Measuring methane
MethaneSAT scientists Dr Sara Mikaloff-Fletcher and Professor David Noone provide insights into how they can ‘see’ invisible greenhouse gases like methane. David uses the analogy of molecules having unique vibrational dance moves with specific frequencies of light exciting the molecules.
Jargon alert:
Infrared light: occurs between the red end of the visible light spectrum (light humans can see) and microwaves. All things over a certain temperature (absolute zero) absorb and emit infrared radiation.
Spectrometer: an instrument that collects information about the different parts of the electromagnetic spectrum. An infrared spectrometer detects frequencies of infrared light that are absorbed by a molecule.
Questions for discussion:
David mentions that methane molecules are sensitive to a specific infrared ‘colour’. What are two methods scientists can use to identify and measure the amount methane being emitted?
Where would be an ideal place to make methane measurements? Why?
Take up the challenge
Can you create a molecule dance?
Which molecule would you choose?
What would your moves look like?
Can identical molecules copy your dance moves?
Can people acting as spectrometers identify which molecule you are by your dance moves?
Transcript
Dr Sara Mikaloff-Fletcher
Principal Scientist (Carbon, Chemistry and Climate), NIWA Science Leader, MethaneSAT
You can’t see greenhouse gases with your eyes, but greenhouse gases do influence infrared light, which we experience not as visible light but actually as heat.
Professor David Noone
Buckley-Glavish Professor of Climate Physics, Department of Physics, University of Auckland
There are very specific colours or frequencies of light that methane absorbs, and it comes with this idea that the molecules dance in different ways and each one of those frequencies is different.
Water is a very simple molecule – H2O. It’s got an O and two Hs hanging off it. And we can imagine that, like your body – I’m the O with little Hs hanging off – and you can imagine there’s different ways I could move. I could vibrate like this or I could vibrate like this or I could wiggle in certain ways. And the way I do that, there’s certain frequencies. I do this with a certain speed, and that’s similar to the way these measurements are made – there’s a certain frequency of light that excites these molecules.
Methane is CH4, and again it’s got a certain shape that vibrates and rotates at certain frequencies.
I am very fond of water. I think water is the most groovy molecule – it’s a little bit asymmetric – so it has some interesting ways that it can wiggle and move around.
Methane is really quite regular. It’s got a stinking big carbon in the middle with four Hs, so it has in some ways a lot more regular ways to move. And so you can build instruments that detect the frequencies of water and methane separately, because they’re very distinct, different frequencies.
You can use that basic physical principle to make different types of instruments called infrared Fourier-transform spectrometers that effectively look at the Sun. And what do they do? They’re spectrometers. All that says is the light is broken up into its composite colours, so like a rainbow. This idea is measuring the different wavelengths or the different colours associated with different gases. There’s a certain colour that methane is connected to.
Another type of instrument is a laser-based system. These are often benchtop instruments that we can put onto certain platforms like aircraft or meteorological weather stations. Lasers are very special in that they have a single frequency – a red laser pointer is red, a green laser pointer is green. You can choose a very specific colour laser that happens to be the colour that methane molecules absorb, and so these laser-based spectrometers choose that one wavelength and just go and hunt down the methane molecules and allow the methane molecules to absorb the light, and therefore we can make a measurement of how much methane is there.
The measurement strategy that you need to adopt for any scientific experiment has to really understand the purpose of those measurements. And so we’re able to utilise the aircraft measurements and other types of measurements we’re making to answer several different questions at once by choosing the most ideal places to do those measurements and when to do those measurements.
Acknowledgements Professor David Noone, University of Auckland Dr Sara Mikaloff-Fletcher, NIWA Methane absorption spectrum, Benjamin J Burger, CC BY-SA 4.0 Bruker EM-27 SUN Fourier-transform spectrometer, lasers in night sky at Lauder, NIWA Scientists fixing instruments in plane and gulf airstream plane, Permian Basin methane mapping project with Scientific Aviation and the University of Wyoming, small aircraft collecting data, all courtesy of MethaneSAT and the Environmental Defense Fund (EDF)
