (see Atmospheric Chemistry and Physics paper).  We find that concentrations of SF6 have increased by more than a factor of 10 since our first measurement in 1973.  We also find that global emissions are now higher than ever, and have increased by almost 50% in the last 5-10 years.

We wanted to find out where this increase in emissions was originating from.  To do this, we used measurements made by the AGAGE and NOAA monitoring networks and a three-dimensional chemical transport model.  The model (called MOZART, developed by the National Center for Atmospheric Research), uses wind speeds and other meteorological information that have been calculated using weather forecasting models, to simulate how pollutants are transported around the world.  By testing how this model responds to changes in emissions from different regions, we can use the measurements to find out where SF6 originated from.

There were two major findings from our work.  Firstly, we find that it is very likely that all of the recent emissions increase is being driven by an increase in emissions from Asian countries that do not report detailed emissions to the United Nations Framework Convention on Climate Change (UNFCCC), such as China, India and South Korea.  Secondly, we find that developed countries that do report emissions to the UNFCCC (e.g. USA, UK, Germany), are likely to be underestimating their emissions.


EDGAR emissions interpolated to 5×5 degrees and scaled in each hemisphere using AGAGE measurements between 1970 – 2008 can be found here in NetCDF format.

Regionally optimized EDGAR emissions at 1.8×1.8 degrees for 2004-2005 and 2006-2008 can be found here in NetCDF format.

Optimized 3D mole fraction fields at 5×5 degrees, and 28 vertical (sigma) levels are here in NetCDF format (10MB).