On screen chemistry with Jonathan Hare
In BBC1 documentary The Climate Wars broadcast last September, the presenter, Iain Stewart, demonstrated that CO2 absorbs infrared (ir) energy and so showed how it can trap heat in the Earth’s atmosphere and contribute to the ‘greenhouse effect’.
The apparatus comprised a 1 m long, 20 cm diameter tube filled with CO2 from a cylinder. A lighted candle was held at one end of the tube while a thermal imaging infrared camera viewed it from the other end. The bright false-colour image of the hot candle on the camera screen slowly disappears as CO2 was introduced into the tube, showing that the gas absorbs in the infrared.
Since I built the apparatus for the programme let me share with you what I learnt about this experiment.
Owing to its molecular rotations and vibrations, CO2 has several absorption bands in the infrared, the main ones being at 4.3 µm (2350 cm–1), 7.5 µm (1388 cm–1) and 15 µm (667 cm–1). The latter band lies very close to the maximum of the Earth’s ir black body emission, making CO2 a very important greenhouse gas.1
The thermal imaging camera we used was sensitive from ca 1 to 5 µm, quite a large part of the infrared spectrum. A lit candle or match produces lots of energy through the infrared to the visible. Consequently, a candle looks very bright (colourful) on the false-colour infrared camera image.
To be able to seal and look through the tube, I covered the ends in cling film. Plastics absorb strongly in the infrared region so it’s hardly as transparent as it looks to the eye but the film was so thin these simple ‘windows’ actually worked quite well in practice. The CO2 is flowed into one end of the tube and vented out the other so that the tube is well flushed with gas at about atmospheric pressure. In the process the thin film windows bulge a little.
A disappearing act
You might think that when you view the candle through the tube using the camera, and you introduce CO2, the bright flame would ‘disappear’, owing to the infrared absorption. However, the flame doesn’t disappear. This is because the CO2 absorptions observable by the infrared camera are weak and are only in a relatively small part of the spectrum.
The only way to get the demonstration to work is to have a ‘CO2 filter’ on the camera, which only lets through infrared at around 4 µm, close to one of the CO2 absorptions (which are broadened a bit at atmospheric pressure). The filter blocks out much of the infrared energy so that the CO2 absorption is not so swamped and this allows us to observe the ‘vanishing candle’.
In another programme on climate change recently broadcast in the US, the same experiment was done using a military state-of-the-art infrared camera and CO2 filter (unfortunately not available for us to use in the UK). It was so sensitive that they used the heat from the presenter’s face rather than a candle to demonstrate the effect.
Originally published in InfoChem
1. Greenhouse gas absorption spectrum in the earth’s atmosphere. www.iitap.iastate.edu/gccourse/forcing/spectrum.html